petrofacies analysis—a petrophysical tool for geologic/engineering reservoir characterization

8/9/2019 Petrofacies AnalysisA Petrophysical Tool for Geologic/Engineering Reservoir Characterization

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Chapter 6

Petrofacies AnalysisA Petrophysical

Tool for Geologic/EngineeringReservoir Characterization

W. L. WatneyW. J. Guy

J. H. DovetonS. BhattacharyaP. M. GerlachG. C. Bohling

T. R. CarrKansas Geological Survey

Lawrence, Kansas, U.S.A.

ABSTRACT

Petrofacies analysis is defined as the characterization and classificationof pore types and fluid saturations as revealed by petrophysical measure-ments of a reservoir. The word petrofacies makes an explicit link

between petroleum engineers concerns wi th pore characterist ics as

arbiters of production performance and the facies paradigm of geologistsas a methodology for genetic understanding and prediction. In petrofaciesanalysis, the porosity and resistivity axes of the classical Pickett plot areused to map water saturation, bulk volume water, and estimated perme-ability, as well as capillary pressure information where it is available.When data points are connected in order of depth within a reservoir, thecharacteristic patterns reflect reservoir rock character and its interplay withthe hydrocarbon column. A third variable can be presented at each pointon the crossplot by assigning a color scale that is based on other well logs,often gamma ray or photoelectric effect, or other derived variables.Contrasts between reservoir pore types and fluid saturations are reflected

in changing patterns on the crossplot and can help discriminate and char-acterize reservoir heterogeneity.Many hundreds of analyses of well logs facilitated by spreadsheet and

object-oriented programming have provided the means to distinguish pat-terns typical of certain complex pore types (size and connectedness) forsandstones and carbonate reservoirs, occurrences of irreducible water satu-ration, and presence of transition zones. The result has been an improvedmeans to evaluate potential production, such as bypassed pay behind pipeand in old exploration wells, or to assess zonation and continuity of thereservoir.

Watney, W.L., et al., Petrofacies analysisa petrophysicaltool for geologic/engineering reservoir characteri-zation, 1999, in R. Schatzinger and J. Jordan, eds.,Reservoir Characterization-Recent Advances,AAPG Memoir 71, p. 7390.


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74 Watney et al.

INTRODUCTION

Reservoir characterization and modeling are ongo-ing procedures used as the reservoir is developed.Well log data and occasional cores provide the funda-mental stratigraphic information critical to delineat-ing flow units, a primary objective of reservoircharacterization. Flow units are correlatable and map-pable regions in the reservoir that control fluid flow.

Their distinction is usually centered on comparingpermeability and porosity information. Flow unitclassification is refined as fluid recovery, pressuredata, or chemical fingerprinting are obtained. Often,particularly in older fields, production is commingledand cannot be used to substantiate flow units. More-over, the costs of extensive fluid and pressure testingare not economical. The question examined in thispaper is whether the traditional definition of flowunits can be modified to include additional informa-tion obtained from basic suites of well logs. Anapproach referred to as petrofacies analysisi sdescribed that extends the use of well logs to maxi-mize information that relates to pore type and fluid

flow. In particular, the utility of distinguishing verti-cal and lateral trends and patterns of irreducible bulkvolume water, water saturation, and porosity is eval-uated as a tool to improve the definition of flow unitsusing well logs. Petrofacies are defined as portions ofthe reservoir that exhibit distinctive geological faciesand petrophysical attributes.

Selecting flow units from core and log data is sub-jective, due to judging whether reservoir conformance(interconnection) and lateral continuity exist withoutactual fluid flow information (Willhite, 1986). Consis-tent, explicitly defined methodological steps must bedeveloped to ensure that each well is treated similarlyto make the approach robust and to help ensure that

the procedure can be repeated and improved as moreinformation becomes available.The initial task is correlating the reservoir interval

and establishing stratigraphic subdivisions and litho-facies. Next, the correlated stratigraphic intervals aremapped with the subsurface control to test coherencyof the data. At this stage, porosity and permeabilitydata are integrated with stratigraphic units and litho-facies to define porous and permeable flow units.This information is then compared with the produc-tion and well-test data to check for consistency andcorrelations.

An intermediate step proposed is to extract furtherinformation about pore types and fluid saturationsusing petrofacies analysis. The analysis is based on thePickett plot and delineation of depth-based trends andpatterns in porosity, resistivity, water saturation, andbulk volume water (BVW). Thousands of analyses ofthis type have demonstrated a well-known fact, thatporosity varies considerably due to varying pore typeand capillarity; furthermore, the use of rules-of-thumb

values for effective porosity and saturation cutoffs hasbeen deemed inadequate to address todays needs forprecise descriptions of reservoirs for use in improvedoil recovery operations.

While porosity may vary little, saturations and pro-ductivity can be considerably different when poretypes change. Alternatively, changes in water satura-tion and BVW may vary closely with elevation of thereservoir, suggesting fluid continuity and reservoirconformance, as well as serving as an additional toolin evaluating lateral reservoir continuity. Added infor-mation on pore types, vertical reservoir conformance,and fluid/reservoir continuity provided by petrofa-cies analysis is important in assessing flow units and

ensuring robust reservoir modeling.Petrofacies analysis is used in this example toextend an initial stratigraphic analysis of a sandstonereservoir in an attempt to define flow units. The ulti-mate objective of this reservoir characterization is toconduct a reservoir simulation of the field to help eval-uate future production options.

STUDY AREA

Petrofacies analysis was applied to a lower Mor-rowan (Lower Pennsylvanian) sandstone reservoir inArroyo field, Stanton County, Kansas (T29S, R41W)(Figure 1). Arroyo field, operated by J.M. Huber Cor-

poration, was discovered in 1992 by subsurface meth-ods. Cumulative production exceeds 651,000 bbl ofoil and 21 Gcf of natural gas. Arroyo field is a combi-nation structural stratigraphic trap, currently con-taining 6240 proven productive acres with 24 oil andgas wells and 3 dry holes. The field contains tworeservoirs, the lower Morrowan sandstone and the St.Louis Limestone (oolite). The lower Morrowan sand-stone is located at approximately 1715 m (5626 ft)below the surface. The sandstone ranges from 0 to 19m (0 to 62 ft) thick and is lenticular throughout thefield (Figure 2). The porosity of the sandstone ranges

Petrofacies analysis in this study was applied to distinguishing flow unitsand including discriminating pore type as an assessment of reservoir confor-mance and continuity. The analysis is facilitated through the use of color-image cross sections depicting depositional sequences, natural gamma ray,porosity, and permeability. Also, cluster analysis was applied to discriminatepetrophysically similar reservoir rock.


