AUTHORS

William B Harrison III Department ofGeosciences Western Michigan UniversityKalamazoo Michigan 49008harrisonwmichedu

William Harrison is a Professor Emeritus andcurator at the Michigan Geological Repositoryfor Research and Education (MGRRE) part of theDepartment of Geosciences at Western MichiganUniversity He received his PhD from the Uni-versity of Cincinnati in 1974 He joined thefaculty at Western Michigan University in 1973and founded MGRRE in 1982 His research in-

Reservoir characteristics ofthe Bass Islands dolomite inOtsego County MichiganResults for a saline reservoirCO2 sequestration demonstrationWilliam B Harrison III G Michael Grammer andDavid A Barnes

terests are stratigraphy subsurface and petro-leum geology of the Michigan Basin

G Michael Grammer Department ofGeosciences Western Michigan UniversityKalamazoo Michigan 49008

Michael Grammer is an associate professorand research scientist at the Michigan Geologi-cal Repository for Research and Education(MGRRE) part of the Department of Geo-sciences at Western Michigan University Hereceived his PhD from the University of Miamiin 1991 He joined the faculty at Western Mich-igan University in 2002 His research interestsare in carbonate sedimentology and sequencestratigraphy

David A Barnes Department of Geo-sciences Western Michigan University Kala-mazoo Michigan 49008

David Barnes is a professor and research sci-entist at the Michigan Geological Repository forResearch and Education (MGRRE) part of theDepartment of Geosciences at Western Michi-gan University He received his PhD from theUniversity of California Santa Barbara in 1982He joined the faculty at Western MichiganUniversity in 1986 His research interests are inclastic sedimentology and diagenesis andgeological carbon sequestration

ACKNOWLEDGEMENTS

The authors thank Kristen Carter Paul Daniels

ABSTRACT

As part of a phase II plan to understand test and evaluate the CO2

sequestration potential for deep saline reservoirs inMichigan a dem-onstration test well was completed in late 2006 in Otsego Countynorthern lower Michigan The well was drilled to 3630 ft (1006 m)and open-hole logged Selected conventional cores totaling 180 ft(55 m) were taken in the saline reservoir (Bass Islands Formation)the immediately overlying confining unit (Bois Blanc Formation)and the overlying seal (Amherstberg Formation) Additionally 24sidewall cores were taken in several uphole formations The wholecore was sampled every foot by drilling 2-in (5-cm)-long and 1-in(25-cm)-diameter test plugs for porosity and permeability (PampP)analyses Seventy-four horizontal plugs 12 vertical plugs 6 wholecores and 17 sidewall core plugs were sent to Core Laboratoriesfor routine PampP analyses Fifteen blue-dyed epoxy-impregnatedthin sections were made from selected PampP plugs The whole corewas slabbed for examination and description of lithology sedimen-tary structures and facies characteristics This Upper Silurian andLowerndashMiddle Devonian stratigraphic section was carefully exam-ined for lithology and facies characteristics that relate to reservoir andseal properties pertinent to CO2 sequestration The overlying primaryseal (Amherstberg Formation) is a low-porosity low-permeabilitylimestone that is highly fossiliferous and densely cemented with cal-cite and chalcedony This unit is the ultimate vertical barrier to thevertical migration of fluids The immediately overlying confiningunit (Bois Blanc Formation) is a very cherty limestone and dolostonewith moderate porosity and low permeability Some fluids maymove into this unit but very low permeability will severely restrictthe vertical flow Thin sections show abundant microporosity The

and Eric Venteris for their thoughtful reviews ofthis manuscript We also thank our colleaguesat the Midwest Regional Carbon SequestrationPartnership and Battelle Memorial for intellec-tual and financial support during this project

Copyright copy2009 The American Association of Petroleum GeologistsDivision of EnvironmentalGeosciences All rights reservedDOI101306eg05080909011

Environmental Geosciences v 16 no 3 (September 2009) pp 139ndash 151 139

Stephen Kelley provided yeoman service inslabbing core and drilling core plugs for porosityand permeability analyses Thanks to LindaHarrison for the high-resolution core photo-graphs Robert G Mannes and Allen Modroo atCore Energy LLC have provided timely discus-sions about the geology in and around the StCharlton 4-30 well The Michigan GeologicalRepository for Research and Education hasprovided research space and facilities for thestudy of the samples and data

140 Reservoir Characteristics of the Bass Islands

target saline reservoir interval (Bass Islands Formation) is a variablyporous and permeable dolostone composed of several tidal flat cyclicpackages The Bass Islands Formation has a gross thickness of 70 ft(21m)with a reservoir interval composed ofmore than 40 ft (12m) ofgreater than 10 porosity and permeability zones exceeding 500 mdAverage porosity over the entire Bass Islands is 125 Average per-meability is 224 md The CO2 injection tests using the Bass Islandssection were completed during February and March 2008 Analysisof the Bass Islands Formation in northernMichigan indicated excellentreservoir quality for injection and storage of CO2 and high-qualitysealing units to prevent vertical migration Monitoring well data con-ducted during and after the injection test validates preinjection reser-voir simulation modeling performed at Battelle Pacific NorthwestLabs using well data and rock observations from this study

INTRODUCTION

As part of a phase II geological study and pilot CO2 injection testprogram the Bass Islands Formation in northern lower Michiganwas evaluated as a possible candidate for geological carbon seques-tration The Bass Islands Formation has long been recognized inMich-igan as a widespread brine-filled carbonate unit that caps the Silurianstratigraphic section (Lilienthal 1978) It is overlain and truncated inplaces by the major interregional unconformity at the Tippecanoe-Kaskaskia sequence boundary (Sloss 1963) Although the Bass Is-lands Formation is mostly known from the subsurface in Michiganscattered outcrops of the Raisin River Dolomite member are ob-served in southeastern Michigan and the St Ignace Dolomite mem-ber near the Straits of Mackinac in northern lower Michigan andMichiganrsquos upper peninsula Most of the data about the Bass IslandsFormation have been derived from well data acquired by the drillingof oil and gas wells throughout the Michigan Basin Most wells havewireline logs that record various lithologic and fluid propertiesalthough it is a common practice for drillers to set casing a few feetinto an anhydritic bed in the Bass Islands Formation Many of thelogs therefore only comprise gamma-ray and cased-hole neutronporosity logs A few open-hole wireline logs through the Bass IslandsFormation exist and they are the most useful in estimating reservoirproperties (Barnes et al in press) A few wells have collected sets ofdrill cuttings so the lithology interpreted from logs can be confirmedAlthough wireline logs and drill cuttings can provide general litho-logic and porosity properties rock core samples can provide additionalcritical data for reservoir evaluation Facies characteristics and rock per-meability are only satisfactorily determined from core samples Veryfew core samples from the Bass Islands Formation throughout theMichigan Basin exist however Gardner (1974) reported core froma part of the Bass Islands Formation in two wells the Sun-Bradley 4Newaygo County Sec 11-T12N-R13W and the Sun-Mancelona 1Antrim County Sec 36-T29N-R5W

Dolomite in Otsego County Michigan

InNovember 2006 an important corewas collectedas part of the Midwest Regional Carbon SequestrationPartnership phase II research program This particular sitewas chosen because it coincided with an ongoing CO2

enhanced oil recovery (EOR) project owned by CoreEnergy LLC This EOR project takes high-purity CO2

separated from produced Devonian Antrim Shale gas atnearby gas processing plants compresses it to approxi-mately 1100 psi (77 kgcm) and pumps it along an 11-mi(18-km)-long pipeline to a series of Silurian NiagaranPinnacle reefs (Grammer et al in press) This researchproject and injection experiment was able to take ad-vantage of the existing infrastructure of the EOR activityand acquire core and logs from the Bass Islands Forma-tion during drilling of a new well for EOR operations TheCore Energy-State Charlton 4-30 well API permit num-ber 21-137-57916-0000 (Figure 1) was drilled in OtsegoCounty of northern lowerMichigan as an additional EORoperation well in a Niagaran reef oil field The well wasinitially drilled and logged to a depth of 3630 ft (1006m)then deepened to the Silurian Niagaran reef zone for fu-ture use in the ongoing EOR activities The shallower partof thiswell is currently being used as a field demonstrationwell to sequester anthropogenic CO2 in the Bass IslandsFormation a deep saline reservoir In early 2008 slightlymore than 10000 tons of CO2 was injected into the BassIslands Formation Additional injections of up to 50000tons CO2 are being completed during the spring and sum-mer of 2009

This studywas undertaken to delineate site-specificlithologic and reservoir properties for the Bass Islands inthe area of the test well and to use the detailed core-to-log correlations to extrapolate reservoir characteristicsand CO2 storage capacity estimates to nearby areas ofthe Michigan Basin using wireline-log data along withvery limited core and sample data Applying a se-quence stratigraphic approach further provides verticaland lateral predictability in the reservoir

Regional Setting

The Michigan Basin is a cratonic basin located on thesouthern margin of the Canadian shield and centeredon the lower peninsula ofMichigan The basin hasmorethan 16000 ft (4877m) of Paleozoic sediments in thecenter and thins dramatically toward themargins It alsooccupies parts of eastern Wisconsin southwestern On-tario Canada northwesternOhio northeastern Illinoisand northern Indiana and coversmore than 100000mi2

(160000 km2) (Catacosinos et al 1991) There have

been more than 58000 wells (Figure 1) drilled for oiland gas exploration and development since the 1920sCumulative production from all formations exceeds125 billion bbl of oil and 65 tcf of natural gas Althoughmore than one-third of the wells drilled in Michiganhave produced hydrocarbons many others have en-countered porous and permeable subsurface forma-tions that contain only brine with salinity values up to400000 ppm (40) These saline reservoirs are consid-ered target reservoirs for CO2 sequestration The BassIslands Formation a saline reservoir with no historic hy-drocarbon production is such a potential reservoir in theMichigan Basin

The Bass Islands Formation in northeastern OtsegoCounty is a porous and permeable dolostone producedby the diagenetic alteration of Late Silurian shallow-water carbonate sedimentary rocks Much of the alter-ation is caused by the diagenetic processes associatedwith the Late SilurianndashEarly Devonian exposure relatedto the sequence-bounding sub-Kaskaskia interregionalunconformity (Sloss 1963) The unconformity is over-lain by the Bois Blanc Formation in this study area butmay be overlain by other formations in other parts of theMichigan Basin (Figure 2) The reservoir properties ofthe Bass Islands Formation are primarily controlled bythe original depositional texture and fabric which isoverprinted by early and late diagenesis

Regional mapping and examination of wireline logsfromwells in other counties in northern and centralMich-igan show thicknesses of the Bass Islands reservoir ofup to 90 ft (27m) (Figure 3) A large area in the north-ern half of Michiganrsquos lower peninsula has a gross reser-voir thickness of greater than 50 ft (15 m) Figure 3 alsoshows the area of a minimum burial depth of 2600 ft(788 m) required to maintain CO2 in its supercriticalphase A slightly more conservative estimate of 2600 ft(788 m) was chosen for mapping purposes instead ofthe typical 2500 ft (762 m) (US Department of En-ergy 2007 p 12) Lateral correlation using wirelinelogs shows that the Bass Islands reservoir interval canbe identified in surrounding counties to the St Charlton4-30 well in Otsego County (Figure 4)

RESULTS AND DISCUSSION

Bass Islands Formation Lithology and Facies

The Bass Islands Formation has its type area in outcropson the Bass Islands near the north-centralOhio coastline

Harrison et al 141

Figure 1 Location map of the Core Energy-State Charlton 4-30 well in northern lower Michigan The map shows oil gas and dry wells with the inset showing Otsego CountyMichigan Southwestndashnortheast-trending oil and gas wells in Manistee to Presque Isle counties are mainly in the northern Niagaran (Silurian) reef trend The eastndashwest swath of gas wellsin Antrim to Alpena counties are predominantly Devonian Antrim Shale gas wells NGRN = Silurian Niagaran pinnacle reef oil and gas fields

142ReservoirCharacteristics

oftheBass

IslandsDolom

iteinOtsego

CountyMichigan

Figure 2 Part of the general-ized Michigan stratigraphic col-umn showing Upper Silurian andLowerndashMiddle Devonian strataThe Silurian-Devonian boundaryis placed at the top of the BassIslands The Mackinac BrecciaGarden Island Formation andSylvania Sandstone are not pre-sent in Otsego County

Figure 3 Isopach map of Bass Islands Dolomite in northern and central Michigan The 2600-ft (788-m) structural contour line marksthe minimum depth for maintaining supercritical-phase CO2 in the subsurface Gridding and contouring were done using ordinary krigingand a spherical variogram model The root-mean-square error was 136 ft (41 m)

Harrison et al 143

Figure 4 Northndashsouth wireline-log cross section showing the Bass Islands Formation (BILD) reservoir interval and the overlying confining unit Bois Blanc Formation (BBLC) and themajor seal unit Amherstberg Formation (AMBG) Well number 57916 is the Core Energy-St Charlton 4-30 well GR = gamma ray RHOB = bulk density NPHI = neutron porosity PEF =photoelectric factor BILD-EVAP = Bass Islands evaporate

144ReservoirCharacteristics

oftheBass

IslandsDolom

iteinOtsego

CountyMichigan

Figure 5 Well logs andcore data from the lowerpart of the Core Energy-St Charlton 4-30 wellsection 30 T31N-R1WOtsego County MichiganBK = base of the Kaskas-kia megasequence andposition of unconformityon top of the Bass IslandsFormation BILD = BassIslands Formation BBLC =Bois Blanc FormationAMBG = AmherstbergFormation The core inter-val reported in this study isthe lower half of core 3and all of core 4 NPHI =neutron porosity PEF =photoelectric factor

Ha

rrison et al 145

of Lake Erie (Lane et al 1909) Sparling (1970) exten-sively described the Bass Islands lithofacies as a series ofshallow-water carbonate deposits that have been dolomi-tized and diagenetically altered by processes associatedwith the Tippecanoe-Kaskaskia sequence-bounding un-conformity Dissolution of primary grains and larger-

146 Reservoir Characteristics of the Bass Islands Dolomite in O

scale dissolution and formation of solution collapse brec-cias are characteristics of the Bass Islands in the outcroparea Although wireline-log correlations show that theBass Islands in the Michigan Basin subsurface continuesthis dolomitic lithology they cannot help to determinethe facies and fabric characteristics seen in outcrop

Figure 6 (A) Core photograph(3442 ft [1049 m]) showing theboundary between the Tippeca-noe and Kaskaskia megase-quences of Sloss (1963) with shelllag followed by condensed inter-val and a chert-rich zone nearthe top of the core (B) Corephotograph (3472 ft [1058 m])showing brecciation and solutionpipes related to karst developedduring subaerial exposure(C) Core photograph (3461 ft[1055 m]) showing well-developedcross-bedding in peloidal grain-stone associated with high-energycarbonate sand shoals (D) Corephotograph (3477 ft [1060 m]) oflaminated cyanobacterial matsassociated with tidal flat depositsPhotos are by Linda Harrison

tsego County Michigan

Limited core observations (Gardner 1974) do suggestshallow-water deposits and diagenetic alteration includ-ing brecciation

Acquisition of a new core from the Core Energy-St Charlton 4-30 well provides an opportunity to care-

fully evaluate the reservoir properties of this formationThe well penetrated at least 188 ft (57 m) of Bass IslandsFormation and cored 78 ft (24 m) of the uppermost BassIslands along with 42 ft (13 m) of the overlying BoisBlanc Formation (Figure 5) This is the most complete

Figure 7 Thin-section photomicrographs (plane light) of reservoir intervals (A) Brecciated (Br) finely crystalline dolomite related tokarst developed in response to subaerial exposure (3469 ft [1057 m]) Well-developed solution-enhanced vugs (V) and intercrystallineporosity The measured porosity and permeability (Klinkenberg) in this zone are gt31 and gt290 md respectively (B) Brecciated (Br)finely crystalline dolomite related to karst developed in response to subaerial exposure (34722 ft [10583 m]) Well-developed solution-enhanced vugs (V) and intercrystalline porosity The measured porosity and permeability (Klinkenberg) in this zone are gt26 and 90 mdrespectively (C) Dolomitized burrowed peloidal skeletal wackestone Pore types include fossil molds (M) solution-enhanced vugs (V) andintercrystalline pores Late saddle dolomite (SD) partially occludes local vugs (3466 ft [1056 m]) (D) Dolomitized peloidal packstone tograinstone Well-developed solution-enhanced vugs (V) small molds and intercrystalline porosity (3461 ft [1055 m]) (E) Dolomitizedbioturbated peloidal wackestone-packstone Well-developed solution-enhanced vugs (V) small molds and intercrystalline porosity (3495 ft[1065 m]) (F) Higher-magnification photograph of sample E showing well-developed solution-enhanced vugs (V) small molds and inter-crystalline porosity

Harrison et al 147

subsurface core sample known for the Bass Islands For-mation inMichigan The Tippecanoe-Kaskaskia sequence-bounding unconformity is apparent in the core at 34424 ft(10493 m) (Figure 6A) Immediately overlying the un-conformity is a bed of brachiopod shells and an abun-dance of siliceous replacement of the original carbonateThe Bass Islands lithology immediately underlying theunconformity is a dolomitized bioturbated skeletalwackestone This package can be subdivided into severalmeter-scale shallowing-upward cycles of stacked faciesThe cycles begin with a shallow subtidal burrowed andbioturbated skeletal and peloidal wackestone to pack-stone (Figure 6B) These subtidal facies shallow upwardinto higher-energy cross-bedded and laminated peloidalgrainstones (Figure 6C) The grainstones are capped bylaminated and crenulated cyanobacterial mat mudstonesthat represent high intertidal or supratidal environmentswith subaerial exposure and represent the top of theshallowing-upward cycle (Figure 6D)

148 Reservoir Characteristics of the Bass Islands Dolomite in O

Reservoir Properties

Although the reservoir properties of shallow-water car-bonate deposits in the Bass Islands of Michigan aregreatly influenced by dissolution and diagenesis strongfabric characteristics which are inherited from theiroriginal depositional facies character influence reser-voir properties This carbonate reservoir has some pre-served primary porosity as well as abundant secondaryporosity Dissolution has produced significant second-ary pores (Figures 6B 7A B) Fossil grains are most vul-nerable because dissolution processes have selectivelyremoved grains instead of the finer micritic matrix(Figure 7C E F) In addition to moldic porosity abun-dant intercrystalline porosity resulting from dolomitiza-tion is observed In the grainstones abundant intergran-ular porosity and significant permeability are observedMuch of the porosity in the grainstones is primary inter-granular porosity although some pores have been modi-fied into vugs by dissolution Figure 7D shows a thin sec-tion of the cross-bedded grainstone seen in Figure 7C at3461 ft (1055 m)

Conventional coreporosity andpermeability (PampP)havebeenmeasured from 2-in (5-cm)-long by 1-in (25-cm)-diameter sample plugs drilled fromwhole core collectedin theCore Energy-St Charlton 4-30well Seventy-fourhorizontal plugs 12 vertical plugs 6 whole cores and 17sidewall core plugs were sent to Core Laboratories forroutine PampP analyses Sixty-six horizontal plugs werecollected in the Bass Islands reservoir interval Differentfacies show substantially different reservoir properties(Figure 8) Intergranular porosity in the grainstone faciesis representative of primary porosity that forms useful po-rosity zones in the Bass Islands in this well Good porosityalso exists as the result of selective dissolution of some car-bonate grains producingmoldic porosity This facies is fur-ther enhanced by karst dissolution and brecciation to be-come the best PampP in the entire formation These faciesare relatively thin beds stacked in a series of shallowing-upward packages that follow a predictable stratigraphicframework This framework can provide a better under-standing of the vertical and lateral distribution of the po-rous reservoir facies

Stratigraphic Framework

The Bass Islands interval exhibits 5-m (16-ft)-scale cy-cles nested within two high-frequency sequences (HFSapproximately 10 m [33 ft]) (Figure 9) Sequence andcycle boundaries were determined from core analysis

Figure 8 Porosity and permeability from core-plug analyses ofthe Core Energy-St Charlton 4-30 well Generalized facies typesare indicated throughout the cored section

tsego County Michigan

and were specifically based on evidence for subaerialexposure caused by a fall in relative sea level the pres-ence of flooding surfaces indicating a transgressive eventor vertical successions of facies indicating an overall flood-ing event followed by a shallowing of facies indicating arelative fall in sea level Subaerial exposure horizons aremarked by brecciation (collapse breccia) solution pipesand local development of terra rosa all of which are com-mon diagnostic indicators of exposure (eg Esteban andKlappa 1983) The highest frequency packages are the cy-cles which are amaximumof approximately 24 ft (73m)thick and have an average thickness of 14 ft (43 m)The HFS range from 28 to 44 ft (85 to 134 m) thickand the Bass Islands interval is approximately 72 ft thick(22 m)

Reservoir Intervals

The best reservoir units in theCore Energy-St Charlton4-30 core are present in two different intervals Thefirst is related to solution brecciation and recrystalliza-tion of limestones caused by subaerial exposure wherewell-developed interparticle intercrystalline and solution-enhanced vugs contribute to the creation of high porositiesand permeabilities The second unit is related to inner plat-form (midramp) bioturbated facies where the burrow gal-leries have been preferentially dolomitized with coarsercrystalline sucrosic dolomite Figure 10 illustrates howthese types of facies commonly referred to as tubulartempestites may be in excellent communication becauseof the nature of the three-dimensional burrow galleries

Figure 9 Stratigraphic hierarchy observed in the Bass Islands interval High-frequency sequences (HFS) along with higher-frequencycycles correlate well to the stratigraphic position of the best reservoir intervals and many of the HFS and cycle boundaries are manifestedby higher gamma-ray log values Regressive hemicycles of both the HFS and higher-frequency cycles correlate to the best developmentof reservoir throughout much of the cored interval NPHI = neutron porosity PEF = photoelectric factor

Harrison et al 149

courtesy of E A Shinn

and the density of burrows commonly found in similartypes of environments

Reservoir development also correlates well with thestratigraphic framework (Figure 9) Sequence and cycleboundaries were identified by the presence of subaerialexposure horizons as indicated by karstic brecciation andsolution flooding surface and facies stacking patternsAs mentioned above a well-constrained hierarchy ex-ists within the Bass Islands interval whereby the higher-frequency cycles exist within two medium-scale HFSFrom a reservoir standpoint the HFS and most of thehigher-frequency cycles constrain the development ofthe best reservoir intervals Regressive hemicycles ofboth the HFS and higher-frequency cycles correlate tothe best development of the reservoir throughout muchof the cored interval Many of the HFS and cycleboundaries are manifested by higher gamma-ray logvalues and can be identified in wells without core dataWe believe that the correlation of depositional faciesobserved in core to stratigraphic packages and wireline-log signatures is a powerful tool to help interpret the geo-logic properties of the Bass Islands interval in otherareas throughout Michigan where only wireline logsare available

