Stratigraphic Architecture and Reservoir Characterization...

Stratigraphic Architecture and Reservoir Characterization of the Silurian Racine Formation, Forsyth Oil Field, Macon County, Central IllinoisYaghoob LasemiIllinois State Geological Survey, Prairie Research Institute, University of Illinois at Urbana-Champaign, Champaign, Illinois

Circular 596 2018

ILLINOIS STATE GEOLOGICAL SURVEYPrairie Research InstituteUniversity of Illinois at Urbana-Champaign

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© 2018 University of Illinois Board of Trustees. All rights reserved. For permissions information, contact the Illinois State Geological Survey.

Front cover: Type log in Forsyth Field (Triple G Oil Co. Ltd. Schwarze Trust No. 2, API No. 121152135800) showing the upper Racine dolomite reservoir interval (in tan) in the regressive package of the upper Racine sequence.

Circular 596 2018

ILLINOIS STATE GEOLOGICAL SURVEYPrairie Research InstituteUniversity of Illinois at Urbana-Champaign615 E. Peabody DriveChampaign, Illinois 61820-6918http://www.isgs.illinois.edu

Stratigraphic Architecture and Reservoir Characterization of the Silurian Racine Formation, Forsyth Oil Field, Macon County, Central IllinoisYaghoob LasemiIllinois State Geological Survey, Prairie Research Institute, University of Illinois at Urbana-Champaign, Champaign, Illinois

Suggested citation:Lasemi, Y., 2018, Stratigraphic architecture and reservoir characterization of the Silurian Racine For-

mation, Forsyth Oil Field, Macon County, central Illinois: Illinois State Geological Survey, Circular 596, 21 p.

Contents

Abstract 1

Introduction 1

Field Discovery and Development 1

Geological Setting 1

Stratigraphy of the Racine Formation in the Study Area 5

Petroleum Reservoirs 7

Facies and Depositional Setting 9

Structure and Petroleum Trap 18

Potential for Improving Recovery 18

Conclusions 20

Acknowledgments 20

References 20

Figures 1 Location of the Illinois Basin in light blue (Buschbach and Kolata 1990) and the Sangamon Arch in west-central Illinois in brown (Whiting and Stevenson 1965) in the northwest area of the Illinois Basin 2 2 Map of the Mt. Auburn trend showing oil fields and producing horizons 3 3 Map showing the location of Forsyth Field and the status of the wells already drilled in the field 4 4 Forsyth Field production decline curve showing stages of field development and production from 1963 through 2013 5 5 Structure contour map of the top of the reservoir interval at Forsyth showing a regional dip toward the southeast 6 6 General stratigraphic column in the Mt. Auburn trend showing the reservoirs in the upper part of the Racine sequences 7 7 Stratigraphic reference section in Forsyth Field (Triple G Oil Co. Ltd. Schwarze No. 2, API No. 121152144400) showing geophysical logs, a lithologic column, and sequences of the Racine Formation 8 8 Stratigraphic nomenclature of the Silurian System in Illinois and Silurian sea level changes 9 9 Type log in Forsyth Field (Triple G Oil Co. Ltd. Schwarze Trust No. 2, API No. 121152135800) showing the upper Racine dolomite reservoir interval (in tan) in the regressive package of the upper Racine sequence 10 10 Cross section A–A′ showing lateral and vertical variability of the upper Racine reservoir compartments in the Mt. Auburn trend from Blackland North Field to Forsyth Field 11 11 Southwest–northeast cross section B–B′ along the Mt. Auburn trend from Blackland Field to Forsyth Field that shows the reservoirs (in tan) in the upper and lower Racine sequences 12 12 Isopach map of the main reservoir interval at Forsyth Field showing two northeast– southwest-trending juxtaposed bodies that formed as ramp margin facies parallel to the shoreline during deposition 13

13 Northwest–southeast correlation C–C′ showing the lateral variation of the Racine reservoirs shown in tan at Forsyth (log porosity based on the limestone matrix) 14 14 Southwest–northeast correlation D–D′ showing the lateral variation in the Racine reservoirs at Forsyth (log porosity based on the limestone matrix) 15 15 South–north correlation E–E′ showing lateral variation in the Racine reservoir shown in tan (log porosity based on the limestone matrix) 16 16 Isopach map of the Racine reservoir modified from the map by IBEX Geological Consultants Inc. (1992) 17 17 (a, b) Core samples and (c, d) photomicrographs of the reservoir showing porous, partially dolomitized crinoid fragments (CR) and (d) medium to coarsely crystalline dolomite. (e, f ) Core sample and photomicrograph of bioturbated dense caprock facies 18 18 Depositional and diagenetic model for the development of a Silurian dolomite reservoir in the Mt. Auburn trend area of the Sangamon Arch (Lasemi 2013) 19

Illinois State Geological Survey Circular 596 1

ABSTRACTThe Silurian Racine Formation (Wenlock- Ludlow) is the major oil-producing unit in a number of oil fields in the Mt. Auburn trend of the Sangamon Arch, central Illinois. This study focuses on geo-logical characterization of the Racine at Forsyth Field to evaluate its potential for future development in the undeveloped areas and through enhanced oil recovery methods. Forsyth Field lies in the extreme northeastern part of the Mt. Auburn trend in the northwest of the Illinois Basin. Dis-covered in 1963, the field has produced more than 750,000 barrels of oil from a compartmentalized dolomite reservoir in the upper part of the Racine Formation.

