hydraulic seals and their origin: evidence from the stable...

ABSTRACT

Regionally extensive, overpressured natural gaspools within the St. Peter Sandstone (MiddleOrdovician) of the Michigan basin are bounded byrock types that include 3–7-m-thick zones of low-permeability, low-porosity carbonates or diagene-tically banded quartz sandstones. Replacivedolomite from an approximately 5-m-thick carbon-ate interval in the east-central portion of theMichigan basin has very low δ13C values that sys-tematically decrease from approximately –5‰ atthe top to –10‰ (PDB) at the base. δ18O values forthe replacive dolomite also decrease systemati-cally with depth from approximately 27 to 23‰(SMOW). These data suggest an upward decreasein isotope exchange between the replacive dolomi-tization fluid and the precursor rock (i.e., the sys-tem was rock dominated at the top of the carbon-ate interval), which implies upward, cross-forma-tional movement of the dolomitizing fluid. Fluid-rock interaction modeling suggests that thedolomitizing f luid had a total dissolved carbon(TDC) content of approximately 4000 ppm and aδ13C value of –27‰, which indicates that the car-bon was primarily derived from organic diagenesis.Sr isotope and major element data suggest that thisdolomitizing fluid had a modified seawater origin.

Carbonate intervals in the St. Peter Sandstonecontain dolomite cement in quartz sandstone lami-nations and fractures that have δ13C values that

closely mimic the large δ13C variations of the adja-cent replacive dolomite with depth. This correla-tion indicates that the replacive dolomite con-trolled the δ13C value of the carbon dissolved inthe parent f luids of the later dolomite cement.Moreover, these data suggest that the parent fluidsof the dolomite cement were not confined to acommon fracture network, but flowed pervasivelythrough the carbonate intervals. The δ13C values ofdolomite cement in diagenetically banded quartzsandstone intervals from three locations in the cen-tral portion of the Michigan basin range from –9 to–4‰ and are relatively invariant at a particularlocality; therefore, the TDC of the dolomitizingfluid in the central Michigan basin is interpreted tohave contained only about 20–30% organic carbon.The fact that all dolomites analyzed in the St. PeterSandstone have much lower δ13C values than car-bonates in adjacent formations indicates thatdolomitization and the formation of hydraulic sealswere related to organic matter diagenesis.

INTRODUCTION

Variations in lithology and cementation withinthick sandstone sequences result in significantporosity and permeability contrasts that can pro-duce zones (i.e., compartments) (Hunt, 1990) thatare abnormally pressured. Laterally extensive, tight-ly cemented zones and/or rock types that have lowpermeability (e.g., argillaceous, evaporite-dominat-ed, or carbonate-dominated intervals) can act ashydraulic seals that impede cross-formational fluidflow on geologic time scales (Hunt, 1990; Powley,1990; Toth et al., 1991). If faulting or lateral varia-tions in lithology produce side seals, interveningporous zones can become hydrologically isolatedso that increased burial will result in abnormallyhigh fluid pressures (Hunt, 1990). Because a signif-icant portion of the world’s hydrocarbons havebeen produced from seal-bound compartments(Hunt, 1990), it is important to understand thegenesis of seals.

30 AAPG Bulletin, V. 79, No. 1 (January 1995), P. 30–48.

©Copyright 1995. The American Association of Petroleum Geologists. Allrights reserved.

1Manuscript received February 28, 1994; revised manuscript receivedAugust 25, 1994; final acceptance September 12, 1994.

2Department of Geology and Geophysics, University of Wisconsin–Madison, Madison, Wisconsin 53706.

This research was funded by the Gas Research Institute under contract5089-260-1810. We appreciate the leadership and encouragement ofcolleagues Bob Dott and Charlie Byers throughout this research project. Weare very grateful to Amoco, Shell Western Exploration and Production,UNOCAL, and Marathon Oil Company for providing technical assistance,access to cores, samples for analysis, and permission to publish the results.We thank William Harrison, Western Michigan University, for providingaccess to cores and samples for analysis. We thank Mike Spicuzza forassistance in the stable isotope laboratory.

Hydraulic Seals and Their Origin: Evidence from theStable Isotope Geochemistry of Dolomites in the MiddleOrdovician St. Peter Sandstone, Michigan Basin1

Bryce L. Winter, John W. Valley, J. A. (Toni) Simo, Gregory C. Nadon, and Clark M. Johnson2

Shales and carbonates become effective seals forpressure compartments when their permeability isreduced by diagenetic processes, such as silicifica-tion, dolomitization, and carbonate cementation(Hunt, 1990). Seals in sandstones are composed ofalternating centimeter- to meter-scale bands thatare carbonate cemented, clay cemented, and/orquartz cemented (Boles and Ramseyer, 1987; Tigertand Al-Shaieb, 1989; Hunt, 1990; Drzewiecki et al.,1994). Although numerous petrographic and geo-chemical studies have been conducted on porositydevelopment and preservation in reservoir rocks,few studies have focused on low-permeabilityseals. Understanding the chemical processesinvolved in seal formation has important implica-tions for understanding fluid movement in sedi-mentary basins and the interplay between organicand inorganic diagenesis.

In the late 1980s, the Middle Ordovician St.Peter Sandstone became a focus for extensive gasexploration in the deep Michigan basin (Harrison,

1987; Catacosinos et al., 1991). Vertical-pressureprofiles and hydraulic head distributions indicatethat the pressures within the St. Peter Sandstoneand overlying Glenwood Formation are up to 650psi above the hydrostatic gradient (Bahr et al.,1994). The largest area of overpressure is locatedwest and north of Saginaw Bay (Figure 1) andencompasses many of the deep gas-producingfields within the two formations (Bahr et al., 1994).

In this paper, we present oxygen and carbon sta-ble isotope data for replacive dolomite anddolomite cement from vertical sections through anatural gas-producing interval, including the low-permeability top and bottom seals, from four dif-ferent wells in the Saginaw Bay area (Figure 1).These data are compared with stable isotope datafor dolomite cement in diagenetically bandedquartz sandstone intervals from two different loca-tions in the central Michigan basin that have litho-logic, diagenetic, and hydrologic characteristics ofa seal. The stable isotope data and fluid-rock inter-action modeling (cf. Banner and Hanson, 1990) arehere used to (1) geochemically document the abili-ty of specific rock types to act as fluid flow barri-ers, (2) determine the direction of fluid flow dur-ing different diagenetic events, and (3) evaluatethe role of organic matter diagenesis in seal forma-tion in the St. Peter Sandstone of the Michiganbasin.

GEOLOGIC AND STRATIGRAPHIC SETTING

The Michigan basin is a North American intra-cratonic basin that began to subside in the EarlyCambrian and continued into the Jurassic(Catacosinos et al., 1991). The Middle OrdovicianSt. Peter Sandstone, the basal formation of theTippecanoe sequence (Sloss, 1963), is in general amature, eolian/marine shelf quartzarenite (Figure2). The St. Peter Sandstone extends across theMichigan basin and achieves a maximum thicknessof approximately 320 m (Figure 1). In the centralMichigan basin, the St. Peter Sandstone overliessilty dolomites and shales (Figure 2) that are infor-mally referred to as the Brazos shale (Catacosinoset al., 1991; Catacosinos and Daniels, 1991).Dolomitized carbonates and dolomitic shales of theLower Ordovician Prairie du Chien Group underliethe Tippecanoe sequence (Figure 2). The shalesand carbonates of the Glenwood Formation markthe upward transition from the clastic-dominatedSt. Peter Sandstone to the overlying transgressivemarine carbonates of the Black River and Trentonformations (Figure 2).