150 Reservoir Characteristics of the Bass Islands Dolomite in O

CONCLUSIONS

The Bass Islands Formation can be mapped throughoutmuch of northern and central lower Michigan where itis an important saline reservoir target for CO2 seques-tration with more than 40 ft (12 m) of greater than 10porosity and permeabilities exceeding 500md Averageporosity in the Bass Islands cored interval in the CoreEnergy-St Charlton 4-30 test well is 125 and averagepermeability is 224 md

The Bass Islands in the area of the Core Energy-StCharlton 4-30 well is composed of a series of shallow-water and tidal flat carbonate facies that are stacked in apredictable shallowing-upward pattern The Bass Is-lands Formation seen in this core can be subdivided intotwoHFS and five higher-frequency cycles Stratigraphicanalysis helps predict vertical and lateral facies patternsand contributes to the evaluation of regional reservoirproperties The best reservoir quality occurs in the sub-tidal burrowed packstones that have some dissolutionof grains and develop moldic porosity This facies is alsoenhanced further by karst dissolution and brecciationrelated to the Tippecanoe-Kaskaskia sequence-boundingunconformity at the top of the Bass Islands Formation

Figure 10 Some of the bestreservoirs in the Bass Islands ex-ists in the thoroughly bioturbatedinner platform facies Burrowsare of the Thalassinoides ichno-facies which is similar to thethree-dimensional galleries formedtoday by burrowing shrimp (egCallianassa) Photograph of theGreat Bahama Bank with the lo-cation of heavily bioturbated innerplatform facies In many modernenvironments these facies mayextend for thousands of squarekilometers Note the high densityof burrows and three-dimensionaldistribution of an individual bur-row gallery preserved in epoxyboth of which illustrate the po-tential for such facies to be lat-erally and vertically connected inthree dimensions commonlyleading to excellent reservoir

potential The plastic castphotograph of the ghost shrimpCallianassa burrow gallery is

tsego County Michigan

Additional porosity can be found in carbonate grainstonefacies that preserve some primary intergranular porosityThe shallowest tidal flat or stromatolitic facies showminimal PampP and are likely to be internal baffles or bar-riers to fluid flow through the formation In addition cor-relation of wireline logs shows that the Bass Islands cyclicporosity seen in the St Charlton 4-30 well is presentthroughout the northern part of lower Michigan

REFERENCES CITED

Barnes D A W B Harrison III and A Wahr in press Assessmentof regional geological carbon sequestration potential in UpperSilurian to Middle Devonian strata of the Michigan Basin inM Grobe J C Pashin and R L Dodge eds Carbon dioxidesequestration in geologic mediamdashState of the science AAPGStudies in Geology 59

Catacosinos P A P A Daniels Jr and W B Harrison III 1991Structure stratigraphy and petroleum geology of the MichiganBasin inMW Leighton D R Kolata D FOltz and J J Eideleds Interior cratonic basins AAPG Memoir 51 p 561ndash602

Esteban M and C F Klappa 1983 Subaerial exposure environ-

ment in P A Scholle D G Bebout and C H Moore edsCarbonate depositional environments AAPG Memoir 33p 1ndash95

Gardner W C 1974 Middle Devonian stratigraphy and deposi-tional environments in the Michigan Basin Michigan BasinGeological Society Special Paper 1 138 p

GrammerGMDABarnesWBHarrison III andAESandomierskiin press Practical synergies for increasing domestic oil produc-tion and geological sequestration of anthropogenic CO2 An ex-ample from the Michigan Basin in M Grobe J C Pashin andR L Dodge eds Carbon dioxide sequestration in geologicmediamdashState of the science AAPG Studies in Geology 59

Lane A C C S Prosser W H Sherzer and A W Grabau 1909Nomenclature and subdivision of the Upper Silurian strata ofMichigan Ohio and western New York Geological Society ofAmerica Bulletin v 19 p 553ndash556

Lilienthal R T 1978 Stratigraphic cross-sections of the MichiganBasin Department of Natural Resources Geological SurveyDivision Report of Investigation No 19 36 p

Sloss L L 1963 Sequences in the cratonic interior of North Amer-ica Geological Society of America Bulletin v 75 p 93ndash113doi1011300016-7606(1963)74[93SITCIO]20CO2

Sparling D R 1970 The Bass Islands Formation in its type regionThe Ohio Journal of Science v 70 no 1 p 1ndash33

US Department of Energy 2007 Carbon sequestration atlas of theUnited States and Canada Pittsburgh Pennsylvania NationalEnergy Technology Laboratory 86 p

Harrison et al 151

Stephen Kelley provided yeoman service inslabbing core and drilling core plugs for porosityand permeability analyses Thanks to LindaHarrison for the high-resolution core photo-graphs Robert G Mannes and Allen Modroo atCore Energy LLC have provided timely discus-sions about the geology in and around the StCharlton 4-30 well The Michigan GeologicalRepository for Research and Education hasprovided research space and facilities for thestudy of the samples and data

140 Reservoir Characteristics of the Bass Islands

target saline reservoir interval (Bass Islands Formation) is a variablyporous and permeable dolostone composed of several tidal flat cyclicpackages The Bass Islands Formation has a gross thickness of 70 ft(21m)with a reservoir interval composed ofmore than 40 ft (12m) ofgreater than 10 porosity and permeability zones exceeding 500 mdAverage porosity over the entire Bass Islands is 125 Average per-meability is 224 md The CO2 injection tests using the Bass Islandssection were completed during February and March 2008 Analysisof the Bass Islands Formation in northernMichigan indicated excellentreservoir quality for injection and storage of CO2 and high-qualitysealing units to prevent vertical migration Monitoring well data con-ducted during and after the injection test validates preinjection reser-voir simulation modeling performed at Battelle Pacific NorthwestLabs using well data and rock observations from this study

INTRODUCTION

As part of a phase II geological study and pilot CO2 injection testprogram the Bass Islands Formation in northern lower Michiganwas evaluated as a possible candidate for geological carbon seques-tration The Bass Islands Formation has long been recognized inMich-igan as a widespread brine-filled carbonate unit that caps the Silurianstratigraphic section (Lilienthal 1978) It is overlain and truncated inplaces by the major interregional unconformity at the Tippecanoe-Kaskaskia sequence boundary (Sloss 1963) Although the Bass Is-lands Formation is mostly known from the subsurface in Michiganscattered outcrops of the Raisin River Dolomite member are ob-served in southeastern Michigan and the St Ignace Dolomite mem-ber near the Straits of Mackinac in northern lower Michigan andMichiganrsquos upper peninsula Most of the data about the Bass IslandsFormation have been derived from well data acquired by the drillingof oil and gas wells throughout the Michigan Basin Most wells havewireline logs that record various lithologic and fluid propertiesalthough it is a common practice for drillers to set casing a few feetinto an anhydritic bed in the Bass Islands Formation Many of thelogs therefore only comprise gamma-ray and cased-hole neutronporosity logs A few open-hole wireline logs through the Bass IslandsFormation exist and they are the most useful in estimating reservoirproperties (Barnes et al in press) A few wells have collected sets ofdrill cuttings so the lithology interpreted from logs can be confirmedAlthough wireline logs and drill cuttings can provide general litho-logic and porosity properties rock core samples can provide additionalcritical data for reservoir evaluation Facies characteristics and rock per-meability are only satisfactorily determined from core samples Veryfew core samples from the Bass Islands Formation throughout theMichigan Basin exist however Gardner (1974) reported core froma part of the Bass Islands Formation in two wells the Sun-Bradley 4Newaygo County Sec 11-T12N-R13W and the Sun-Mancelona 1Antrim County Sec 36-T29N-R5W

Dolomite in Otsego County Michigan

InNovember 2006 an important corewas collectedas part of the Midwest Regional Carbon SequestrationPartnership phase II research program This particular sitewas chosen because it coincided with an ongoing CO2

enhanced oil recovery (EOR) project owned by CoreEnergy LLC This EOR project takes high-purity CO2

separated from produced Devonian Antrim Shale gas atnearby gas processing plants compresses it to approxi-mately 1100 psi (77 kgcm) and pumps it along an 11-mi(18-km)-long pipeline to a series of Silurian NiagaranPinnacle reefs (Grammer et al in press) This researchproject and injection experiment was able to take ad-vantage of the existing infrastructure of the EOR activityand acquire core and logs from the Bass Islands Forma-tion during drilling of a new well for EOR operations TheCore Energy-State Charlton 4-30 well API permit num-ber 21-137-57916-0000 (Figure 1) was drilled in OtsegoCounty of northern lowerMichigan as an additional EORoperation well in a Niagaran reef oil field The well wasinitially drilled and logged to a depth of 3630 ft (1006m)then deepened to the Silurian Niagaran reef zone for fu-ture use in the ongoing EOR activities The shallower partof thiswell is currently being used as a field demonstrationwell to sequester anthropogenic CO2 in the Bass IslandsFormation a deep saline reservoir In early 2008 slightlymore than 10000 tons of CO2 was injected into the BassIslands Formation Additional injections of up to 50000tons CO2 are being completed during the spring and sum-mer of 2009

This studywas undertaken to delineate site-specificlithologic and reservoir properties for the Bass Islands inthe area of the test well and to use the detailed core-to-log correlations to extrapolate reservoir characteristicsand CO2 storage capacity estimates to nearby areas ofthe Michigan Basin using wireline-log data along withvery limited core and sample data Applying a se-quence stratigraphic approach further provides verticaland lateral predictability in the reservoir

Regional Setting

The Michigan Basin is a cratonic basin located on thesouthern margin of the Canadian shield and centeredon the lower peninsula ofMichigan The basin hasmorethan 16000 ft (4877m) of Paleozoic sediments in thecenter and thins dramatically toward themargins It alsooccupies parts of eastern Wisconsin southwestern On-tario Canada northwesternOhio northeastern Illinoisand northern Indiana and coversmore than 100000mi2

(160000 km2) (Catacosinos et al 1991) There have

been more than 58000 wells (Figure 1) drilled for oiland gas exploration and development since the 1920sCumulative production from all formations exceeds125 billion bbl of oil and 65 tcf of natural gas Althoughmore than one-third of the wells drilled in Michiganhave produced hydrocarbons many others have en-countered porous and permeable subsurface forma-tions that contain only brine with salinity values up to400000 ppm (40) These saline reservoirs are consid-ered target reservoirs for CO2 sequestration The BassIslands Formation a saline reservoir with no historic hy-drocarbon production is such a potential reservoir in theMichigan Basin

The Bass Islands Formation in northeastern OtsegoCounty is a porous and permeable dolostone producedby the diagenetic alteration of Late Silurian shallow-water carbonate sedimentary rocks Much of the alter-ation is caused by the diagenetic processes associatedwith the Late SilurianndashEarly Devonian exposure relatedto the sequence-bounding sub-Kaskaskia interregionalunconformity (Sloss 1963) The unconformity is over-lain by the Bois Blanc Formation in this study area butmay be overlain by other formations in other parts of theMichigan Basin (Figure 2) The reservoir properties ofthe Bass Islands Formation are primarily controlled bythe original depositional texture and fabric which isoverprinted by early and late diagenesis

Regional mapping and examination of wireline logsfromwells in other counties in northern and centralMich-igan show thicknesses of the Bass Islands reservoir ofup to 90 ft (27m) (Figure 3) A large area in the north-ern half of Michiganrsquos lower peninsula has a gross reser-voir thickness of greater than 50 ft (15 m) Figure 3 alsoshows the area of a minimum burial depth of 2600 ft(788 m) required to maintain CO2 in its supercriticalphase A slightly more conservative estimate of 2600 ft(788 m) was chosen for mapping purposes instead ofthe typical 2500 ft (762 m) (US Department of En-ergy 2007 p 12) Lateral correlation using wirelinelogs shows that the Bass Islands reservoir interval canbe identified in surrounding counties to the St Charlton4-30 well in Otsego County (Figure 4)

RESULTS AND DISCUSSION

Bass Islands Formation Lithology and Facies

The Bass Islands Formation has its type area in outcropson the Bass Islands near the north-centralOhio coastline

Harrison et al 141

Figure 1 Location map of the Core Energy-State Charlton 4-30 well in northern lower Michigan The map shows oil gas and dry wells with the inset showing Otsego CountyMichigan Southwestndashnortheast-trending oil and gas wells in Manistee to Presque Isle counties are mainly in the northern Niagaran (Silurian) reef trend The eastndashwest swath of gas wellsin Antrim to Alpena counties are predominantly Devonian Antrim Shale gas wells NGRN = Silurian Niagaran pinnacle reef oil and gas fields

142ReservoirCharacteristics

oftheBass

IslandsDolom

iteinOtsego

CountyMichigan

Figure 2 Part of the general-ized Michigan stratigraphic col-umn showing Upper Silurian andLowerndashMiddle Devonian strataThe Silurian-Devonian boundaryis placed at the top of the BassIslands The Mackinac BrecciaGarden Island Formation andSylvania Sandstone are not pre-sent in Otsego County

Figure 3 Isopach map of Bass Islands Dolomite in northern and central Michigan The 2600-ft (788-m) structural contour line marksthe minimum depth for maintaining supercritical-phase CO2 in the subsurface Gridding and contouring were done using ordinary krigingand a spherical variogram model The root-mean-square error was 136 ft (41 m)

Harrison et al 143

Figure 4 Northndashsouth wireline-log cross section showing the Bass Islands Formation (BILD) reservoir interval and the overlying confining unit Bois Blanc Formation (BBLC) and themajor seal unit Amherstberg Formation (AMBG) Well number 57916 is the Core Energy-St Charlton 4-30 well GR = gamma ray RHOB = bulk density NPHI = neutron porosity PEF =photoelectric factor BILD-EVAP = Bass Islands evaporate

144ReservoirCharacteristics

oftheBass

IslandsDolom

iteinOtsego

CountyMichigan

Figure 5 Well logs andcore data from the lowerpart of the Core Energy-St Charlton 4-30 wellsection 30 T31N-R1WOtsego County MichiganBK = base of the Kaskas-kia megasequence andposition of unconformityon top of the Bass IslandsFormation BILD = BassIslands Formation BBLC =Bois Blanc FormationAMBG = AmherstbergFormation The core inter-val reported in this study isthe lower half of core 3and all of core 4 NPHI =neutron porosity PEF =photoelectric factor

Ha

rrison et al 145

of Lake Erie (Lane et al 1909) Sparling (1970) exten-sively described the Bass Islands lithofacies as a series ofshallow-water carbonate deposits that have been dolomi-tized and diagenetically altered by processes associatedwith the Tippecanoe-Kaskaskia sequence-bounding un-conformity Dissolution of primary grains and larger-

146 Reservoir Characteristics of the Bass Islands Dolomite in O

scale dissolution and formation of solution collapse brec-cias are characteristics of the Bass Islands in the outcroparea Although wireline-log correlations show that theBass Islands in the Michigan Basin subsurface continuesthis dolomitic lithology they cannot help to determinethe facies and fabric characteristics seen in outcrop

Figure 6 (A) Core photograph(3442 ft [1049 m]) showing theboundary between the Tippeca-noe and Kaskaskia megase-quences of Sloss (1963) with shelllag followed by condensed inter-val and a chert-rich zone nearthe top of the core (B) Corephotograph (3472 ft [1058 m])showing brecciation and solutionpipes related to karst developedduring subaerial exposure(C) Core photograph (3461 ft[1055 m]) showing well-developedcross-bedding in peloidal grain-stone associated with high-energycarbonate sand shoals (D) Corephotograph (3477 ft [1060 m]) oflaminated cyanobacterial matsassociated with tidal flat depositsPhotos are by Linda Harrison

tsego County Michigan

Limited core observations (Gardner 1974) do suggestshallow-water deposits and diagenetic alteration includ-ing brecciation

Acquisition of a new core from the Core Energy-St Charlton 4-30 well provides an opportunity to care-

fully evaluate the reservoir properties of this formationThe well penetrated at least 188 ft (57 m) of Bass IslandsFormation and cored 78 ft (24 m) of the uppermost BassIslands along with 42 ft (13 m) of the overlying BoisBlanc Formation (Figure 5) This is the most complete

Figure 7 Thin-section photomicrographs (plane light) of reservoir intervals (A) Brecciated (Br) finely crystalline dolomite related tokarst developed in response to subaerial exposure (3469 ft [1057 m]) Well-developed solution-enhanced vugs (V) and intercrystallineporosity The measured porosity and permeability (Klinkenberg) in this zone are gt31 and gt290 md respectively (B) Brecciated (Br)finely crystalline dolomite related to karst developed in response to subaerial exposure (34722 ft [10583 m]) Well-developed solution-enhanced vugs (V) and intercrystalline porosity The measured porosity and permeability (Klinkenberg) in this zone are gt26 and 90 mdrespectively (C) Dolomitized burrowed peloidal skeletal wackestone Pore types include fossil molds (M) solution-enhanced vugs (V) andintercrystalline pores Late saddle dolomite (SD) partially occludes local vugs (3466 ft [1056 m]) (D) Dolomitized peloidal packstone tograinstone Well-developed solution-enhanced vugs (V) small molds and intercrystalline porosity (3461 ft [1055 m]) (E) Dolomitizedbioturbated peloidal wackestone-packstone Well-developed solution-enhanced vugs (V) small molds and intercrystalline porosity (3495 ft[1065 m]) (F) Higher-magnification photograph of sample E showing well-developed solution-enhanced vugs (V) small molds and inter-crystalline porosity

Harrison et al 147

subsurface core sample known for the Bass Islands For-mation inMichigan The Tippecanoe-Kaskaskia sequence-bounding unconformity is apparent in the core at 34424 ft(10493 m) (Figure 6A) Immediately overlying the un-conformity is a bed of brachiopod shells and an abun-dance of siliceous replacement of the original carbonateThe Bass Islands lithology immediately underlying theunconformity is a dolomitized bioturbated skeletalwackestone This package can be subdivided into severalmeter-scale shallowing-upward cycles of stacked faciesThe cycles begin with a shallow subtidal burrowed andbioturbated skeletal and peloidal wackestone to pack-stone (Figure 6B) These subtidal facies shallow upwardinto higher-energy cross-bedded and laminated peloidalgrainstones (Figure 6C) The grainstones are capped bylaminated and crenulated cyanobacterial mat mudstonesthat represent high intertidal or supratidal environmentswith subaerial exposure and represent the top of theshallowing-upward cycle (Figure 6D)

148 Reservoir Characteristics of the Bass Islands Dolomite in O

Reservoir Properties

Although the reservoir properties of shallow-water car-bonate deposits in the Bass Islands of Michigan aregreatly influenced by dissolution and diagenesis strongfabric characteristics which are inherited from theiroriginal depositional facies character influence reser-voir properties This carbonate reservoir has some pre-served primary porosity as well as abundant secondaryporosity Dissolution has produced significant second-ary pores (Figures 6B 7A B) Fossil grains are most vul-nerable because dissolution processes have selectivelyremoved grains instead of the finer micritic matrix(Figure 7C E F) In addition to moldic porosity abun-dant intercrystalline porosity resulting from dolomitiza-tion is observed In the grainstones abundant intergran-ular porosity and significant permeability are observedMuch of the porosity in the grainstones is primary inter-granular porosity although some pores have been modi-fied into vugs by dissolution Figure 7D shows a thin sec-tion of the cross-bedded grainstone seen in Figure 7C at3461 ft (1055 m)

Conventional coreporosity andpermeability (PampP)havebeenmeasured from 2-in (5-cm)-long by 1-in (25-cm)-diameter sample plugs drilled fromwhole core collectedin theCore Energy-St Charlton 4-30well Seventy-fourhorizontal plugs 12 vertical plugs 6 whole cores and 17sidewall core plugs were sent to Core Laboratories forroutine PampP analyses Sixty-six horizontal plugs werecollected in the Bass Islands reservoir interval Differentfacies show substantially different reservoir properties(Figure 8) Intergranular porosity in the grainstone faciesis representative of primary porosity that forms useful po-rosity zones in the Bass Islands in this well Good porosityalso exists as the result of selective dissolution of some car-bonate grains producingmoldic porosity This facies is fur-ther enhanced by karst dissolution and brecciation to be-come the best PampP in the entire formation These faciesare relatively thin beds stacked in a series of shallowing-upward packages that follow a predictable stratigraphicframework This framework can provide a better under-standing of the vertical and lateral distribution of the po-rous reservoir facies

Stratigraphic Framework

The Bass Islands interval exhibits 5-m (16-ft)-scale cy-cles nested within two high-frequency sequences (HFSapproximately 10 m [33 ft]) (Figure 9) Sequence andcycle boundaries were determined from core analysis

Figure 8 Porosity and permeability from core-plug analyses ofthe Core Energy-St Charlton 4-30 well Generalized facies typesare indicated throughout the cored section

tsego County Michigan

and were specifically based on evidence for subaerialexposure caused by a fall in relative sea level the pres-ence of flooding surfaces indicating a transgressive eventor vertical successions of facies indicating an overall flood-ing event followed by a shallowing of facies indicating arelative fall in sea level Subaerial exposure horizons aremarked by brecciation (collapse breccia) solution pipesand local development of terra rosa all of which are com-mon diagnostic indicators of exposure (eg Esteban andKlappa 1983) The highest frequency packages are the cy-cles which are amaximumof approximately 24 ft (73m)thick and have an average thickness of 14 ft (43 m)The HFS range from 28 to 44 ft (85 to 134 m) thickand the Bass Islands interval is approximately 72 ft thick(22 m)

Reservoir Intervals

The best reservoir units in theCore Energy-St Charlton4-30 core are present in two different intervals Thefirst is related to solution brecciation and recrystalliza-tion of limestones caused by subaerial exposure wherewell-developed interparticle intercrystalline and solution-enhanced vugs contribute to the creation of high porositiesand permeabilities The second unit is related to inner plat-form (midramp) bioturbated facies where the burrow gal-leries have been preferentially dolomitized with coarsercrystalline sucrosic dolomite Figure 10 illustrates howthese types of facies commonly referred to as tubulartempestites may be in excellent communication becauseof the nature of the three-dimensional burrow galleries

Figure 9 Stratigraphic hierarchy observed in the Bass Islands interval High-frequency sequences (HFS) along with higher-frequencycycles correlate well to the stratigraphic position of the best reservoir intervals and many of the HFS and cycle boundaries are manifestedby higher gamma-ray log values Regressive hemicycles of both the HFS and higher-frequency cycles correlate to the best developmentof reservoir throughout much of the cored interval NPHI = neutron porosity PEF = photoelectric factor