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Petrofacies AnalysisA Petrophysical Tool for Geologic/Engineering Reservoir Characterization 75

of this field on the Kansas Geological Surveys DigitalPetroleum Atlas. This atlas is located on the Surveys

Internet Home Page (http://crude2.kgs.ukans.edu/DPA/Arroyo/arroyoMain.html).The lenticular lower Morrowan sandstone is com-

prised of a series of upward-coarsening, marginalmarine shoreface deposits that are mostly confined towithin an 0.8 km (0.5 mi) wide meandering valley up to48 m (157 ft) deep (Figure 1). The sandstone was previ-ously correlated and subdivided into five separatesandstone-dominated genetic units (1, 3, 5, 9, 11) usinggamma ray, porosity, and resistivity logs and one spec-tral gamma ray log. Each genetic unit is delineated bybounding surfaces usually characterized by abrupt

13141516

17

2021 22

23 24

2526

272829

14-1 KENDRICKARNOLD 1

21-1 SANTA FE

SMITH TRUST 22-1

22-2 SANTA FE

ARNOLD 23-1

ARNOLD 23-2KENDRICK 23-2

IRWIN TRUST

23-1 KENDRICK

UNEY 24-1

26-1 PRO FARMS

26-1 SANTA FE

26-2 PRO FARMS

27-1 PRO FARMS

28-1 LAUMAN

SPIKES 1-29

1 mile

Kansas

ARNOLD 13-1

22-1 KENDRICK

27-2 PRO FARMS

22-1 SANTA FE

SCOTT 21-1

FRETZ 16-2

Figure 1. (a) Index map of Arroyo field identifying cross section (shown in Figure 2) and with well names anddistribution.

up to 20% and averages 14%. All positions of the sand-stone have been perforated in the field, with some

wells reported as only gross intervals. Initial reservoirpressure was 1434 psi.The upper portion of the sandstone has produced

only natural gas, and the lowest portion has producedsignificant amounts of both oil and gas. No water hasbeen produced in any of the wells. Also, no oil-watercontact has been recognized. The reservoir driveappears to be gas expansion.

A considerable amount of supporting data onArroyo field, including digital well logs, completionreports, and interpretive maps, cross sections, and syn-thetic seismograms, are included in a digital publication


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76 Watney et al.

changes in lithofacies. The surfaces either representsubaerial exposure or flooding surfaces, or both. Eachgenetic unit is believed to represent temporally distinctepisodic deposition. Only several of the genetic unitsare developed at any particular location in the field(Figure 2).

The sandstones were deposited in a meandering val-ley system during overall rise in sea level. Maps of eachgenetic unit record episodes of infilling of this valley,each unit with varying geometries and sand abundanceand quality. The lowest sandstones are more limited indistribution, filling only the lowest (deepest) portions ofthe valley, while the higher sandstones locally extendbeyond the confines of the valley. For these reasons, the

stratigraphic distribution was believed to be a control-ling factor on flow unit definition.

METHODOLOGY

Volumetric properties of pore space and fluid satu-ration can be calculated from porosity logs (density,neutron, or sonic) and resistivity logs using the stan-dard Archie equations. When plotted on a double-logarithmic plot of porosity versus resistivity (aPickett plot), additional information on pore size and

fluid producibility may be deduced by the use of pat-tern recognition informed by basic reservoir engineer-ing principles. A template Pickett plot is shown inFigure 3 for the upper Morrowan in the Arroyo field.A water line (Ro) expresses the theoretical resistivity-porosity coordinates of all zones that are completelysaturated with water. The water line is established bythe first Archie equation that links the formation fac-tor, F, to the ratio of the resistivity of the completelywater-saturated rock, Ro, to the resistivity of the for-mation water, Rw, to the porosity of the rock:

using an Arroyo field formation water resistivity, Rw,of 0.04 ohm-m and Archie parameter values of a = 1and m = 1.8, which express pore geometry in the Mor-rowan sandstone. Contours for different values ofwater saturation parallel the water line, with spacingdetermined by the saturation exponent, n (generallywith a value of about 2 in water-wet rocks) in the sec-ond Archie equation:

I R R St o wn

= = 1

F R R ao wm

= =

Figure 1. (b) An isopachous map of the lower Morrowan interval (top middle Morrowan limestone to topMississippian). Contour interval is 25 ft.


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where Iis the resistivity index and Rt is the actualresistivity of the rock. Lines can also be drawn on the

plot that are contours of bulk volume water (BVW),where water content is expressed as a proportion ofthe total rock, rather than in terms of the pore space asis the case with saturation.

The disposition of crossplotted zones with respectto the log axes of resistivity-porosity and the com-puted reference axes of water saturation and bulk vol-ume water (BVW) gives useful clues on both pore typeand producibility. These properties can be seen whenrelating Pickett plots to production histories (Figure 4)from some example wells in the Arroyo field. Noticehow overall well performance is determined to a largeextent by higher porosities and lower water satura-tions; however, the location of the data-cloud with

respect to the BVW contours reflects the pore size andlikely water-cut. Lower values of BVW are matchedwith coarser pores; higher values of BVW are linkedwith either finer pores or zones with coarse pores andproducible water. In terms of data-cloud shape, a clas-sic reservoir profile would show zones high in thereservoir at irreducible water saturation and relativelylow BVWs with a progressive increase in bulk volumewater with increasing depth and descent into the tran-sition zone. Some aspects of this ideal character areshown in the plots in Figure 4, where the four wellshave been arranged from most productive at the top to

Figure 2. West-to-east stratigraphic well log cross section through Arroyo field containing Lauman 28-1, SantaFe 21-1, Santa Fe 22-1, Santa Fe 22-2, and Kendrick 22-1 wells. Datum of section is middle Morrowan lime-stone. Correlated stratigraphic intervals are correlated through the lower Morrowan sandstone interval. Lineof cross section is shown on Figure 1a.

Figure 3. A template Pickett plot for the upperMorrowan sandstone in Arroyo field.

BVW

0.040.03

Sw=50%Sw=100%

Resistivity, ohm-m1 10 100

100

10

1

Porosity%

Rw=0.04

a=1

m=1.8

n=2

10 1001

least productive at the bottom. Notice that the bottomexample was not completed for production, but aban-

doned because a DST (drill-stem test) yielded nothingbut saltwater. The associated Pickett plot shows arather ragged scatter of mostly low-porosity zoneswith high water saturations that probably reflect resid-ual hydrocarbon saturations. This pattern is commonat the margins of fields, as is this well, and contrasts


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78 Watney et al.

with an idealized situation far away from fields where

zones are completely water-saturated and form atrend on a Pickett plot that conforms closely to thewater line (Ro).

The Pickett (porosity-resistivity) crossplots are thefundamental components in the petrofacies analysis.The connection of data points by depth and the abil-ity to annotate the data points with a third variablehelp establish relationships between the petrophysi-cal response and the geologylithologies, strati-graphic units, and structure; i.e., the petrofacies.Template lines identify minimum BVW and associ-ated water saturations and porosities on the Pickett

plot. This, in turn, helps to correlate the geology to

fluid-related parameters and to delineate specificchanges in fluids and variations in the pores betweenthe different wells.