The St. Peter Sandstone ranges from exclusivelyquartz sandstones in the northwest part of thebasin to a mixed carbonate-siliciclastic succession

Winter et al. 31

60 kilometers

50 miles

120

160

200

240

80

Areas ofOver-Pressure

12

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Sagi

naw B

ay

State Joseph1

State Foster2

Hunt Winterfield3

Ballentine4

5

Walczak6

Whyte

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7 Ruppert

40

5 6

Erosional Edgeof St. PeterSandstone

240200

160

120

280

320

C A N A D A

U N I T E D S T A T E S

Figure 1—Isopach map (40-m contour interval) of the St.Peter Sandstone in the Michigan basin showing the areaof overpressure (Bahr et al., 1994) and the localities ofthe sampled cores. Specific localities of the cores are (1)Unocal State Joseph, Crawford County, Sec. 7-T25N-R4W;(2) Mobile State Foster 1-12, Ogemaw County, Sec. 12-T24N-R4E; (3) Hunt Winterfield, Clare County, Sec. 30-T20N-R6W; (4) Amoco Ballentine 1-35A, Gladwin Coun-ty, Sec. 35-T18N-R1W; (5) Shell Whyte 1-33, Bay County,Sec. 33-T15N-R4E; (6) Shell Walczak 1-7, Bay County, Sec.7-T14N-R5E; (7) Shell Ruppert 1-25, Tuscola County, Sec.25-T14N-R7E.

along the southern margin of the basin. The litho-logic variation within the St. Peter Sandstoneallows subdivision into as many as 18 depositionalsequences, most of which can be traced through-out the overpressured region (Nadon et al., 1991).In particular, there are seven, 3–7-m-thick, low-porosity, depositional carbonate intervals com-posed of replacive dolomite mudstones (which are

commonly peloidal), wackestones, and grain-stones; minor dolomitic shale; and minor dolomite-cemented quartz sandstone laminations (cf.Drzewiecki et al., 1994). Oolitic and fossiliferous(brachiopods, ostrocods, trilobites) grainstones arean important component (~10–35%), and minorstromatolitic heads and laminations occur through-out the carbonate lithofacies. Anhydrite most com-monly occurs as cement in fractures and voids thatcrosscut depositional structures, whereas anhy-drite nodules are comparatively rare. Based onthese observations, we interpret the carbonatelithofacies of the St. Peter Sandstone in theMichigan basin to have been deposited in a shal-low, subtidal, low- to high-energy shelf environ-ment with possible intermittent circulation restric-tion, rather than a highly restricted, sabkha-likeenvironment as envisioned by Barnes et al. (1992).

PETROGRAPHY AND DIAGENETIC HISTORY

The diagenetic history of the St. Peter Sandstoneis complex and includes minor, early marine, cryp-tocrystalline carbonate cementation; mechanicalcompaction; multiple episodes of dolomitizationand dolomite cementation; K-feldspar and quartzovergrowth cementation; intergranular pressuresolution; authigenic illite precipitation; anhydritecementation; and pyrite precipitation (Barnes etal., 1992; Drzewiecki, 1992; Drzewiecki et al., 1994).Minor, early marine, isopachous rim cements(0.05–0.3 mm thick), which are now dolomitized,are observed in sandy intervals near and within thecarbonate lithofacies (Barnes et al., 1992;Drzewiecki, 1992; Drzewiecki et al., 1994). Earlydiagenetic features and evidence for the precursormineralogy of the carbonate lithofacies in the St.Peter Sandstone, however, are characteristicallyrare, the latter having been virtually obliterated byburial dolomitization (Figure 3).

Dolomite that has replaced mud-dominatedlimestone (Figure 3a) consists of very cloudy,subhedral crystals that range in size from 10 to50 µm and have nonplanar boundaries (terminol-ogy of Sibley and Gregg, 1987). Dolomite thathas replaced grain-supported limestone consistsof cloudy, subhedral crystals that range in sizefrom 50 to 175 µm and have nonplanar to morerarely planar boundaries (Figure 3b). The allo-chems in the replacive dolomite grainstones arepresent as ghosts, where inclusions in thedolomite crystals define the outline and less com-monly the internal structure of the original grains(Figure 3b). Under cathodoluminescence, bothreplacive dolomite mudstone and grainstonehave dull to bright orange luminescence that iscommonly mottled with tan and purple-orange

32 Hydraulic Seals and Their Origin

Amoco Ballentine 1-35 (Gladwin Co.)

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3400

3500

3600

3700

Comp.

Neutron Por.

0 150Gamma Ray

2.0 3.0-.5 .45

45 -15

Bulk Density

Gle

nwoo

dB

R

Lithology

Depth (m)

Electric Logs

Sampled Intervals

1200

0 ft

1150

0 ft

St. P

eter

San

dsto

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orm

atio

nSH

State JosephState FosterHunt Winterfield

BallentineWalczakRuppertWhyte

Hunt Winterfield

Quartzose Sandstone

��CarbonateBioturbated Sandstone

��Dolomitic Shale

*

*

*

Figure 2—Lithologic and electric logs for the Amoco Bal-lentine well, Gladwin County, in the Michigan basin. BR= Black River; SH = Shakopee Formation of the Prairie duChien Group. The uppermost portion of the Shakopee inthis log incorporates the Brazos shale. The dolomitefrom the Brazos shale was analyzed from the Hunt Win-terfield core. Correlative intervals sampled in other wellsare indicated by the solid blocks on the right side. Thelocations of the wells are shown in Figure 1. * = produc-ing intervals.

luminescence. Based on the similar luminescencecharacter and the similar geochemical signature ofadjacent samples (discussed in a following sec-tion), we interpret both the mudstones and grain-stones to have been dolomitized by the same fluidduring a single event (i.e., replacive dolomite mud-stones and grainstones are genetically related).

Dolomite cement that fills fractures and voids inthe carbonate lithofacies primarily consists of clear,planar crystals that have uniform optical extinctionand range up to 1 mm in diameter (Figure 3b). Thisdolomite cement is zoned and predominantly non-luminescent with the zoning defined by dullorange subzones. The replacive dolomite grain-stone in the Brazos shale is similar to that in the St.

Peter Sandstone except that crystal sizes can rangeup to 300 µm and all dolomite is nonluminescent.Dolomite cement in quartz sandstone lithofacies inthe St. Peter Sandstone can comprise from 3 to 35wt. % of the sample and is usually composed ofnonplanar crystals having uniform optical extinc-tion and is very dull orange or nonluminescent.

ANALYTICAL METHODS

Approximately 1–5 mg of the replacive and frac-ture-fill dolomites, each predominantly composedof a single cathodoluminescence type (i.e., mottledorange luminescent or zoned nonluminescent),

Winter et al. 33

a

b

Figure 3—Photomicrographs(plane transmitted light) of thedifferent types of burial dolomitein the carbonate lithofacies of theSt. Peter Sandstone, Michiganbasin. (a) Replacive dolomitemudstone (25–50-µm crystals)with displaced organic matter.Crosscutting fracture is filled withdolomite cement that undercathodoluminescence is zonedand primarily nonluminescent;the zoning is defined by dullorange subzones. SampleW10938.3 from the Walczak core,Bay County. Photo is 1.3 mmacross. (b) Replacive dolomitegrainstone with carbonate grains(skeletal grains and oolite) pres-ent as ghosts. White grains arequartz. Sample W10945.1 from theWalczak core, Bay County. Photois 2.6 mm across.

were obtained from thin section stubs using a 0.25mm microdrill. Approximately 10–30 mg samplesof dolomite-cemented sandstone were obtainedfrom slabbed cores using a 5 mm drill. The samplepowders were reacted with concentrated H3PO4 at50°C in separate off-line vessels for 8–12 hr, andCO2 was extracted using standard cryogenic proce-dures. Stable isotope ratios were measured on aFinnigan MAT 251™ in the Department of Geologyand Geophysics at the University of Wisconsin–Madison (UW). A fractionation factor (α) of1.01007 was used to correct for the oxygen iso-tope fractionation that occurs during dolomitedigestion in phosphoric acid at 50°C. Nineteenanalyses of the UW calcite standard run throughoutthe course of this study yielded a cumulative meanδ18O (SMOW) value of 19.11 ±0.07 (1σ) and amean δ13C (PDB) value of –15.96 ±0.08 (1σ). Fiveanalyses of NBS-19 yielded δ18O (PDB) = –2.11±0.02 (1σ) and δ13C (PDB) = +1.96 ±0.01 (1σ), andfive analyses of NBS-20 yielded δ18O (PDB) = –4.12±0.02 (1σ) and δ13C (PDB) = –1.06 ±0.01 (1σ).

GENERAL RESULTS

δ18O (SMOW) and δ13C (PDB) values for re-placive dolomites in the St. Peter Sandstone rangefrom 20.9 to 27.3‰ and –10.2 to –4.9‰, respec-tively, whereas replacive dolomites in the Brazosshale have δ18Ο and δ13C values that are less vari-able, ranging from 21.1 to 21.9‰ and –3.8 to–3.2‰, respectively (Table 1, Figure 4). Dolomitecement in quartz sandstone intervals from the St.Peter Sandstone and from the lower GlenwoodFormation has δ18O values ranging from 12.8 to23.1‰ and δ13C values ranging from –8.9 to –3.9‰(Table 1, Figure 5). δ13C values for dolomitecement in quartz sandstone from individual locali-ties are relatively invariant (Table 1, Figure 5). Theδ13C values of all the dolomites in the St. PeterSandstone are substantially lower than those ofdolomite in the overlying (Trenton/Black River,δ13C = –1.2 to 1.2‰; Taylor and Sibley, 1986; Budaiand Wilson, 1991; Glenwood, δ13C = –3.5 to–2.0‰; Vandrey, 1991) and underlying (Brazosshale, δ13C = –3.8 to –3.2‰) formations in theTippecanoe sequence.