The Racine is more than 240 ft (73 m) thick and consists of small- and large-scale interbedding of limestone, dolo-mite, silty argillaceous dolomite or lime-stone, and calcareous shale. An uncon-formity subdivides the formation into two sequences. The reservoir interval at For-syth occurs in the regressive package of the upper Racine sequence and is a len-ticular, commonly compartmentalized, dolomite body reaching a maximum net thickness of nearly 12 ft (3.66 m) with an average porosity of 16%. Each reservoir compartment constitutes the upper part of a small-scale shallowing-upward cycle capped with a transgressive limestone. The reservoir interval, which correlates with reservoirs in the upper part of the Racine in neighboring fields, generally is a nonreef dolomitized fossiliferous grain-stone to wackestone. The combination of depositional and diagenetic processes and updip pinch-out of the reservoir against the Sangamon Arch was respon-sible for petroleum entrapment.

The original oil in place at Forsyth is cal-culated at more than 9 million barrels, and the field has produced approximately 8% of its original oil in place. Poor res-ervoir performance and below-average cumulative primary production (nearly 10,000 barrels per well) suggest poor permeability for the dolomite reservoir interval at Forsyth. However, great poten-tial exists for improving petroleum recov-ery from the field. Several locations are undrilled, and in the developed areas, the reservoir was stimulated with relatively small-volume fracturing. Development of the undrilled areas, infill drilling, and larger volume hydraulic fracturing will

undoubtedly improve recovery from the field. The reservoir at Forsyth Field has never been waterflooded; the field is close to a commercial source of carbon dioxide (CO

2) and is a potential candidate

for CO2 enhanced oil recovery, which

could result in storage of anthropogenic CO

2 and increased oil production.

INTRODUCTIONThe Middle Silurian Racine Formation is a major oil-producing unit in the Mt. Auburn trend of the Sangamon Arch, central Illinois (Figure 1). More than 30 million barrels of oil have been produced from the Racine in Christian, Macon, and Sangamon Counties since commercial oil discovery in 1921. Forsyth Field lies in the extreme northeastern part of the Mt. Auburn trend, in the northwest of the Illi-nois Basin (Figures 1 and 2), and covers an area of nearly 2,000 acres (809 ha). It is located in north-central Macon County, central Illinois; the main part of the field underlies Secs. 23, 24, 25, and 26, T17N, R2E (Figure 3). The field has produced essentially from a dolomite reservoir in the upper part of the Middle Silurian suc-cession.

Lasemi et al. (2010) carried out regional work on sedimentology, stratigraphy, and reservoir characterization of the Silurian reservoirs along the Mt. Auburn trend, but the oil fields were not the subject of a detailed study. This study focuses on stratigraphic architecture and reservoir characterization of the Racine at Forsyth Field. Geological characterization is vital for evaluating the remaining recover-able oil from the undeveloped areas and by waterflooding or enhanced oil recovery (EOR) methods such as carbon dioxide (CO

2) flooding. A short summary

of this work was included in a paper by Damico et al. (2014). Parts of this work were prepared as unpublished reports for Illinois Department of Commerce and Economic Opportunity Grant 13-484001 and for U.S. Department of Energy Grant DE-FE0009612.

For this study, a number of maps and stratigraphic cross sections were pre-pared. Cores and well cuttings were not available for the entire field. However, a complete suite of both open- and cased-hole geophysical logs were available for most wells. Therefore, geophysical

log interpretation and correlation with equivalent intervals in other Mt. Auburn trend fields were the basis for geological characterization. Subsurface maps and cross sections were prepared by using wells with more reliable logs, including density, sidewall neutron, microresistivity (microlog), and induction log.

FIELD DISCOVERY AND DEVELOPMENTAtkins & Hale discovered Forsyth Field in 1963 when the company reentered an old dry hole, the Schwartz No. 1 (API No. 121150005100) in Sec. 24, T17N, R2E. After a relatively small fracture treatment, Atkins & Hale completed the well in November 1963 with an initial production of 168 barrels (about 42 gal [159 L]) per day from the Silurian Racine reservoir, the top of which was encountered at 2,118 ft (646 m). Initially, M. Mazzarino drilled Schwartz No. 1 in 1956 to a total depth of 2,215 ft (675 m), which was completed as a dry hole.

Subsequent to early development of the field in the 1960s and 1970s, a second phase of development took place in the early 1980s when operators drilled a large number of wells, which resulted in an upsurge in production (Figure 4). In 1981, the production rate peaked at approximately 300 barrels per day from 75 producing wells; however, production declined steeply because of the charac-teristically poor reservoir permeability. Eighty-nine wells were drilled, of which 82 wells were completed as oil produc-ers. In all completed wells, casing was set through the formation and fractured the porous intervals with low-volume (about 20,000 gal [75,708 L]) water/nitrogen-foam sand hydraulic fracturing. Of the 82 completed oil wells, 42 wells are currently on pump, and the field is producing at a rate of less than 15 barrels per day. For-syth Field has accumulated more than 750,000 barrels of oil.