Harrison et al 149

courtesy of E A Shinn

and the density of burrows commonly found in similartypes of environments

Reservoir development also correlates well with thestratigraphic framework (Figure 9) Sequence and cycleboundaries were identified by the presence of subaerialexposure horizons as indicated by karstic brecciation andsolution flooding surface and facies stacking patternsAs mentioned above a well-constrained hierarchy ex-ists within the Bass Islands interval whereby the higher-frequency cycles exist within two medium-scale HFSFrom a reservoir standpoint the HFS and most of thehigher-frequency cycles constrain the development ofthe best reservoir intervals Regressive hemicycles ofboth the HFS and higher-frequency cycles correlate tothe best development of the reservoir throughout muchof the cored interval Many of the HFS and cycleboundaries are manifested by higher gamma-ray logvalues and can be identified in wells without core dataWe believe that the correlation of depositional faciesobserved in core to stratigraphic packages and wireline-log signatures is a powerful tool to help interpret the geo-logic properties of the Bass Islands interval in otherareas throughout Michigan where only wireline logsare available

150 Reservoir Characteristics of the Bass Islands Dolomite in O

CONCLUSIONS

The Bass Islands Formation can be mapped throughoutmuch of northern and central lower Michigan where itis an important saline reservoir target for CO2 seques-tration with more than 40 ft (12 m) of greater than 10porosity and permeabilities exceeding 500md Averageporosity in the Bass Islands cored interval in the CoreEnergy-St Charlton 4-30 test well is 125 and averagepermeability is 224 md

The Bass Islands in the area of the Core Energy-StCharlton 4-30 well is composed of a series of shallow-water and tidal flat carbonate facies that are stacked in apredictable shallowing-upward pattern The Bass Is-lands Formation seen in this core can be subdivided intotwoHFS and five higher-frequency cycles Stratigraphicanalysis helps predict vertical and lateral facies patternsand contributes to the evaluation of regional reservoirproperties The best reservoir quality occurs in the sub-tidal burrowed packstones that have some dissolutionof grains and develop moldic porosity This facies is alsoenhanced further by karst dissolution and brecciationrelated to the Tippecanoe-Kaskaskia sequence-boundingunconformity at the top of the Bass Islands Formation

Figure 10 Some of the bestreservoirs in the Bass Islands ex-ists in the thoroughly bioturbatedinner platform facies Burrowsare of the Thalassinoides ichno-facies which is similar to thethree-dimensional galleries formedtoday by burrowing shrimp (egCallianassa) Photograph of theGreat Bahama Bank with the lo-cation of heavily bioturbated innerplatform facies In many modernenvironments these facies mayextend for thousands of squarekilometers Note the high densityof burrows and three-dimensionaldistribution of an individual bur-row gallery preserved in epoxyboth of which illustrate the po-tential for such facies to be lat-erally and vertically connected inthree dimensions commonlyleading to excellent reservoir

potential The plastic castphotograph of the ghost shrimpCallianassa burrow gallery is

tsego County Michigan

Additional porosity can be found in carbonate grainstonefacies that preserve some primary intergranular porosityThe shallowest tidal flat or stromatolitic facies showminimal PampP and are likely to be internal baffles or bar-riers to fluid flow through the formation In addition cor-relation of wireline logs shows that the Bass Islands cyclicporosity seen in the St Charlton 4-30 well is presentthroughout the northern part of lower Michigan

REFERENCES CITED

Barnes D A W B Harrison III and A Wahr in press Assessmentof regional geological carbon sequestration potential in UpperSilurian to Middle Devonian strata of the Michigan Basin inM Grobe J C Pashin and R L Dodge eds Carbon dioxidesequestration in geologic mediamdashState of the science AAPGStudies in Geology 59

Catacosinos P A P A Daniels Jr and W B Harrison III 1991Structure stratigraphy and petroleum geology of the MichiganBasin inMW Leighton D R Kolata D FOltz and J J Eideleds Interior cratonic basins AAPG Memoir 51 p 561ndash602

Esteban M and C F Klappa 1983 Subaerial exposure environ-

ment in P A Scholle D G Bebout and C H Moore edsCarbonate depositional environments AAPG Memoir 33p 1ndash95

Gardner W C 1974 Middle Devonian stratigraphy and deposi-tional environments in the Michigan Basin Michigan BasinGeological Society Special Paper 1 138 p

GrammerGMDABarnesWBHarrison III andAESandomierskiin press Practical synergies for increasing domestic oil produc-tion and geological sequestration of anthropogenic CO2 An ex-ample from the Michigan Basin in M Grobe J C Pashin andR L Dodge eds Carbon dioxide sequestration in geologicmediamdashState of the science AAPG Studies in Geology 59

Lane A C C S Prosser W H Sherzer and A W Grabau 1909Nomenclature and subdivision of the Upper Silurian strata ofMichigan Ohio and western New York Geological Society ofAmerica Bulletin v 19 p 553ndash556

Lilienthal R T 1978 Stratigraphic cross-sections of the MichiganBasin Department of Natural Resources Geological SurveyDivision Report of Investigation No 19 36 p

Sloss L L 1963 Sequences in the cratonic interior of North Amer-ica Geological Society of America Bulletin v 75 p 93ndash113doi1011300016-7606(1963)74[93SITCIO]20CO2

Sparling D R 1970 The Bass Islands Formation in its type regionThe Ohio Journal of Science v 70 no 1 p 1ndash33

US Department of Energy 2007 Carbon sequestration atlas of theUnited States and Canada Pittsburgh Pennsylvania NationalEnergy Technology Laboratory 86 p

Harrison et al 151

InNovember 2006 an important corewas collectedas part of the Midwest Regional Carbon SequestrationPartnership phase II research program This particular sitewas chosen because it coincided with an ongoing CO2

enhanced oil recovery (EOR) project owned by CoreEnergy LLC This EOR project takes high-purity CO2

separated from produced Devonian Antrim Shale gas atnearby gas processing plants compresses it to approxi-mately 1100 psi (77 kgcm) and pumps it along an 11-mi(18-km)-long pipeline to a series of Silurian NiagaranPinnacle reefs (Grammer et al in press) This researchproject and injection experiment was able to take ad-vantage of the existing infrastructure of the EOR activityand acquire core and logs from the Bass Islands Forma-tion during drilling of a new well for EOR operations TheCore Energy-State Charlton 4-30 well API permit num-ber 21-137-57916-0000 (Figure 1) was drilled in OtsegoCounty of northern lowerMichigan as an additional EORoperation well in a Niagaran reef oil field The well wasinitially drilled and logged to a depth of 3630 ft (1006m)then deepened to the Silurian Niagaran reef zone for fu-ture use in the ongoing EOR activities The shallower partof thiswell is currently being used as a field demonstrationwell to sequester anthropogenic CO2 in the Bass IslandsFormation a deep saline reservoir In early 2008 slightlymore than 10000 tons of CO2 was injected into the BassIslands Formation Additional injections of up to 50000tons CO2 are being completed during the spring and sum-mer of 2009

This studywas undertaken to delineate site-specificlithologic and reservoir properties for the Bass Islands inthe area of the test well and to use the detailed core-to-log correlations to extrapolate reservoir characteristicsand CO2 storage capacity estimates to nearby areas ofthe Michigan Basin using wireline-log data along withvery limited core and sample data Applying a se-quence stratigraphic approach further provides verticaland lateral predictability in the reservoir

Regional Setting

The Michigan Basin is a cratonic basin located on thesouthern margin of the Canadian shield and centeredon the lower peninsula ofMichigan The basin hasmorethan 16000 ft (4877m) of Paleozoic sediments in thecenter and thins dramatically toward themargins It alsooccupies parts of eastern Wisconsin southwestern On-tario Canada northwesternOhio northeastern Illinoisand northern Indiana and coversmore than 100000mi2

(160000 km2) (Catacosinos et al 1991) There have

been more than 58000 wells (Figure 1) drilled for oiland gas exploration and development since the 1920sCumulative production from all formations exceeds125 billion bbl of oil and 65 tcf of natural gas Althoughmore than one-third of the wells drilled in Michiganhave produced hydrocarbons many others have en-countered porous and permeable subsurface forma-tions that contain only brine with salinity values up to400000 ppm (40) These saline reservoirs are consid-ered target reservoirs for CO2 sequestration The BassIslands Formation a saline reservoir with no historic hy-drocarbon production is such a potential reservoir in theMichigan Basin

The Bass Islands Formation in northeastern OtsegoCounty is a porous and permeable dolostone producedby the diagenetic alteration of Late Silurian shallow-water carbonate sedimentary rocks Much of the alter-ation is caused by the diagenetic processes associatedwith the Late SilurianndashEarly Devonian exposure relatedto the sequence-bounding sub-Kaskaskia interregionalunconformity (Sloss 1963) The unconformity is over-lain by the Bois Blanc Formation in this study area butmay be overlain by other formations in other parts of theMichigan Basin (Figure 2) The reservoir properties ofthe Bass Islands Formation are primarily controlled bythe original depositional texture and fabric which isoverprinted by early and late diagenesis

Regional mapping and examination of wireline logsfromwells in other counties in northern and centralMich-igan show thicknesses of the Bass Islands reservoir ofup to 90 ft (27m) (Figure 3) A large area in the north-ern half of Michiganrsquos lower peninsula has a gross reser-voir thickness of greater than 50 ft (15 m) Figure 3 alsoshows the area of a minimum burial depth of 2600 ft(788 m) required to maintain CO2 in its supercriticalphase A slightly more conservative estimate of 2600 ft(788 m) was chosen for mapping purposes instead ofthe typical 2500 ft (762 m) (US Department of En-ergy 2007 p 12) Lateral correlation using wirelinelogs shows that the Bass Islands reservoir interval canbe identified in surrounding counties to the St Charlton4-30 well in Otsego County (Figure 4)

RESULTS AND DISCUSSION

Bass Islands Formation Lithology and Facies

The Bass Islands Formation has its type area in outcropson the Bass Islands near the north-centralOhio coastline

Harrison et al 141

Figure 1 Location map of the Core Energy-State Charlton 4-30 well in northern lower Michigan The map shows oil gas and dry wells with the inset showing Otsego CountyMichigan Southwestndashnortheast-trending oil and gas wells in Manistee to Presque Isle counties are mainly in the northern Niagaran (Silurian) reef trend The eastndashwest swath of gas wellsin Antrim to Alpena counties are predominantly Devonian Antrim Shale gas wells NGRN = Silurian Niagaran pinnacle reef oil and gas fields

142ReservoirCharacteristics

oftheBass

IslandsDolom

iteinOtsego

CountyMichigan

Figure 2 Part of the general-ized Michigan stratigraphic col-umn showing Upper Silurian andLowerndashMiddle Devonian strataThe Silurian-Devonian boundaryis placed at the top of the BassIslands The Mackinac BrecciaGarden Island Formation andSylvania Sandstone are not pre-sent in Otsego County

Figure 3 Isopach map of Bass Islands Dolomite in northern and central Michigan The 2600-ft (788-m) structural contour line marksthe minimum depth for maintaining supercritical-phase CO2 in the subsurface Gridding and contouring were done using ordinary krigingand a spherical variogram model The root-mean-square error was 136 ft (41 m)

Harrison et al 143

Figure 4 Northndashsouth wireline-log cross section showing the Bass Islands Formation (BILD) reservoir interval and the overlying confining unit Bois Blanc Formation (BBLC) and themajor seal unit Amherstberg Formation (AMBG) Well number 57916 is the Core Energy-St Charlton 4-30 well GR = gamma ray RHOB = bulk density NPHI = neutron porosity PEF =photoelectric factor BILD-EVAP = Bass Islands evaporate

144ReservoirCharacteristics

oftheBass

IslandsDolom

iteinOtsego

CountyMichigan

Figure 5 Well logs andcore data from the lowerpart of the Core Energy-St Charlton 4-30 wellsection 30 T31N-R1WOtsego County MichiganBK = base of the Kaskas-kia megasequence andposition of unconformityon top of the Bass IslandsFormation BILD = BassIslands Formation BBLC =Bois Blanc FormationAMBG = AmherstbergFormation The core inter-val reported in this study isthe lower half of core 3and all of core 4 NPHI =neutron porosity PEF =photoelectric factor

Ha

rrison et al 145

of Lake Erie (Lane et al 1909) Sparling (1970) exten-sively described the Bass Islands lithofacies as a series ofshallow-water carbonate deposits that have been dolomi-tized and diagenetically altered by processes associatedwith the Tippecanoe-Kaskaskia sequence-bounding un-conformity Dissolution of primary grains and larger-

146 Reservoir Characteristics of the Bass Islands Dolomite in O

scale dissolution and formation of solution collapse brec-cias are characteristics of the Bass Islands in the outcroparea Although wireline-log correlations show that theBass Islands in the Michigan Basin subsurface continuesthis dolomitic lithology they cannot help to determinethe facies and fabric characteristics seen in outcrop

Figure 6 (A) Core photograph(3442 ft [1049 m]) showing theboundary between the Tippeca-noe and Kaskaskia megase-quences of Sloss (1963) with shelllag followed by condensed inter-val and a chert-rich zone nearthe top of the core (B) Corephotograph (3472 ft [1058 m])showing brecciation and solutionpipes related to karst developedduring subaerial exposure(C) Core photograph (3461 ft[1055 m]) showing well-developedcross-bedding in peloidal grain-stone associated with high-energycarbonate sand shoals (D) Corephotograph (3477 ft [1060 m]) oflaminated cyanobacterial matsassociated with tidal flat depositsPhotos are by Linda Harrison

tsego County Michigan

Limited core observations (Gardner 1974) do suggestshallow-water deposits and diagenetic alteration includ-ing brecciation

Acquisition of a new core from the Core Energy-St Charlton 4-30 well provides an opportunity to care-

fully evaluate the reservoir properties of this formationThe well penetrated at least 188 ft (57 m) of Bass IslandsFormation and cored 78 ft (24 m) of the uppermost BassIslands along with 42 ft (13 m) of the overlying BoisBlanc Formation (Figure 5) This is the most complete

Figure 7 Thin-section photomicrographs (plane light) of reservoir intervals (A) Brecciated (Br) finely crystalline dolomite related tokarst developed in response to subaerial exposure (3469 ft [1057 m]) Well-developed solution-enhanced vugs (V) and intercrystallineporosity The measured porosity and permeability (Klinkenberg) in this zone are gt31 and gt290 md respectively (B) Brecciated (Br)finely crystalline dolomite related to karst developed in response to subaerial exposure (34722 ft [10583 m]) Well-developed solution-enhanced vugs (V) and intercrystalline porosity The measured porosity and permeability (Klinkenberg) in this zone are gt26 and 90 mdrespectively (C) Dolomitized burrowed peloidal skeletal wackestone Pore types include fossil molds (M) solution-enhanced vugs (V) andintercrystalline pores Late saddle dolomite (SD) partially occludes local vugs (3466 ft [1056 m]) (D) Dolomitized peloidal packstone tograinstone Well-developed solution-enhanced vugs (V) small molds and intercrystalline porosity (3461 ft [1055 m]) (E) Dolomitizedbioturbated peloidal wackestone-packstone Well-developed solution-enhanced vugs (V) small molds and intercrystalline porosity (3495 ft[1065 m]) (F) Higher-magnification photograph of sample E showing well-developed solution-enhanced vugs (V) small molds and inter-crystalline porosity

Harrison et al 147

subsurface core sample known for the Bass Islands For-mation inMichigan The Tippecanoe-Kaskaskia sequence-bounding unconformity is apparent in the core at 34424 ft(10493 m) (Figure 6A) Immediately overlying the un-conformity is a bed of brachiopod shells and an abun-dance of siliceous replacement of the original carbonateThe Bass Islands lithology immediately underlying theunconformity is a dolomitized bioturbated skeletalwackestone This package can be subdivided into severalmeter-scale shallowing-upward cycles of stacked faciesThe cycles begin with a shallow subtidal burrowed andbioturbated skeletal and peloidal wackestone to pack-stone (Figure 6B) These subtidal facies shallow upwardinto higher-energy cross-bedded and laminated peloidalgrainstones (Figure 6C) The grainstones are capped bylaminated and crenulated cyanobacterial mat mudstonesthat represent high intertidal or supratidal environmentswith subaerial exposure and represent the top of theshallowing-upward cycle (Figure 6D)

148 Reservoir Characteristics of the Bass Islands Dolomite in O

Reservoir Properties

Although the reservoir properties of shallow-water car-bonate deposits in the Bass Islands of Michigan aregreatly influenced by dissolution and diagenesis strongfabric characteristics which are inherited from theiroriginal depositional facies character influence reser-voir properties This carbonate reservoir has some pre-served primary porosity as well as abundant secondaryporosity Dissolution has produced significant second-ary pores (Figures 6B 7A B) Fossil grains are most vul-nerable because dissolution processes have selectivelyremoved grains instead of the finer micritic matrix(Figure 7C E F) In addition to moldic porosity abun-dant intercrystalline porosity resulting from dolomitiza-tion is observed In the grainstones abundant intergran-ular porosity and significant permeability are observedMuch of the porosity in the grainstones is primary inter-granular porosity although some pores have been modi-fied into vugs by dissolution Figure 7D shows a thin sec-tion of the cross-bedded grainstone seen in Figure 7C at3461 ft (1055 m)

Conventional coreporosity andpermeability (PampP)havebeenmeasured from 2-in (5-cm)-long by 1-in (25-cm)-diameter sample plugs drilled fromwhole core collectedin theCore Energy-St Charlton 4-30well Seventy-fourhorizontal plugs 12 vertical plugs 6 whole cores and 17sidewall core plugs were sent to Core Laboratories forroutine PampP analyses Sixty-six horizontal plugs werecollected in the Bass Islands reservoir interval Differentfacies show substantially different reservoir properties(Figure 8) Intergranular porosity in the grainstone faciesis representative of primary porosity that forms useful po-rosity zones in the Bass Islands in this well Good porosityalso exists as the result of selective dissolution of some car-bonate grains producingmoldic porosity This facies is fur-ther enhanced by karst dissolution and brecciation to be-come the best PampP in the entire formation These faciesare relatively thin beds stacked in a series of shallowing-upward packages that follow a predictable stratigraphicframework This framework can provide a better under-standing of the vertical and lateral distribution of the po-rous reservoir facies

Stratigraphic Framework

The Bass Islands interval exhibits 5-m (16-ft)-scale cy-cles nested within two high-frequency sequences (HFSapproximately 10 m [33 ft]) (Figure 9) Sequence andcycle boundaries were determined from core analysis

Figure 8 Porosity and permeability from core-plug analyses ofthe Core Energy-St Charlton 4-30 well Generalized facies typesare indicated throughout the cored section

tsego County Michigan

and were specifically based on evidence for subaerialexposure caused by a fall in relative sea level the pres-ence of flooding surfaces indicating a transgressive eventor vertical successions of facies indicating an overall flood-ing event followed by a shallowing of facies indicating arelative fall in sea level Subaerial exposure horizons aremarked by brecciation (collapse breccia) solution pipesand local development of terra rosa all of which are com-mon diagnostic indicators of exposure (eg Esteban andKlappa 1983) The highest frequency packages are the cy-cles which are amaximumof approximately 24 ft (73m)thick and have an average thickness of 14 ft (43 m)The HFS range from 28 to 44 ft (85 to 134 m) thickand the Bass Islands interval is approximately 72 ft thick(22 m)

Reservoir Intervals

The best reservoir units in theCore Energy-St Charlton4-30 core are present in two different intervals Thefirst is related to solution brecciation and recrystalliza-tion of limestones caused by subaerial exposure wherewell-developed interparticle intercrystalline and solution-enhanced vugs contribute to the creation of high porositiesand permeabilities The second unit is related to inner plat-form (midramp) bioturbated facies where the burrow gal-leries have been preferentially dolomitized with coarsercrystalline sucrosic dolomite Figure 10 illustrates howthese types of facies commonly referred to as tubulartempestites may be in excellent communication becauseof the nature of the three-dimensional burrow galleries

Figure 9 Stratigraphic hierarchy observed in the Bass Islands interval High-frequency sequences (HFS) along with higher-frequencycycles correlate well to the stratigraphic position of the best reservoir intervals and many of the HFS and cycle boundaries are manifestedby higher gamma-ray log values Regressive hemicycles of both the HFS and higher-frequency cycles correlate to the best developmentof reservoir throughout much of the cored interval NPHI = neutron porosity PEF = photoelectric factor

Harrison et al 149

courtesy of E A Shinn

and the density of burrows commonly found in similartypes of environments

Reservoir development also correlates well with thestratigraphic framework (Figure 9) Sequence and cycleboundaries were identified by the presence of subaerialexposure horizons as indicated by karstic brecciation andsolution flooding surface and facies stacking patternsAs mentioned above a well-constrained hierarchy ex-ists within the Bass Islands interval whereby the higher-frequency cycles exist within two medium-scale HFSFrom a reservoir standpoint the HFS and most of thehigher-frequency cycles constrain the development ofthe best reservoir intervals Regressive hemicycles ofboth the HFS and higher-frequency cycles correlate tothe best development of the reservoir throughout muchof the cored interval Many of the HFS and cycleboundaries are manifested by higher gamma-ray logvalues and can be identified in wells without core dataWe believe that the correlation of depositional faciesobserved in core to stratigraphic packages and wireline-log signatures is a powerful tool to help interpret the geo-logic properties of the Bass Islands interval in otherareas throughout Michigan where only wireline logsare available

150 Reservoir Characteristics of the Bass Islands Dolomite in O

CONCLUSIONS

The Bass Islands Formation can be mapped throughoutmuch of northern and central lower Michigan where itis an important saline reservoir target for CO2 seques-tration with more than 40 ft (12 m) of greater than 10porosity and permeabilities exceeding 500md Averageporosity in the Bass Islands cored interval in the CoreEnergy-St Charlton 4-30 test well is 125 and averagepermeability is 224 md

The Bass Islands in the area of the Core Energy-StCharlton 4-30 well is composed of a series of shallow-water and tidal flat carbonate facies that are stacked in apredictable shallowing-upward pattern The Bass Is-lands Formation seen in this core can be subdivided intotwoHFS and five higher-frequency cycles Stratigraphicanalysis helps predict vertical and lateral facies patternsand contributes to the evaluation of regional reservoirproperties The best reservoir quality occurs in the sub-tidal burrowed packstones that have some dissolutionof grains and develop moldic porosity This facies is alsoenhanced further by karst dissolution and brecciationrelated to the Tippecanoe-Kaskaskia sequence-boundingunconformity at the top of the Bass Islands Formation

Figure 10 Some of the bestreservoirs in the Bass Islands ex-ists in the thoroughly bioturbatedinner platform facies Burrowsare of the Thalassinoides ichno-facies which is similar to thethree-dimensional galleries formedtoday by burrowing shrimp (egCallianassa) Photograph of theGreat Bahama Bank with the lo-cation of heavily bioturbated innerplatform facies In many modernenvironments these facies mayextend for thousands of squarekilometers Note the high densityof burrows and three-dimensionaldistribution of an individual bur-row gallery preserved in epoxyboth of which illustrate the po-tential for such facies to be lat-erally and vertically connected inthree dimensions commonlyleading to excellent reservoir

potential The plastic castphotograph of the ghost shrimpCallianassa burrow gallery is

tsego County Michigan

Additional porosity can be found in carbonate grainstonefacies that preserve some primary intergranular porosityThe shallowest tidal flat or stromatolitic facies showminimal PampP and are likely to be internal baffles or bar-riers to fluid flow through the formation In addition cor-relation of wireline logs shows that the Bass Islands cyclicporosity seen in the St Charlton 4-30 well is presentthroughout the northern part of lower Michigan