REPLACIVE DOLOMITE IN CARBONATELITHOFACIES

Lithology of Gas-Producing Intervals

The lithology and stable isotope variations ofdolomites in vertical sections of the same strati-graphic interval from three different localities in

the east-central portion of the Michigan basin nearSaginaw Bay (Figure 1) are shown in Figures 6–8.The datum from which all of the vertical sections(Figures 6–8) are measured is a clay-rich layer thatproduces a distinct spike on the gamma log. Thisgamma spike can be correlated throughout theMichigan basin and is interpreted as a chronostrati-graphic marker (Nadon et al., 1991). The strati-graphic sections in Figures 6–8 are divided intofour intervals (A, B, C, and D) based on the litholo-gy. Core analysis (Moline, 1992) indicates that theporosity and permeability of sandstone intervals Aand C range from 2 to 8% and 0.1–6.0 md, respec-tively; sandstone interval C is a producing gasreservoir. Carbonate intervals B and D are, respec-tively, the top and bottom seals for the gas reser-voir, and have 0.6–1.6% porosity and permeabili-ties ranging from 0.01 to 0.74 md (Moline, 1992).

Carbon Isotope Variations

The general variation in the dolomite δ13C val-ues with depth is directly comparable in all threevertical sections (Figures 6b, 7b, 8b). In the lowercarbonate interval (interval B), δ13C systematicallydecreases from approximately –5.5‰ at the top toapproximately –10‰ at the bottom (Figures 6b,7b, 8b). δ13C values for dolomite in the lowerquartz sandstone (interval A) increase dramaticallywith depth (Figures 6b, 7b, 8b), whereas δ13C val-ues of the dolomite cement in the upper, gas-pro-ducing sandstone (interval C) are similar to theδ13C values of the dolomite in the upper portion ofinterval B (δ13C = –5.5 to –4.5‰) and display novariation with depth. With the data available, δ13Cvalues of dolomite in the upper carbonate (intervalD) appear to increase upward from about –6.5‰ atthe base to a maximum of –5‰. The replacivedolomite samples from the Ballentine core arefrom the lower portion of an approximately 3-m-thick carbonate interval (Figure 2). The verticalcoverage from the Ballentine core is not as exten-sive as the other cores, but the δ13C variations arecomparable (Figure 4, Table 1).

The very low δ13C values of the replacivedolomites in the St. Peter Sandstone (Figures 6b,7b, 8b) are unusual and indicate a major contribu-tion of 13C-depleted carbon from organic matter.The contribution of organic carbon during dolomi-tization systematically increased with depth ininterval B in all three sections (Figures 6b, 7b, 8b).The δ13C values of most replacive dolomites areusually similar to those of the precursor limestone,ranging from –4‰ to +4‰ throughout geologictime, and have previously been used to infer theδ13C values of paleo-oceans (e.g., Holser et al.,1986). The small change in δ13C values that usually


Walczak, St. Peter SandstoneW10935.8-1 26.17 –5.92 RDMW10935.8-2 24.63 –6.34 RDMW10936-1** 25.58 –6.29 RDMW10936-2** 24.70 –6.38 RDMW10937.2-1 24.09 –7.90 RDMW10937.2-2 23.94 –7.62 RDGW10937.4-1** 22.99 –8.53 RDMW10938.0-2 24.95 –7.76 RDMW10938.0-3** 25.78 –7.79 RDMW10938.3-1 25.74 –7.68 RDMW10938.3-2 25.12 –7.81 RDMW10938.3-2 25.23 –7.79 RDMW10938.5 25.64 –7.82 RDMW10938.5 26.06 –7.58 RDMW10938.7-1 21.33 –7.06 DFFW10938.7-2** 23.65 –6.96 RDGW10938.9-1** 25.80 –6.90 RDGW10938.9-3 18.50 –7.57 DFFW10939.4B1** 21.75 –6.65 DFFW10939.4B2 21.59 –6.82 DFFW10939.4W 18.46 –8.01 DFFW10939.4BL 24.61 –7.09 RDMW10939.4G 23.27 –6.44 RDMW10939.8-1 22.39 –7.16 RDGW10939.9 21.87 –7.38 RDGW10941.9** 19.70 –9.19 DFFW10942-1** 19.64 –9.22 DFFW10942-2 23.86 –9.63 RDMW10942.2 23.42 –9.51 RDMW10942.4-1 23.02 –9.58 RDMW10942.4-2 23.57 –9.63 RDMW10942.4-4** 20.47 –9.23 DFFW10942.4-5** 19.48 –9.54 DFFW10942.6-1 19.69 –9.41 DFFW10942.6-2 21.55 –8.95 DFFW10942.6-3** 22.54 –9.26 RDGW10942.8** 22.86 –9.47 RDGW10943.6 23.73 –10.23 RDMW10943.8 24.08 –10.14 RDMW10944.3 20.27 –9.53 DSLW10945.1-1 22.79 –9.66 RDMW10945.1-2** 22.47 –9.54 DSLW10946.3 22.90 –9.72 RDMW10947.2-1 23.38 –9.55 RDMW10947.2-2 19.27 –9.38 DSLW10948.2-1 23.07 –10.13 RDMW10948.2-2 23.02 –10.21 RDGW10948.3-1 22.90 –10.08 RDMW10948.3-2 23.18 –10.22 RDGW10948.8-1** 20.27 –10.47 DFFW10948.8-2 22.03 –10.06 RDGW10948.8-1B 22.04 –9.88 RDGW10948.8W 20.99 –9.86 DFF

W10949.5W** 19.62 –9.60 DFFW10949.5B** 22.76 –9.85 DFFW10949.5B 22.27 –9.62 RDGW10950.1A** 18.05 –9.24 DCSW10950.3A 23.55 –9.88 RDGW10950.3B 23.78 –9.91 RDGW10950.4W** 19.69 –9.51 DFFW10950.5W** 19.24 –9.18 DFFW10950.5 22.98 –9.73 DCSW10950.7W** 20.41 –9.26 DFFW10950.7 22.71 –9.66 DCSW10959.8W** 18.38 –8.67 DFFW10950.9 20.73 –8.53 DCSW10951 20.50 –8.77 DCSW10951.3 19.54 –9.03 DCSW10951.7 17.21 –8.46 DCS

State Foster, St. Peter SandstoneSF11272.3 20.00 –7.60 DCSSF11273.7 18.10 –7.60 DCSSF11274.1-1 21.20 –7.40 DCSSF11274.1-2 21.30 –7.40 DCSSF11274.1-3 19.80 –7.60 DCSSF11274.2-1** 20.90 –7.50 DCSSF11274.2-2** 17.30 –7.80 DCSSF11274.2-3** 20.80 –7.60 DCSSF11284.5** 18.10 –7.40 DCSSF11295.0 17.70 –7.50 DCSSF11304.3 17.80 –7.50 DCSSF11304.6** 17.20 –7.60 DCSSF11306.5-1** 19.30 –7.40 DCSSF11306.5-2** 18.60 –7.50 DCSSF11306.5-3** 20.80 –7.30 DCSSF11307.2** 17.50 –7.60 DCSSF11308.3** 19.30 –7.40 DCSSF11308.5** 20.10 –7.30 DCSSF11310.3** 23.10 –7.10 DCSSF11310.4** 18.30 –7.60 DCS

Ballentine, St. Peter SandstoneAB11769-1** 25.50 –11.10 RDMAB11769.4-1 26.10 –6.10 RDMAB11769.4-2** 26.50 –6.50 RDMAB11769.4-3** 26.60 –6.40 RDMAB11770.1-1 25.40 –10.40 RDMAB11770.1-2** 26.00 –10.70 RDMAB11770.1-3 25.70 –10.80 RDMAB11770.1-4 25.40 –10.60 RDMAB11771.4** 25.80 –10.10 RDMAB11772-1 24.80 –12.00 RDMAB11772-2 24.90 –11.10 RDMAB11778.5-1 23.50 –8.00 RDMAB11778.2A-3 22.70 –7.80 RDMAB11778.5-1 23.50 –8.00 RDM

Table 1. Oxygen (SMOW) and Carbon (PDB) StableIsotope Data (‰) for Carbonates from MiddleOrdovician Formations, Michigan Basin

Sample No. δ18O δ13C Type* Sample No. δ18O δ13C Type*


AB11769-2** 22.00 –9.20 DSLAB11778.2A-1 20.30 –7.90 DSLAB11778.2B-1 19.60 –8.00 DSLAB11778.2B-2** 19.50 –8.00 DSLAB11778.5-2B** 17.40 –6.50 DSLAB11783.5** 18.80 –7.70 DSLAB11783.5B 15.10 –8.50 DSL