GEOLOGICAL SETTINGThe Illinois Basin (Figure 1) formed in the Late Cambrian on the northeast exten-sion of the Late Proterozoic to Middle Cambrian Reelfoot Rift–Rough Creek Graben system (Kolata and Nelson 1990). During the Silurian, a reef bank, the Terre

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Figure 1 Location of the Illinois Basin in light blue (Buschbach and Kolata 1990) and the Sangamon Arch in west-central Illinois in brown (Whiting and Stevenson 1965) in the northwest area of the Illinois Basin. Forsyth Field is located in the extreme northeast of the Mt. Auburn trend along the southeastern flank of the arch. A slope break in southern Illinois and southwestern Indiana (Droste and Shaver 1980, 1987) separated the gently sloping ramp from the deep Vincennes Basin in the southeastern part of the Illinois Basin during Silurian time (modified from Lasemi et al. 2010). RR, Reelfoot Rift; RCG, Rough Creek Graben; VB, Vincennes Basin.

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Figure 4 Forsyth Field production decline curve showing stages of field development and production from 1963 through 2013. Mstb, thousand stock tank barrels.

Haute reef bank (Figure 1), formed a plat-form margin in southwestern Indiana and southern Illinois that was facing the deep proto-Illinois Vincennes Basin (Droste and Shaver 1980, 1987). Northward, the Illinois Basin was a southeastward-dip-ping, gently sloping ramp platform.

A broad northeast–southwest-trending structure, the Sangamon Arch, existed in west-central Illinois (Figure 1) that formed because of upward warping of strata during Middle Silurian through Middle Devonian time (Whiting and Ste-venson 1965). It is located on the gently sloping ramp area in the northwest of the Illinois Basin. The southern flank of the Sangamon Arch encompasses a number of oil fields that display a northeast–southwest trend (Figures 1 and 2), namely the Mt. Auburn trend (Lasemi 2009c). A structure contour map of the base of a limestone marker near the top of the Silurian deposits (Figure 5) indicates that the overall direction of regional dip in the study area is toward the southeast.

The petroleum reservoirs in the Mt. Auburn trend area occur in the upper part of Silurian deposits. The encompass-ing interval, adapting the stratigraphic classification for western Illinois (Will-man and Atherton 1975), is classified as

the Middle Silurian Racine Formation (Figure 6). Named for quarry exposures at Racine, Wisconsin, the formation is up to 300 ft (91 m) thick and consists of reef and inter-reef silty argillaceous dolo-mite or limestone facies (Willman and Atherton 1975). Lasemi (2009a, 2009b) recognized an intra-Racine erosional unconformity that subdivides the Racine Formation into two sequences (Figures 6 and 7). The unconformity could be asso-ciated with a global eustatic event on top of the Wenlock Series, possibly enhanced by upwarping in the Sangamon Arch. It corresponds to the medium-scale sea level fall (25–75 m) that Haq and Schutter (2008) reported at the Wenlock–Ludlow boundary (Figure 8).

Deposition of the Racine Formation in the study area was coeval with deposition of the lower part of the Moccasin Springs Formation (Figure 8) of southern Illinois (Willman and Atherton 1975; Mikulic 1990; Norby 1990). The Racine overlies, with a conformable contact, the upper Llandovery–lower Wenlock Joliet Forma-tion. Except for the deep part of the basin, where the Silurian–Devonian boundary is conformable, the Silurian succession unconformably underlies the Middle or Upper Devonian deposits with the promi-nent sub-Kaskaskia erosional unconfor-

mity (Willman and Atherton 1975; Droste and Shaver 1987). In the Sangamon Arch area, Lower and Middle Devonian depos-its are absent (Whiting and Stevenson 1965; North 1969) and the Silurian rocks unconformably underlie the Upper Devo-nian–lowermost Mississippian organic-rich New Albany Shale.

STRATIGRAPHY OF THE RACINE FORMATION IN THE STUDY AREAIn the study area, the Racine Formation overlies, with a sharp contact, the Joliet Formation and unconformably underlies the New Albany Shale (Figure 6). The Racine consists of small- and large-scale interbedding of limestone, dolomite, silty argillaceous dolomite or limestone, and calcareous shale (Figure 7). To define key stratigraphic surfaces for correlation and for detailed stratigraphy, the deep-est well at Forsyth Field, the Triple G Oil Company Ltd. Schwarze-Pense Com-munity No. 2 in Sec. 24, T17N, R2E, has been selected to serve as a stratigraphic reference section (Figure 7). Several geo-physical discontinuities, which represent key stratigraphic surfaces, are present within the Racine Formation. These sur-faces are isochronous and correspond to

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Figure 5 Structure contour map of the top of the reservoir interval at Forsyth showing a regional dip toward the southeast. Note that except for a few southeast-trending noses, no structural closure is present.


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Figure 6 General stratigraphic column in the Mt. Auburn trend showing the reservoirs in the upper part of the Racine sequences. Sys., system; Ser., series; Fm., formation; LRS, lower Racine sequence; URS, upper Racine sequence.

chronostratigraphic events that resulted in major changes in lithology and deposi-tional setting. A few of these surfaces are listed below (Figure 7):

1. The base of the Racine Formation at 2,335 ft (712 m); from this depth, gamma-ray and resistivity logs display a characteristic overall decrease in resistivity and increase in radioac- tivity upward.

2. The base of a radioactive calcareous shale at 2,270 ft (692 m).

3. The unconformable upper boundary of the lower Racine interval at 2,182 ft (665 m).

4. The base of a transgressive limestone marker that overlies the Racine reservoir near the top of Silurian deposits at 2,114 ft (644 m).