REFERENCES CITED

Barnes D A W B Harrison III and A Wahr in press Assessmentof regional geological carbon sequestration potential in UpperSilurian to Middle Devonian strata of the Michigan Basin inM Grobe J C Pashin and R L Dodge eds Carbon dioxidesequestration in geologic mediamdashState of the science AAPGStudies in Geology 59

Catacosinos P A P A Daniels Jr and W B Harrison III 1991Structure stratigraphy and petroleum geology of the MichiganBasin inMW Leighton D R Kolata D FOltz and J J Eideleds Interior cratonic basins AAPG Memoir 51 p 561ndash602

Esteban M and C F Klappa 1983 Subaerial exposure environ-

ment in P A Scholle D G Bebout and C H Moore edsCarbonate depositional environments AAPG Memoir 33p 1ndash95

Gardner W C 1974 Middle Devonian stratigraphy and deposi-tional environments in the Michigan Basin Michigan BasinGeological Society Special Paper 1 138 p

GrammerGMDABarnesWBHarrison III andAESandomierskiin press Practical synergies for increasing domestic oil produc-tion and geological sequestration of anthropogenic CO2 An ex-ample from the Michigan Basin in M Grobe J C Pashin andR L Dodge eds Carbon dioxide sequestration in geologicmediamdashState of the science AAPG Studies in Geology 59

Lane A C C S Prosser W H Sherzer and A W Grabau 1909Nomenclature and subdivision of the Upper Silurian strata ofMichigan Ohio and western New York Geological Society ofAmerica Bulletin v 19 p 553ndash556

Lilienthal R T 1978 Stratigraphic cross-sections of the MichiganBasin Department of Natural Resources Geological SurveyDivision Report of Investigation No 19 36 p

Sloss L L 1963 Sequences in the cratonic interior of North Amer-ica Geological Society of America Bulletin v 75 p 93ndash113doi1011300016-7606(1963)74[93SITCIO]20CO2

Sparling D R 1970 The Bass Islands Formation in its type regionThe Ohio Journal of Science v 70 no 1 p 1ndash33

US Department of Energy 2007 Carbon sequestration atlas of theUnited States and Canada Pittsburgh Pennsylvania NationalEnergy Technology Laboratory 86 p

Harrison et al 151

Figure 1 Location map of the Core Energy-State Charlton 4-30 well in northern lower Michigan The map shows oil gas and dry wells with the inset showing Otsego CountyMichigan Southwestndashnortheast-trending oil and gas wells in Manistee to Presque Isle counties are mainly in the northern Niagaran (Silurian) reef trend The eastndashwest swath of gas wellsin Antrim to Alpena counties are predominantly Devonian Antrim Shale gas wells NGRN = Silurian Niagaran pinnacle reef oil and gas fields

142ReservoirCharacteristics

oftheBass

IslandsDolom

iteinOtsego

CountyMichigan

Figure 2 Part of the general-ized Michigan stratigraphic col-umn showing Upper Silurian andLowerndashMiddle Devonian strataThe Silurian-Devonian boundaryis placed at the top of the BassIslands The Mackinac BrecciaGarden Island Formation andSylvania Sandstone are not pre-sent in Otsego County

Figure 3 Isopach map of Bass Islands Dolomite in northern and central Michigan The 2600-ft (788-m) structural contour line marksthe minimum depth for maintaining supercritical-phase CO2 in the subsurface Gridding and contouring were done using ordinary krigingand a spherical variogram model The root-mean-square error was 136 ft (41 m)

Harrison et al 143

Figure 4 Northndashsouth wireline-log cross section showing the Bass Islands Formation (BILD) reservoir interval and the overlying confining unit Bois Blanc Formation (BBLC) and themajor seal unit Amherstberg Formation (AMBG) Well number 57916 is the Core Energy-St Charlton 4-30 well GR = gamma ray RHOB = bulk density NPHI = neutron porosity PEF =photoelectric factor BILD-EVAP = Bass Islands evaporate

144ReservoirCharacteristics

oftheBass

IslandsDolom

iteinOtsego

CountyMichigan

Figure 5 Well logs andcore data from the lowerpart of the Core Energy-St Charlton 4-30 wellsection 30 T31N-R1WOtsego County MichiganBK = base of the Kaskas-kia megasequence andposition of unconformityon top of the Bass IslandsFormation BILD = BassIslands Formation BBLC =Bois Blanc FormationAMBG = AmherstbergFormation The core inter-val reported in this study isthe lower half of core 3and all of core 4 NPHI =neutron porosity PEF =photoelectric factor

Ha

rrison et al 145

of Lake Erie (Lane et al 1909) Sparling (1970) exten-sively described the Bass Islands lithofacies as a series ofshallow-water carbonate deposits that have been dolomi-tized and diagenetically altered by processes associatedwith the Tippecanoe-Kaskaskia sequence-bounding un-conformity Dissolution of primary grains and larger-

146 Reservoir Characteristics of the Bass Islands Dolomite in O

scale dissolution and formation of solution collapse brec-cias are characteristics of the Bass Islands in the outcroparea Although wireline-log correlations show that theBass Islands in the Michigan Basin subsurface continuesthis dolomitic lithology they cannot help to determinethe facies and fabric characteristics seen in outcrop

Figure 6 (A) Core photograph(3442 ft [1049 m]) showing theboundary between the Tippeca-noe and Kaskaskia megase-quences of Sloss (1963) with shelllag followed by condensed inter-val and a chert-rich zone nearthe top of the core (B) Corephotograph (3472 ft [1058 m])showing brecciation and solutionpipes related to karst developedduring subaerial exposure(C) Core photograph (3461 ft[1055 m]) showing well-developedcross-bedding in peloidal grain-stone associated with high-energycarbonate sand shoals (D) Corephotograph (3477 ft [1060 m]) oflaminated cyanobacterial matsassociated with tidal flat depositsPhotos are by Linda Harrison

tsego County Michigan

Limited core observations (Gardner 1974) do suggestshallow-water deposits and diagenetic alteration includ-ing brecciation

Acquisition of a new core from the Core Energy-St Charlton 4-30 well provides an opportunity to care-

fully evaluate the reservoir properties of this formationThe well penetrated at least 188 ft (57 m) of Bass IslandsFormation and cored 78 ft (24 m) of the uppermost BassIslands along with 42 ft (13 m) of the overlying BoisBlanc Formation (Figure 5) This is the most complete

Figure 7 Thin-section photomicrographs (plane light) of reservoir intervals (A) Brecciated (Br) finely crystalline dolomite related tokarst developed in response to subaerial exposure (3469 ft [1057 m]) Well-developed solution-enhanced vugs (V) and intercrystallineporosity The measured porosity and permeability (Klinkenberg) in this zone are gt31 and gt290 md respectively (B) Brecciated (Br)finely crystalline dolomite related to karst developed in response to subaerial exposure (34722 ft [10583 m]) Well-developed solution-enhanced vugs (V) and intercrystalline porosity The measured porosity and permeability (Klinkenberg) in this zone are gt26 and 90 mdrespectively (C) Dolomitized burrowed peloidal skeletal wackestone Pore types include fossil molds (M) solution-enhanced vugs (V) andintercrystalline pores Late saddle dolomite (SD) partially occludes local vugs (3466 ft [1056 m]) (D) Dolomitized peloidal packstone tograinstone Well-developed solution-enhanced vugs (V) small molds and intercrystalline porosity (3461 ft [1055 m]) (E) Dolomitizedbioturbated peloidal wackestone-packstone Well-developed solution-enhanced vugs (V) small molds and intercrystalline porosity (3495 ft[1065 m]) (F) Higher-magnification photograph of sample E showing well-developed solution-enhanced vugs (V) small molds and inter-crystalline porosity

Harrison et al 147

subsurface core sample known for the Bass Islands For-mation inMichigan The Tippecanoe-Kaskaskia sequence-bounding unconformity is apparent in the core at 34424 ft(10493 m) (Figure 6A) Immediately overlying the un-conformity is a bed of brachiopod shells and an abun-dance of siliceous replacement of the original carbonateThe Bass Islands lithology immediately underlying theunconformity is a dolomitized bioturbated skeletalwackestone This package can be subdivided into severalmeter-scale shallowing-upward cycles of stacked faciesThe cycles begin with a shallow subtidal burrowed andbioturbated skeletal and peloidal wackestone to pack-stone (Figure 6B) These subtidal facies shallow upwardinto higher-energy cross-bedded and laminated peloidalgrainstones (Figure 6C) The grainstones are capped bylaminated and crenulated cyanobacterial mat mudstonesthat represent high intertidal or supratidal environmentswith subaerial exposure and represent the top of theshallowing-upward cycle (Figure 6D)

148 Reservoir Characteristics of the Bass Islands Dolomite in O

Reservoir Properties

Although the reservoir properties of shallow-water car-bonate deposits in the Bass Islands of Michigan aregreatly influenced by dissolution and diagenesis strongfabric characteristics which are inherited from theiroriginal depositional facies character influence reser-voir properties This carbonate reservoir has some pre-served primary porosity as well as abundant secondaryporosity Dissolution has produced significant second-ary pores (Figures 6B 7A B) Fossil grains are most vul-nerable because dissolution processes have selectivelyremoved grains instead of the finer micritic matrix(Figure 7C E F) In addition to moldic porosity abun-dant intercrystalline porosity resulting from dolomitiza-tion is observed In the grainstones abundant intergran-ular porosity and significant permeability are observedMuch of the porosity in the grainstones is primary inter-granular porosity although some pores have been modi-fied into vugs by dissolution Figure 7D shows a thin sec-tion of the cross-bedded grainstone seen in Figure 7C at3461 ft (1055 m)

Conventional coreporosity andpermeability (PampP)havebeenmeasured from 2-in (5-cm)-long by 1-in (25-cm)-diameter sample plugs drilled fromwhole core collectedin theCore Energy-St Charlton 4-30well Seventy-fourhorizontal plugs 12 vertical plugs 6 whole cores and 17sidewall core plugs were sent to Core Laboratories forroutine PampP analyses Sixty-six horizontal plugs werecollected in the Bass Islands reservoir interval Differentfacies show substantially different reservoir properties(Figure 8) Intergranular porosity in the grainstone faciesis representative of primary porosity that forms useful po-rosity zones in the Bass Islands in this well Good porosityalso exists as the result of selective dissolution of some car-bonate grains producingmoldic porosity This facies is fur-ther enhanced by karst dissolution and brecciation to be-come the best PampP in the entire formation These faciesare relatively thin beds stacked in a series of shallowing-upward packages that follow a predictable stratigraphicframework This framework can provide a better under-standing of the vertical and lateral distribution of the po-rous reservoir facies

Stratigraphic Framework

The Bass Islands interval exhibits 5-m (16-ft)-scale cy-cles nested within two high-frequency sequences (HFSapproximately 10 m [33 ft]) (Figure 9) Sequence andcycle boundaries were determined from core analysis

Figure 8 Porosity and permeability from core-plug analyses ofthe Core Energy-St Charlton 4-30 well Generalized facies typesare indicated throughout the cored section

tsego County Michigan

and were specifically based on evidence for subaerialexposure caused by a fall in relative sea level the pres-ence of flooding surfaces indicating a transgressive eventor vertical successions of facies indicating an overall flood-ing event followed by a shallowing of facies indicating arelative fall in sea level Subaerial exposure horizons aremarked by brecciation (collapse breccia) solution pipesand local development of terra rosa all of which are com-mon diagnostic indicators of exposure (eg Esteban andKlappa 1983) The highest frequency packages are the cy-cles which are amaximumof approximately 24 ft (73m)thick and have an average thickness of 14 ft (43 m)The HFS range from 28 to 44 ft (85 to 134 m) thickand the Bass Islands interval is approximately 72 ft thick(22 m)

Reservoir Intervals

The best reservoir units in theCore Energy-St Charlton4-30 core are present in two different intervals Thefirst is related to solution brecciation and recrystalliza-tion of limestones caused by subaerial exposure wherewell-developed interparticle intercrystalline and solution-enhanced vugs contribute to the creation of high porositiesand permeabilities The second unit is related to inner plat-form (midramp) bioturbated facies where the burrow gal-leries have been preferentially dolomitized with coarsercrystalline sucrosic dolomite Figure 10 illustrates howthese types of facies commonly referred to as tubulartempestites may be in excellent communication becauseof the nature of the three-dimensional burrow galleries

Figure 9 Stratigraphic hierarchy observed in the Bass Islands interval High-frequency sequences (HFS) along with higher-frequencycycles correlate well to the stratigraphic position of the best reservoir intervals and many of the HFS and cycle boundaries are manifestedby higher gamma-ray log values Regressive hemicycles of both the HFS and higher-frequency cycles correlate to the best developmentof reservoir throughout much of the cored interval NPHI = neutron porosity PEF = photoelectric factor

Harrison et al 149

courtesy of E A Shinn

and the density of burrows commonly found in similartypes of environments

Reservoir development also correlates well with thestratigraphic framework (Figure 9) Sequence and cycleboundaries were identified by the presence of subaerialexposure horizons as indicated by karstic brecciation andsolution flooding surface and facies stacking patternsAs mentioned above a well-constrained hierarchy ex-ists within the Bass Islands interval whereby the higher-frequency cycles exist within two medium-scale HFSFrom a reservoir standpoint the HFS and most of thehigher-frequency cycles constrain the development ofthe best reservoir intervals Regressive hemicycles ofboth the HFS and higher-frequency cycles correlate tothe best development of the reservoir throughout muchof the cored interval Many of the HFS and cycleboundaries are manifested by higher gamma-ray logvalues and can be identified in wells without core dataWe believe that the correlation of depositional faciesobserved in core to stratigraphic packages and wireline-log signatures is a powerful tool to help interpret the geo-logic properties of the Bass Islands interval in otherareas throughout Michigan where only wireline logsare available

150 Reservoir Characteristics of the Bass Islands Dolomite in O

CONCLUSIONS

The Bass Islands Formation can be mapped throughoutmuch of northern and central lower Michigan where itis an important saline reservoir target for CO2 seques-tration with more than 40 ft (12 m) of greater than 10porosity and permeabilities exceeding 500md Averageporosity in the Bass Islands cored interval in the CoreEnergy-St Charlton 4-30 test well is 125 and averagepermeability is 224 md

The Bass Islands in the area of the Core Energy-StCharlton 4-30 well is composed of a series of shallow-water and tidal flat carbonate facies that are stacked in apredictable shallowing-upward pattern The Bass Is-lands Formation seen in this core can be subdivided intotwoHFS and five higher-frequency cycles Stratigraphicanalysis helps predict vertical and lateral facies patternsand contributes to the evaluation of regional reservoirproperties The best reservoir quality occurs in the sub-tidal burrowed packstones that have some dissolutionof grains and develop moldic porosity This facies is alsoenhanced further by karst dissolution and brecciationrelated to the Tippecanoe-Kaskaskia sequence-boundingunconformity at the top of the Bass Islands Formation

Figure 10 Some of the bestreservoirs in the Bass Islands ex-ists in the thoroughly bioturbatedinner platform facies Burrowsare of the Thalassinoides ichno-facies which is similar to thethree-dimensional galleries formedtoday by burrowing shrimp (egCallianassa) Photograph of theGreat Bahama Bank with the lo-cation of heavily bioturbated innerplatform facies In many modernenvironments these facies mayextend for thousands of squarekilometers Note the high densityof burrows and three-dimensionaldistribution of an individual bur-row gallery preserved in epoxyboth of which illustrate the po-tential for such facies to be lat-erally and vertically connected inthree dimensions commonlyleading to excellent reservoir

potential The plastic castphotograph of the ghost shrimpCallianassa burrow gallery is

tsego County Michigan

Additional porosity can be found in carbonate grainstonefacies that preserve some primary intergranular porosityThe shallowest tidal flat or stromatolitic facies showminimal PampP and are likely to be internal baffles or bar-riers to fluid flow through the formation In addition cor-relation of wireline logs shows that the Bass Islands cyclicporosity seen in the St Charlton 4-30 well is presentthroughout the northern part of lower Michigan

REFERENCES CITED

Barnes D A W B Harrison III and A Wahr in press Assessmentof regional geological carbon sequestration potential in UpperSilurian to Middle Devonian strata of the Michigan Basin inM Grobe J C Pashin and R L Dodge eds Carbon dioxidesequestration in geologic mediamdashState of the science AAPGStudies in Geology 59

Catacosinos P A P A Daniels Jr and W B Harrison III 1991Structure stratigraphy and petroleum geology of the MichiganBasin inMW Leighton D R Kolata D FOltz and J J Eideleds Interior cratonic basins AAPG Memoir 51 p 561ndash602

Esteban M and C F Klappa 1983 Subaerial exposure environ-

ment in P A Scholle D G Bebout and C H Moore edsCarbonate depositional environments AAPG Memoir 33p 1ndash95

Gardner W C 1974 Middle Devonian stratigraphy and deposi-tional environments in the Michigan Basin Michigan BasinGeological Society Special Paper 1 138 p

GrammerGMDABarnesWBHarrison III andAESandomierskiin press Practical synergies for increasing domestic oil produc-tion and geological sequestration of anthropogenic CO2 An ex-ample from the Michigan Basin in M Grobe J C Pashin andR L Dodge eds Carbon dioxide sequestration in geologicmediamdashState of the science AAPG Studies in Geology 59

Lane A C C S Prosser W H Sherzer and A W Grabau 1909Nomenclature and subdivision of the Upper Silurian strata ofMichigan Ohio and western New York Geological Society ofAmerica Bulletin v 19 p 553ndash556

Lilienthal R T 1978 Stratigraphic cross-sections of the MichiganBasin Department of Natural Resources Geological SurveyDivision Report of Investigation No 19 36 p

Sloss L L 1963 Sequences in the cratonic interior of North Amer-ica Geological Society of America Bulletin v 75 p 93ndash113doi1011300016-7606(1963)74[93SITCIO]20CO2

Sparling D R 1970 The Bass Islands Formation in its type regionThe Ohio Journal of Science v 70 no 1 p 1ndash33

US Department of Energy 2007 Carbon sequestration atlas of theUnited States and Canada Pittsburgh Pennsylvania NationalEnergy Technology Laboratory 86 p

Harrison et al 151

Figure 2 Part of the general-ized Michigan stratigraphic col-umn showing Upper Silurian andLowerndashMiddle Devonian strataThe Silurian-Devonian boundaryis placed at the top of the BassIslands The Mackinac BrecciaGarden Island Formation andSylvania Sandstone are not pre-sent in Otsego County

Figure 3 Isopach map of Bass Islands Dolomite in northern and central Michigan The 2600-ft (788-m) structural contour line marksthe minimum depth for maintaining supercritical-phase CO2 in the subsurface Gridding and contouring were done using ordinary krigingand a spherical variogram model The root-mean-square error was 136 ft (41 m)

Harrison et al 143

Figure 4 Northndashsouth wireline-log cross section showing the Bass Islands Formation (BILD) reservoir interval and the overlying confining unit Bois Blanc Formation (BBLC) and themajor seal unit Amherstberg Formation (AMBG) Well number 57916 is the Core Energy-St Charlton 4-30 well GR = gamma ray RHOB = bulk density NPHI = neutron porosity PEF =photoelectric factor BILD-EVAP = Bass Islands evaporate

144ReservoirCharacteristics

oftheBass

IslandsDolom

iteinOtsego

CountyMichigan

Figure 5 Well logs andcore data from the lowerpart of the Core Energy-St Charlton 4-30 wellsection 30 T31N-R1WOtsego County MichiganBK = base of the Kaskas-kia megasequence andposition of unconformityon top of the Bass IslandsFormation BILD = BassIslands Formation BBLC =Bois Blanc FormationAMBG = AmherstbergFormation The core inter-val reported in this study isthe lower half of core 3and all of core 4 NPHI =neutron porosity PEF =photoelectric factor

Ha

rrison et al 145

of Lake Erie (Lane et al 1909) Sparling (1970) exten-sively described the Bass Islands lithofacies as a series ofshallow-water carbonate deposits that have been dolomi-tized and diagenetically altered by processes associatedwith the Tippecanoe-Kaskaskia sequence-bounding un-conformity Dissolution of primary grains and larger-

146 Reservoir Characteristics of the Bass Islands Dolomite in O

scale dissolution and formation of solution collapse brec-cias are characteristics of the Bass Islands in the outcroparea Although wireline-log correlations show that theBass Islands in the Michigan Basin subsurface continuesthis dolomitic lithology they cannot help to determinethe facies and fabric characteristics seen in outcrop

Figure 6 (A) Core photograph(3442 ft [1049 m]) showing theboundary between the Tippeca-noe and Kaskaskia megase-quences of Sloss (1963) with shelllag followed by condensed inter-val and a chert-rich zone nearthe top of the core (B) Corephotograph (3472 ft [1058 m])showing brecciation and solutionpipes related to karst developedduring subaerial exposure(C) Core photograph (3461 ft[1055 m]) showing well-developedcross-bedding in peloidal grain-stone associated with high-energycarbonate sand shoals (D) Corephotograph (3477 ft [1060 m]) oflaminated cyanobacterial matsassociated with tidal flat depositsPhotos are by Linda Harrison

tsego County Michigan

Limited core observations (Gardner 1974) do suggestshallow-water deposits and diagenetic alteration includ-ing brecciation

Acquisition of a new core from the Core Energy-St Charlton 4-30 well provides an opportunity to care-

fully evaluate the reservoir properties of this formationThe well penetrated at least 188 ft (57 m) of Bass IslandsFormation and cored 78 ft (24 m) of the uppermost BassIslands along with 42 ft (13 m) of the overlying BoisBlanc Formation (Figure 5) This is the most complete

Figure 7 Thin-section photomicrographs (plane light) of reservoir intervals (A) Brecciated (Br) finely crystalline dolomite related tokarst developed in response to subaerial exposure (3469 ft [1057 m]) Well-developed solution-enhanced vugs (V) and intercrystallineporosity The measured porosity and permeability (Klinkenberg) in this zone are gt31 and gt290 md respectively (B) Brecciated (Br)finely crystalline dolomite related to karst developed in response to subaerial exposure (34722 ft [10583 m]) Well-developed solution-enhanced vugs (V) and intercrystalline porosity The measured porosity and permeability (Klinkenberg) in this zone are gt26 and 90 mdrespectively (C) Dolomitized burrowed peloidal skeletal wackestone Pore types include fossil molds (M) solution-enhanced vugs (V) andintercrystalline pores Late saddle dolomite (SD) partially occludes local vugs (3466 ft [1056 m]) (D) Dolomitized peloidal packstone tograinstone Well-developed solution-enhanced vugs (V) small molds and intercrystalline porosity (3461 ft [1055 m]) (E) Dolomitizedbioturbated peloidal wackestone-packstone Well-developed solution-enhanced vugs (V) small molds and intercrystalline porosity (3495 ft[1065 m]) (F) Higher-magnification photograph of sample E showing well-developed solution-enhanced vugs (V) small molds and inter-crystalline porosity