Hunt Winterfield, Brazos ShaleHW11627-2 21.88 –3.78 RDGHW11628-1** 21.71 –3.53 RDGHW11628-2** 21.66 –3.69 RDGHW11628-3 21.09 –3.79 RDGHW11628-4 21.51 –3.89 RDGHW11630-1** 21.45 –3.47 RDGHW11630-2** 21.66 –3.25 RDGHW11631-2** 21.84 –3.41 RDG

Ruppert, St. Peter SandstoneSR10467 25.60 –5.11 RDMSR10468 27.27 –5.56 RDMSR10469 26.11 –5.47 RDMSR10469.5 24.87 –4.87 RDMSR10470.3 24.65 –5.43 RDMSR10471.2 24.65 –5.54 RDMSR10471.7 25.37 –6.44 RDMSR10472.6 23.74 –6.33 RDMSR10473.5 22.68 –5.60 DSLSR10475 20.90 –5.66 DSLSR10480.2 23.62 –5.79 RDGSR10484.8 23.40 –6.84 RDMSR10486.2 23.09 –7.21 RDMSR10486.8 22.75 –7.10 RDMSR10487.6 25.15 –7.99 RDMSR10488.2 25.60 –6.79 RDGSR10488.5 19.27 –7.78 DFFSR10488.6 24.97 –7.37 RDMSR10488.8 24.26 –7.91 RDGSR10489.2 23.71 –8.33 RDMSR10489.6 23.43 –8.16 RDGSR10490.3 24.32 –8.45 RDMSR10491 24.14 –8.99 RDMSR10491.8 23.70 –9.15 RDMSR10492.5 24.05 –9.42 RDMSR10493.2 24.06 –9.37 RDMSR10493.6 23.29 –9.41 RDMSR10494.5 24.74 –9.93 RDMSR10495.6 23.77 –10.03 RDMSR10502.4 21.18 –9.70 DFFSR10502.6 21.34 –9.54 RDGSR10506 22.23 –8.50 RDM

State Joseph, St. Peter SandstoneSJ10943.6 21.02 –8.77 DCSSJ10943.64 21.67 –8.87 DCS

SJ10945.3 19.19 –8.38 DCSSJ10945.5 19.34 –8.46 DCSSJ10945.92 19.39 –8.39 DCSSJ10946.0 19.40 –8.39 DCSSJ10946.06 18.07 –8.25 DCSSJ10949.5** 18.28 –8.33 DCSSJ10949.9** 20.91 –8.41 DCSSJ10950.3** 17.06 –8.19 DCSSJ10951.64 17.76 –8.13 DCSSJ10952.1** 20.54 –8.34 DCSSJ10952.3 17.98 –8.15 DCSSJ10952.6** 20.16 –8.22 DCSSJ10955.18 20.50 –7.92 DCSSJ10955.22 20.78 –7.83 DCSSJ10955.26 19.28 –8.03 DCSSJ10960.92 19.91 –7.95 DCSSJ10960.95** 21.18 –7.89 DCSSJ10960.96 21.45 –7.98 DCSSJ10961.02 22.26 –7.93 DCSSJ10961.56 19.78 –7.88 DCSSJ10972.12 18.48 –7.79 DCSSJ10972.16 19.21 –7.71 DCSSJ10972.17** 19.29 –7.73 DCSSJ10972.24 19.74 –7.73 DCSSJ10979.14 19.16 –7.65 DCSSJ10979.16 19.16 –7.72 DCSSJ10979.19 19.35 –7.59 DCSSJ10980.86 19.04 –7.62 DCSSJ10983.1 18.49 –7.61 DCSSJ10983.18 19.20 –7.52 DCSSJ10983.67 18.37 –7.42 DCS

Hunt Winterfield, Glenwood Formation†

HW10588.4 16.21 –4.53 DCSHW10588.1 12.84 –5.46 DCSHW10594.75 16.44 –4.51 DCSHW10954.9 18.38 –3.96 DCSHW10952.2 15.79 –4.52 DCSHW10589 17.43 –4.10 DCSHW10587.6 16.57 –4.41 DCSHW10593.9 19.01 –3.86 DCSHW10588.3 13.89 –5.05 DCSHW10592.6 14.10 –4.20 DCSHW10588.6 14.25 –4.70 DCS

Whyte, St. Peter SandstoneSW10983.1 25.95 –5.96 RDMSW10983.7 24.58 –5.99 RDMSW10983.9 25.30 –5.87 RDMSW10987.4 23.78 –6.03 RDMSW10987.4 18.91 –6.19 DFF

Sample No. δ18O δ13C Type* Sample No. δ18O δ13C Type*

Table 1. Continued.

accompanies replacive dolomitization indicatesthat the carbon system is usually rock dominated(cf. Banner and Hanson, 1990). Replacive dolo-mites that are spatially associated with subaerialexposure (i.e., karsted) surfaces can display large,systematic δ13C variations with depth (e.g.,Beeunas and Knauth, 1985). There are, however,no sedimentologic, petrographic, or stratigraphicrelations that suggest the replacive dolomites inthe St. Peter Sandstone are associated with a sub-aerial exposure surface. Authigenic dolomitecement, in the form of concretions, lenses, andfracture fills, that precipitates in response to bacte-rial processes, thermal decarboxylation of organicmatter, or methane oxidation in organic-rich envi-ronments can have extreme and systematic spatialvariability in δ13C values (–40 to +25‰), but thedominant dolomite in the carbonate intervals inthe St. Peter Sandstone is clearly replacive in origin(Figure 3b). We are not aware of any unmetamor-phosed replacive dolomites that have comparablylow and systematically variable δ13C values.

Total organic carbon contents and δ13C values oftotal organic carbon (δ13Corg) in the Walczak sec-tion were measured to assess the effects of in situorganic matter degradation during dolomitization.The weight percent of total organic carbon in theWalczak section ranges between 0.05 and 0.55%and does not systematically vary with depth(Figure 9a). δ13Corg values in carbonate interval Bdisplay no systematic variation with depth, andrange between –31 and –29‰ (Figure 9b), whichis typical for Middle Ordovician organic matter(e.g., Hatch et al., 1987). δ13Corg values in thequartz sandstone (interval A) underlying carbonateinterval B are comparatively enriched in 13C (δ13C= –26 to –25‰; Figure 9b), indicating that thisorganic carbon was probably altered (i.e., partiallyoxidized) during deposition and/or diagenesis.

In situ thermal decarboxylation of organic mat-ter could have produced the δ13C systematics ofthe replacive dolomite in interval B (Figures 6–8)only if (1) the original organic carbon content sys-tematically increased with depth in interval B, or(2) there was a vertical temperature gradientacross interval B such that organic matter in thelower portion was decarboxylating to a greaterextent than in the upper portion during dolomiti-zation. The latter is improbable considering thatinterval B is only approximately 5.5 m thick; it ishighly unlikely that the lower portion of interval Bwas ever at a significantly different temperaturethan the upper portion. The absence of a lithologicvariation (e.g., the basal portion being more argilla-ceous) and the absence of any systematic variationin δ13Corg or weight percent organic carbon withdepth (Figure 9) indicate that the δ13C variation ofthe replacive dolomite is not the result of thermal

Winter et al. 37

SW10989.3 19.27 –5.16 DCSSW10991.1 18.11 –5.38 DCSSW10991.0 20.02 –4.93 DCSSW10992.7 19.59 –4.94 DCSSW10994.9 19.98 –4.69 DCSSW10997.3 21.31 –5.20 DCSSW10997.9 18.79 –5.11 DCSSW11001.5 22.27 –5.12 DCSSW11002.9 22.26 –5.10 RDGSW11002.9 18.96 –5.59 DFFSW11003.4 22.92 –5.09 RDGSW11003.6 18.22 –6.28 DFFSW11003.6 22.23 –5.23 RDGSW11003.8-1 24.26 –5.26 RDMSW11003.8-2 23.29 –5.61 RDGSW11003.8 19.54 –6.51 DFFSW11004.7-1 23.58 –6.00 RDGSW11005.9 22.12 –8.14 RDGSW11008.5-1 20.90 –8.50 RDGSW11008.5-2 20.88 –8.48 RDGSW11008.5A 18.67 –8.84 DFFSW11008.5B 18.62 –8.97 DFFSW11009.9 23.41 –9.35 RDMSW11010.1 22.72 –9.32 RDMSW11010.7-1 22.94 –9.82 RDMSW11010.7-2 22.00 –9.77 RDGSW11010.7A 20.37 –9.15 DFFSW11010.7B 20.95 –9.32 DFFSW11012.8 23.67 –10.04 RDMSW11013.1 22.03 –10.04 RDMSW11013.3 19.98 –9.75 DCSSW11014.8 19.22 –9.53 DFFSW11014.8 22.36 –9.23 RDMSW11015.0 21.86 –9.34 RDMSW11015.1 22.80 –9.15 RDMSW11015.5 22.27 –9.11 RDMSW11017.4 18.04 –9.32 DFFSW11017.6-1 22.94 –9.26 RDMSW11017.6 18.95 –9.39 DFFSW11019.7 18.78 –8.76 DCSSW11019.7 18.64 –8.87 DFFSW11026 22.19 –8.24 RDMSW11027.6 18.51 –7.83 DCSSW11034.4 17.69 –7.43 DCS

* RDG = replacive dolomite grainstone, RDM = replacive dolomitemudstone, DCS = dolomite cement in quartz sandstone, DFF = dolomitefilling fractures/vugs, DSL = dolomite cement in sandstone laminations.