5. The unconformable upper boundary of the upper Racine interval (base of the New Albany Shale at 2,101 ft, 640 m).

The base of the limestone marker serves as a datum for the preparation of maps and cross sections in the study area. It is of variable thickness and is absent in the northwest of the Mt. Auburn trend because of pre-Devonian erosion. Some workers in the oil industry consider this

limestone as being of Devonian age and depict the upper contact of the Racine reservoir as a Devonian–Silurian contact. Nevertheless, the Middle Devonian strata pinch out and are absent in the study area of the Sangamon Arch (Whiting and Stevenson 1965; North 1969). In addition, the basal part of Middle Devonian trans-gressive strata commonly displays a high gamma-ray log signature (Lasemi 2013), which is absent in the Forsyth area. Only in the Triple G Oil Company Ltd. Hocka-day No. 7 and Hockaday No. 9 (eastern half of the NW NW of Sec. 23, T17N, R2E) in the extreme northwest of Forsyth Field, a laterally discontinuous uppermost hori-zon may belong to an erosional remnant of Middle Devonian Lingle Formation (see cross section C–C′ below).

The Racine sequences (third-order cycles) consist of transgressive and regressive (highstand) packages (Figure 7) and are superimposed by smaller scale fifth- to fourth-order shallowing-upward cycles (Figures 9 and 10). The thickness of the Racine Formation varies from place to place because of the lack of deposition or erosion at the sequence boundaries. In the study area, the Racine is more than 240 ft (73 m) thick, but its thickness decreases southwestward. It reaches a

thickness of 200 ft (61 m) in Sun Oil Com-pany Damery No. 1, the discovery well of Blackland Field (see cross section A–A′ below).

PETROLEUM RESERVOIRSAlong the Mt. Auburn trend, the Racine Formation generally comprises several permeability pinch-out zones at different horizons. These zones consist of dolo-mitized limestone intervals that occur in the upper part of the upper Racine and also in the upper part of the lower Racine sequences (Figures 6 and 11). In the northwest margin of the trend, the upper Racine reservoirs are absent because of pre-Devonian erosional unconformity; the oil fields in this area produce from the reservoirs developed in the upper part of the lower Racine sequence (Figure 2).

At Forsyth Field, the main producing interval is a lenticular dolomite reser-voir body (Figure 12) that occurs in the regressive package of the upper Racine sequence near the top of the formation (Figures 7 and 9). The reservoir underlies, with a sharp contact, an impermeable limestone (Figure 7), which is persistent throughout the Mt. Auburn trend area (Figures 11 and 13–15). It is a lenticular, commonly compartmentalized dolomite body reaching a maximum net thick-ness of nearly 12 ft (3.66 m) and is com-posed of two juxtaposed lenses trend-ing in a northeast–southwest direction (Figure 12), paralleling the trend of the Sangamon Arch. Neither core nor core analysis data are available for the Forsyth Field; however, reservoir porosity is in the range of 8%–27% based on porosity logs. The average porosity of the main reservoir (corrected for dolomite) when using den-sity and sidewall neutron logs is nearly 16%. The porosity types observed in thin sections and core samples of the Racine from the neighboring fields are secondary intercrystalline, vugular, and moldic (dis-solved fossil) porosities.

A thin limestone bed that is commonly recognizable in microresistivity logs (up to 2 ft, 0.61 m) divides the reservoir into two compartments (Figure 9). This intervening limestone may locally reach a thickness of up to 6 ft (1.83 m) in parts of the Mt. Auburn trend (see the Roth-well No. 1 well, API No. 121152155700 in Figure 10 for an example). The reservoir

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Figure 7 Stratigraphic reference section in Forsyth Field (Triple G Oil Co. Ltd. Schwarze No. 2, API No. 121152144400) showing geo-physical logs, a lithologic column, and sequences of the Racine Formation. Note that the neutron porosity displays limestone porosity and that the reservoir (in tan) underlies a dense limestone marker in the upper Racine sequence. N. Am. Ser., North American series; Global Ser., global series; N. A. Shale, New Albany Shale; Seq. Strat., sequence stratigraphy; API, American Petroleum Institute units; NPHI, neutron porosity; OM, ohmmeter; LRS, lower Racine sequence; URS, upper Racine sequence; TST, transgressive systems tract; mfs, maximum flooding surface; HST, highstand systems tract; U. Racine reservoir, upper Racine reservoir.


Figure 8 Stratigraphic nomenclature of the Silurian System in Illinois and Silurian sea level changes. Stratigraphic column modified from Willman and Atherton (1975) and Norby (1990); Racine subdivision based on Lasemi (2009a, 2009b). N. America, North America; W. Illinois, western Illinois; S. Illinois, southern Illinois; Seq. boundary, sequence boundary; m above PD, meters above present day; Ludlo., Ludlow; Pr., Pridolian; Cayu., Cayugan; URS, upper Racine sequence; LRS, lower Racine sequence.

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Haq and Schutter (2008)Modified from Willman and Atherton (1975) and Norby (1990)

URS

constitutes the upper part of small-scale, fifth- to fourth-order shallowing-upward cycles, which are capped, with a sharp contact, by a transgressive limestone facies (Figures 9 and 10). The reservoir interval is coeval with the combined reservoir units B and C of Lasemi et al. (2010), in which the intervening lime-stone between the reservoir units was beyond the resolution of the geophysical log.