The definition of flow units might be refined toinclude regions of similar or related BVW and poretype using the petrofacies analysis. Often, permeabil-ity data are either lacking or are limited to averagingfrom core-log porosity and permeability correlations.In these cases, assessment of pore type using petrofa-cies analysis may help to provide novel constraints toflow units lacking other substantial data. Of course,production and transient test data and geochemical

Well#20649

Well#20671

Well#20686

100

1K

10K

100K

1000K

MCF

rate

rate

10

0

1K

10K

100K

1000K

MCF

100

1K

10K

100K

1000K

MCF

rate

0 1 2 3 4year

1 10 100Resistivity, ohm-m

100

10

1

Porosity%

100

10

1

Porosity%

100

10

1

Porosity%

100

10

1

Porosity%

Well#20692

No productionDST: 1196 feetsalt water

cumulative

Perforated 28 ft

cumulative

Perforated 33 ft

cumulative

Perforated 16 ft

10 1001

10 1001

10 1001

10 1001

Figure 4. Pickett plots andcorresponding productionhistories for the upperMorrowan sandstone inArroyo field, StantonCounty, Kansas. Each Pickettplot is identified with a five-digit well number. The corre-

sponding well names are asfollows: Well #20686: Huber#10-1 Cockreham, SE NE SESec. 10-T29S-R41W; Well#20649: Huber #26-2 ProFarms, SW NW NW Sec. 26-T29S-R41W; Well #20671:Huber #23-2 Kendrick, C NWSec. 23-T29S-R41W; Well#20692: Petroleum Inc. #1-29Spikes, NENESE Sec. 29-T29S-R41W.


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and (4) integrate this information to define flowunits by comparison with stratigraphic zonation,Pickett plots, color log cross sections, cluster analy-sis, and well productivities.

RESULTS

The digital data from five well logs comprising awest-to-east cross section in Arroyo field were examinedusing petrofacies analysis. The objective was to comparewell data on the cross section to test for evidence of lat-eral continuity. Correlations shown in Figure 2 suggestthat most of the units are continuous. Units that are notshown as continuous may also be connected from out-

side of the plane of the cross section.

Pickett Plots

LAS (log ASCII standard) digital well log files wereread into an Excel-Visual Basic program called PfEF-FERto generate the Pickett crossplots. The initialPickett crossplots provide a visual differentiation ofthe variation in the porosity, resistivity, water satura-tion, and bulk volume water. Permeability lines areannotated on the crossplots, estimated using theempirical relationship between water saturation and

tests are necessary to more definitively constrain thedefinition of flow units.A west-to-east cross section was chosen to further

characterize the sandstone reservoir using the petrofa-cies analysis approach (Figure 2). The cross sectioncrosses the valley in two places, separated by an inter-vening high area.

The questions addressed in this analysis include thefollowing: (1) Is additional evidence available to con-firm or reject the continuity of sandstones across theintervening high region residing between the valleys?(2) What is the evidence of vertical conformance andlateral continuity? (3) How do properties of the sand-stones compare on either side of the valley? (4) Can the

definition of flow units be improved? (5) How do theflow units compare with the detailed stratigraphicsubdivisions?

In addition to the stratigraphic analysis, the pro-cedure included four operations: (1) construct Pick-ett plots for each of the wells on the cross section, (2)perform a cluster analysis of basic petrophysicaldata to independently define similar reservoir prop-erties, (3) prepare a series of color cross sections ofselected petrophysical variables with datums on sealevel elevation and a stratigraphic marker (middleMorrowan limestone located above the sandstone),

Figure 5. Pickett plot of lower Morrowan sandstone in the Santa Fe 22-1 annotated with gamma ray. Notethat this Pickett plot has contours of bulk volume water and permeability, the latter estimated from Timurequation.


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ILD

5360

5380

5400

5420

5440

5460

5480

5500

5520

1 10 100

PHI

5360

5380

5400

5420

5440

5460

5480

5500

5520

- 0. 1 0 . 1 0 .3 0 .5

GR

5360

5380

5400

5420

5440

5460

5480

5500

5520

0 50 100

PEF

5360

5380

5400

5420

5440

5460

5480

5500

5520

2 3 4

SW

5360

5380

5400

5420

5440

5460

5480

5500

5520

0 0.5 1

BVW

5360

5380

5400

5420

5440

5460

5480

5500

5520

0 0.05 0.1

Cluster anal.

5360

5380

5400

5420

5440

5460

5480

5500

5520

1 2 3 4 5 6Strat. sequence

5360

5380

5400

5420

5440

5460

5480

5500

5520

1 3 5 7 9 11Timur k

5360

5380

5400

5420

5440

5460

5480

5500

5520

0.1 1 10 100

Figure 7. Display of well log suite from the Santa Fe 22-1 well accompanied by derived information includingwater saturation (Sw), bulk volume water (BVW), permeability derived from Timur equation (Timur k), strati-graphic units, and amalgamation groups from cluster analysis.

80 Watney et al.

porosity developed by Timur. The relationship holdsfor clean sandstones when water saturations are atirreducible values. We believe that to be the case here.

The crossplot (Figure 5) with points annotated withgamma ray values indicates that points with higher

gamma ray values are located on the left side of thecrossplot at BVWs in excess of 0.06 (Figure 5). Thislocation presumably represents more shaly and finerpores. The permeability lines are not applicable tothese points.

Sante Fe 22-1

BVW=0.02BVW=0.0

3

BVW=0.0

4

BVW=0.0

5

0.01

0.1

1

1 10 100

RESISTIVITY, Ohm-m

POROSITY,

%

13

5

9

11

StratigraphicSequence

Sw=20%

Sw=40%

Sw=60%

Sw=80%Sw=100%

BVW=0.02

Figure 6. Pickett plot of lower Morrowan sandstone in the Santa Fe 22-1 annotated with stratigraphic unitsshown in type log in Figure 2.


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The reservoir has no water leg, and no water has

been produced. The points are annotated by strati-graphic interval and form rather tight clusters orbands for each stratigraphic unit (Figure 6). The clus-tering of points in distinct stratigraphic intervals atlower water saturations suggests that these zones arenear their respective minimum BVWs and closelycorrespond to particular stratigraphic zones. Also,the bands parallel water saturation lines. This pat-tern is ascribed to changing minimum BVW andpore size within a zone, which has implications tofluid flow. If any portions of the reservoir were in agas or oil/water transition zone, the bands of pointsmay have more likely paralleled porosity lines, if thepore type were not changing; however, this is not

seen and no wells have experienced any water-cutoil or gas production. The variations suggest possi-ble changes in pore type and evidence for reservoircontinuity or lack thereof.

Cluster Analysis

Some of the boundaries between stratigraphic unitsinvolve sandstone on sandstone and may not presentbarriers to flow, but do cause changes in transmissi-bi li ty . Also , the in ternal va riabil ity in sandst oneunits may create additional heterogeneity that canretard fluid flow. Cluster analysis was used to examine


the similarity among petrophysical data. The

method provides a consistent automated treatmentof the data to aid in comparing considerable amountsof data among the zones and wells. Wards Methodwas selected as the clustering technique. The methodconsists of a series of clustering steps that beginswith tclusters, each containing one object. Theclustering ends with one cluster containing allobjects. At each step, a merger of two clusters ismade that results in the smallest increase in the vari-ance (Romesburg, 1984).