** Oxygen isotope data presented in Winter et al. (1994).† Data from Drzewiecki (1992).

Sample No. δ18O δ13C Type*

Table 1. Continued.

decarboxylation of in situ, depositional organicmatter.

The approximately 5‰ decrease in replacivedolomite δ13C values with depth over approxi-mately 5.5 m in interval B (Figures 6b, 7b, 8b) canbe explained by a decrease in the amount of car-bon isotope exchange between f luid and rocktoward the top of interval B (i.e., the system wasrock dominated at the top of interval B) duringdolomitization. Two sources for the dissolved,organic-derived carbon are possible: (1) oxidation

of in situ organic matter, or (2) the organic-derivedcarbon was allochthonous and probably transport-ed to the site of dolomitization by the diageneticfluid. If the diagenetic fluid was relatively oxidiz-ing and the amount of isotope exchange betweenfluid and rock decreased upward in interval B dur-ing dolomitization, there probably would havebeen a concomitant upward decrease in the oxida-tion of the in situ organic matter, which could haveproduced the measured δ13C variations of thedolomite with depth (Figures 6b, 7b, 8b).


-14

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-8

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δ 18O (SMOW)

d. Whyte

δC

(PD

B)

-14

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-8

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δ 18O (SMOW)

b. Ruppert

δC

(PD

B)

1313

-14

-12

-10

-8

-6

-4

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12 14 16 18 20 22 24 26 28

δ 18O (SMOW)

a.

BrazosHunt Winterfield

Ballentine

-14

-12

-10

-8

-6

-4

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12 14 16 18 20 22 24 26 28

δ 18O (SMOW)

c. Walczak

Replacive Dolomite MudstoneReplacive Dolomite GrainstoneDolomite Cement in FracturesDolomite Cement in Sandstone

δC

(PD

B)

13δ

C(P

DB

)13

Figure 4—δ18O–δ13C diagrams for replacive dolomite and associated dolomite cements from depositional carbonateintervals in the St. Peter Sandstone in the Michigan basin.

Thermal chemical sulfate reduction is a commonreaction by which depositional organic matter isoxidized during diagenesis in a burial setting (e.g.,Machel, 1987). Thermal chemical sulfate reductioncan produce 13C-depleted dolomite (e.g., Andersonand Garven, 1987) with the level of 13C depletionbeing dependent upon the amount of SO4

= deliv-ered to react with organic matter, which, in turn,depends upon the fluid/rock ratio. Thermal chemi-cal sulfate reduction also produces H2S, whicheither dissolves carbonate or combines with metalcations to produce sulfides, such as pyrite. Theintensity of dissolution or the abundance of pyriteshould broadly correlate with the amount of SO4

=

reduction and, therefore, the δ13C of the dolomite.Minor dissolution features contemporaneous withreplacive dolomitization (e.g. dissolution/trunca-tion of minor luminescence zones) in the carbon-ate intervals of the St. Peter Sandstone are recog-nized under cathodoluminescence, but variation inthe intensity of these dissolution features withdepth is not observed. Sulfur isotope and quantita-tive abundance data for framboidal pyrite dissemi-nated in the replacive dolomite from the St. PeterSandstone are not available, but qualitatively, petro-graphic study does not reveal a systematic variationin the abundance of pyrite with depth. It is proba-ble, therefore, that the dolomitizing fluid itself,

rather than in situ (i.e., depositional) organic car-bon, was the major source of the 13C-depleted car-bon incorporated in the replacive dolomite.

Oxygen Isotope Variations

The vertical variations in δ18O values for thereplacive dolomite in intervals B and D from thethree different sample localities (Figures 6a, 7a, 8a)are generally comparable. δ18O values for replacivedolomite in interval B in the Walczak section(Figure 7a) increase dramatically over a small verti-cal distance (<1 m) from a relatively constant valueof 23 ±1‰ to a maximum value of approximately27‰ at the top, and define a general inverted L-shaped trajectory. The replacive dolomite in theRuppert and Whyte sections does not display anincrease in δ18O values toward the top of intervalB, but δ18O values do dramatically increase upsec-tion to a value of approximately 27‰ over a shortvertical distance in the lower portion of carbonateinterval D (Figures 6a, 8a); therefore, the replacivedolomite δ18O data from the Ruppert and Whytesections also define a general inverted L-shaped tra-jectory when the data from both of the carbonateintervals (i.e., intervals B and D) are viewed incomposite.

FLUID-ROCK INTERACTION MODELING

The variation in the amount of isotope exchangebetween fluid and rock with depth in the carbon-ate intervals during dolomitization can be quantita-tively estimated by simultaneously modeling thevertical variations in the replacive dolomite δ13Cand δ18O values. Fluid-rock interaction modelingcan place constraints on the δ13C value and theconcentration of total carbon dissolved in thedolomitizing fluid. The modeling employed hereuses the mass balance equations and approach ofBanner and Hanson (1990), where it is assumedthat a thermodynamic drive exists for fluid-mineralreaction, and that each fluid increment reacts withthe rock until isotope equilibrium is obtainedbefore it is displaced by the next increment ofunreacted fluid. This process is then repeated forany number of iterations. The absolute value of thecalculated fluid/rock (F/R) ratios may not be accu-rate, but the relative information can be very usefulfor constraining the composition and origin of dia-genetic f luids (Baumgartner and Rumble, 1988;Banner and Hanson, 1990). Because isotope equi-librium is assumed and only reactive f luids aredetected, the calculated F/R ratios represent mini-mum values and it is likely that, in reality, a muchlarger proportion of f luid migrated through the

Winter et al. 39

-12

-10

-8

-6

-4

-2

0

12 14 16 18 20 22 24

δ13

C(P

DB

)

δ 18O (SMOW)

Hunt Winterfield

State Foster

State Joseph

Figure 5—δ18O vs. δ13C for dolomite cement in quartzsandstone intervals from the lower Glenwood Forma-tion (Hunt Winterfield core, Clare County) and the St.Peter Sandstone (State Foster core, Ogemaw County andState Joseph core, Crawford County) that are diageneti-cally banded and have hydrologic characteristics ofseals.

rock (i.e., not all f luid participates in isotopeexchange reactions).

The fluid-rock interaction modeling in this casemust be initiated with the oxygen isotope system.The δ18O value (23 ±1‰) defined by the verticalportion of the inverted L-shaped trajectories inFigures 6a, 7a, and 8a represents the value of thereplacive dolomite in equilibrium with the porewater (i.e., at comparatively high F/R ratios). The±1‰ variation in the equilibrium δ18O value indi-cates that not all oxygen isotope exchange reac-tions reached equilibrium, or that there were subtlevariations in the δ18O value of the pore waterand/or the temperature during dolomitization. Thehorizontal portion of the inverted L-shaped δ18Odata trajectories (Figures 6a, 7a, 8a) is sensitive tochanges in the F/R ratio with depth, which can bemodeled if the δ18O of the initial rock (i.e., just

prior to dolomitization) is known. Based on (1)reported δ18O values of least-altered MiddleOrdovician marine calcite cements (22.5–23.5‰;Lohmann, 1987) and brachiopods (24–25‰;Wadleigh and Veizer, 1992), and (2) measured δ18Ovalues of skeletal limestone in the overlying BlackRiver and Trenton formations in the Michigan basin(23–26‰, cf. Budai and Wilson, 1991), we suggestthat 24‰ is a good approximation for the δ18Ovalue of the limestone prior to replacement bydolomite. To model the fluid-rock interaction pro-cess, the δ18O value of the initial rock must be rep-resented as dolomite (cf. Banner and Hanson,1990). Using ∆dolomite-calcite = 3‰ (Land, 1980), theδ18O value of the initial rock (as dolomite) is esti-mated to be 27‰, in good agreement with the max-imum measured δ18O values (i.e., presumably theleast altered) of replacive dolomite from this study.