Harrison et al 147

subsurface core sample known for the Bass Islands For-mation inMichigan The Tippecanoe-Kaskaskia sequence-bounding unconformity is apparent in the core at 34424 ft(10493 m) (Figure 6A) Immediately overlying the un-conformity is a bed of brachiopod shells and an abun-dance of siliceous replacement of the original carbonateThe Bass Islands lithology immediately underlying theunconformity is a dolomitized bioturbated skeletalwackestone This package can be subdivided into severalmeter-scale shallowing-upward cycles of stacked faciesThe cycles begin with a shallow subtidal burrowed andbioturbated skeletal and peloidal wackestone to pack-stone (Figure 6B) These subtidal facies shallow upwardinto higher-energy cross-bedded and laminated peloidalgrainstones (Figure 6C) The grainstones are capped bylaminated and crenulated cyanobacterial mat mudstonesthat represent high intertidal or supratidal environmentswith subaerial exposure and represent the top of theshallowing-upward cycle (Figure 6D)

148 Reservoir Characteristics of the Bass Islands Dolomite in O

Reservoir Properties

Although the reservoir properties of shallow-water car-bonate deposits in the Bass Islands of Michigan aregreatly influenced by dissolution and diagenesis strongfabric characteristics which are inherited from theiroriginal depositional facies character influence reser-voir properties This carbonate reservoir has some pre-served primary porosity as well as abundant secondaryporosity Dissolution has produced significant second-ary pores (Figures 6B 7A B) Fossil grains are most vul-nerable because dissolution processes have selectivelyremoved grains instead of the finer micritic matrix(Figure 7C E F) In addition to moldic porosity abun-dant intercrystalline porosity resulting from dolomitiza-tion is observed In the grainstones abundant intergran-ular porosity and significant permeability are observedMuch of the porosity in the grainstones is primary inter-granular porosity although some pores have been modi-fied into vugs by dissolution Figure 7D shows a thin sec-tion of the cross-bedded grainstone seen in Figure 7C at3461 ft (1055 m)

Conventional coreporosity andpermeability (PampP)havebeenmeasured from 2-in (5-cm)-long by 1-in (25-cm)-diameter sample plugs drilled fromwhole core collectedin theCore Energy-St Charlton 4-30well Seventy-fourhorizontal plugs 12 vertical plugs 6 whole cores and 17sidewall core plugs were sent to Core Laboratories forroutine PampP analyses Sixty-six horizontal plugs werecollected in the Bass Islands reservoir interval Differentfacies show substantially different reservoir properties(Figure 8) Intergranular porosity in the grainstone faciesis representative of primary porosity that forms useful po-rosity zones in the Bass Islands in this well Good porosityalso exists as the result of selective dissolution of some car-bonate grains producingmoldic porosity This facies is fur-ther enhanced by karst dissolution and brecciation to be-come the best PampP in the entire formation These faciesare relatively thin beds stacked in a series of shallowing-upward packages that follow a predictable stratigraphicframework This framework can provide a better under-standing of the vertical and lateral distribution of the po-rous reservoir facies

Stratigraphic Framework

The Bass Islands interval exhibits 5-m (16-ft)-scale cy-cles nested within two high-frequency sequences (HFSapproximately 10 m [33 ft]) (Figure 9) Sequence andcycle boundaries were determined from core analysis

Figure 8 Porosity and permeability from core-plug analyses ofthe Core Energy-St Charlton 4-30 well Generalized facies typesare indicated throughout the cored section

tsego County Michigan

and were specifically based on evidence for subaerialexposure caused by a fall in relative sea level the pres-ence of flooding surfaces indicating a transgressive eventor vertical successions of facies indicating an overall flood-ing event followed by a shallowing of facies indicating arelative fall in sea level Subaerial exposure horizons aremarked by brecciation (collapse breccia) solution pipesand local development of terra rosa all of which are com-mon diagnostic indicators of exposure (eg Esteban andKlappa 1983) The highest frequency packages are the cy-cles which are amaximumof approximately 24 ft (73m)thick and have an average thickness of 14 ft (43 m)The HFS range from 28 to 44 ft (85 to 134 m) thickand the Bass Islands interval is approximately 72 ft thick(22 m)

Reservoir Intervals

The best reservoir units in theCore Energy-St Charlton4-30 core are present in two different intervals Thefirst is related to solution brecciation and recrystalliza-tion of limestones caused by subaerial exposure wherewell-developed interparticle intercrystalline and solution-enhanced vugs contribute to the creation of high porositiesand permeabilities The second unit is related to inner plat-form (midramp) bioturbated facies where the burrow gal-leries have been preferentially dolomitized with coarsercrystalline sucrosic dolomite Figure 10 illustrates howthese types of facies commonly referred to as tubulartempestites may be in excellent communication becauseof the nature of the three-dimensional burrow galleries

Figure 9 Stratigraphic hierarchy observed in the Bass Islands interval High-frequency sequences (HFS) along with higher-frequencycycles correlate well to the stratigraphic position of the best reservoir intervals and many of the HFS and cycle boundaries are manifestedby higher gamma-ray log values Regressive hemicycles of both the HFS and higher-frequency cycles correlate to the best developmentof reservoir throughout much of the cored interval NPHI = neutron porosity PEF = photoelectric factor

Harrison et al 149

courtesy of E A Shinn

and the density of burrows commonly found in similartypes of environments

Reservoir development also correlates well with thestratigraphic framework (Figure 9) Sequence and cycleboundaries were identified by the presence of subaerialexposure horizons as indicated by karstic brecciation andsolution flooding surface and facies stacking patternsAs mentioned above a well-constrained hierarchy ex-ists within the Bass Islands interval whereby the higher-frequency cycles exist within two medium-scale HFSFrom a reservoir standpoint the HFS and most of thehigher-frequency cycles constrain the development ofthe best reservoir intervals Regressive hemicycles ofboth the HFS and higher-frequency cycles correlate tothe best development of the reservoir throughout muchof the cored interval Many of the HFS and cycleboundaries are manifested by higher gamma-ray logvalues and can be identified in wells without core dataWe believe that the correlation of depositional faciesobserved in core to stratigraphic packages and wireline-log signatures is a powerful tool to help interpret the geo-logic properties of the Bass Islands interval in otherareas throughout Michigan where only wireline logsare available

150 Reservoir Characteristics of the Bass Islands Dolomite in O

CONCLUSIONS

The Bass Islands Formation can be mapped throughoutmuch of northern and central lower Michigan where itis an important saline reservoir target for CO2 seques-tration with more than 40 ft (12 m) of greater than 10porosity and permeabilities exceeding 500md Averageporosity in the Bass Islands cored interval in the CoreEnergy-St Charlton 4-30 test well is 125 and averagepermeability is 224 md

The Bass Islands in the area of the Core Energy-StCharlton 4-30 well is composed of a series of shallow-water and tidal flat carbonate facies that are stacked in apredictable shallowing-upward pattern The Bass Is-lands Formation seen in this core can be subdivided intotwoHFS and five higher-frequency cycles Stratigraphicanalysis helps predict vertical and lateral facies patternsand contributes to the evaluation of regional reservoirproperties The best reservoir quality occurs in the sub-tidal burrowed packstones that have some dissolutionof grains and develop moldic porosity This facies is alsoenhanced further by karst dissolution and brecciationrelated to the Tippecanoe-Kaskaskia sequence-boundingunconformity at the top of the Bass Islands Formation

Figure 10 Some of the bestreservoirs in the Bass Islands ex-ists in the thoroughly bioturbatedinner platform facies Burrowsare of the Thalassinoides ichno-facies which is similar to thethree-dimensional galleries formedtoday by burrowing shrimp (egCallianassa) Photograph of theGreat Bahama Bank with the lo-cation of heavily bioturbated innerplatform facies In many modernenvironments these facies mayextend for thousands of squarekilometers Note the high densityof burrows and three-dimensionaldistribution of an individual bur-row gallery preserved in epoxyboth of which illustrate the po-tential for such facies to be lat-erally and vertically connected inthree dimensions commonlyleading to excellent reservoir

potential The plastic castphotograph of the ghost shrimpCallianassa burrow gallery is

tsego County Michigan

Additional porosity can be found in carbonate grainstonefacies that preserve some primary intergranular porosityThe shallowest tidal flat or stromatolitic facies showminimal PampP and are likely to be internal baffles or bar-riers to fluid flow through the formation In addition cor-relation of wireline logs shows that the Bass Islands cyclicporosity seen in the St Charlton 4-30 well is presentthroughout the northern part of lower Michigan

REFERENCES CITED

Barnes D A W B Harrison III and A Wahr in press Assessmentof regional geological carbon sequestration potential in UpperSilurian to Middle Devonian strata of the Michigan Basin inM Grobe J C Pashin and R L Dodge eds Carbon dioxidesequestration in geologic mediamdashState of the science AAPGStudies in Geology 59

Catacosinos P A P A Daniels Jr and W B Harrison III 1991Structure stratigraphy and petroleum geology of the MichiganBasin inMW Leighton D R Kolata D FOltz and J J Eideleds Interior cratonic basins AAPG Memoir 51 p 561ndash602

Esteban M and C F Klappa 1983 Subaerial exposure environ-

ment in P A Scholle D G Bebout and C H Moore edsCarbonate depositional environments AAPG Memoir 33p 1ndash95

Gardner W C 1974 Middle Devonian stratigraphy and deposi-tional environments in the Michigan Basin Michigan BasinGeological Society Special Paper 1 138 p

GrammerGMDABarnesWBHarrison III andAESandomierskiin press Practical synergies for increasing domestic oil produc-tion and geological sequestration of anthropogenic CO2 An ex-ample from the Michigan Basin in M Grobe J C Pashin andR L Dodge eds Carbon dioxide sequestration in geologicmediamdashState of the science AAPG Studies in Geology 59

Lane A C C S Prosser W H Sherzer and A W Grabau 1909Nomenclature and subdivision of the Upper Silurian strata ofMichigan Ohio and western New York Geological Society ofAmerica Bulletin v 19 p 553ndash556

Lilienthal R T 1978 Stratigraphic cross-sections of the MichiganBasin Department of Natural Resources Geological SurveyDivision Report of Investigation No 19 36 p

Sloss L L 1963 Sequences in the cratonic interior of North Amer-ica Geological Society of America Bulletin v 75 p 93ndash113doi1011300016-7606(1963)74[93SITCIO]20CO2

Sparling D R 1970 The Bass Islands Formation in its type regionThe Ohio Journal of Science v 70 no 1 p 1ndash33

US Department of Energy 2007 Carbon sequestration atlas of theUnited States and Canada Pittsburgh Pennsylvania NationalEnergy Technology Laboratory 86 p

Harrison et al 151

Figure 4 Northndashsouth wireline-log cross section showing the Bass Islands Formation (BILD) reservoir interval and the overlying confining unit Bois Blanc Formation (BBLC) and themajor seal unit Amherstberg Formation (AMBG) Well number 57916 is the Core Energy-St Charlton 4-30 well GR = gamma ray RHOB = bulk density NPHI = neutron porosity PEF =photoelectric factor BILD-EVAP = Bass Islands evaporate

144ReservoirCharacteristics

oftheBass

IslandsDolom

iteinOtsego

CountyMichigan

Figure 5 Well logs andcore data from the lowerpart of the Core Energy-St Charlton 4-30 wellsection 30 T31N-R1WOtsego County MichiganBK = base of the Kaskas-kia megasequence andposition of unconformityon top of the Bass IslandsFormation BILD = BassIslands Formation BBLC =Bois Blanc FormationAMBG = AmherstbergFormation The core inter-val reported in this study isthe lower half of core 3and all of core 4 NPHI =neutron porosity PEF =photoelectric factor

Ha

rrison et al 145

of Lake Erie (Lane et al 1909) Sparling (1970) exten-sively described the Bass Islands lithofacies as a series ofshallow-water carbonate deposits that have been dolomi-tized and diagenetically altered by processes associatedwith the Tippecanoe-Kaskaskia sequence-bounding un-conformity Dissolution of primary grains and larger-

146 Reservoir Characteristics of the Bass Islands Dolomite in O

scale dissolution and formation of solution collapse brec-cias are characteristics of the Bass Islands in the outcroparea Although wireline-log correlations show that theBass Islands in the Michigan Basin subsurface continuesthis dolomitic lithology they cannot help to determinethe facies and fabric characteristics seen in outcrop

Figure 6 (A) Core photograph(3442 ft [1049 m]) showing theboundary between the Tippeca-noe and Kaskaskia megase-quences of Sloss (1963) with shelllag followed by condensed inter-val and a chert-rich zone nearthe top of the core (B) Corephotograph (3472 ft [1058 m])showing brecciation and solutionpipes related to karst developedduring subaerial exposure(C) Core photograph (3461 ft[1055 m]) showing well-developedcross-bedding in peloidal grain-stone associated with high-energycarbonate sand shoals (D) Corephotograph (3477 ft [1060 m]) oflaminated cyanobacterial matsassociated with tidal flat depositsPhotos are by Linda Harrison

tsego County Michigan

Limited core observations (Gardner 1974) do suggestshallow-water deposits and diagenetic alteration includ-ing brecciation

Acquisition of a new core from the Core Energy-St Charlton 4-30 well provides an opportunity to care-

fully evaluate the reservoir properties of this formationThe well penetrated at least 188 ft (57 m) of Bass IslandsFormation and cored 78 ft (24 m) of the uppermost BassIslands along with 42 ft (13 m) of the overlying BoisBlanc Formation (Figure 5) This is the most complete

Figure 7 Thin-section photomicrographs (plane light) of reservoir intervals (A) Brecciated (Br) finely crystalline dolomite related tokarst developed in response to subaerial exposure (3469 ft [1057 m]) Well-developed solution-enhanced vugs (V) and intercrystallineporosity The measured porosity and permeability (Klinkenberg) in this zone are gt31 and gt290 md respectively (B) Brecciated (Br)finely crystalline dolomite related to karst developed in response to subaerial exposure (34722 ft [10583 m]) Well-developed solution-enhanced vugs (V) and intercrystalline porosity The measured porosity and permeability (Klinkenberg) in this zone are gt26 and 90 mdrespectively (C) Dolomitized burrowed peloidal skeletal wackestone Pore types include fossil molds (M) solution-enhanced vugs (V) andintercrystalline pores Late saddle dolomite (SD) partially occludes local vugs (3466 ft [1056 m]) (D) Dolomitized peloidal packstone tograinstone Well-developed solution-enhanced vugs (V) small molds and intercrystalline porosity (3461 ft [1055 m]) (E) Dolomitizedbioturbated peloidal wackestone-packstone Well-developed solution-enhanced vugs (V) small molds and intercrystalline porosity (3495 ft[1065 m]) (F) Higher-magnification photograph of sample E showing well-developed solution-enhanced vugs (V) small molds and inter-crystalline porosity

Harrison et al 147

subsurface core sample known for the Bass Islands For-mation inMichigan The Tippecanoe-Kaskaskia sequence-bounding unconformity is apparent in the core at 34424 ft(10493 m) (Figure 6A) Immediately overlying the un-conformity is a bed of brachiopod shells and an abun-dance of siliceous replacement of the original carbonateThe Bass Islands lithology immediately underlying theunconformity is a dolomitized bioturbated skeletalwackestone This package can be subdivided into severalmeter-scale shallowing-upward cycles of stacked faciesThe cycles begin with a shallow subtidal burrowed andbioturbated skeletal and peloidal wackestone to pack-stone (Figure 6B) These subtidal facies shallow upwardinto higher-energy cross-bedded and laminated peloidalgrainstones (Figure 6C) The grainstones are capped bylaminated and crenulated cyanobacterial mat mudstonesthat represent high intertidal or supratidal environmentswith subaerial exposure and represent the top of theshallowing-upward cycle (Figure 6D)

148 Reservoir Characteristics of the Bass Islands Dolomite in O

Reservoir Properties

Although the reservoir properties of shallow-water car-bonate deposits in the Bass Islands of Michigan aregreatly influenced by dissolution and diagenesis strongfabric characteristics which are inherited from theiroriginal depositional facies character influence reser-voir properties This carbonate reservoir has some pre-served primary porosity as well as abundant secondaryporosity Dissolution has produced significant second-ary pores (Figures 6B 7A B) Fossil grains are most vul-nerable because dissolution processes have selectivelyremoved grains instead of the finer micritic matrix(Figure 7C E F) In addition to moldic porosity abun-dant intercrystalline porosity resulting from dolomitiza-tion is observed In the grainstones abundant intergran-ular porosity and significant permeability are observedMuch of the porosity in the grainstones is primary inter-granular porosity although some pores have been modi-fied into vugs by dissolution Figure 7D shows a thin sec-tion of the cross-bedded grainstone seen in Figure 7C at3461 ft (1055 m)

Conventional coreporosity andpermeability (PampP)havebeenmeasured from 2-in (5-cm)-long by 1-in (25-cm)-diameter sample plugs drilled fromwhole core collectedin theCore Energy-St Charlton 4-30well Seventy-fourhorizontal plugs 12 vertical plugs 6 whole cores and 17sidewall core plugs were sent to Core Laboratories forroutine PampP analyses Sixty-six horizontal plugs werecollected in the Bass Islands reservoir interval Differentfacies show substantially different reservoir properties(Figure 8) Intergranular porosity in the grainstone faciesis representative of primary porosity that forms useful po-rosity zones in the Bass Islands in this well Good porosityalso exists as the result of selective dissolution of some car-bonate grains producingmoldic porosity This facies is fur-ther enhanced by karst dissolution and brecciation to be-come the best PampP in the entire formation These faciesare relatively thin beds stacked in a series of shallowing-upward packages that follow a predictable stratigraphicframework This framework can provide a better under-standing of the vertical and lateral distribution of the po-rous reservoir facies

Stratigraphic Framework

The Bass Islands interval exhibits 5-m (16-ft)-scale cy-cles nested within two high-frequency sequences (HFSapproximately 10 m [33 ft]) (Figure 9) Sequence andcycle boundaries were determined from core analysis

Figure 8 Porosity and permeability from core-plug analyses ofthe Core Energy-St Charlton 4-30 well Generalized facies typesare indicated throughout the cored section

tsego County Michigan

and were specifically based on evidence for subaerialexposure caused by a fall in relative sea level the pres-ence of flooding surfaces indicating a transgressive eventor vertical successions of facies indicating an overall flood-ing event followed by a shallowing of facies indicating arelative fall in sea level Subaerial exposure horizons aremarked by brecciation (collapse breccia) solution pipesand local development of terra rosa all of which are com-mon diagnostic indicators of exposure (eg Esteban andKlappa 1983) The highest frequency packages are the cy-cles which are amaximumof approximately 24 ft (73m)thick and have an average thickness of 14 ft (43 m)The HFS range from 28 to 44 ft (85 to 134 m) thickand the Bass Islands interval is approximately 72 ft thick(22 m)

Reservoir Intervals

The best reservoir units in theCore Energy-St Charlton4-30 core are present in two different intervals Thefirst is related to solution brecciation and recrystalliza-tion of limestones caused by subaerial exposure wherewell-developed interparticle intercrystalline and solution-enhanced vugs contribute to the creation of high porositiesand permeabilities The second unit is related to inner plat-form (midramp) bioturbated facies where the burrow gal-leries have been preferentially dolomitized with coarsercrystalline sucrosic dolomite Figure 10 illustrates howthese types of facies commonly referred to as tubulartempestites may be in excellent communication becauseof the nature of the three-dimensional burrow galleries

Figure 9 Stratigraphic hierarchy observed in the Bass Islands interval High-frequency sequences (HFS) along with higher-frequencycycles correlate well to the stratigraphic position of the best reservoir intervals and many of the HFS and cycle boundaries are manifestedby higher gamma-ray log values Regressive hemicycles of both the HFS and higher-frequency cycles correlate to the best developmentof reservoir throughout much of the cored interval NPHI = neutron porosity PEF = photoelectric factor

Harrison et al 149

courtesy of E A Shinn

and the density of burrows commonly found in similartypes of environments

Reservoir development also correlates well with thestratigraphic framework (Figure 9) Sequence and cycleboundaries were identified by the presence of subaerialexposure horizons as indicated by karstic brecciation andsolution flooding surface and facies stacking patternsAs mentioned above a well-constrained hierarchy ex-ists within the Bass Islands interval whereby the higher-frequency cycles exist within two medium-scale HFSFrom a reservoir standpoint the HFS and most of thehigher-frequency cycles constrain the development ofthe best reservoir intervals Regressive hemicycles ofboth the HFS and higher-frequency cycles correlate tothe best development of the reservoir throughout muchof the cored interval Many of the HFS and cycleboundaries are manifested by higher gamma-ray logvalues and can be identified in wells without core dataWe believe that the correlation of depositional faciesobserved in core to stratigraphic packages and wireline-log signatures is a powerful tool to help interpret the geo-logic properties of the Bass Islands interval in otherareas throughout Michigan where only wireline logsare available

150 Reservoir Characteristics of the Bass Islands Dolomite in O

CONCLUSIONS

The Bass Islands Formation can be mapped throughoutmuch of northern and central lower Michigan where itis an important saline reservoir target for CO2 seques-tration with more than 40 ft (12 m) of greater than 10porosity and permeabilities exceeding 500md Averageporosity in the Bass Islands cored interval in the CoreEnergy-St Charlton 4-30 test well is 125 and averagepermeability is 224 md

The Bass Islands in the area of the Core Energy-StCharlton 4-30 well is composed of a series of shallow-water and tidal flat carbonate facies that are stacked in apredictable shallowing-upward pattern The Bass Is-lands Formation seen in this core can be subdivided intotwoHFS and five higher-frequency cycles Stratigraphicanalysis helps predict vertical and lateral facies patternsand contributes to the evaluation of regional reservoirproperties The best reservoir quality occurs in the sub-tidal burrowed packstones that have some dissolutionof grains and develop moldic porosity This facies is alsoenhanced further by karst dissolution and brecciationrelated to the Tippecanoe-Kaskaskia sequence-boundingunconformity at the top of the Bass Islands Formation

Figure 10 Some of the bestreservoirs in the Bass Islands ex-ists in the thoroughly bioturbatedinner platform facies Burrowsare of the Thalassinoides ichno-facies which is similar to thethree-dimensional galleries formedtoday by burrowing shrimp (egCallianassa) Photograph of theGreat Bahama Bank with the lo-cation of heavily bioturbated innerplatform facies In many modernenvironments these facies mayextend for thousands of squarekilometers Note the high densityof burrows and three-dimensionaldistribution of an individual bur-row gallery preserved in epoxyboth of which illustrate the po-tential for such facies to be lat-erally and vertically connected inthree dimensions commonlyleading to excellent reservoir

potential The plastic castphotograph of the ghost shrimpCallianassa burrow gallery is

tsego County Michigan

Additional porosity can be found in carbonate grainstonefacies that preserve some primary intergranular porosityThe shallowest tidal flat or stromatolitic facies showminimal PampP and are likely to be internal baffles or bar-riers to fluid flow through the formation In addition cor-relation of wireline logs shows that the Bass Islands cyclicporosity seen in the St Charlton 4-30 well is presentthroughout the northern part of lower Michigan