The petrophysical variables included in the clusteranalysis are gamma ray, deep induction resistivity, Pe(photoelectric index), Sw (water saturation), BVW(bulk volume water), and apparent permeability using

the Timur equation

This apparent permeability, ka, is a minimum estimatewhen Sw is greater than irreducible Sw. Porosity () andSw are fractional values in this equation. Shaly intervalswere assigned to zero permeability (removing depthintervals where gamma ray exceeded 60 API units andneutron minus density porosity was greater than one).Depth was also included within the cluster analysis as anadjacency constraint to enhance spatial continuity.

k Sa w= 1 104 4 5 2

.

Sante Fe 22-1

Sw=20%

Sw=40%

Sw=60%

Sw=80%Sw=100%

BVW=0.0

3

BVW=0.0

4

BVW=0.0

5

0.01

0.1

1

1 10 100

RESISTIVITY, Ohm-m

POROSITY,

%

1

2

3

4

5

Cluster Analysis

BVW=0.02

Figure 8. Pickett plot of lower Morrowan sandstone in the Santa Fe 22-1 annotated with amalgamation groupsfrom cluster analysis.


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82 Watney et al.

Stratigraphic Sequences IndexDatum = Subsea Elevation 1-2 2-4 4-6 6-10 10-12

l . 28-1 s . f . 21-1 s . f . 22-1 s . f . 22-2 k . 22-1-1923.5 11

-1924 11

-1924.5 11

-1925 11

-1925.5 11

-1926 11

-1926.5 11

-1927 11

-1927.5 11

-1928 11

-1928.5 11

-1929 11

-1929.5 11

-1930 11

-1930.5 11

-1931 11

-1931.5 11

-1932 11

-1932.5 11

-1933 11

-1933.5 11

-1934 11

-1934.5 11

-1935 11

-1935.5 11

-1936 11

-1936.5 11

-1937 11

-1937.5 11

-1938 11

-1938.5 11

-1939 11

-1939.5 11

-1940 11

-1940.5 11

-1941 11

-1941.5 11

-1942 11

-1942.5 11

-1943 11

-1943.5 11-1944 11

-1944.5 11

-1945 11

-1945.5 11

-1946 11

-1946.5 11

-1947 11

-1947.5 11

-1948 11 11

-1948.5 9

-1949 9

-1949.5 9

-1950 9 9

-1950.5 9

-1951 9

-1951.5 9

-1952 9

-1952.5 9 11

-1953 9 11

-1953.5 9 11

-1954 9 11

-1954.5 9 11

-1955 9 11

-1955.5 9 11

-1956 9 11

-1956.5 9 11

-1957 9 11

-1957.5 9 11

-1958 9 11

-1958.5 9 11

-1959 9 11

-1959.5 9 11

-1960 9 11

-1960.5 9 11

-1961 9 11

-1961.5 9 11

-1962 9 11 11 11

-1962.5 9 11 11 11

-1963 9 11 11 11

-1963.5 9 11 11 11

-1964 9 11 11 11

-1964.5 9 11 11 11

-1965 9 11 11 11

-1965.5 9 11 11 11

-1966 9 11 11 11

-1966.5 9 11 11 11 11

-1967 9 11 11 11 11

-1967.5 9 11 11 11 11

-1968 9 11 11 11 11

-1968.5 9 11 11 11 11

-1969 9 11 11 11 11

-1969.5 9 11 11 11 11

-1970 9 11 11 11 11

-1970.5 9 11 11 11 11

-1971 9 11 11 11 11

-1971.5 9 11 11 11 11

-1972 9 11 11 11 11

-1972.5 9 11 11 11 11

-1973 9 11 11 11 11

-1973.5 9 11 11 11 11

-1974 9 11 11 11 11

-1974.5 9 11 11 11 11

-1975 9 11 11 11 11 11

-1975.5 9 9 11 11 11

-1976 9 9 9 11 11 11

-1976.5 5 9 11 11 11

-1977 5 9 9 11 11 11

-1977.5 5 9 11 11 11

-1978 5 5 9 11 11 11 11

-1978.5 5 9 11 11 9

-1979 5 9 11 11 9

-1979.5 5 9 11 11 9

-1980 5 9 11 11 11 9 9

-1980.5 5 9 9 11 9

-1981 5 9 9 11 9

-1981.5 5 9 9 11 9

-1982 5 9 9 9 11 9

-1982.5 5 9 9 11 9

-1983 5 9 9 11 9

-1983.5 5 9 9 11 9

-1984 5 9 9 11 11 9

-1984.5 5 9 9 9 9

-1985 5 9 9 9 9

-1985.5 5 9 9 9 9

-1986 5 9 9 9 9 9

-1986.5 5 9 9 9 9

-1987 5 9 9 9 9

-1987.5 5 9 9 9 9

-1988 5 9 9 9 9

-1988.5 5 9 9 9 9

-1989 5 9 9 9 9

-1989.5 5 9 9 9 9

-1990 5 9 9 9 9

-1990.5 5 9 9 9 9

-1991 5 9 9 9 9

-1991.5 5 9 9 9 9

-1992 5 9 9 9 9 9

-1992.5 5 5 9 9 9

-1993 5 5 9 9 9

-1993.5 5 5 9 9 9

-1994 5 5 5 9 9 9

-1994.5 5 5 9 9 9

-1995 5 5 9 9 9

-1995.5 5 5 9 9 9

-1996 5 5 9 9 9

-1996.5 5 5 9 9 9

-1997 5 5 9 9 9

-1997.5 5 5 9 9 9

-1998 5 5 9 9 9

-1998.5 5 5 9 9 9

-1999 5 5 9 9 9

-1999.5 5 5 9 9 9

-2000 5 5 9 9 9

-2000.5 5 5 9 9 9

-2001 5 5 9 9 9

-2001.5 5 5 9 9 9

-2002 5 5 9 9 9

-2002.5 5 5 9 9 9

-2003 5 5 5 9 9 9-2003.5 3 5 9 9 9

-2004 3 5 9 9 9 9

-2004.5 3 5 9 5 9

-2005 3 3 5 9 5 9

-2005.5 3 5 9 5 9

-2006 3 5 9 5 5 9

-2006.5 3 5 9 5 9

-2007 3 5 9 5 9

-2007.5 3 5 9 5 9

-2008 3 5 9 5 9

-2008.5 3 5 9 5 9

-2009 3 5 9 5 9

-2009.5 3 5 9 5 9

-2010 3 5 9 5 9 9

-2010.5 3 5 9 5 5

-2011 3 5 9 5 5

-2011.5 3 5 9 5 5

-2012 3 5 9 9 5 5 5

-2012.5 3 5 5 5 5

-2013 3 5 5 5 5

-2013.5 3 5 5 5 5

-2014 3 5 5 5 5 5

-2014.5 3 5 5 5 5

-2015 3 5 5 5 5

-2015.5 3 5 5 5 5