In the extreme northwest of Forsyth Field, two laterally discontinuous younger horizons produced in the Triple G Oil Company Ltd. Hockaday No. 7 (Figure 13) and Hockaday No. 9 in the eastern half of the NW NW of Sec. 23, T17N, R2E. In these wells, the uppermost reservoir may be an erosional remnant belonging to the Middle Devonian Lingle Forma-tion, although its affinity to the Upper

Devonian Hardin (Sylamore) Sandstone, which locally occurs at the base of the New Albany Shale, cannot be ruled out.

A report by IBEX Geological Consultants Inc. (1992) indicated a Silurian reservoir thickness of more than 15 ft (4.6 m) in parts of Forsyth Field (Figure 16). How-ever, the validity of the thickness map in this report is questionable. The report shows an abnormally high reservoir thickness for parts of the field because it considers the three pay horizons in the northwest of the field mentioned above (Figure 16) as a single reservoir. In addi-tion, wells with a conventional electric log/cased-hole gamma-ray log, which are not very reliable for the recognition of contacts, were included in preparing the thickness map. IBEX Geological Consul-tants Inc. (1992) envisioned four separate lenticular reservoir pinch-outs within the

field (Figure 16). Well-to-well correlation and the isopach map of the main produc-ing interval (Figure 12), however, indicate a lenticular body composed of two jux-taposed lenses, which do not appear to show any intervening porosity barrier.

FACIES AND DEPOSITIONAL SETTINGCores and well samples are missing for the entire area of Forsyth Field. According to the Scout Check report cards and notes written on a few old electric logs, reser-voir intervals were cored in a number of wells, but the cores were not saved. Geophysical log cross sections indicate that the Silurian reservoir interval at For-syth correlates with the reservoirs in the upper part of the Racine Formation in the neighboring fields of the Mt. Auburn

10 Circular 596 Illinois State Geological Survey

2,10

02,

150

New

Alb

any

Sha

leR

acin

e

Mid

dle

Dev

onia

n

Upp

er

Silu

rian

For

mat

ion

Ser

ies

Sys

tem

TD = 2,177

Seq

uenc

e S

trat

igra

phy

3rd-

orde

r (s

eque

nce)

5th-

4th-

orde

rC

ycle

Ord

er

GAMMA RAY (API)

CALIPER (IN)

SELF POTENTIAL (MV) MINV (OM)

SNR (OM)

LNR (OM)

MNOR (OM)

NEUTRON POROSITY (%)

CONDUCTIVITY (MMHO)

Shale Dolomite LimestoneArgillaceousdolomite

U. Racinereservoir Transgressive

Regressive

2,10

02,

150

Dep

th (f

t)

1500

166

–200 0

150 300

1000

1000

0

150

15

–1040

01,000

Figure 9 Type log in Forsyth Field (Triple G Oil Co. Ltd. Schwarze Trust No. 2, API No. 121152135800) showing the upper Racine dolomite reservoir interval (in tan) in the regressive package of the upper Racine sequence. Note that a thin dolomitic limestone horizon (<1 ft, <0.3 m) divides the reservoir interval into two compartments. Note also the sharp contact between the reservoir and the overlying transgressive limestone. This well penetrated the highstand (regressive) part of the sequence; for the complete upper Racine sequence stratigraphy, see Figures 7 and 10. API, American Petroleum Institute units; MV, mil-livolts; IN, inches; SNR, short normal resistivity; LNR, long normal resistivity, MINV, microinverse; MNOR, micronormal; OM, ohmmeter; MMHO, milli-mohs; TD, total depth; U. Racine reservoir, upper Racine reservoir.

area. In these fields, the reservoir facies is commonly a nonreef dolomitized fossil-iferous grainstone to wackestone (Figure 17a–d) capped, with a sharp contact, by a dense limestone facies (Figures 7 and 10). This facies is generally a bioturbated fossiliferous lime mudstone to packstone (Figure 17e,f) interpreted as a transgres-sive horizon.

The depositional model of the Racine Formation is a gently sloping ramp (ramp platform of Ahr 1973, 1998) that existed during deposition in the Sangamon Arch

area (Figure 18). Because the deposi-tional wave energy was not strong in the homoclinal ramp setting on the arch, only bioclast grainstone shoal facies and small patch reefs were developed (Lasemi 2009c; Lasemi et al. 2010), as opposed to the large pinnacle reefs that developed along the steeper slope of the deep Illi-nois Basin margin (Bristol 1974; Droste and Shaver 1987). These facies graded to lime wackestone and mudstone basin-ward (Lasemi 2009c, 2014; Lasemi et al. 2010), similar to numerous modern and ancient examples (Wilson 1975; Tucker

and Wright 1990; Flügel 2010). Dolomite reservoirs formed because slightly saltier magnesium-rich seawater percolated through the previously deposited lime sediment (Figure 18) during the regres-sive phase of a small-scale shallowing-upward cycle and was capped by deeper water-impermeable limestone beds fol-lowing transgression (Lasemi 2010, 2012, 2014; Lasemi et al. 2010). During burial diagenesis, undolomitized limestone was compacted but the dolomite resisted compaction, thus retaining its porosity.