REFERENCES CITED

Barnes D A W B Harrison III and A Wahr in press Assessmentof regional geological carbon sequestration potential in UpperSilurian to Middle Devonian strata of the Michigan Basin inM Grobe J C Pashin and R L Dodge eds Carbon dioxidesequestration in geologic mediamdashState of the science AAPGStudies in Geology 59

Catacosinos P A P A Daniels Jr and W B Harrison III 1991Structure stratigraphy and petroleum geology of the MichiganBasin inMW Leighton D R Kolata D FOltz and J J Eideleds Interior cratonic basins AAPG Memoir 51 p 561ndash602

Esteban M and C F Klappa 1983 Subaerial exposure environ-

ment in P A Scholle D G Bebout and C H Moore edsCarbonate depositional environments AAPG Memoir 33p 1ndash95

Gardner W C 1974 Middle Devonian stratigraphy and deposi-tional environments in the Michigan Basin Michigan BasinGeological Society Special Paper 1 138 p

GrammerGMDABarnesWBHarrison III andAESandomierskiin press Practical synergies for increasing domestic oil produc-tion and geological sequestration of anthropogenic CO2 An ex-ample from the Michigan Basin in M Grobe J C Pashin andR L Dodge eds Carbon dioxide sequestration in geologicmediamdashState of the science AAPG Studies in Geology 59

Lane A C C S Prosser W H Sherzer and A W Grabau 1909Nomenclature and subdivision of the Upper Silurian strata ofMichigan Ohio and western New York Geological Society ofAmerica Bulletin v 19 p 553ndash556

Lilienthal R T 1978 Stratigraphic cross-sections of the MichiganBasin Department of Natural Resources Geological SurveyDivision Report of Investigation No 19 36 p

Sloss L L 1963 Sequences in the cratonic interior of North Amer-ica Geological Society of America Bulletin v 75 p 93ndash113doi1011300016-7606(1963)74[93SITCIO]20CO2

Sparling D R 1970 The Bass Islands Formation in its type regionThe Ohio Journal of Science v 70 no 1 p 1ndash33

US Department of Energy 2007 Carbon sequestration atlas of theUnited States and Canada Pittsburgh Pennsylvania NationalEnergy Technology Laboratory 86 p

Harrison et al 151

Figure 5 Well logs andcore data from the lowerpart of the Core Energy-St Charlton 4-30 wellsection 30 T31N-R1WOtsego County MichiganBK = base of the Kaskas-kia megasequence andposition of unconformityon top of the Bass IslandsFormation BILD = BassIslands Formation BBLC =Bois Blanc FormationAMBG = AmherstbergFormation The core inter-val reported in this study isthe lower half of core 3and all of core 4 NPHI =neutron porosity PEF =photoelectric factor

Ha

rrison et al 145

of Lake Erie (Lane et al 1909) Sparling (1970) exten-sively described the Bass Islands lithofacies as a series ofshallow-water carbonate deposits that have been dolomi-tized and diagenetically altered by processes associatedwith the Tippecanoe-Kaskaskia sequence-bounding un-conformity Dissolution of primary grains and larger-

146 Reservoir Characteristics of the Bass Islands Dolomite in O

scale dissolution and formation of solution collapse brec-cias are characteristics of the Bass Islands in the outcroparea Although wireline-log correlations show that theBass Islands in the Michigan Basin subsurface continuesthis dolomitic lithology they cannot help to determinethe facies and fabric characteristics seen in outcrop

Figure 6 (A) Core photograph(3442 ft [1049 m]) showing theboundary between the Tippeca-noe and Kaskaskia megase-quences of Sloss (1963) with shelllag followed by condensed inter-val and a chert-rich zone nearthe top of the core (B) Corephotograph (3472 ft [1058 m])showing brecciation and solutionpipes related to karst developedduring subaerial exposure(C) Core photograph (3461 ft[1055 m]) showing well-developedcross-bedding in peloidal grain-stone associated with high-energycarbonate sand shoals (D) Corephotograph (3477 ft [1060 m]) oflaminated cyanobacterial matsassociated with tidal flat depositsPhotos are by Linda Harrison

tsego County Michigan

Limited core observations (Gardner 1974) do suggestshallow-water deposits and diagenetic alteration includ-ing brecciation

Acquisition of a new core from the Core Energy-St Charlton 4-30 well provides an opportunity to care-

fully evaluate the reservoir properties of this formationThe well penetrated at least 188 ft (57 m) of Bass IslandsFormation and cored 78 ft (24 m) of the uppermost BassIslands along with 42 ft (13 m) of the overlying BoisBlanc Formation (Figure 5) This is the most complete

Figure 7 Thin-section photomicrographs (plane light) of reservoir intervals (A) Brecciated (Br) finely crystalline dolomite related tokarst developed in response to subaerial exposure (3469 ft [1057 m]) Well-developed solution-enhanced vugs (V) and intercrystallineporosity The measured porosity and permeability (Klinkenberg) in this zone are gt31 and gt290 md respectively (B) Brecciated (Br)finely crystalline dolomite related to karst developed in response to subaerial exposure (34722 ft [10583 m]) Well-developed solution-enhanced vugs (V) and intercrystalline porosity The measured porosity and permeability (Klinkenberg) in this zone are gt26 and 90 mdrespectively (C) Dolomitized burrowed peloidal skeletal wackestone Pore types include fossil molds (M) solution-enhanced vugs (V) andintercrystalline pores Late saddle dolomite (SD) partially occludes local vugs (3466 ft [1056 m]) (D) Dolomitized peloidal packstone tograinstone Well-developed solution-enhanced vugs (V) small molds and intercrystalline porosity (3461 ft [1055 m]) (E) Dolomitizedbioturbated peloidal wackestone-packstone Well-developed solution-enhanced vugs (V) small molds and intercrystalline porosity (3495 ft[1065 m]) (F) Higher-magnification photograph of sample E showing well-developed solution-enhanced vugs (V) small molds and inter-crystalline porosity

Harrison et al 147

subsurface core sample known for the Bass Islands For-mation inMichigan The Tippecanoe-Kaskaskia sequence-bounding unconformity is apparent in the core at 34424 ft(10493 m) (Figure 6A) Immediately overlying the un-conformity is a bed of brachiopod shells and an abun-dance of siliceous replacement of the original carbonateThe Bass Islands lithology immediately underlying theunconformity is a dolomitized bioturbated skeletalwackestone This package can be subdivided into severalmeter-scale shallowing-upward cycles of stacked faciesThe cycles begin with a shallow subtidal burrowed andbioturbated skeletal and peloidal wackestone to pack-stone (Figure 6B) These subtidal facies shallow upwardinto higher-energy cross-bedded and laminated peloidalgrainstones (Figure 6C) The grainstones are capped bylaminated and crenulated cyanobacterial mat mudstonesthat represent high intertidal or supratidal environmentswith subaerial exposure and represent the top of theshallowing-upward cycle (Figure 6D)

148 Reservoir Characteristics of the Bass Islands Dolomite in O

Reservoir Properties

Although the reservoir properties of shallow-water car-bonate deposits in the Bass Islands of Michigan aregreatly influenced by dissolution and diagenesis strongfabric characteristics which are inherited from theiroriginal depositional facies character influence reser-voir properties This carbonate reservoir has some pre-served primary porosity as well as abundant secondaryporosity Dissolution has produced significant second-ary pores (Figures 6B 7A B) Fossil grains are most vul-nerable because dissolution processes have selectivelyremoved grains instead of the finer micritic matrix(Figure 7C E F) In addition to moldic porosity abun-dant intercrystalline porosity resulting from dolomitiza-tion is observed In the grainstones abundant intergran-ular porosity and significant permeability are observedMuch of the porosity in the grainstones is primary inter-granular porosity although some pores have been modi-fied into vugs by dissolution Figure 7D shows a thin sec-tion of the cross-bedded grainstone seen in Figure 7C at3461 ft (1055 m)

Conventional coreporosity andpermeability (PampP)havebeenmeasured from 2-in (5-cm)-long by 1-in (25-cm)-diameter sample plugs drilled fromwhole core collectedin theCore Energy-St Charlton 4-30well Seventy-fourhorizontal plugs 12 vertical plugs 6 whole cores and 17sidewall core plugs were sent to Core Laboratories forroutine PampP analyses Sixty-six horizontal plugs werecollected in the Bass Islands reservoir interval Differentfacies show substantially different reservoir properties(Figure 8) Intergranular porosity in the grainstone faciesis representative of primary porosity that forms useful po-rosity zones in the Bass Islands in this well Good porosityalso exists as the result of selective dissolution of some car-bonate grains producingmoldic porosity This facies is fur-ther enhanced by karst dissolution and brecciation to be-come the best PampP in the entire formation These faciesare relatively thin beds stacked in a series of shallowing-upward packages that follow a predictable stratigraphicframework This framework can provide a better under-standing of the vertical and lateral distribution of the po-rous reservoir facies

Stratigraphic Framework

The Bass Islands interval exhibits 5-m (16-ft)-scale cy-cles nested within two high-frequency sequences (HFSapproximately 10 m [33 ft]) (Figure 9) Sequence andcycle boundaries were determined from core analysis

Figure 8 Porosity and permeability from core-plug analyses ofthe Core Energy-St Charlton 4-30 well Generalized facies typesare indicated throughout the cored section

tsego County Michigan

and were specifically based on evidence for subaerialexposure caused by a fall in relative sea level the pres-ence of flooding surfaces indicating a transgressive eventor vertical successions of facies indicating an overall flood-ing event followed by a shallowing of facies indicating arelative fall in sea level Subaerial exposure horizons aremarked by brecciation (collapse breccia) solution pipesand local development of terra rosa all of which are com-mon diagnostic indicators of exposure (eg Esteban andKlappa 1983) The highest frequency packages are the cy-cles which are amaximumof approximately 24 ft (73m)thick and have an average thickness of 14 ft (43 m)The HFS range from 28 to 44 ft (85 to 134 m) thickand the Bass Islands interval is approximately 72 ft thick(22 m)

Reservoir Intervals

The best reservoir units in theCore Energy-St Charlton4-30 core are present in two different intervals Thefirst is related to solution brecciation and recrystalliza-tion of limestones caused by subaerial exposure wherewell-developed interparticle intercrystalline and solution-enhanced vugs contribute to the creation of high porositiesand permeabilities The second unit is related to inner plat-form (midramp) bioturbated facies where the burrow gal-leries have been preferentially dolomitized with coarsercrystalline sucrosic dolomite Figure 10 illustrates howthese types of facies commonly referred to as tubulartempestites may be in excellent communication becauseof the nature of the three-dimensional burrow galleries

Figure 9 Stratigraphic hierarchy observed in the Bass Islands interval High-frequency sequences (HFS) along with higher-frequencycycles correlate well to the stratigraphic position of the best reservoir intervals and many of the HFS and cycle boundaries are manifestedby higher gamma-ray log values Regressive hemicycles of both the HFS and higher-frequency cycles correlate to the best developmentof reservoir throughout much of the cored interval NPHI = neutron porosity PEF = photoelectric factor

Harrison et al 149

courtesy of E A Shinn

and the density of burrows commonly found in similartypes of environments

Reservoir development also correlates well with thestratigraphic framework (Figure 9) Sequence and cycleboundaries were identified by the presence of subaerialexposure horizons as indicated by karstic brecciation andsolution flooding surface and facies stacking patternsAs mentioned above a well-constrained hierarchy ex-ists within the Bass Islands interval whereby the higher-frequency cycles exist within two medium-scale HFSFrom a reservoir standpoint the HFS and most of thehigher-frequency cycles constrain the development ofthe best reservoir intervals Regressive hemicycles ofboth the HFS and higher-frequency cycles correlate tothe best development of the reservoir throughout muchof the cored interval Many of the HFS and cycleboundaries are manifested by higher gamma-ray logvalues and can be identified in wells without core dataWe believe that the correlation of depositional faciesobserved in core to stratigraphic packages and wireline-log signatures is a powerful tool to help interpret the geo-logic properties of the Bass Islands interval in otherareas throughout Michigan where only wireline logsare available

150 Reservoir Characteristics of the Bass Islands Dolomite in O

CONCLUSIONS

The Bass Islands Formation can be mapped throughoutmuch of northern and central lower Michigan where itis an important saline reservoir target for CO2 seques-tration with more than 40 ft (12 m) of greater than 10porosity and permeabilities exceeding 500md Averageporosity in the Bass Islands cored interval in the CoreEnergy-St Charlton 4-30 test well is 125 and averagepermeability is 224 md

The Bass Islands in the area of the Core Energy-StCharlton 4-30 well is composed of a series of shallow-water and tidal flat carbonate facies that are stacked in apredictable shallowing-upward pattern The Bass Is-lands Formation seen in this core can be subdivided intotwoHFS and five higher-frequency cycles Stratigraphicanalysis helps predict vertical and lateral facies patternsand contributes to the evaluation of regional reservoirproperties The best reservoir quality occurs in the sub-tidal burrowed packstones that have some dissolutionof grains and develop moldic porosity This facies is alsoenhanced further by karst dissolution and brecciationrelated to the Tippecanoe-Kaskaskia sequence-boundingunconformity at the top of the Bass Islands Formation

Figure 10 Some of the bestreservoirs in the Bass Islands ex-ists in the thoroughly bioturbatedinner platform facies Burrowsare of the Thalassinoides ichno-facies which is similar to thethree-dimensional galleries formedtoday by burrowing shrimp (egCallianassa) Photograph of theGreat Bahama Bank with the lo-cation of heavily bioturbated innerplatform facies In many modernenvironments these facies mayextend for thousands of squarekilometers Note the high densityof burrows and three-dimensionaldistribution of an individual bur-row gallery preserved in epoxyboth of which illustrate the po-tential for such facies to be lat-erally and vertically connected inthree dimensions commonlyleading to excellent reservoir

potential The plastic castphotograph of the ghost shrimpCallianassa burrow gallery is

tsego County Michigan

Additional porosity can be found in carbonate grainstonefacies that preserve some primary intergranular porosityThe shallowest tidal flat or stromatolitic facies showminimal PampP and are likely to be internal baffles or bar-riers to fluid flow through the formation In addition cor-relation of wireline logs shows that the Bass Islands cyclicporosity seen in the St Charlton 4-30 well is presentthroughout the northern part of lower Michigan

REFERENCES CITED

Barnes D A W B Harrison III and A Wahr in press Assessmentof regional geological carbon sequestration potential in UpperSilurian to Middle Devonian strata of the Michigan Basin inM Grobe J C Pashin and R L Dodge eds Carbon dioxidesequestration in geologic mediamdashState of the science AAPGStudies in Geology 59

Catacosinos P A P A Daniels Jr and W B Harrison III 1991Structure stratigraphy and petroleum geology of the MichiganBasin inMW Leighton D R Kolata D FOltz and J J Eideleds Interior cratonic basins AAPG Memoir 51 p 561ndash602

Esteban M and C F Klappa 1983 Subaerial exposure environ-

ment in P A Scholle D G Bebout and C H Moore edsCarbonate depositional environments AAPG Memoir 33p 1ndash95

Gardner W C 1974 Middle Devonian stratigraphy and deposi-tional environments in the Michigan Basin Michigan BasinGeological Society Special Paper 1 138 p

GrammerGMDABarnesWBHarrison III andAESandomierskiin press Practical synergies for increasing domestic oil produc-tion and geological sequestration of anthropogenic CO2 An ex-ample from the Michigan Basin in M Grobe J C Pashin andR L Dodge eds Carbon dioxide sequestration in geologicmediamdashState of the science AAPG Studies in Geology 59

Lane A C C S Prosser W H Sherzer and A W Grabau 1909Nomenclature and subdivision of the Upper Silurian strata ofMichigan Ohio and western New York Geological Society ofAmerica Bulletin v 19 p 553ndash556

Lilienthal R T 1978 Stratigraphic cross-sections of the MichiganBasin Department of Natural Resources Geological SurveyDivision Report of Investigation No 19 36 p

Sloss L L 1963 Sequences in the cratonic interior of North Amer-ica Geological Society of America Bulletin v 75 p 93ndash113doi1011300016-7606(1963)74[93SITCIO]20CO2

Sparling D R 1970 The Bass Islands Formation in its type regionThe Ohio Journal of Science v 70 no 1 p 1ndash33

US Department of Energy 2007 Carbon sequestration atlas of theUnited States and Canada Pittsburgh Pennsylvania NationalEnergy Technology Laboratory 86 p

Harrison et al 151

of Lake Erie (Lane et al 1909) Sparling (1970) exten-sively described the Bass Islands lithofacies as a series ofshallow-water carbonate deposits that have been dolomi-tized and diagenetically altered by processes associatedwith the Tippecanoe-Kaskaskia sequence-bounding un-conformity Dissolution of primary grains and larger-

146 Reservoir Characteristics of the Bass Islands Dolomite in O

scale dissolution and formation of solution collapse brec-cias are characteristics of the Bass Islands in the outcroparea Although wireline-log correlations show that theBass Islands in the Michigan Basin subsurface continuesthis dolomitic lithology they cannot help to determinethe facies and fabric characteristics seen in outcrop

Figure 6 (A) Core photograph(3442 ft [1049 m]) showing theboundary between the Tippeca-noe and Kaskaskia megase-quences of Sloss (1963) with shelllag followed by condensed inter-val and a chert-rich zone nearthe top of the core (B) Corephotograph (3472 ft [1058 m])showing brecciation and solutionpipes related to karst developedduring subaerial exposure(C) Core photograph (3461 ft[1055 m]) showing well-developedcross-bedding in peloidal grain-stone associated with high-energycarbonate sand shoals (D) Corephotograph (3477 ft [1060 m]) oflaminated cyanobacterial matsassociated with tidal flat depositsPhotos are by Linda Harrison

tsego County Michigan

Limited core observations (Gardner 1974) do suggestshallow-water deposits and diagenetic alteration includ-ing brecciation

Acquisition of a new core from the Core Energy-St Charlton 4-30 well provides an opportunity to care-

fully evaluate the reservoir properties of this formationThe well penetrated at least 188 ft (57 m) of Bass IslandsFormation and cored 78 ft (24 m) of the uppermost BassIslands along with 42 ft (13 m) of the overlying BoisBlanc Formation (Figure 5) This is the most complete

Figure 7 Thin-section photomicrographs (plane light) of reservoir intervals (A) Brecciated (Br) finely crystalline dolomite related tokarst developed in response to subaerial exposure (3469 ft [1057 m]) Well-developed solution-enhanced vugs (V) and intercrystallineporosity The measured porosity and permeability (Klinkenberg) in this zone are gt31 and gt290 md respectively (B) Brecciated (Br)finely crystalline dolomite related to karst developed in response to subaerial exposure (34722 ft [10583 m]) Well-developed solution-enhanced vugs (V) and intercrystalline porosity The measured porosity and permeability (Klinkenberg) in this zone are gt26 and 90 mdrespectively (C) Dolomitized burrowed peloidal skeletal wackestone Pore types include fossil molds (M) solution-enhanced vugs (V) andintercrystalline pores Late saddle dolomite (SD) partially occludes local vugs (3466 ft [1056 m]) (D) Dolomitized peloidal packstone tograinstone Well-developed solution-enhanced vugs (V) small molds and intercrystalline porosity (3461 ft [1055 m]) (E) Dolomitizedbioturbated peloidal wackestone-packstone Well-developed solution-enhanced vugs (V) small molds and intercrystalline porosity (3495 ft[1065 m]) (F) Higher-magnification photograph of sample E showing well-developed solution-enhanced vugs (V) small molds and inter-crystalline porosity

Harrison et al 147

subsurface core sample known for the Bass Islands For-mation inMichigan The Tippecanoe-Kaskaskia sequence-bounding unconformity is apparent in the core at 34424 ft(10493 m) (Figure 6A) Immediately overlying the un-conformity is a bed of brachiopod shells and an abun-dance of siliceous replacement of the original carbonateThe Bass Islands lithology immediately underlying theunconformity is a dolomitized bioturbated skeletalwackestone This package can be subdivided into severalmeter-scale shallowing-upward cycles of stacked faciesThe cycles begin with a shallow subtidal burrowed andbioturbated skeletal and peloidal wackestone to pack-stone (Figure 6B) These subtidal facies shallow upwardinto higher-energy cross-bedded and laminated peloidalgrainstones (Figure 6C) The grainstones are capped bylaminated and crenulated cyanobacterial mat mudstonesthat represent high intertidal or supratidal environmentswith subaerial exposure and represent the top of theshallowing-upward cycle (Figure 6D)

148 Reservoir Characteristics of the Bass Islands Dolomite in O

Reservoir Properties

Although the reservoir properties of shallow-water car-bonate deposits in the Bass Islands of Michigan aregreatly influenced by dissolution and diagenesis strongfabric characteristics which are inherited from theiroriginal depositional facies character influence reser-voir properties This carbonate reservoir has some pre-served primary porosity as well as abundant secondaryporosity Dissolution has produced significant second-ary pores (Figures 6B 7A B) Fossil grains are most vul-nerable because dissolution processes have selectivelyremoved grains instead of the finer micritic matrix(Figure 7C E F) In addition to moldic porosity abun-dant intercrystalline porosity resulting from dolomitiza-tion is observed In the grainstones abundant intergran-ular porosity and significant permeability are observedMuch of the porosity in the grainstones is primary inter-granular porosity although some pores have been modi-fied into vugs by dissolution Figure 7D shows a thin sec-tion of the cross-bedded grainstone seen in Figure 7C at3461 ft (1055 m)

Conventional coreporosity andpermeability (PampP)havebeenmeasured from 2-in (5-cm)-long by 1-in (25-cm)-diameter sample plugs drilled fromwhole core collectedin theCore Energy-St Charlton 4-30well Seventy-fourhorizontal plugs 12 vertical plugs 6 whole cores and 17sidewall core plugs were sent to Core Laboratories forroutine PampP analyses Sixty-six horizontal plugs werecollected in the Bass Islands reservoir interval Differentfacies show substantially different reservoir properties(Figure 8) Intergranular porosity in the grainstone faciesis representative of primary porosity that forms useful po-rosity zones in the Bass Islands in this well Good porosityalso exists as the result of selective dissolution of some car-bonate grains producingmoldic porosity This facies is fur-ther enhanced by karst dissolution and brecciation to be-come the best PampP in the entire formation These faciesare relatively thin beds stacked in a series of shallowing-upward packages that follow a predictable stratigraphicframework This framework can provide a better under-standing of the vertical and lateral distribution of the po-rous reservoir facies