-2

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18-11 -10 -9 -8 -7 -6 -5 -4

δ 18O(SMOW) δ 13 C (PDB)

Dep

th (

met

ers)

InitialRock

InitialRock

3.03.0

1.00.5W/R =

3.0

3.0 3.0

1.0W/R = 0.5

0.1

5.05.0

7.010

10

4,000 ppmTDCC 1,500 ppm

TDC

δ13CFluid = -27‰

D

B

A Replacive Dolomite MudstoneReplacive Dolomite GrainstoneDolomite Cement in FracturesDolomite Cement in Sandstone

RUPPERTa. b.

DolomiteCement

Sandstone

Dolomite

Figure 6—Lithology and vertical variations of (a) δ18O and (b) δ13C values for dolomites in the St. Peter Sandstonefrom the Ruppert core, Tuscola County. The depth is given relative to a distinct spike on the gamma log (3189.5 mdepth), which can be correlated throughout the Michigan basin and is interpreted to represent a chronostrati-graphic marker. Interval C is the gas-producing sandstone interval. The solid lines through the data illustrate thecalculated variation in the F/R ratio (by weight) during the replacive dolomitization event as determined by pro-gressive, open-system fluid-rock interaction modeling (cf. Banner and Hanson, 1990) assuming a constant porosityof 12%. See text for further details and assumptions used in the modeling. The modeling suggests that the replacivedolomitization fluid had a TDC concentration of 4000 ppm (if all carbon is present as HCO3

– this is equivalent to20,333 ppm HCO3

–) and a δ13C value of –27‰.

Ruppert Section

Fluid-rock interaction modeling of the δ18O datafor the replacive dolomites in interval D from theRuppert section indicates that the F/R ratioincreases with depth (Figure 6a). The relativelyconstant δ18O value of replacive dolomite withdepth in interval B suggests that the equilibriumδ18O value was obtained at F/Rweight ratio ofapproximately 3 (Figure 6a). Using the F/R-depthrelation determined from the δ18O data for intervalD in the Ruppert section (Figure 6a), the variationin the replacive dolomite δ13C values with depth inthis same interval can be modeled by adjusting theδ13C value and total dissolved carbon (TDC) con-centration of the dolomitizing fluid. The δ13C dataindicate that the initial rock had a δ13C value veryclose to –5‰, and the TDC content of the dolomi-tizing fluid was very high and had a low δ13C value(i.e., the TDC was dominated by organic carbon).The variation in the measured δ13C values of thereplacive dolomites with depth in interval D of theRuppert section (where F/R <3) is closely modeledusing a fluid having 4000 ppm total dissolved car-bon and a δ13C value of –27‰ (Figure 6b). Usingthese values for the composition of the modelfluid, the δ13C–F/R relation can now be calculatedfor interval B (i.e., where F/R >3, Figure 6b)because the measured δ13C values of the replacive

dolomite define a systematic trend with depth. Theminimum δ13C values of –10‰ are achieved at anF/Rweight ratio of about 10.

The δ13C value of –5‰ estimated for the initialrock is much lower than the –1‰ measured forMiddle Ordovician brachiopods (Wadleigh andVeizer, 1992) and marine cements (Lohmann, 1987),and the –1 to +1‰ measured for Trenton/BlackRiver limestone (Taylor and Sibley, 1986; Budai andWilson, 1991). This difference may indicate that theδ13C value of the marine limestone in the St. PeterSandstone was lower than published values formarine calcite from other Middle Ordovician forma-tions, or alternatively, the δ13C value of the marinelimestone in the St. Peter Sandstone may have beenlowered during early diagenesis (i.e., prior to burialdolomitization), possibly via some organic matteroxidation/degradation process.

Walczak Section

In the Walczak section (Figure 7a), the replacivedolomite δ18O values decrease from the initial rockδ18O value (i.e., 27‰ at F/R = 0) to the equilibriumδ18O value (i.e., 23‰ at F/R ≈ 3) over a very smalldepth interval (i.e., <0.3 m) near the top of inter-val B. The coincident, large decrease in δ13C valueswith depth (Figure 7b) is consistent with a large

Winter et al. 41

14 16 18 20 22 24 26 28 30

2

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14

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2

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10

12

14

Dep

th (

met

ers)

δ 18O(SMOW) δ 13 C (PDB)

Replacive Dolomite MudstoneReplacive Dolomite GrainstoneDolomite Cement in FracturesDolomite Cement in SandstoneC

B

A

0.5 0.11.0

3.0

5.0

8.0

W/R =InitialRock

InitialRock1.02.03.0

3.28.0 6.0

5.0 4.0 3.01.5 0.5

4,000 ppm TDC

7,000 ppm TDC

δ13CFluid = -40‰

Dolomite Cement

a. b.

WALCZAK

Dolomite

Sandstone

δ13CFluid = -27‰

Figure 7—Lithology and vertical variations of (a) δ18O and (b) δ13C values for dolomites in the St. Peter Sandstonefrom the Walczak core, Bay County. The depth is given relative to a distinct spike on the gamma log (3326.6 mdepth), which is correlated with the gamma spike designated as the datum in Figures 6 and 8. Interval C is the gas-producing interval. The solid lines through the data illustrate the calculated variation in the F/R ratio (by weight)during the replacive dolomitization event as determined by progressive, open-system fluid-rock interaction model-ing (cf. Banner and Hanson, 1990) assuming a constant porosity of 12%. See text for further details and assumptionsused in the modeling. The two curves in (b) illustrate how changes in the concentration and δ13C value of TDC in thediagenetic fluid would affect the δ13C value of the replacive dolomite and the vertical variation in the F/Rweight ratio.

change in F/R over this small depth interval.Employing the F/R-depth relations suggested bythe δ18O data and modeling of the upper portionof carbonate interval B (Figure 7a), the variation inthe δ13C of the replacive dolomite with depth canbe closely modeled (Figure 7b) with the same fluid(TDC = 4000 ppm, δ13C = –27‰) and initial rock(δ13C = –5‰) compositions used in Figure 7b. Asignificant increase in the TDC content or adecrease in the δ13C value of the model fluid drasti-cally decreases the maximum allowable F/R ratio atthe base of interval B; moreover, the model trend isnot consistent with the δ13C depth trend definedby the replacive dolomite data (see Figure 7b).

Whyte Section

The δ18O data of the replacive dolomites from theupper carbonate interval (interval D) in the Whytesection are broadly consistent with the model trendillustrated in Figure 8a, decreasing by about 4‰from the initial rock to the estimated equilibrium

δ18O value of 23‰ over a small depth interval.Using the F/R-depth relations suggested by the δ18Omodeling in Figure 8a, and using the previouslyemployed compositions for the initial rock anddolomitizing fluid (Figures 6b, 7b), the δ13C–F/Rrelation is calculated for interval D in Figure 8b.

The δ18O data from interval B (i.e., the lowercarbonate interval) in the Whyte section are allclose to the inferred equilibrium δ18O value (i.e.,δ18O = 23‰), similar to interval B in the Ruppertsection (Figure 6a). These data suggest that the F/Rratio at the top of interval B in the Whyte sectionshould be approximately 3, as in the Ruppert sec-tion. However, the δ13C values at the top of inter-val B in the Whyte section are approximately 2‰higher than those in the Ruppert section (compareFigures 6b and 8b) and approach the inferred δ13Cvalue of the initial rock (i.e., δ13C = –5‰), whichsuggests that the limestone at the top of interval Bin the Whyte section was dolomitized at F/R ratiosof less than 3.

Most of the replacive dolomite at the top ofinterval B in the Whyte section (Figure 8) is


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18-12 -11 -10 -9 -8 -7 -6 -5 -4

δ 13 C (PDB)

Dep

th (

met

ers)

δ 18O(SMOW)

Replacive Dolomite MudstoneReplacive Dolomite GrainstoneDolomite Cement in FracturesDolomite Cement in Sandstone

A

C

B

D0.10.5

1.0

3.0

0.10.51.0

3.0

7.0

9.0

0.51.0

3.0

0.51.53.0

7.0

9.0

InitialRock δ13CFluid = -27‰

4,000 ppm TDC

DolomiteCement

a. b.