2,000

2,100 2,150

Sha

leD

olom

iteLi

mes

tone

Arg

illac

eous

dolo

mite

Unc

onfo

rmity

Tran

sgre

ssiv

e

Reg

ress

ive

New Albany Shale Racine

Middle

Devonian

Upper

Silurian

Formation

Series

System

Depth (ft)

DE

NS

ITY

PO

RO

SIT

Y (

%)

NE

UT

RO

N P

OR

OS

ITY

(%

)

GA

MM

A R

AY

(A

PI)

Rot

hwel

l No.

1 (

AP

I 121

1521

5570

0), B

lack

land

Fie

ld

Seq. Strat.3rd-order (sequence)

5th-/4th-orderCycle Order

GA

MM

A R

AY

(A

PI)

CA

LIP

ER

(IN

)

SP

(M

V)

MIN

V (

OM

)

SN

R (

OM

)

LNR

(O

M)

MN

OR

(O

M)

Sch

war

ze T

rust

No.

2 (

AP

I 121

1521

3580

0), F

orsy

th F

ield

GA

MM

A R

AY

(A

PI)

Dat

um

10 m

i

1,900 2,000

30–1

0

30–1

0

015

0 TD

= 2

,040

A

150

0

166–2

000

150

300

100

0

100

0 0

150

15

2,100 2,150

TD

= 2

,177

A′

CH

RIS

TIA

N

MA

CO

N

SA

NG

AM

ON

LOG

AN

Bla

ckla

nd N

.

For

syth

5 m

i

5 km

0 0N

A′

A

Depth (ft)

Fig

ure

10

Cro

ss s

ectio

n A

–A′

show

ing

late

ral a

nd v

ertic

al v

aria

bilit

y of

the

upp

er R

acin

e re

serv

oir

com

part

men

ts in

the

Mt.

Aub

urn

tren

d fr

om B

lack

land

Nor

th F

ield

to

For

syth

Fie

ld. N

ote

that

the

res

ervo

ir ho

rizon

s (in

tan)

con

stitu

te t

he u

pper

par

t of

sm

all-s

cale

sha

llow

ing-

upw

ard

cycl

es in

the

reg

ress

ive

pack

age

of t

he u

pper

Rac

ine

sequ

ence

. Not

e al

so t

he s

harp

con

tact

bet

wee

n de

nse

tran

sgre

ssiv

e lim

esto

ne a

nd t

he u

nder

lyin

g re

serv

oir

units

. Seq

. Str

at.,

sequ

ence

str

atig

raph

y; A

PI,

Am

eric

an

Pet

role

um I

nstit

ute

units

; SP,

sel

f-po

tent

ial;

MV,

mill

ivol

ts; I

N,

inch

es; S

NR

, sh

ort

norm

al r

esis

tivity

; LN

R,

long

nor

mal

res

istiv

ity,

MIN

V, m

icro

inve

rse;

MN

OR

, m

icro

norm

al;

OM

, oh

mm

eter

; TD

, to

tal d

epth

.

Dat

um

Formation

M. Silurian

New AlbanyShale

Series U. Dev.

Racine

NE

UT

RO

NP

OR

OS

ITY

(%

)

LNR

(O

M)

SN

R (

OM

)

GA

MM

A R

AY

AP

I

Sch

war

ze-P

ense

Com

. 2 (

AP

I 121

1521

4440

0)

Depth (ft)

LNR

(O

M)

SN

R (

OM

)S

ELF

-P

OT

EN

TIA

L

Dam

ery

No.

1 (

AP

I 121

1500

1030

0)

Depth (ft)

NE

UT

RO

NG

AM

MA

RA

Y

AP

I

Nol

an N

o. 2

(A

PI 1

2115

2167

900)

AP

I

9 m

i0

100

100

0

0–2

000

150

300

150

2,00

00

2,000

TD

= 2

,074

015

0

300

150

Depth (ft) 0

040

100

100

0

2,100

B′

2,150

TD

= 2

,892

2,050

1,900 2,0001,950B

TD

= 3

,780

LRSURS

5.8

mi

MO

IA

CH

RIS

TIA

N

MA

CO

N

SA

NG

AM

ON

LOG

AN

Dec

atur

For

syth 5

mi

5 km

0 0N

B

B′

Bla

ckla

nd

MV

Fig

ure

11

Sou

thw

est–

nort

heas

t cr

oss

sect

ion

B–B

′ al

ong

the

Mt.

Aub

urn

tren

d fr

om B

lack

land

Fie

ld to

For

syth

Fie

ld t

hat

show

s th

e re

serv

oirs

(in

tan)

in t

he u

pper

and

low

er

Rac

ine

sequ

ence

s. N

ote

that

the

res

ervo

ir at

For

syth

and

Bla

ckla

nd F

ield

s oc

curs

in t

he u

pper

seq

uenc

e. T

he r

eser

voir

in t

he lo

wer

seq

uenc

e at

Dec

atur

Fie

ld c

orre

late

s w

ith

a pa

tch

reef

res

ervo

ir fu

rthe

r to

the

sou

thw

est.

Not

e th

e sh

arp

cont

act

betw

een

the

base

of

the

datu

m (

limes

tone

mar

ker)

and

the

upp

erm

ost

Rac

ine

rese

rvoi

r in

thi

s fig

ure.