-2016 3 5 5 5 5

-2016.5 3 5 5 5 5

-2017 3 5 5 5 5

-2017.5 3 5 5 5 5

-2018 3 5 5 5 5

-2018.5 3 5 5 5 5

-2019 3 5 5 5 5

-2019.5 3 5 5 5 5

-2020 3 5 5 5 5

-2020.5 3 5 5 5 5

-2021 3 5 5 5 5

-2021.5 3 5 5 5 5

-2022 3 5 5 5 5

-2022.5 3 5 5 5 5

-2023 3 5 5 5 5

-2023.5 3 5 5 5 5

-2024 3 5 5 5 5 5

-2024.5 3 5 5 3 5

-2025 3 5 5 3 5

-2025.5 3 5 5 3 5

-2026 3 5 5 5 3 3 5

-2026.5 3 3 5 3 5

-2027 3 3 5 3 5

-2027.5 3 3 5 3 5

-2028 3 3 3 5 3 5

-2028.5 3 3 5 3 5

-2029 3 3 5 3 5

-2029.5 3 3 5 3 5

-2030 3 3 5 3 5

-2030.5 3 3 5 3 5

-2031 3 3 5 3 5

-2031.5 3 3 5 3 5

-2032 3 3 5 3 5

-2032.5 3 3 5 3 5

-2033 3 3 5 3 5

-2033.5 3 3 5 3 5

-2034 3 3 5 3 5

-2034.5 3 3 5 3 5

-2035 3 3 5 3 5

-2035.5 3 3 5 3 5

-2036 3 3 5 3 5

-2036.5 3 3 5 3 5

-2037 3 3 5 3 5

-2037.5 3 3 5 3 5

-2038 3 3 5 3 5

-2038.5 3 3 5 3 5

-2039 3 3 5 3 5

-2039.5 3 3 5 3 5

-2040 3 3 5 3 5 5

-2040.5 3 3 5 3 3

-2041 3 3 5 3 3

-2041.5 3 3 5 3 3

-2042 3 3 5 3 3 3

-2042.5 3 3 5 3 3

-2043 3 3 5 3 3

-2043.5 3 3 5 3 3

-2044 3 3 3 5 3 3

-2044.5 3 1 5 3 3

-2045 3 1 5 3 3

-2045.5 3 1 5 3 3

-2046 3 3 1 1 5 3 3

-2046.5 1 1 5 3 3

-2047 1 1 5 3 3

-2047.5 1 1 5 3 3

-2048 1 1 1 5 3 3

-2048.5 1 1 5 3 3

-2049 1 1 5 3 3

-2049.5 1 1 5 3 3

-2050 1 1 5 3 3

-2050.5 1 1 0 5 3 3

-2051 1 0 5 3 3

-2051.5 1 0 5 3 3

-2052 1 0 5 5 3 3

-2052.5 1 0 0 3 3 3

-2053 1 0 3 3 3

-2053.5 1 0 3 3 3

-2054 1 0 3 3 3 3

-2054.5 1 0 3 3 3

-2055 1 0 3 3 3

-2055.5 1 0 3 3 3

-2056 1 0 3 3 3

-2056.5 1 0 3 3 3

-2057 1 0 3 3 3

-2057.5 1 0 3 3 3

-2058 1 0 3 3 3

-2058.5 1 0 3 3 3

-2059 1 0 3 3 3

-2059.5 1 0 3 3 3

-2060 1 0 3 3 3

-2060.5 1 0 3 3 3

-2061 1 0 3 3 3

-2061.5 1 0 3 3 3

-2062 1 0 3 3 3

-2062.5 1 3 3 3-2063 1 3 3 3

-2063.5 1 3 3 3

-2064 1 3 3 3

-2064.5 1 3 3 3

-2065 1 3 3 3

-2065.5 1 3 3 3

-2066 1 3 3 3

-2066.5 1 3 3 3

-2067 1 3 3 3

-2067.5 1 3 3 3

-2068 1 3 3 3

-2068.5 1 3 3 3

-2069 1 3 3 3

-2069.5 1 3 3 3

-2070 1 3 3 3

-2070.5 1 3 3 3

-2071 1 3 3 3

-2071.5 1 3 3 3

-2072 1 3 3 3 3

-2072.5 1 3 0 3

-2073 1 3 0 3

-2073.5 1 3 0 3

-2074 1 1 3 0 0 3

-2074.5 3 0 3

-2075 3 0 3

-2075.5 3 0 3

-2076 3 0 3

-2076.5 3 0 3

-2077 3 0 3

-2077.5 3 0 3

-2078 3 0 3

-2078.5 3 0 3

-2079 3 0 3

-2079.5 3 0 3

-2080 3 3 3

-2080.5 3 1

-2081 3 1

-2081.5 3 1

-2082 3 1

-2082.5 3 1

-2083 3 3 1

-2083.5 1 1

-2084 1 1

-2084.5 1 1

-2085 1 1 1

-2085.5 1 1

-2086 1 1

-2086.5 1 1

-2087 1 1

-2087.5 1 1

-2088 1 1

-2088.5 1 1

-2089 1 1

-2089.5 1 1

-2090 1 1

-2090.5 1 1

-2091 1 1

-2091.5 1 1

-2092 1 1 1

-2092.5 1

-2093 1

-2093.5 1

-2094 1

-2094.5 1

-2095 1

-2095.5 1

-2096 1

-2096.5 1

-2097 1

-2097.5 1

-2098 1

-2098.5 1

-2099 1

-2099.5 1

-2100 1

-2100.5 1

-2101 1

-2101.5 1

-2102 1

-2102.5 1

-2103 1

-2103.5 1

-2104 1

-2104.5 1

-2105 1

-2105.5 1

-2106 1

-2106.5 1

-2107 1

-2107.5 1

-2108 1

-2108.5 1

-2109 1

-2109.5 1

-2110 1

-2110.5 1

-2111 1

-2111.5 1

-2112 1

-2112.5 1

-2113 1

-2113.5 1

-2114 1

-2114.5 1

-2115 1

-2115.5 1

-2116 1

-2116.5 1

-2117 1

-2117.5 1

-2118 1

-2118.5 1

-2119 1

-2119.5 1

-2120 1

-2120.5 1

-2121 1

-2121.5 1

-2122 1-2122.5 1

-2123 1

-2123.5 1

-2124 1

-2124.5 1

-2125 1

-2125.5 1

-2126 1

-2126.5 1

-2127 1

-2127.5 1

-2128 1

-2128.5 1

-2129 1

-2129.5 1

-2130 1

-2130.5 1

-2131 1

-2131.5 1

-2132 1

-2132.5 1

-2133 1

-2133.5 1

-2134 1 1

-2134.5 0

0

11

9

5

3

1

11

9

5

3

1

Subsea

Figure 9. East-to-west colorcross section depictingstratigraphic sequences (1, 3,5, 9, and 11) correspondingwith same sequences asdefined in Figure 2.Sections are annotated withperforations as bars along

the right margin of eachwell. Index map in Figure 1.(a) [left] Structural presenta-tion with sea level datum.

Six separate groups of points were selected fromthe cluster analysis of each well. Several criteriawere used to determine this number. First, the num-ber is not large enough to produce too many groups,which could complicate reservoir modeling. Second,the cluster dendrogram for each well showed good

separation of groups at this level. Third, the numberis comparable to the stratigraphic divisions andmight show useful groupings and comparison.