WHYTE

Dolomite

Sandstone

Figure 8—Lithology and vertical variations of (a) δ18O and (b) δ13C values for dolomites in St. Peter Sandstone fromthe Whyte core, Bay County. The depth is given relative to a distinct spike on the gamma log (3348.2 m depth),which is correlated with the gamma spike designated as the datum in Figures 6 and 7. Interval C is the gas-produc-ing sandstone interval. The solid lines through the data illustrate the calculated variation in the F/R ratio (byweight) during the replacive dolomitization event as determined by progressive, open-system fluid-rock interactionmodeling (cf. Banner and Hanson, 1990) assuming a constant porosity of 12%. See text for further details andassumptions used in the modeling.

replacive dolomite grainstone, which containsminor nonluminescent, relatively 18O-depleteddolomite that formed during later, higher-temper-ature diagenetic events (see following para-graphs). Replacive dolomite grainstone common-ly has δ18O values lower than coexisting replacivedolomite mudstone (see Figures 6a, 7a, 8a). Aswill be discussed in a following section, δ13C val-ues of relatively late, nonluminescent dolomite inthe carbonate intervals of the St. Peter Sandstoneare virtually identical to coexisting luminescentdolomite. These observations may explain whythe δ13C values of the replacive dolomite in theupper portion of interval B in the Whyte sectionapproach the inferred δ13C value of the initialrock (–5‰), whereas the measured δ18O valuesare lower than the inferred value of the initialrock (i.e., 27‰). It is also possible that the initialrock at this particular locality and depth may havehad a relatively higher δ13C value of approximate-ly –3‰. In Figure 8 we have modeled the δ18Odata for interval B in the Whyte section similarlyto the δ18O data in the Walczak core, where theF/R approaches zero toward the top of the car-bonate interval (Figure 8a).

IMPLICATIONS FOR FLUID ORIGIN AND THESOURCE OF ORGANIC CARBON

The general δ18O and δ13C systematics of thereplacive dolomites in the vertical sections fromlocalities 5–7 in Figure 1 are best explained by a sys-tematic increase in the amount of oxygen and car-bon isotope exchange between fluid and rock withdepth (i.e., the system became more fluid dominat-ed with depth) during dolomitization. The resultsof the fluid-rock interaction modeling indicate thatthe vertical carbon isotope variations could havebeen produced by a dolomitizing f luid having aTDC concentration of approximately 4000 ppmand a δ13C value of –27‰, which indicates that thedissolved carbon was dominated by organic car-bon. Dolomitization by such a f luid would alsoexplain the very low δ13C values of the replacivedolomite in the Ballentine core (see Figure 4). Thedifferences between the δ13C data and the modeltrends (Figures 6b, 7b, 8b) may be the result ofporosity and permeability variations that could pro-duce nonsystematic variations in isotope exchangebetween fluid and rock with depth. Moreover, δ13Cvalues lower than the model trends (Figures 6b, 7b,

Winter et al. 43

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Figure 9—Vertical variations in (a) the weight percent total organic carbon and (b) the δ13C of the total organiccarbon in interval B from the Walczak core, Bay County. The depth is given relative to the same gamma spike as inFigure 7.

8b) at a given depth may result from local in situorganic matter oxidation or thermal decarboxyla-tion, which would increase the concentration of13C-depleted carbon dissolved in the fluid.

Based on the results of simultaneous fluid-rockinteraction modeling of minor element, Sr isotope,and oxygen isotope variations, Winter et al. (inpress) suggested that the replacive dolomites fromthe Walczak section formed from a fluid having thefollowing composition: 87Sr/86Sr = 0.7093 ±1, Sr =30 ±10 ppm, and Sr/Ca = 0.019. Winter et al. (inpress) proposed that the composition of this fluid(their fluid 1) is most consistent with that of older(pre–Middle Ordovician) seawater that was modi-fied under the reducing conditions of a basinal set-ting and as a result of rock interaction. The stableisotope systematics and modeling presented hereindicate that in the east-central portion of theMichigan basin, near Saginaw Bay (Figure 1), fluid1 had a high concentration of dissolved, organic-derived carbon.

The high concentrations of organic carbon influid 1 may be related to the high concentrations ofcarboxylic acid anions in formation waters generat-ed by thermalcatalytic reactions in hydrocarbonsource rocks (cf. Carothers and Kharaka, 1978).Kharaka et al. (1986) reported dissolved organiccarbon concentrations as high as 10,000 ppm in oil-field waters between 80 and 100°C. Surdam et al.(1989) suggested that within this 80–100°C temper-ature interval carboxylic acid anions can buffer thealkalinity to relatively low pH values at low PCO2,which increases carbonate solubility. As tempera-tures increase above 100°C, the carboxylic acidanions begin to decarboxylate, which increases thePCO2. With the pH still being externally buffered bythe carboxylic acid anions (and not by the carbon-ate system), this PCO2 increase results in a decreasein the solubility of carbonate (i.e., leading to car-bonate precipitation) (Surdam et al., 1989).

The modified, older (pre–Middle Ordovician)seawater origin of fluid 1 indicates that its general,large-scale flow direction in the Michigan basinwas upward (Winter et al., in press). The stable iso-tope data and fluid-rock interaction modeling dis-cussed previously (Figures 6–8) place constraintson the smaller scale f low dynamics of f luid 1.Figure 10 illustrates the flow direction and the rela-tive hydraulic heads of different depth intervalswithin the St. Peter Sandstone. The systematicdecrease in the calculated F/R ratios toward thetop of both carbonate intervals indicates that fluidflow during dolomitization was primarily upwardrather than downward, and that the hydraulic headin sandstone interval C was less than that in sand-stone interval A (Figure 10, scenario 1). Moreover,the data suggest that the depositional carbonatelayers impeded the upward flow of the dolomitizing

fluid. Dolomitization of the depositional carbonateintervals must have further enhanced the ability ofthese carbonate intervals to impede later cross-for-mational fluid flow.

DOLOMITE CEMENT IN CARBONATEINTERVALS

δ18O values for most of the dolomite fracture filland dolomite cement in quartz sandstone lamina-tions from localities 4–7 (Figure 1) range fromapproximately 17 to 21‰, and are substantiallylower than the equilibrium δ18O value (approxi-mately 23‰) of the replacive dolomite (Figures 4,6a, 7a, 8a). These data are consistent with the laterdolomite cements precipitating at relatively highertemperatures than the replacive dolomite.

The δ13C values of the dolomite fracture fill inthe carbonate lithofacies mimic the vertical δ13Cvariation of the adjacent host rock (Figures 6b, 7b8b) and suggest that the parent fluids of the frac-ture-filling dolomite flowed pervasively throughthe replacive dolomites and not through a com-mon fracture network. The fact that the δ13C val-ues of the dolomite fracture fill are within 0.5‰ ofadjacent replacive dolomite indicates that the δ13Cvalue of the fluid from which the dolomite fracturefill precipitated was controlled by the carbon inthe adjacent host dolomite.

Moreover, the data suggest that the parent fluidsof the dolomite fracture fill must have interactedwith the replacive dolomites and that these fluidsmust have had a relatively low TDC content. If theparent fluids of the dolomite fracture fill were con-fined to a common fracture network, the δ13C val-ues of the dolomite fracture fill would presumablybe more homogeneous and would not mimic thelarge vertical δ13C variations of the adjacentreplacive dolomites. Pervasive flow of the parentfluid of the fracture-filling dolomite through thecarbonate intervals suggests that the first (i.e., thereplacive) dolomitization event was insufficient atreducing the permeability to the point of com-pletely stopping cross-formational fluid flow. Inthis instance, two major burial diagenetic eventswere apparently necessary to enhance the sealingcapabilities of the carbonate intervals and trap nat-ural gas in the intervening sandstone.

DOLOMITE CEMENT IN QUARTZ SANDSTONEINTERVALS

Porous and Permeable Sandstone Intervals

δ18O and δ13C values for the dolomite cement ininterval A (lower quartz sandstone interval) of the


Winter et al. 45

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itho

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t

Figure 10—Schematic illustration of several possible scenarios for fluid flow during dolomitization of the deposi-tional carbonate intervals in the St. Peter Sandstone in the south-central Michigan basin. Also shown is the resultingvertical variation in the δ13C values of the replacive dolomite for each fluid-flow scenario, assuming that the dolomi-tizing fluid had approximately 4000 ppm TDC and a δ13C value of approximately –27‰, and assuming the originalrock prior to dolomitization had a δ13C value of –5‰. In each case, the arrows indicate the direction of fluid flow,and the thickness of each arrow conveys the relative amount of fluid-rock interaction. The fluid flow in the morepermeable sandstone intervals is dominantly horizontal, but the direction is not known. The first scenario illus-trates the preferred interpretation of the flow character of the dolomitizing fluid. The dramatic upward decrease influid-rock interaction during dolomitization suggests that the depositional carbonate intervals strongly impededcross-formational fluid flow. Moreover, the systematic upward increase in the δ13C values of the replacive dolomitesuggests that the primary direction of fluid flow was upward rather than downward, which implies that thehydraulic head in sandstone interval C (PC) was less than the hydraulic head in interval A (PA). Scenario 2 illustratesanother case where PC<PA (i.e., the primary direction of fluid flow is upward), but here, the flow character throughthe limestone resulted in more uniform and greater amounts of fluid-rock interaction during dolomitization. Thisscenario would have produced dolomite having lower and more uniform δ13C values in the middle and upper por-tions of the carbonate intervals. Scenario 3 illustrates the case where the carbonate intervals strongly impede cross-formational fluid flow and PC>PA (i.e., fluid flows downward across interval B). These flow characteristics result inthe δ13C values of dolomite systematically decreasing upward in interval B (i.e., the converse of scenario 1). Sce-nario 4 illustrates the case where PC≈PA, which may result in the reversal of the direction of fluid flow across inter-val B in response, for example, to temperature-induced (i.e., burial-induced) diagenetic reactions.