A

PI,

Am

eric

an P

etro

leum

Ins

titut

e un

its; M

V, m

illiv

olts

; SN

R,

shor

t no

rmal

res

istiv

ity; L

NR

, lo

ng n

orm

al r

esis

tivity

; OM

, oh

mm

eter

; U. D

ev.,

Upp

er D

evon

ian;

M. S

iluria

n, M

iddl

e S

iluria

n; L

RS

, lo

wer

Rac

ine

sequ

ence

; UR

S,

uppe

r R

acin

e se

quen

ce; T

D,

tota

l dep

th.

T17

N

R3ER2E

31

30

36

19

1314

24

35

26 25

23

2

6

8

10

2

4

6

10

8

4

0.25 mi

0.25 km

0

0NContour interval: 2 ft

Township boundarySection lineControl point

Figure 12 Isopach map of the main reservoir interval at Forsyth Field showing two northeast–southwest-trending juxtaposed bodies that formed as ramp margin facies parallel to the shoreline during deposition.

19

NW

Phi

llips

No.

2 (

AP

I 121

1521

2030

0)H

ocka

day

No.

8 (

AP

I 121

1521

3970

0)P

hilli

ps N

o. 1

4 (A

PI 1

2115

2156

100)

Hoc

kada

y N

o. 7

(A

PI 1

2115

2139

600)

Formation

M. Silurian

New Albany Shale

Series U. Devonian

Racine

GA

MM

A R

AY

SP

(M

V)

LNR

(O

M)

SN

R (

OM

)

NE

UT

RO

N (

AP

I)

AP

I0

150 0

–200

100

0

100

0

2,00

00

150

300

2,050 2,100Depth (ft)

M. D

ev. L

ingl

e F

m?

TD

= 2

,140

C

LNR

(O

M)

NP

HI (

%)

SN

R (

OM

)

GA

MM

A R

AY

CO

ND

(M

MH

O)

CA

LI (

IN)

SP

(M

V)

AP

I

–10

40

100

0 0

150

0

0

166–2

000

150

300

100

910

ft2,

080

ft

Depth (ft) 2,050 2,100

Dat

um

TD

= 2

,146

LNR

(O

M)

DP

HI (

%)

SN

R (

OM

)

GA

MM

A R

AY

CO

ND

(M

MH

O)

CA

LI (

IN)

SP

(M

V)

AP

I

–10

30

100

0 0

150

050

0

0

166–2

000

150

300

100

1,14

2 ft

2,050 2,100

TD

= 2

,165

LNR

(O

M)

NP

HI (

%)

SN

R (

OM

)

GA

MM

A R

AY

CA

LI (

IN)

SP

(M

V)

CO

ND

(M

MH

O)–1

040

100

0

100

0

020

0 166–2

000

01,

000

Depth (ft)

Depth (ft)

2,050 2,100

TD

= 2

,150

Phi

llips

No.

7 (

AP

I 121

1521

2050

0)H

ende

rson

Com

. 2 (

AP

I 121

1521

5580

0)S

E

LNR

(O

M)

NP

HI (

%)

SN

R (

OM

)

GA

MM

A R

AY

CO

ND

(M

MH

O)

CA

LI (

IN)

SP

(M

V)

AP

I

–10

40

100

0 0

150

01,

000

0

166–2

000

150

300

100

1,00

0

Depth (ft)

4,15

7 ft

TD

= 2

,159

2,050 2,100 2,150

LNR

(O

M)

DP

HI (

%)

SN

R (

OM

)

GA

MM

A R

AY

CO

ND

(M

MH

O)

CA

LI (

IN)

SP

(M

V)

AP

I

TD

= 2

,185

2,100 2,150

Formation

M. Silurian

New Albany Shale

Series U. Devonian

Racine

C′

LOG

AN

CH

RIS

TIA

N

MA

CO

N

SANGAMON

For

syth

Fie

ld

5 m

i

5 km

0 0N

R2

ER

3E

T17N

C

C′

26

3135

30

36251314 23

24

Depth (ft)–1

030

100

0 0

150

050

0

0

166–2

000

150

300

100

Fig

ure

13

Nor

thw

est–

sout

heas

t co

rrel

atio

n C

–C′

show

ing

the

late

ral v

aria

tion

of t

he R

acin

e re

serv

oirs

sho

wn

in ta

n at

For

syth

(lo

g po

rosi

ty b

ased

on

the

limes

tone

mat

rix).

The

var

ia-

tion

in t

hick

ness

of

the

limes

tone

mar

ker

is d

ue to

dol

omiti

zatio

n an

d pr

e-D

evon

ian

eros

ion.

Not

e th

e pr

esen

ce o

f tw

o re

serv

oirs

abo

ve t

he m

ain

rese

rvoi

r in

Hoc

kada

y N

o. 7

tha

t pi

nch

out

in o

nly

one

loca

tion

to t

he s

outh

east

. AP

I, A

mer

ican

Pet

role

um I

nstit

ute

units

; SP,

sel

f-po

tent

ial;

MV,

mill

ivol

ts; S

NR

, sh

ort

norm

al r

esis

tivity

; LN

R,

long

nor

mal

res

istiv

ity; O

M,

ohm

-m

eter

; CA

LI,

calip

er; I

N,

inch

es; N

PH

I, ne

utro

n po

rosi

ty; C

ON

D,

cond

uctiv

ity; M

MH

O,

mill

i-moh

s; D

PH

I, de

nsity

por

osity

; U. D

evon

ian,

Upp

er D

evon

ian;

M. D

ev. L

ingl

e F

m.,

Mid

dle

Dev

onia

n Li

ngle

For

mat

ion;

M. S

iluria

n, M

iddl

e S

iluria

n; T

D,

tota

l dep

th.