The assigned groupings derived from clusteranalysis were first compared by depth w ith thepetrophysical data and stratigraphic zonation. The


11/18


boundaries between the stratigraphic intervals andthe cluster assigned groupings generally coincide.The clustering identified a moderate amount ofsmaller scale heterogeneity within each stratigraphicinterval (Figure 7). This internal variation includeshaving the same cluster group in different stratigraphic

units, e.g., groups 4 and 5 in stratigraphic sequences5 and 9. It would be anticipated that similar sandstoneproperties would transcend sandstone intervals.However, the general finding is that each stratigraphicinterval is dominated by only one or two assignedcluster groupings.

Figure 9. (b) Stratigraphiccross section with datumat top lower Morrowansandstone.


12/18

84 Watney et al.

Gamma Ray IndexDatum = Subsea Elevation 0-20 20-40 40-60 60-80 80-100

100-120.01 120.01-140.01 140.01-160.01 160.01-180.01 180.01-200.01bsea L. 28-1 s. f. 21 -1 s. f. 22-1 s. f. 22 -2 k. 22-1

-1923.5 56

-1924 50

-1924.5 47

-1925 43

-1925.5 42

-1926 42

-1926.5 45

-1927 42

-1927.5 37

-1928 33

-1928.5 34

-1929 38

-1929.5 38

-1930 37

-1930.5 33

-1931 37

-1931.5 37

-1932 40

-1932.5 37

-1933 38

-1933.5 35

-1934 34

-1934.5 33

-1935 36

-1935.5 35

-1936 36

-1936.5 35

-1937 36

-1937.5 37

-1938 38

-1938.5 38

-1939 35

-1939.5 32

-1940 33

-1940.5 36

-1941 39

-1941.5 40

-1942 42

-1942.5 41

-1943 40

-1943.5 40

-1944 42

-1944.5 43

-1945 42

-1945.5 39

-1946 36

-1946.5 33

-1947 35

-1947.5 50

-1948 11 74

-1948.5 95

-1949 104

-1949.5 104

-1950 9 106

-1950.5 111

-1951 117

-1951.5 116

-1952 120

-1952.5 122 62

-1953 124 56

-1953.5 107 57

-1954 87 63

-1954.5 80 69

-1955 103 65

-1955.5 117 60

-1956 104 54

-1956.5 72 55

-1957 53 54

-1957.5 63 51

-1958 83 48

-1958.5 105 50

-1959 110 51

-1959.5 97 54

-1960 72 50

-1960.5 50 48

-1961 38 42

-1961.5 36 41

-1962 40 45 110 111

-1962.5 47 49 110 101

-1963 55 51 111 96

-1963.5 61 46 110 93

-1964 61 43 109 102

-1964.5 53 43 112 103

-1965 50 42 110 108

-1965.5 49 45 112 106

-1966 66 43 107 105

-1966.5 86 46 99 102 104

-1967 118 50 99 99 106

-1967.5 140 51 98 103 108

-1968 153 52 96 110 112

-1968.5 149 44 93 111 114

-1969 129 39 93 115 115

-1969.5 96 35 94 111 122

-1970 62 38 93 109 121

-1970.5 44 42 96 104 119

-1971 37 48 94 105 112

-1971.5 36 49 93 105 110

-1972 37 50 89 107 113

-1972.5 39 49 88 109 114

-1973 39 48 88 111 118

-1973.5 38 46 92 110 120

-1974 38 41 93 109 120

-1974.5 42 43 93 108 117

-1975 43 11 50 90 111 115

-1975.5 46 68 91 112 122

-1976 9 45 89 92 114 124

-1976.5 46 103 92 114 125

-1977 41 9 112 91 117 121

-1977.5 45 111 93 119 124

-1978 5 43 97 91 116 1 1 124

-1978.5 45 76 94 115 127

-1979 42 57 88 116 123

-1979.5 40 48 92 117 129

-1980 38 45 1 1 90 115 9 129

-1980.5 37 42 91 117 133

-1981 42 45 87 116 127

-1981.5 41 43 86 122 129

-1982 41 42 9 91 121 126

-1982.5 39 50 94 120 124

-1983 40 69 91 119 119

-1983.5 38 87 90 123 118

-1984 37 108 94 11 127 124

-1984.5 37 120 97 122 130

-1985 41 115 99 122 132

-1985.5 41 100 95 124 129

-1986 42 86 98 9 125 122

-1986.5 39 83 95 125 119

-1987 40 80 96 122 121

-1987.5 40 76 90 123 125

-1988 42 74 86 124 126

-1988.5 44 70 86 127 127

-1989 57 68 93 126 126

-1989.5 62 66 101 122 126

-1990 61 61 102 126 128

-1990.5 47 56 103 129 128

-1991 43 56 100 129 138

-1991.5 39 71 103 118 140

-1992 35 9 89 100 114 147

-1992.5 30 101 103 117 141

-1993 28 116 106 125 118

-1993.5 29 127 105 127 94

-1994 29 5 132 94 130 80

-1994.5 29 110 74 130 86

-1995 31 80 60 132 96

-1995.5 32 62 60 130 95

-1996 35 56 66 121 85

-1996.5 35 61 66 99 72

-1997 32 61 55 67 58

-1997.5 30 66 43 45 52

-1998 29 70 36 38 40

-1998.5 28 72 30 40 39

-1999 29 67 30 42 37

-1999.5 29 56 28 42 39

-2000 30 43 28 41 35

-2000.5 30 40 24 39 34

-2001 32 41 26 41 35

-2001.5 33 42 25 44 37

-2002 32 40 30 49 38

-2002.5 30 41 29 63 37

-2003 5 31 42 34 101 39

-2003.5 31 40 34 176 44

-2004 32 39 37 9 243 49

-2004.5 37 37 34 270 50

-2005 3 39 39 35 234 45

-2005.5 38 37 34 159 41

-2006 30 38 33 5 91 41

-2006.5 29 39 28 48 53

-2007 29 41 26 39 67

-2007.5 29 38 27 36 79

-2008 29 38 29 35 70

-2008.5 28 36 30 39 50

-2009 33 36 31 44 36

-2009.5 33 35 29 58 29

-2010 32 32 28 65 9 34

-2010.5 29 32 26 63 42

-2011 29 33 26 48 63

-2011.5 28 36 26 35 74

-2012 26 42 9 26 36 5 79

-2012.5 26 47 27 41 69

-2013 29 54 28 48 58

-2013.5 33 53 32 52 48

-2014 37 50 5 33 49 39

-2014.5 38 42 33 45 33

-2015 39 39 32 37 28

-2015.5 37 38 31 35 27

-2016 39 41 30 32 27

-2016.5 37 44 31 31 29

-2017 36 46 33 31 27

-2017.5 28 46 36 33 26

-2018 25 48 36 35 26

-2018.5 25 53 37 35 28

-2019 29 53 34 34 29

-2019.5 34 52 34 37 30

-2020 35 47 32 51 38

-2020.5 38 45 33 74 52

-2021 40 42 32 96 67

-2021.5 41 44 32 110 74

-2022 41 42 29 110 74

-2022.5 40 43 29 108 60

-2023 44 41 30 93 46

-2023.5 49 41 33 83 35

-2024 55 33 33 5 73 35

-2024.5 53 25 34 70 42

-2025 46 18 32 63 51

-2025.5 36 21 32 52 63

-2026 34 5 30 33 3 42 63

-2026.5 39 44 35 36 62