Stratigraphic Framework

The Bass Islands interval exhibits 5-m (16-ft)-scale cy-cles nested within two high-frequency sequences (HFSapproximately 10 m [33 ft]) (Figure 9) Sequence andcycle boundaries were determined from core analysis

Figure 8 Porosity and permeability from core-plug analyses ofthe Core Energy-St Charlton 4-30 well Generalized facies typesare indicated throughout the cored section

tsego County Michigan

and were specifically based on evidence for subaerialexposure caused by a fall in relative sea level the pres-ence of flooding surfaces indicating a transgressive eventor vertical successions of facies indicating an overall flood-ing event followed by a shallowing of facies indicating arelative fall in sea level Subaerial exposure horizons aremarked by brecciation (collapse breccia) solution pipesand local development of terra rosa all of which are com-mon diagnostic indicators of exposure (eg Esteban andKlappa 1983) The highest frequency packages are the cy-cles which are amaximumof approximately 24 ft (73m)thick and have an average thickness of 14 ft (43 m)The HFS range from 28 to 44 ft (85 to 134 m) thickand the Bass Islands interval is approximately 72 ft thick(22 m)

Reservoir Intervals

The best reservoir units in theCore Energy-St Charlton4-30 core are present in two different intervals Thefirst is related to solution brecciation and recrystalliza-tion of limestones caused by subaerial exposure wherewell-developed interparticle intercrystalline and solution-enhanced vugs contribute to the creation of high porositiesand permeabilities The second unit is related to inner plat-form (midramp) bioturbated facies where the burrow gal-leries have been preferentially dolomitized with coarsercrystalline sucrosic dolomite Figure 10 illustrates howthese types of facies commonly referred to as tubulartempestites may be in excellent communication becauseof the nature of the three-dimensional burrow galleries

Figure 9 Stratigraphic hierarchy observed in the Bass Islands interval High-frequency sequences (HFS) along with higher-frequencycycles correlate well to the stratigraphic position of the best reservoir intervals and many of the HFS and cycle boundaries are manifestedby higher gamma-ray log values Regressive hemicycles of both the HFS and higher-frequency cycles correlate to the best developmentof reservoir throughout much of the cored interval NPHI = neutron porosity PEF = photoelectric factor

Harrison et al 149

courtesy of E A Shinn

and the density of burrows commonly found in similartypes of environments

Reservoir development also correlates well with thestratigraphic framework (Figure 9) Sequence and cycleboundaries were identified by the presence of subaerialexposure horizons as indicated by karstic brecciation andsolution flooding surface and facies stacking patternsAs mentioned above a well-constrained hierarchy ex-ists within the Bass Islands interval whereby the higher-frequency cycles exist within two medium-scale HFSFrom a reservoir standpoint the HFS and most of thehigher-frequency cycles constrain the development ofthe best reservoir intervals Regressive hemicycles ofboth the HFS and higher-frequency cycles correlate tothe best development of the reservoir throughout muchof the cored interval Many of the HFS and cycleboundaries are manifested by higher gamma-ray logvalues and can be identified in wells without core dataWe believe that the correlation of depositional faciesobserved in core to stratigraphic packages and wireline-log signatures is a powerful tool to help interpret the geo-logic properties of the Bass Islands interval in otherareas throughout Michigan where only wireline logsare available

150 Reservoir Characteristics of the Bass Islands Dolomite in O

CONCLUSIONS

The Bass Islands Formation can be mapped throughoutmuch of northern and central lower Michigan where itis an important saline reservoir target for CO2 seques-tration with more than 40 ft (12 m) of greater than 10porosity and permeabilities exceeding 500md Averageporosity in the Bass Islands cored interval in the CoreEnergy-St Charlton 4-30 test well is 125 and averagepermeability is 224 md

The Bass Islands in the area of the Core Energy-StCharlton 4-30 well is composed of a series of shallow-water and tidal flat carbonate facies that are stacked in apredictable shallowing-upward pattern The Bass Is-lands Formation seen in this core can be subdivided intotwoHFS and five higher-frequency cycles Stratigraphicanalysis helps predict vertical and lateral facies patternsand contributes to the evaluation of regional reservoirproperties The best reservoir quality occurs in the sub-tidal burrowed packstones that have some dissolutionof grains and develop moldic porosity This facies is alsoenhanced further by karst dissolution and brecciationrelated to the Tippecanoe-Kaskaskia sequence-boundingunconformity at the top of the Bass Islands Formation

Figure 10 Some of the bestreservoirs in the Bass Islands ex-ists in the thoroughly bioturbatedinner platform facies Burrowsare of the Thalassinoides ichno-facies which is similar to thethree-dimensional galleries formedtoday by burrowing shrimp (egCallianassa) Photograph of theGreat Bahama Bank with the lo-cation of heavily bioturbated innerplatform facies In many modernenvironments these facies mayextend for thousands of squarekilometers Note the high densityof burrows and three-dimensionaldistribution of an individual bur-row gallery preserved in epoxyboth of which illustrate the po-tential for such facies to be lat-erally and vertically connected inthree dimensions commonlyleading to excellent reservoir

potential The plastic castphotograph of the ghost shrimpCallianassa burrow gallery is

tsego County Michigan

Additional porosity can be found in carbonate grainstonefacies that preserve some primary intergranular porosityThe shallowest tidal flat or stromatolitic facies showminimal PampP and are likely to be internal baffles or bar-riers to fluid flow through the formation In addition cor-relation of wireline logs shows that the Bass Islands cyclicporosity seen in the St Charlton 4-30 well is presentthroughout the northern part of lower Michigan

REFERENCES CITED

Barnes D A W B Harrison III and A Wahr in press Assessmentof regional geological carbon sequestration potential in UpperSilurian to Middle Devonian strata of the Michigan Basin inM Grobe J C Pashin and R L Dodge eds Carbon dioxidesequestration in geologic mediamdashState of the science AAPGStudies in Geology 59

Catacosinos P A P A Daniels Jr and W B Harrison III 1991Structure stratigraphy and petroleum geology of the MichiganBasin inMW Leighton D R Kolata D FOltz and J J Eideleds Interior cratonic basins AAPG Memoir 51 p 561ndash602

Esteban M and C F Klappa 1983 Subaerial exposure environ-

ment in P A Scholle D G Bebout and C H Moore edsCarbonate depositional environments AAPG Memoir 33p 1ndash95

Gardner W C 1974 Middle Devonian stratigraphy and deposi-tional environments in the Michigan Basin Michigan BasinGeological Society Special Paper 1 138 p

GrammerGMDABarnesWBHarrison III andAESandomierskiin press Practical synergies for increasing domestic oil produc-tion and geological sequestration of anthropogenic CO2 An ex-ample from the Michigan Basin in M Grobe J C Pashin andR L Dodge eds Carbon dioxide sequestration in geologicmediamdashState of the science AAPG Studies in Geology 59

Lane A C C S Prosser W H Sherzer and A W Grabau 1909Nomenclature and subdivision of the Upper Silurian strata ofMichigan Ohio and western New York Geological Society ofAmerica Bulletin v 19 p 553ndash556

Lilienthal R T 1978 Stratigraphic cross-sections of the MichiganBasin Department of Natural Resources Geological SurveyDivision Report of Investigation No 19 36 p

Sloss L L 1963 Sequences in the cratonic interior of North Amer-ica Geological Society of America Bulletin v 75 p 93ndash113doi1011300016-7606(1963)74[93SITCIO]20CO2

Sparling D R 1970 The Bass Islands Formation in its type regionThe Ohio Journal of Science v 70 no 1 p 1ndash33

US Department of Energy 2007 Carbon sequestration atlas of theUnited States and Canada Pittsburgh Pennsylvania NationalEnergy Technology Laboratory 86 p

Harrison et al 151

Limited core observations (Gardner 1974) do suggestshallow-water deposits and diagenetic alteration includ-ing brecciation

Acquisition of a new core from the Core Energy-St Charlton 4-30 well provides an opportunity to care-

fully evaluate the reservoir properties of this formationThe well penetrated at least 188 ft (57 m) of Bass IslandsFormation and cored 78 ft (24 m) of the uppermost BassIslands along with 42 ft (13 m) of the overlying BoisBlanc Formation (Figure 5) This is the most complete

Figure 7 Thin-section photomicrographs (plane light) of reservoir intervals (A) Brecciated (Br) finely crystalline dolomite related tokarst developed in response to subaerial exposure (3469 ft [1057 m]) Well-developed solution-enhanced vugs (V) and intercrystallineporosity The measured porosity and permeability (Klinkenberg) in this zone are gt31 and gt290 md respectively (B) Brecciated (Br)finely crystalline dolomite related to karst developed in response to subaerial exposure (34722 ft [10583 m]) Well-developed solution-enhanced vugs (V) and intercrystalline porosity The measured porosity and permeability (Klinkenberg) in this zone are gt26 and 90 mdrespectively (C) Dolomitized burrowed peloidal skeletal wackestone Pore types include fossil molds (M) solution-enhanced vugs (V) andintercrystalline pores Late saddle dolomite (SD) partially occludes local vugs (3466 ft [1056 m]) (D) Dolomitized peloidal packstone tograinstone Well-developed solution-enhanced vugs (V) small molds and intercrystalline porosity (3461 ft [1055 m]) (E) Dolomitizedbioturbated peloidal wackestone-packstone Well-developed solution-enhanced vugs (V) small molds and intercrystalline porosity (3495 ft[1065 m]) (F) Higher-magnification photograph of sample E showing well-developed solution-enhanced vugs (V) small molds and inter-crystalline porosity

Harrison et al 147

subsurface core sample known for the Bass Islands For-mation inMichigan The Tippecanoe-Kaskaskia sequence-bounding unconformity is apparent in the core at 34424 ft(10493 m) (Figure 6A) Immediately overlying the un-conformity is a bed of brachiopod shells and an abun-dance of siliceous replacement of the original carbonateThe Bass Islands lithology immediately underlying theunconformity is a dolomitized bioturbated skeletalwackestone This package can be subdivided into severalmeter-scale shallowing-upward cycles of stacked faciesThe cycles begin with a shallow subtidal burrowed andbioturbated skeletal and peloidal wackestone to pack-stone (Figure 6B) These subtidal facies shallow upwardinto higher-energy cross-bedded and laminated peloidalgrainstones (Figure 6C) The grainstones are capped bylaminated and crenulated cyanobacterial mat mudstonesthat represent high intertidal or supratidal environmentswith subaerial exposure and represent the top of theshallowing-upward cycle (Figure 6D)

148 Reservoir Characteristics of the Bass Islands Dolomite in O

Reservoir Properties

Although the reservoir properties of shallow-water car-bonate deposits in the Bass Islands of Michigan aregreatly influenced by dissolution and diagenesis strongfabric characteristics which are inherited from theiroriginal depositional facies character influence reser-voir properties This carbonate reservoir has some pre-served primary porosity as well as abundant secondaryporosity Dissolution has produced significant second-ary pores (Figures 6B 7A B) Fossil grains are most vul-nerable because dissolution processes have selectivelyremoved grains instead of the finer micritic matrix(Figure 7C E F) In addition to moldic porosity abun-dant intercrystalline porosity resulting from dolomitiza-tion is observed In the grainstones abundant intergran-ular porosity and significant permeability are observedMuch of the porosity in the grainstones is primary inter-granular porosity although some pores have been modi-fied into vugs by dissolution Figure 7D shows a thin sec-tion of the cross-bedded grainstone seen in Figure 7C at3461 ft (1055 m)

Conventional coreporosity andpermeability (PampP)havebeenmeasured from 2-in (5-cm)-long by 1-in (25-cm)-diameter sample plugs drilled fromwhole core collectedin theCore Energy-St Charlton 4-30well Seventy-fourhorizontal plugs 12 vertical plugs 6 whole cores and 17sidewall core plugs were sent to Core Laboratories forroutine PampP analyses Sixty-six horizontal plugs werecollected in the Bass Islands reservoir interval Differentfacies show substantially different reservoir properties(Figure 8) Intergranular porosity in the grainstone faciesis representative of primary porosity that forms useful po-rosity zones in the Bass Islands in this well Good porosityalso exists as the result of selective dissolution of some car-bonate grains producingmoldic porosity This facies is fur-ther enhanced by karst dissolution and brecciation to be-come the best PampP in the entire formation These faciesare relatively thin beds stacked in a series of shallowing-upward packages that follow a predictable stratigraphicframework This framework can provide a better under-standing of the vertical and lateral distribution of the po-rous reservoir facies

Stratigraphic Framework

The Bass Islands interval exhibits 5-m (16-ft)-scale cy-cles nested within two high-frequency sequences (HFSapproximately 10 m [33 ft]) (Figure 9) Sequence andcycle boundaries were determined from core analysis

Figure 8 Porosity and permeability from core-plug analyses ofthe Core Energy-St Charlton 4-30 well Generalized facies typesare indicated throughout the cored section

tsego County Michigan

and were specifically based on evidence for subaerialexposure caused by a fall in relative sea level the pres-ence of flooding surfaces indicating a transgressive eventor vertical successions of facies indicating an overall flood-ing event followed by a shallowing of facies indicating arelative fall in sea level Subaerial exposure horizons aremarked by brecciation (collapse breccia) solution pipesand local development of terra rosa all of which are com-mon diagnostic indicators of exposure (eg Esteban andKlappa 1983) The highest frequency packages are the cy-cles which are amaximumof approximately 24 ft (73m)thick and have an average thickness of 14 ft (43 m)The HFS range from 28 to 44 ft (85 to 134 m) thickand the Bass Islands interval is approximately 72 ft thick(22 m)

Reservoir Intervals

The best reservoir units in theCore Energy-St Charlton4-30 core are present in two different intervals Thefirst is related to solution brecciation and recrystalliza-tion of limestones caused by subaerial exposure wherewell-developed interparticle intercrystalline and solution-enhanced vugs contribute to the creation of high porositiesand permeabilities The second unit is related to inner plat-form (midramp) bioturbated facies where the burrow gal-leries have been preferentially dolomitized with coarsercrystalline sucrosic dolomite Figure 10 illustrates howthese types of facies commonly referred to as tubulartempestites may be in excellent communication becauseof the nature of the three-dimensional burrow galleries

Figure 9 Stratigraphic hierarchy observed in the Bass Islands interval High-frequency sequences (HFS) along with higher-frequencycycles correlate well to the stratigraphic position of the best reservoir intervals and many of the HFS and cycle boundaries are manifestedby higher gamma-ray log values Regressive hemicycles of both the HFS and higher-frequency cycles correlate to the best developmentof reservoir throughout much of the cored interval NPHI = neutron porosity PEF = photoelectric factor

Harrison et al 149

courtesy of E A Shinn

and the density of burrows commonly found in similartypes of environments

Reservoir development also correlates well with thestratigraphic framework (Figure 9) Sequence and cycleboundaries were identified by the presence of subaerialexposure horizons as indicated by karstic brecciation andsolution flooding surface and facies stacking patternsAs mentioned above a well-constrained hierarchy ex-ists within the Bass Islands interval whereby the higher-frequency cycles exist within two medium-scale HFSFrom a reservoir standpoint the HFS and most of thehigher-frequency cycles constrain the development ofthe best reservoir intervals Regressive hemicycles ofboth the HFS and higher-frequency cycles correlate tothe best development of the reservoir throughout muchof the cored interval Many of the HFS and cycleboundaries are manifested by higher gamma-ray logvalues and can be identified in wells without core dataWe believe that the correlation of depositional faciesobserved in core to stratigraphic packages and wireline-log signatures is a powerful tool to help interpret the geo-logic properties of the Bass Islands interval in otherareas throughout Michigan where only wireline logsare available

150 Reservoir Characteristics of the Bass Islands Dolomite in O

CONCLUSIONS

The Bass Islands Formation can be mapped throughoutmuch of northern and central lower Michigan where itis an important saline reservoir target for CO2 seques-tration with more than 40 ft (12 m) of greater than 10porosity and permeabilities exceeding 500md Averageporosity in the Bass Islands cored interval in the CoreEnergy-St Charlton 4-30 test well is 125 and averagepermeability is 224 md

The Bass Islands in the area of the Core Energy-StCharlton 4-30 well is composed of a series of shallow-water and tidal flat carbonate facies that are stacked in apredictable shallowing-upward pattern The Bass Is-lands Formation seen in this core can be subdivided intotwoHFS and five higher-frequency cycles Stratigraphicanalysis helps predict vertical and lateral facies patternsand contributes to the evaluation of regional reservoirproperties The best reservoir quality occurs in the sub-tidal burrowed packstones that have some dissolutionof grains and develop moldic porosity This facies is alsoenhanced further by karst dissolution and brecciationrelated to the Tippecanoe-Kaskaskia sequence-boundingunconformity at the top of the Bass Islands Formation

Figure 10 Some of the bestreservoirs in the Bass Islands ex-ists in the thoroughly bioturbatedinner platform facies Burrowsare of the Thalassinoides ichno-facies which is similar to thethree-dimensional galleries formedtoday by burrowing shrimp (egCallianassa) Photograph of theGreat Bahama Bank with the lo-cation of heavily bioturbated innerplatform facies In many modernenvironments these facies mayextend for thousands of squarekilometers Note the high densityof burrows and three-dimensionaldistribution of an individual bur-row gallery preserved in epoxyboth of which illustrate the po-tential for such facies to be lat-erally and vertically connected inthree dimensions commonlyleading to excellent reservoir

potential The plastic castphotograph of the ghost shrimpCallianassa burrow gallery is

tsego County Michigan

Additional porosity can be found in carbonate grainstonefacies that preserve some primary intergranular porosityThe shallowest tidal flat or stromatolitic facies showminimal PampP and are likely to be internal baffles or bar-riers to fluid flow through the formation In addition cor-relation of wireline logs shows that the Bass Islands cyclicporosity seen in the St Charlton 4-30 well is presentthroughout the northern part of lower Michigan

REFERENCES CITED

Barnes D A W B Harrison III and A Wahr in press Assessmentof regional geological carbon sequestration potential in UpperSilurian to Middle Devonian strata of the Michigan Basin inM Grobe J C Pashin and R L Dodge eds Carbon dioxidesequestration in geologic mediamdashState of the science AAPGStudies in Geology 59

Catacosinos P A P A Daniels Jr and W B Harrison III 1991Structure stratigraphy and petroleum geology of the MichiganBasin inMW Leighton D R Kolata D FOltz and J J Eideleds Interior cratonic basins AAPG Memoir 51 p 561ndash602

Esteban M and C F Klappa 1983 Subaerial exposure environ-

ment in P A Scholle D G Bebout and C H Moore edsCarbonate depositional environments AAPG Memoir 33p 1ndash95

Gardner W C 1974 Middle Devonian stratigraphy and deposi-tional environments in the Michigan Basin Michigan BasinGeological Society Special Paper 1 138 p

GrammerGMDABarnesWBHarrison III andAESandomierskiin press Practical synergies for increasing domestic oil produc-tion and geological sequestration of anthropogenic CO2 An ex-ample from the Michigan Basin in M Grobe J C Pashin andR L Dodge eds Carbon dioxide sequestration in geologicmediamdashState of the science AAPG Studies in Geology 59

Lane A C C S Prosser W H Sherzer and A W Grabau 1909Nomenclature and subdivision of the Upper Silurian strata ofMichigan Ohio and western New York Geological Society ofAmerica Bulletin v 19 p 553ndash556

Lilienthal R T 1978 Stratigraphic cross-sections of the MichiganBasin Department of Natural Resources Geological SurveyDivision Report of Investigation No 19 36 p

Sloss L L 1963 Sequences in the cratonic interior of North Amer-ica Geological Society of America Bulletin v 75 p 93ndash113doi1011300016-7606(1963)74[93SITCIO]20CO2

Sparling D R 1970 The Bass Islands Formation in its type regionThe Ohio Journal of Science v 70 no 1 p 1ndash33

US Department of Energy 2007 Carbon sequestration atlas of theUnited States and Canada Pittsburgh Pennsylvania NationalEnergy Technology Laboratory 86 p

Harrison et al 151

subsurface core sample known for the Bass Islands For-mation inMichigan The Tippecanoe-Kaskaskia sequence-bounding unconformity is apparent in the core at 34424 ft(10493 m) (Figure 6A) Immediately overlying the un-conformity is a bed of brachiopod shells and an abun-dance of siliceous replacement of the original carbonateThe Bass Islands lithology immediately underlying theunconformity is a dolomitized bioturbated skeletalwackestone This package can be subdivided into severalmeter-scale shallowing-upward cycles of stacked faciesThe cycles begin with a shallow subtidal burrowed andbioturbated skeletal and peloidal wackestone to pack-stone (Figure 6B) These subtidal facies shallow upwardinto higher-energy cross-bedded and laminated peloidalgrainstones (Figure 6C) The grainstones are capped bylaminated and crenulated cyanobacterial mat mudstonesthat represent high intertidal or supratidal environmentswith subaerial exposure and represent the top of theshallowing-upward cycle (Figure 6D)

148 Reservoir Characteristics of the Bass Islands Dolomite in O

Reservoir Properties

Although the reservoir properties of shallow-water car-bonate deposits in the Bass Islands of Michigan aregreatly influenced by dissolution and diagenesis strongfabric characteristics which are inherited from theiroriginal depositional facies character influence reser-voir properties This carbonate reservoir has some pre-served primary porosity as well as abundant secondaryporosity Dissolution has produced significant second-ary pores (Figures 6B 7A B) Fossil grains are most vul-nerable because dissolution processes have selectivelyremoved grains instead of the finer micritic matrix(Figure 7C E F) In addition to moldic porosity abun-dant intercrystalline porosity resulting from dolomitiza-tion is observed In the grainstones abundant intergran-ular porosity and significant permeability are observedMuch of the porosity in the grainstones is primary inter-granular porosity although some pores have been modi-fied into vugs by dissolution Figure 7D shows a thin sec-tion of the cross-bedded grainstone seen in Figure 7C at3461 ft (1055 m)

Conventional coreporosity andpermeability (PampP)havebeenmeasured from 2-in (5-cm)-long by 1-in (25-cm)-diameter sample plugs drilled fromwhole core collectedin theCore Energy-St Charlton 4-30well Seventy-fourhorizontal plugs 12 vertical plugs 6 whole cores and 17sidewall core plugs were sent to Core Laboratories forroutine PampP analyses Sixty-six horizontal plugs werecollected in the Bass Islands reservoir interval Differentfacies show substantially different reservoir properties(Figure 8) Intergranular porosity in the grainstone faciesis representative of primary porosity that forms useful po-rosity zones in the Bass Islands in this well Good porosityalso exists as the result of selective dissolution of some car-bonate grains producingmoldic porosity This facies is fur-ther enhanced by karst dissolution and brecciation to be-come the best PampP in the entire formation These faciesare relatively thin beds stacked in a series of shallowing-upward packages that follow a predictable stratigraphicframework This framework can provide a better under-standing of the vertical and lateral distribution of the po-rous reservoir facies