Walczak and Whyte vertical sections decrease andincrease, respectively, with depth (Figures 7, 8).Because δ13C values of carbonate cements inquartz sandstones, in general, very closely reflectthe δ13C of the carbon dissolved in the pore fluid,it is unlikely that the δ18O and δ13C variations inthis dolomite cement can be explained by varia-tions in fluid-rock interaction. The isotope data forthe dolomite cement in interval A can be interpret-ed as a variable mixture between two end-memberdolomite components. One end-member dolomitewould have an isotopic composition similar to thatof the directly overlying replacive dolomite ininterval B (δ13C ≈ –10‰, δ18O ≈ 19–23‰) and thesecond end member, which becomes proportional-ly more abundant with depth, precipitated later(i.e., at higher temperatures) from a fluid having acomparatively higher δ13C value (δ13C ≈ –7 to–6‰).

Low-Permeability Sandstone Intervals

The sandstone intervals from the Hunt Winter-field, State Foster, and State Joseph cores are diage-netically banded (Drzewiecki et al., 1994) and haveporosity and permeabilities that are characteristicof seals (Moline, 1992). Based on the systematiccovariation of minor elements, 87Sr/86Sr ratios, andδ18O values, Winter et al. (in press) suggested thatthe dolomite cement in quartz sandstone fromthese cores is composed of two compositionallydistinct end-member dolomites. The 5–7‰ rangein δ18O values for the samples of dolomite cementin sandstone from all three localities (Figure 5) isconsistent with mixing of two end-memberdolomites that precipitated at two different tem-peratures (Winter et al., in press).

The δ13C values of the dolomite cement inquartz sandstone should be nearly equivalent tothe δ13C value of the bicarbonate dissolved in theparent fluid. The relatively uniform δ13C values ofthe dolomite cement in low-permeability sand-stones from individual localities (Figure 5) indicatethat both the higher-temperature and lower-tem-perature end-member dolomites composing thesesamples (Winter et al., in press), and their respec-tive parent fluids had the same δ13C value. Winteret al. (in press) interpreted isotope and elementalvariations of the dolomite cement in quartz sand-stone from the State Joseph and State Foster coresto indicate that the earlier, lower-temperaturedolomite end member, which has a relatively high-er δ18O value, precipitated from fluid 1. Therefore,f luid 1 at these two localities in the centralMichigan basin (i.e., the State Foster and StateJoseph core localities, see Figure 1) had a higherδ13C value (δ13C = –7 to –8‰) than in the east-

central portion of the Michigan basin (δ13C ≈–27‰), where it produced replacive dolomite inthe carbonate lithofacies of the St. Peter Sandstone.In the east-central portion of the Michigan basin,the dissolved carbon in fluid 1 was dominated byorganic carbon, but in the central Michigan basinwhere fluid 1 produced dolomite cement in sand-stone, organic carbon (with δ13C = –27‰) com-prised only approximately 25–30% of the total dis-solved carbon. This regional variability in δ13C val-ues and organic carbon content of f luid 1 ofWinter et al. (in press) suggests that the east-cen-tral portion of the Michigan basin, near SaginawBay, was in closer proximity to thermally mature,organic-rich source rocks.

BRAZOS SHALE

The average δ18O value of the replacive dolo-mite grainstone from the Brazos shale (δ18O =21.6‰, Figure 4) is close to the lower limit of theestimated equilibrium δ18O value of the St. Peterreplacive dolomite grainstone (δ18O = 23 ±1‰).This relationship, in conjunction with the data andarguments presented by Winter et al. (in press),suggests that the replacive dolomite grainstonefrom the Brazos shale precipitated from fluid 1(composition previously given). The Brazosreplacive dolomite grainstone from the HuntWinterfield core (central Michigan basin, Figure 1)has much higher and less variable δ13C values (–3.8to –3.2‰) compared to the replacive dolomitefrom the St. Peter Sandstone in the east-central por-tion of the Michigan basin, which is consistentwith the arguments given that fluid 1 had a com-paratively lower dissolved organic carbon contentand δ13C value in the central Michigan basin.

CONCLUSIONS

Dolomitized carbonate intervals (approximately5 m thick) in the St. Peter Sandstone in the east-central portion of the Michigan basin are the low-permeability seals that trap natural gas producedfrom the intervening porous and permeable quartzsandstone. δ13C values for replacive dolomite sys-tematically decrease from –5‰ at the top of thecarbonate interval to –10‰ at the bottom. Theδ18O and δ13C variations of replacive dolomite inthe carbonate intervals are best interpreted to bethe result of a systematic increase in the amount ofisotope exchange between f luid and rock withincreasing depth during dolomitization. Stable iso-tope data and fluid-rock interaction modeling indi-cate that the F/Rweight ratio during dolomitization in-creased from less than 0.1 at the top to approximately


10 at the base of the carbonate intervals. This verti-cal variation in f luid-rock interaction suggestsupward movement of the dolomitizing fluid acrossthe carbonate intervals in the St. Peter Sandstone.Moreover, the large vertical variation in theamounts of isotope exchange between fluid androck during dolomitization indicates that the depo-sitional carbonate intervals substantially impededvertical, cross-formational flow of the dolomitizingfluid. Dolomitization of these carbonate intervalsmust have further impeded later cross-formationalfluid flow and enhanced their capacity to trap nat-ural gas in the intervening quartz sandstone in-terval.

All dolomites in the St. Peter Sandstone havesubstantially lower δ13C values (δ13C = –10.2 to–4.9‰) than carbonates from other formations inthe Tippecanoe sequence in the Michigan basin; amajor portion of the carbon incorporated in thedolomites from the St. Peter Sandstone must havebeen derived from organic matter. This indicatesthat replacive dolomitization and dolomite cemen-tation, and, therefore, hydraulic seal formation inthe St. Peter Sandstone in the Michigan basin wasintimately related to organic matter diagenesis.Fluid-rock interaction modeling of the stable iso-tope variations in the carbonate intervals in theeast-central portion of the Michigan basin indicatesthat the replacive dolomitization fluid (fluid 1) hadapproximately 4000 ppm TDC and a δ13C value of–27‰. Based on fluid-rock interaction modeling ofelemental, Sr isotope, and δ18O variations of thisdolomite, Winter et al. (in press) proposed that thecomposition of fluid 1 is consistent with modifiedolder (i.e., pre–Middle Ordovician) seawater.

The δ13C values of dolomite cement in the depo-sitional carbonate intervals closely mimic those ofthe directly adjacent replacive dolomite (i.e., δ13Cvaries systematically with depth), whereas theδ18O values are approximately 3–7‰ lower thanthe equilibrium value of the replacive dolomite.These data indicate that the diagenetically laterdolomite cement probably precipitated at compar-atively higher temperatures, and that localreplacive dolomite was the major source of carbonfor the dolomite cement. That the δ13C values ofthe fracture-filling dolomite are not uniform withdepth suggests the parent fluids were not confinedto a common fracture network, but rather thesefluids migrated pervasively through the deposition-al carbonate intervals after the replacive dolomiti-zation event.

δ18O values for dolomite cement in diagenetical-ly banded quartz sandstone intervals, which havelow porosity and permeability, vary by 5–7‰ at aparticular locality, whereas the δ13C values vary byless than 1.5‰. The δ13C values of the dolomitecement in the State Foster and State Joseph cores

and the replacive dolomite in the Brazos Formationfrom the Hunt Winterfield core suggest that the ini-tial dolomitizing f luid (f luid 1) in the centralMichigan basin had a lower concentration of dis-solved organic-derived carbon than it did in theeast-central portion of the Michigan basin (wherefluid 1 produced replacive dolomite in the St. PeterSandstone). These data suggest a potential for apre–Middle Ordovician organic source rock in theeast-central portion of the Michigan basin.

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Bryce Winter

Bryce Winter is an assistant sci-entist in the Department of Geo-logy and Geophysics at the Univer-sity of Wisconsin–Madison. Hereceived a B.S. degree from KansasState University, and an M.S. degreeand a Ph.D. from Arizona StateUniversity. His research focuses onusing geochemistry to betterunderstand low-temperature/sedi-mentary systems, including paleo-oceanography, provenance studies, the fluid history ofsedimentary basins, ocean chemistry, and bacterialprocesses.


ABOUT THE AUTHOR

hydraulic seals and their origin: evidence from the stable...

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