-2027 43 54 37 33 59

-2027.5 43 50 36 33 60

-2028 41 3 43 37 35 59

-2028.5 38 36 35 38 58

-2029 40 36 34 37 65

-2029.5 38 35 33 36 77

-2030 36 36 32 36 85

-2030.5 34 43 31 40 77

-2031 41 52 29 46 58

-2031.5 45 65 31 45 44

-2032 46 82 31 42 39

-2032.5 41 100 30 35 42

-2033 39 115 28 36 43

-2033.5 39 115 32 34 58

-2034 43 97 34 37 80

-2034.5 46 67 33 39 92

-2035 46 40 34 38 92

-2035.5 42 28 33 36 85

-2036 38 24 32 35 84

-2036.5 38 23 30 37 83

-2037 39 23 28 38 72

-2037.5 41 24 29 37 59

-2038 39 25 28 37 46

-2038.5 35 25 30 35 42

-2039 31 25 31 35 39

-2039.5 31 25 33 37 37

-2040 32 24 36 39 5 36

-2040.5 36 23 37 40 38

-2041 38 23 36 42 38

-2041.5 43 23 34 41 36

-2042 40 24 34 41 3 35

-2042.5 41 25 34 40 38

-2043 38 32 39 42 40

-2043.5 37 38 39 40 38

-2044 34 3 55 37 38 37

-2044.5 33 77 32 34 37

-2045 36 99 31 34 40

-2045.5 42 102 32 34 42

-2046 3 43 1 91 32 37 43

-2046.5 43 87 34 38 38

-2047 48 100 38 39 34

-2047.5 53 117 41 37 29

-2048 1 54 121 39 39 32

-2048.5 48 107 37 41 36

-2049 44 90 34 40 40

-2049.5 42 75 33 38 40

-2050 39 70 34 36 35

-2050.5 42 1 75 36 38 32

-2051 47 78 39 39 31

-2051.5 49 80 39 36 34

-2052 45 90 5 41 33 37

-2052.5 37 0 120 51 29 39

-2053 35 135 64 31 39

-2053.5 40 134 76 31 37

-2054 43 116 3 72 33 35

-2054.5 43 105 64 30 34

-2055 42 93 58 29 35

-2055.5 42 84 64 29 37

-2056 40 75 93 30 38

-2056.5 35 72 117 31 37

-2057 33 78 128 36 35

-2057.5 31 82 103 45 35

-2058 30 76 70 52 35

-2058.5 31 62 48 49 36

-2059 32 46 50 44 36

-2059.5 32 39 65 42 37

-2060 37 33 67 43 36

-2060.5 55 30 59 42 33

-2061 61 28 46 38 33

-2061.5 56 30 48 38 37

-2062 39 32 62 41 41

-2062.5 36 70 44 40

-2063 37 67 49 40

-2063.5 49 59 50 41

-2064 69 68 47 43

-2064.5 86 88 41 46

-2065 85 95 35 46

-2065.5 67 83 36 49

-2066 51 68 35 51

-2066.5 40 63 40 57

-2067 35 66 49 59

-2067.5 30 67 58 62

-2068 27 64 56 59

-2068.5 24 61 51 56

-2069 25 61 46 52

-2069.5 28 61 47 52

-2070 29 67 45 56

-2070.5 29 67 54 56

-2071 30 72 67 55

-2071.5 36 78 88 52

-2072 49 82 3 104 53

-2072.5 61 87 111 54

-2073 71 84 101 55

-2073.5 73 90 93 54

-2074 1 74 92 0 90 53

-2074.5 95 95 52

-2075 84 91 51

-2075.5 63 83 53

-2076 39 88 54

-2076.5 26 99 52

-2077 24 119 46

-2077.5 24 117 45

-2078 26 105 41

-2078.5 38 81 44

-2079 58 70 43

-2079.5 75 66 49

-2080 78 3 55

-2080.5 78 64

-2081 72 78

-2081.5 74 91

-2082 76 93

-2082.5 89 89

-2083 3 98 78

-2083.5 108 80

-2084 110 69

-2084.5 110 57

-2085 1 108 41

-2085.5 105 31

-2086 95 26

-2086.5 78 26

-2087 71 27

-2087.5 77 28

-2088 83 30

-2088.5 85 32

-2089 86 35

-2089.5 94 36

-2090 104 35

-2090.5 102 32

-2091 97 30

-2091.5 79 30

-2092 1 66 31

-2092.5 34

-2093 34

-2093.5 32

-2094 27

-2094.5 28

-2095 27

-2095.5 30

-2096 29

-2096.5 32

-2097 32

-2097.5 33

-2098 34

-2098.5 36

-2099 35

-2099.5 33

-2100 34

-2100.5 34

-2101 36

-2101.5 35

-2102 34

-2102.5 33

-2103 33

-2103.5 32

-2104 33

-2104.5 33

-2105 35

-2105.5 34

-2106 34

-2106.5 34

-2107 36

-2107.5 37

-2108 37

-2108.5 37

-2109 38

-2109.5 40

-2110 40

-2110.5 41

-2111 41

-2111.5 40

-2112 37

-2112.5 36

-2113 36

-2113.5 39

-2114 38

-2114.5 38

-2115 41

-2115.5 44

-2116 46

-2116.5 46

-2117 45

-2117.5 44

-2118 39

-2118.5 37

-2119 35

-2119.5 33

-2120 31

-2120.5 32

-2121 33

-2121.5 34

-2122 34

-2122.5 37

-2123 40

-2123.5 42

-2124 44

-2124.5 45

-2125 45

-2125.5 44

-2126 44

-2126.5 48

-2127 47

-2127.5 47

-2128 44

-2128.5 44

-2129 46

-2129.5 46

-2130 50

-2130.5 54

-2131 54

-2131.5 52

-2132 47

-2132.5 48

-2133 48

-2133.5 49

-2134 1 61

-2134.5 82

0

Figure 10. East-to-west color-image cross section depicting gamma ray variation across Arroyo field. Thesection includes same wells as in Figure 2. Wells are annotated with perforations as bar along right side ofeach well. Section also shows correlations of stratigraphic sequences. Cross section is part of an Excelspreadsheet and is at the resolution of the digitized data (0.5 ft in this example). Vertical scale bar shown onthis and ensuing structural sections. No horizontal scale (wells are equally spaced). (a) [above] Structuralversion of cross section with a sea level datum.


13/18


conformance. The question remains as to the extentof lateral continuity. Flow units are not fieldwide inextent, but are anticipated to be correlatable to somedegree. This continuity is ultimately establishedusing petrophysical data, fluid recovery, pressure,and fluid chemistry. The suites of petrophysical vari-ables including BVW can be used to evaluate lateral

continuity. Continuous trends or constancy of prop-erties of the sandstone and correlations with struc-tural elevation suggest possible fluid continuity inthe reservoir.

Computer-assisted generation of color cross sec-tions based on original digital well log sampling of0.16 m (0.5 ft) provides the means to observe andevaluate detailed subtle changes in reservoir charac-ter and substantially assists in assessing continuityand assigning flow units. The cross sections are gen-erated with an elevation (subsea) or stratigraphicdatum.