Stratigraphic Framework

The Bass Islands interval exhibits 5-m (16-ft)-scale cy-cles nested within two high-frequency sequences (HFSapproximately 10 m [33 ft]) (Figure 9) Sequence andcycle boundaries were determined from core analysis

Figure 8 Porosity and permeability from core-plug analyses ofthe Core Energy-St Charlton 4-30 well Generalized facies typesare indicated throughout the cored section

tsego County Michigan

and were specifically based on evidence for subaerialexposure caused by a fall in relative sea level the pres-ence of flooding surfaces indicating a transgressive eventor vertical successions of facies indicating an overall flood-ing event followed by a shallowing of facies indicating arelative fall in sea level Subaerial exposure horizons aremarked by brecciation (collapse breccia) solution pipesand local development of terra rosa all of which are com-mon diagnostic indicators of exposure (eg Esteban andKlappa 1983) The highest frequency packages are the cy-cles which are amaximumof approximately 24 ft (73m)thick and have an average thickness of 14 ft (43 m)The HFS range from 28 to 44 ft (85 to 134 m) thickand the Bass Islands interval is approximately 72 ft thick(22 m)

Reservoir Intervals

The best reservoir units in theCore Energy-St Charlton4-30 core are present in two different intervals Thefirst is related to solution brecciation and recrystalliza-tion of limestones caused by subaerial exposure wherewell-developed interparticle intercrystalline and solution-enhanced vugs contribute to the creation of high porositiesand permeabilities The second unit is related to inner plat-form (midramp) bioturbated facies where the burrow gal-leries have been preferentially dolomitized with coarsercrystalline sucrosic dolomite Figure 10 illustrates howthese types of facies commonly referred to as tubulartempestites may be in excellent communication becauseof the nature of the three-dimensional burrow galleries

Figure 9 Stratigraphic hierarchy observed in the Bass Islands interval High-frequency sequences (HFS) along with higher-frequencycycles correlate well to the stratigraphic position of the best reservoir intervals and many of the HFS and cycle boundaries are manifestedby higher gamma-ray log values Regressive hemicycles of both the HFS and higher-frequency cycles correlate to the best developmentof reservoir throughout much of the cored interval NPHI = neutron porosity PEF = photoelectric factor

Harrison et al 149

courtesy of E A Shinn

and the density of burrows commonly found in similartypes of environments

Reservoir development also correlates well with thestratigraphic framework (Figure 9) Sequence and cycleboundaries were identified by the presence of subaerialexposure horizons as indicated by karstic brecciation andsolution flooding surface and facies stacking patternsAs mentioned above a well-constrained hierarchy ex-ists within the Bass Islands interval whereby the higher-frequency cycles exist within two medium-scale HFSFrom a reservoir standpoint the HFS and most of thehigher-frequency cycles constrain the development ofthe best reservoir intervals Regressive hemicycles ofboth the HFS and higher-frequency cycles correlate tothe best development of the reservoir throughout muchof the cored interval Many of the HFS and cycleboundaries are manifested by higher gamma-ray logvalues and can be identified in wells without core dataWe believe that the correlation of depositional faciesobserved in core to stratigraphic packages and wireline-log signatures is a powerful tool to help interpret the geo-logic properties of the Bass Islands interval in otherareas throughout Michigan where only wireline logsare available

150 Reservoir Characteristics of the Bass Islands Dolomite in O

CONCLUSIONS

The Bass Islands Formation can be mapped throughoutmuch of northern and central lower Michigan where itis an important saline reservoir target for CO2 seques-tration with more than 40 ft (12 m) of greater than 10porosity and permeabilities exceeding 500md Averageporosity in the Bass Islands cored interval in the CoreEnergy-St Charlton 4-30 test well is 125 and averagepermeability is 224 md

The Bass Islands in the area of the Core Energy-StCharlton 4-30 well is composed of a series of shallow-water and tidal flat carbonate facies that are stacked in apredictable shallowing-upward pattern The Bass Is-lands Formation seen in this core can be subdivided intotwoHFS and five higher-frequency cycles Stratigraphicanalysis helps predict vertical and lateral facies patternsand contributes to the evaluation of regional reservoirproperties The best reservoir quality occurs in the sub-tidal burrowed packstones that have some dissolutionof grains and develop moldic porosity This facies is alsoenhanced further by karst dissolution and brecciationrelated to the Tippecanoe-Kaskaskia sequence-boundingunconformity at the top of the Bass Islands Formation

Figure 10 Some of the bestreservoirs in the Bass Islands ex-ists in the thoroughly bioturbatedinner platform facies Burrowsare of the Thalassinoides ichno-facies which is similar to thethree-dimensional galleries formedtoday by burrowing shrimp (egCallianassa) Photograph of theGreat Bahama Bank with the lo-cation of heavily bioturbated innerplatform facies In many modernenvironments these facies mayextend for thousands of squarekilometers Note the high densityof burrows and three-dimensionaldistribution of an individual bur-row gallery preserved in epoxyboth of which illustrate the po-tential for such facies to be lat-erally and vertically connected inthree dimensions commonlyleading to excellent reservoir

potential The plastic castphotograph of the ghost shrimpCallianassa burrow gallery is

tsego County Michigan

Additional porosity can be found in carbonate grainstonefacies that preserve some primary intergranular porosityThe shallowest tidal flat or stromatolitic facies showminimal PampP and are likely to be internal baffles or bar-riers to fluid flow through the formation In addition cor-relation of wireline logs shows that the Bass Islands cyclicporosity seen in the St Charlton 4-30 well is presentthroughout the northern part of lower Michigan

REFERENCES CITED

Barnes D A W B Harrison III and A Wahr in press Assessmentof regional geological carbon sequestration potential in UpperSilurian to Middle Devonian strata of the Michigan Basin inM Grobe J C Pashin and R L Dodge eds Carbon dioxidesequestration in geologic mediamdashState of the science AAPGStudies in Geology 59

Catacosinos P A P A Daniels Jr and W B Harrison III 1991Structure stratigraphy and petroleum geology of the MichiganBasin inMW Leighton D R Kolata D FOltz and J J Eideleds Interior cratonic basins AAPG Memoir 51 p 561ndash602

Esteban M and C F Klappa 1983 Subaerial exposure environ-

ment in P A Scholle D G Bebout and C H Moore edsCarbonate depositional environments AAPG Memoir 33p 1ndash95

Gardner W C 1974 Middle Devonian stratigraphy and deposi-tional environments in the Michigan Basin Michigan BasinGeological Society Special Paper 1 138 p

GrammerGMDABarnesWBHarrison III andAESandomierskiin press Practical synergies for increasing domestic oil produc-tion and geological sequestration of anthropogenic CO2 An ex-ample from the Michigan Basin in M Grobe J C Pashin andR L Dodge eds Carbon dioxide sequestration in geologicmediamdashState of the science AAPG Studies in Geology 59

Lane A C C S Prosser W H Sherzer and A W Grabau 1909Nomenclature and subdivision of the Upper Silurian strata ofMichigan Ohio and western New York Geological Society ofAmerica Bulletin v 19 p 553ndash556

Lilienthal R T 1978 Stratigraphic cross-sections of the MichiganBasin Department of Natural Resources Geological SurveyDivision Report of Investigation No 19 36 p

Sloss L L 1963 Sequences in the cratonic interior of North Amer-ica Geological Society of America Bulletin v 75 p 93ndash113doi1011300016-7606(1963)74[93SITCIO]20CO2

Sparling D R 1970 The Bass Islands Formation in its type regionThe Ohio Journal of Science v 70 no 1 p 1ndash33

US Department of Energy 2007 Carbon sequestration atlas of theUnited States and Canada Pittsburgh Pennsylvania NationalEnergy Technology Laboratory 86 p

Harrison et al 151

and were specifically based on evidence for subaerialexposure caused by a fall in relative sea level the pres-ence of flooding surfaces indicating a transgressive eventor vertical successions of facies indicating an overall flood-ing event followed by a shallowing of facies indicating arelative fall in sea level Subaerial exposure horizons aremarked by brecciation (collapse breccia) solution pipesand local development of terra rosa all of which are com-mon diagnostic indicators of exposure (eg Esteban andKlappa 1983) The highest frequency packages are the cy-cles which are amaximumof approximately 24 ft (73m)thick and have an average thickness of 14 ft (43 m)The HFS range from 28 to 44 ft (85 to 134 m) thickand the Bass Islands interval is approximately 72 ft thick(22 m)

Reservoir Intervals

The best reservoir units in theCore Energy-St Charlton4-30 core are present in two different intervals Thefirst is related to solution brecciation and recrystalliza-tion of limestones caused by subaerial exposure wherewell-developed interparticle intercrystalline and solution-enhanced vugs contribute to the creation of high porositiesand permeabilities The second unit is related to inner plat-form (midramp) bioturbated facies where the burrow gal-leries have been preferentially dolomitized with coarsercrystalline sucrosic dolomite Figure 10 illustrates howthese types of facies commonly referred to as tubulartempestites may be in excellent communication becauseof the nature of the three-dimensional burrow galleries

Figure 9 Stratigraphic hierarchy observed in the Bass Islands interval High-frequency sequences (HFS) along with higher-frequencycycles correlate well to the stratigraphic position of the best reservoir intervals and many of the HFS and cycle boundaries are manifestedby higher gamma-ray log values Regressive hemicycles of both the HFS and higher-frequency cycles correlate to the best developmentof reservoir throughout much of the cored interval NPHI = neutron porosity PEF = photoelectric factor

Harrison et al 149

courtesy of E A Shinn

and the density of burrows commonly found in similartypes of environments

Reservoir development also correlates well with thestratigraphic framework (Figure 9) Sequence and cycleboundaries were identified by the presence of subaerialexposure horizons as indicated by karstic brecciation andsolution flooding surface and facies stacking patternsAs mentioned above a well-constrained hierarchy ex-ists within the Bass Islands interval whereby the higher-frequency cycles exist within two medium-scale HFSFrom a reservoir standpoint the HFS and most of thehigher-frequency cycles constrain the development ofthe best reservoir intervals Regressive hemicycles ofboth the HFS and higher-frequency cycles correlate tothe best development of the reservoir throughout muchof the cored interval Many of the HFS and cycleboundaries are manifested by higher gamma-ray logvalues and can be identified in wells without core dataWe believe that the correlation of depositional faciesobserved in core to stratigraphic packages and wireline-log signatures is a powerful tool to help interpret the geo-logic properties of the Bass Islands interval in otherareas throughout Michigan where only wireline logsare available

150 Reservoir Characteristics of the Bass Islands Dolomite in O

CONCLUSIONS

The Bass Islands Formation can be mapped throughoutmuch of northern and central lower Michigan where itis an important saline reservoir target for CO2 seques-tration with more than 40 ft (12 m) of greater than 10porosity and permeabilities exceeding 500md Averageporosity in the Bass Islands cored interval in the CoreEnergy-St Charlton 4-30 test well is 125 and averagepermeability is 224 md

The Bass Islands in the area of the Core Energy-StCharlton 4-30 well is composed of a series of shallow-water and tidal flat carbonate facies that are stacked in apredictable shallowing-upward pattern The Bass Is-lands Formation seen in this core can be subdivided intotwoHFS and five higher-frequency cycles Stratigraphicanalysis helps predict vertical and lateral facies patternsand contributes to the evaluation of regional reservoirproperties The best reservoir quality occurs in the sub-tidal burrowed packstones that have some dissolutionof grains and develop moldic porosity This facies is alsoenhanced further by karst dissolution and brecciationrelated to the Tippecanoe-Kaskaskia sequence-boundingunconformity at the top of the Bass Islands Formation

Figure 10 Some of the bestreservoirs in the Bass Islands ex-ists in the thoroughly bioturbatedinner platform facies Burrowsare of the Thalassinoides ichno-facies which is similar to thethree-dimensional galleries formedtoday by burrowing shrimp (egCallianassa) Photograph of theGreat Bahama Bank with the lo-cation of heavily bioturbated innerplatform facies In many modernenvironments these facies mayextend for thousands of squarekilometers Note the high densityof burrows and three-dimensionaldistribution of an individual bur-row gallery preserved in epoxyboth of which illustrate the po-tential for such facies to be lat-erally and vertically connected inthree dimensions commonlyleading to excellent reservoir

potential The plastic castphotograph of the ghost shrimpCallianassa burrow gallery is

tsego County Michigan

Additional porosity can be found in carbonate grainstonefacies that preserve some primary intergranular porosityThe shallowest tidal flat or stromatolitic facies showminimal PampP and are likely to be internal baffles or bar-riers to fluid flow through the formation In addition cor-relation of wireline logs shows that the Bass Islands cyclicporosity seen in the St Charlton 4-30 well is presentthroughout the northern part of lower Michigan

REFERENCES CITED

Barnes D A W B Harrison III and A Wahr in press Assessmentof regional geological carbon sequestration potential in UpperSilurian to Middle Devonian strata of the Michigan Basin inM Grobe J C Pashin and R L Dodge eds Carbon dioxidesequestration in geologic mediamdashState of the science AAPGStudies in Geology 59

Catacosinos P A P A Daniels Jr and W B Harrison III 1991Structure stratigraphy and petroleum geology of the MichiganBasin inMW Leighton D R Kolata D FOltz and J J Eideleds Interior cratonic basins AAPG Memoir 51 p 561ndash602

Esteban M and C F Klappa 1983 Subaerial exposure environ-

ment in P A Scholle D G Bebout and C H Moore edsCarbonate depositional environments AAPG Memoir 33p 1ndash95

Gardner W C 1974 Middle Devonian stratigraphy and deposi-tional environments in the Michigan Basin Michigan BasinGeological Society Special Paper 1 138 p

GrammerGMDABarnesWBHarrison III andAESandomierskiin press Practical synergies for increasing domestic oil produc-tion and geological sequestration of anthropogenic CO2 An ex-ample from the Michigan Basin in M Grobe J C Pashin andR L Dodge eds Carbon dioxide sequestration in geologicmediamdashState of the science AAPG Studies in Geology 59

Lane A C C S Prosser W H Sherzer and A W Grabau 1909Nomenclature and subdivision of the Upper Silurian strata ofMichigan Ohio and western New York Geological Society ofAmerica Bulletin v 19 p 553ndash556

Lilienthal R T 1978 Stratigraphic cross-sections of the MichiganBasin Department of Natural Resources Geological SurveyDivision Report of Investigation No 19 36 p

Sloss L L 1963 Sequences in the cratonic interior of North Amer-ica Geological Society of America Bulletin v 75 p 93ndash113doi1011300016-7606(1963)74[93SITCIO]20CO2

Sparling D R 1970 The Bass Islands Formation in its type regionThe Ohio Journal of Science v 70 no 1 p 1ndash33

US Department of Energy 2007 Carbon sequestration atlas of theUnited States and Canada Pittsburgh Pennsylvania NationalEnergy Technology Laboratory 86 p

Harrison et al 151

courtesy of E A Shinn

and the density of burrows commonly found in similartypes of environments

Reservoir development also correlates well with thestratigraphic framework (Figure 9) Sequence and cycleboundaries were identified by the presence of subaerialexposure horizons as indicated by karstic brecciation andsolution flooding surface and facies stacking patternsAs mentioned above a well-constrained hierarchy ex-ists within the Bass Islands interval whereby the higher-frequency cycles exist within two medium-scale HFSFrom a reservoir standpoint the HFS and most of thehigher-frequency cycles constrain the development ofthe best reservoir intervals Regressive hemicycles ofboth the HFS and higher-frequency cycles correlate tothe best development of the reservoir throughout muchof the cored interval Many of the HFS and cycleboundaries are manifested by higher gamma-ray logvalues and can be identified in wells without core dataWe believe that the correlation of depositional faciesobserved in core to stratigraphic packages and wireline-log signatures is a powerful tool to help interpret the geo-logic properties of the Bass Islands interval in otherareas throughout Michigan where only wireline logsare available

150 Reservoir Characteristics of the Bass Islands Dolomite in O

CONCLUSIONS

The Bass Islands Formation can be mapped throughoutmuch of northern and central lower Michigan where itis an important saline reservoir target for CO2 seques-tration with more than 40 ft (12 m) of greater than 10porosity and permeabilities exceeding 500md Averageporosity in the Bass Islands cored interval in the CoreEnergy-St Charlton 4-30 test well is 125 and averagepermeability is 224 md

The Bass Islands in the area of the Core Energy-StCharlton 4-30 well is composed of a series of shallow-water and tidal flat carbonate facies that are stacked in apredictable shallowing-upward pattern The Bass Is-lands Formation seen in this core can be subdivided intotwoHFS and five higher-frequency cycles Stratigraphicanalysis helps predict vertical and lateral facies patternsand contributes to the evaluation of regional reservoirproperties The best reservoir quality occurs in the sub-tidal burrowed packstones that have some dissolutionof grains and develop moldic porosity This facies is alsoenhanced further by karst dissolution and brecciationrelated to the Tippecanoe-Kaskaskia sequence-boundingunconformity at the top of the Bass Islands Formation

Figure 10 Some of the bestreservoirs in the Bass Islands ex-ists in the thoroughly bioturbatedinner platform facies Burrowsare of the Thalassinoides ichno-facies which is similar to thethree-dimensional galleries formedtoday by burrowing shrimp (egCallianassa) Photograph of theGreat Bahama Bank with the lo-cation of heavily bioturbated innerplatform facies In many modernenvironments these facies mayextend for thousands of squarekilometers Note the high densityof burrows and three-dimensionaldistribution of an individual bur-row gallery preserved in epoxyboth of which illustrate the po-tential for such facies to be lat-erally and vertically connected inthree dimensions commonlyleading to excellent reservoir

potential The plastic castphotograph of the ghost shrimpCallianassa burrow gallery is

tsego County Michigan

Additional porosity can be found in carbonate grainstonefacies that preserve some primary intergranular porosityThe shallowest tidal flat or stromatolitic facies showminimal PampP and are likely to be internal baffles or bar-riers to fluid flow through the formation In addition cor-relation of wireline logs shows that the Bass Islands cyclicporosity seen in the St Charlton 4-30 well is presentthroughout the northern part of lower Michigan

REFERENCES CITED

Barnes D A W B Harrison III and A Wahr in press Assessmentof regional geological carbon sequestration potential in UpperSilurian to Middle Devonian strata of the Michigan Basin inM Grobe J C Pashin and R L Dodge eds Carbon dioxidesequestration in geologic mediamdashState of the science AAPGStudies in Geology 59

Catacosinos P A P A Daniels Jr and W B Harrison III 1991Structure stratigraphy and petroleum geology of the MichiganBasin inMW Leighton D R Kolata D FOltz and J J Eideleds Interior cratonic basins AAPG Memoir 51 p 561ndash602

Esteban M and C F Klappa 1983 Subaerial exposure environ-

ment in P A Scholle D G Bebout and C H Moore edsCarbonate depositional environments AAPG Memoir 33p 1ndash95

Gardner W C 1974 Middle Devonian stratigraphy and deposi-tional environments in the Michigan Basin Michigan BasinGeological Society Special Paper 1 138 p

GrammerGMDABarnesWBHarrison III andAESandomierskiin press Practical synergies for increasing domestic oil produc-tion and geological sequestration of anthropogenic CO2 An ex-ample from the Michigan Basin in M Grobe J C Pashin andR L Dodge eds Carbon dioxide sequestration in geologicmediamdashState of the science AAPG Studies in Geology 59

Lane A C C S Prosser W H Sherzer and A W Grabau 1909Nomenclature and subdivision of the Upper Silurian strata ofMichigan Ohio and western New York Geological Society ofAmerica Bulletin v 19 p 553ndash556

Lilienthal R T 1978 Stratigraphic cross-sections of the MichiganBasin Department of Natural Resources Geological SurveyDivision Report of Investigation No 19 36 p

Sloss L L 1963 Sequences in the cratonic interior of North Amer-ica Geological Society of America Bulletin v 75 p 93ndash113doi1011300016-7606(1963)74[93SITCIO]20CO2

Sparling D R 1970 The Bass Islands Formation in its type regionThe Ohio Journal of Science v 70 no 1 p 1ndash33

US Department of Energy 2007 Carbon sequestration atlas of theUnited States and Canada Pittsburgh Pennsylvania NationalEnergy Technology Laboratory 86 p

Harrison et al 151

Additional porosity can be found in carbonate grainstonefacies that preserve some primary intergranular porosityThe shallowest tidal flat or stromatolitic facies showminimal PampP and are likely to be internal baffles or bar-riers to fluid flow through the formation In addition cor-relation of wireline logs shows that the Bass Islands cyclicporosity seen in the St Charlton 4-30 well is presentthroughout the northern part of lower Michigan

REFERENCES CITED

Barnes D A W B Harrison III and A Wahr in press Assessmentof regional geological carbon sequestration potential in UpperSilurian to Middle Devonian strata of the Michigan Basin inM Grobe J C Pashin and R L Dodge eds Carbon dioxidesequestration in geologic mediamdashState of the science AAPGStudies in Geology 59

Catacosinos P A P A Daniels Jr and W B Harrison III 1991Structure stratigraphy and petroleum geology of the MichiganBasin inMW Leighton D R Kolata D FOltz and J J Eideleds Interior cratonic basins AAPG Memoir 51 p 561ndash602

Esteban M and C F Klappa 1983 Subaerial exposure environ-

ment in P A Scholle D G Bebout and C H Moore edsCarbonate depositional environments AAPG Memoir 33p 1ndash95

Gardner W C 1974 Middle Devonian stratigraphy and deposi-tional environments in the Michigan Basin Michigan BasinGeological Society Special Paper 1 138 p

GrammerGMDABarnesWBHarrison III andAESandomierskiin press Practical synergies for increasing domestic oil produc-tion and geological sequestration of anthropogenic CO2 An ex-ample from the Michigan Basin in M Grobe J C Pashin andR L Dodge eds Carbon dioxide sequestration in geologicmediamdashState of the science AAPG Studies in Geology 59

Lane A C C S Prosser W H Sherzer and A W Grabau 1909Nomenclature and subdivision of the Upper Silurian strata ofMichigan Ohio and western New York Geological Society ofAmerica Bulletin v 19 p 553ndash556

Lilienthal R T 1978 Stratigraphic cross-sections of the MichiganBasin Department of Natural Resources Geological SurveyDivision Report of Investigation No 19 36 p

Sloss L L 1963 Sequences in the cratonic interior of North Amer-ica Geological Society of America Bulletin v 75 p 93ndash113doi1011300016-7606(1963)74[93SITCIO]20CO2

Sparling D R 1970 The Bass Islands Formation in its type regionThe Ohio Journal of Science v 70 no 1 p 1ndash33

US Department of Energy 2007 Carbon sequestration atlas of theUnited States and Canada Pittsburgh Pennsylvania NationalEnergy Technology Laboratory 86 p

Harrison et al 151