Figure 10. (b) Crosssection with a stratigraphicdatum on the top ofthe lower Morrowansandstone.

The posting of assigned cluster groups as anattribute on the Pickett plot further indicates a closecorrespondence between stratigraphic units andassigned cluster grouping (as in Figure 8). The clus-ter analysis can be adapted in the spreadsheet envi-ronment to help facilitate consistent, rapidassignment of cluster groups and further aid in flow

unit assessment.

Color Cross Sections

In general, flow units are assigned to zones in thereservoir with similar permeability and porosity,and that also exhibit lateral continuity. Flow unitsare inferred to control fluid f low, and confirmationwas sought to substantiate these units. Petrophysi-cal variation within individual well profiles hasbeen described up to this point, focused on vertical


14/18

86 Watney et al.

induction resistivity (Figures 911). Each petrophysicalvariable is presented as a structural and stratigraphiccross section placed side by side, the latter with a datumat the top of the middle Morrowan limestone. Strati-graphic units are identified and correlated. Perforatedintervals are shown alongside each well profile.

Figure 11. East-to-westcolor image cross sectiondepicting porosity variationacross Arroyo field. Thesection includes samewells as Figure 2. Wells areannotated with perforationsas bar along right side of

each well. Section also showcorrelations of stratigraphicsequences. (a) [left] Crosssection is a structural ver-sion with a sea level datum.

Cross sections for key petrophysical parameterswere generated, including gamma ray, permeabilitycalculated with the Timur equation (apparent perme-ability filtered on gamma ray and neutron-density shaleindicators), Pe (photoelectric effect) from the lithoden-sity log, porosity, water saturation, BVW, and deep


15/18


The five wells in the cross section are perforatedin two distinct intervals, a lower interval restrictedto stratigraphic unit #1 in the Lauman 28-1 andKendrick 22-1. Also, the lowest part of unit #3 in theSanta Fe 2-21 is suggested to be part of the lowerinterval and may possibly be recorrelated with unit#1 (Figure 9).

The lower interval produces significant amounts ofoil and gas. The upper perforated interval includesstratigraphic units #3, #5, #9, and #11. The upper inter-

val produces natural gas and minor amounts of oilfrom the Santa Fe 2-11 and Santa Fe 2-22. This differ-ence suggests that the reservoirs are separate. Thelower and upper sandstones are isolated by a promi-nent shaly interval, according to the gamma ray andphotoelectric logs.

On closer inspection of the gamma ray cross section,the stratigraphic units can be distinguished with thehelp of the correlation lines; however, there is consider-able variation in the internal properties of the strati-graphic units (Figure 10). This variation persists in theother parameters. The stratigraphic units generally

appear to delineate most of the petrophysical variationexcept for several possible re-correlations. These re-correlations are based on further analysis.

Porosity varies from 15 to 20% in the Lauman well to0 to 8% in the Kendrick (Figure 11). Apparent perme-ability calculated from the Timur equation and filteredon gamma ray and neutron shale indicators shows con-siderable changes on the cross section (Figure 12). Thepermeability and porosity are both higher on the west.Permeability ranges between 10 and 100 millidarcies

(md) in the Lauman 28-1 on the west side to between0.1 to 1 md in the Kendrick 22-1 on the east side.High permeability and porosity correlate well

with a trend of increased natural gas production anddecreased oil production in the Lauman 28-1, with2.5 Gcf of gas and 75,000 bbl of oil. In comparison, theKendrick 22-1 well on the east and lowest side of thecross section recovered less gas, 1 Gcf, but more oil,over 120,000 bbl. Permeability varies considerably inthin streaks near the base of the Kendrick 22-1 well.Unit #1 is separated from the overlying sandstonesby a thicker shaly interval.

Figure 11. (b) Cross sectionwith a stratigraphic datum onthe top of the lower Morrowansandstone.


16/18

88 Watney et al.

but the mixed production is similar to the recoveriesnoted in unit #1, suggesting that they are a commonreservoir in the deeper portions of the paleovalleys.

Production from perforations in the upper intervalin Santa Fe 21-1 and Santa Fe 22-2 is notably different.Santa Fe 21-1 has realized 570 Mcf of gas from the

Unit #3 is also thick in the paleovalleys on either sideof a central high. Santa Fe 2-21 is perforated in the basalpart of a thin sandstone that is in close proximity to thelower interval, unit #1. This zone in Santa Fe 2-21 hasproduced 450 Mcf of gas and nearly 50,000 bbl of oil.Production values are less than Lauman and Kendrick,

Figure 12. East-to-westcolor image cross sectiondepicting permeabilityvariation across Arroyofield. The section includessame wells as Figure 2.Wells are annotated withperforations as bar along

right side of each well.Section also showcorrelations of stratigraphicsequences. (a) [left] Crosssection is a structural versionwith a sea level datum.


17/18


CONCLUSIONS

The stratigraphic units serve as adequate means toclassify flow units in this reservoir, with added refine-ments using petrofacies analysis. Petrofacies analysisuses Pickett plots to decipher reservoir properties of

Figure 12. (b) Crosssection with a stratigraphicdatum on the top ofthe lower Morrowansandstone.

upper zone and has been declining relatively rapidlyto 1 Mcf per month. In contrast, the Santa Fe 22-2 wellhas produced nearly three times more gas at 1.8 Gcf,and its production has declined to only 18 Mcf permonth. Both wells had produced for nearly 3 yr at thetime the production figures were reported.


18/18

90 Watney et al.

each stratigraphic unit. Cluster analysis provides aconsistent means to further delineate reservoir proper-ties. The boundaries of the clustered groups are com-monly those of the stratigraphic units. The clusteredgroups provide further subdivisions of the reservoirrock that could be used to classify finer scale flow units.Color cross sections further substantiate the use of thestratigraphic divisions as basic templates for distin-guishing flow units. The color cross sections are repre-sentative of the original digitized well log data andprovide the means to precisely subdivide the strati-graphic units. Petrofacies analysis should prove usefulfor evaluating improved petroleum recovery options.

ACKNOWLEDGMENTS

Several grants have supported the PfEFFER loganalysis project at the Kansas Geological Survey andused its developments over the past few years,including the Petrofacies Analysis Project with the

Kansas Technology Enterprise Corporation and theindustrial consortium, Development and Demon-stration of An Enhanced, Integrated Spreadsheet-based Well Log Analysis Software,Subcontract No.G4S60821 with BDM-Oklahoma and industry consor-tium. PfEFFER application and testing has been con-ducted with support from Shaben FieldClass IIField Demonstration Project,Contract No. DE-FC22-

94PC14987, and Digital Petroleum Atlas,ContractNo. DE-FG22-95BC14817, both supported by theDepartment of Energy.

REFERENCES CITED

Romesburg, H.C., 1984, Cluster Analysis forResearchers: Belmont, California, Lifetime LearningPublications, 334 p.

Willhite, G.P., 1986, Waterflooding: SPE TextbookSeries Volume 3: Richardson, Texas, Society ofPetroleum Engineers, Richardson, 326 p.

petrofacies analysis—a petrophysical tool for geologic/engineering reservoir characterization

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