0

SW

NE

Sch

war

ze T

r. N

o. 6

(A

PI 1

2115

2153

000)

Cav

ende

r N

o. 2

(A

PI 1

2115

2134

300)

Phi

llips

No.

7 (

AP

I 121

1521

2050

0)S

chw

arze

Tr.

No.

2 (

AP

I 121

1521

3580

0)

Sch

war

ze N

o. 5

(A

PI 1

2115

2140

200)

Sch

war

ze P

ense

Com

. 2 (

AP

I 121

1521

4440

0)

LNR

(O

M)

DP

HI (

%)

SN

R (

OM

)

GA

MM

A R

AY

CO

ND

(M

MH

O)

CA

LI (

IN)

SP

(M

V)

AP

I

2,050 2,100Depth (ft)

TD

= 2

,170

–10

30

100

0 0

015

0

050

016

6–200

0

150

300

100

1,51

0 ft

LNR

(O

M)

NP

HI (

%)

SN

R (

OM

)

GA

MM

AR

AY

CO

ND

(M

MH

O)

CA

LI (

IN)

SP

(M

V)

AP

I

–10

40

100

0 0

0

050

06–2

00

150

100

TD

= 2

,892

2,050 2,100Depth (ft)

Formation

M. Silurian

New Albany Shale

Series Upper Devonian

Racine

LOG

AN

CH

RIS

TIA

N

MA

CO

N

SANGAMON

For

syth

Fie

ld

5 m

i

5 km

0 0N

150 160

300

R2E

R3E

T17N

26

3135

30

3625

19

1314 23

24

D

D′

Formation

M. Silurian

New Albany Shale

Series U. Devonian

Racine

LNR

(O

M)

NP

HI (

%)

SN

R (

OM

)

GA

MM

A R

AY

CO

ND

(M

MH

O)

CA

LI (

IN)

SP

(M

V)

AP

I

–10

40

100

0 0

015

0

01,

000

166–2

000

150

300

100

2,98

7 ft

2,050 2,100 2,150

TD

= 2

,152

D

LNR

(O

M)

NP

HI (

%)

SN

R (

OM

)

GA

MM

A R

AY

CO

ND

(M

MH

O)

CA

LI (

IN)

SP

(M

V)

AP

I

–10

40

100

0 0

015

0

01,

000

166–2

000

150

300

100

1,65

0 ft

TD

= 2

,159

2,100 2,150Depth (ft)

Dat

um

LN (

OM

)

NP

HI (

%)

SN

(O

M)

GA

MM

A R

AY

CO

ND

(M

MH

O)

CA

LI (

IN)

SP

(M

V)

AP

I

–10

40

100

0 0

150

01,

000

0

166–2

000

150

300

100

1,65

0 ft

TD

= 2

,177

2,100 2,150Depth (ft)

LNR

(O

M)

DP

HI (

%)

SN

R (

OM

)

GA

MM

A R

AY

CO

ND

(M

MH

O)

CA

LI (

IN)

SP

(M

V)

AP

I

–10

30

100

0 0

015

0

050

016

6–200

0

150

300

100

TD

= 2

,180

2,100 2,l150Depth (ft)

D′

Depth (ft)

Fig

ure

14

Sou

thw

est–

nort

heas

t co

rrel

atio

n D

–D′

show

ing

the

late

ral v

aria

tion

in t

he R

acin

e re

serv

oirs

at

For

syth

(lo

g po

rosi

ty b

ased

on

the

limes

tone

mat

rix).

The

var

iatio

n in

thi

ckne

ss

of t

he li

mes

tone

mar

ker

is d

ue to

pre

-Dev

onia

n er

osio

n. A

PI,

Am

eric

an P

etro

leum

Ins

titut

e un

its; S

P, s

elf-

pote

ntia

l; M

V, m

illiv

olts

; CA

LI,

calip

er; I

N,

inch

es; D

PH

I, de

nsity

por

osity

; SN

R,

shor

t no

rmal

res

istiv

ity; L

NR

, lo

ng n

orm

al r

esis

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Formation New Albany Shale

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Depth (ft)

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150

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rix).

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ican

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R2E R3E

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Contour interval 5 ft10

Section line Township boundary

Dry holeAbandoned oil wellOil well 0.5 mi

0.5 km

0

0N

Figure 16 Isopach map of the Racine reservoir modified from the map by IBEX Geological Consultants Inc. (1992). In Sec. 23 (northwest of the map), the thicknesses of the main reservoir and two younger reservoir units are combined, giving rise to an abnormally large thickness (cf. Figures 12 and 13).


STRUCTURE AND PETROLEUM TRAPBecause of pre-Upper Devonian uncon-formity and the formation of an erosional remnant on the Silurian deposits, a structure map prepared on this diachron-ous surface may not represent the true structural attributes of the Racine res-ervoir. Therefore, the structure contour map (Figure 5) was prepared by using the base of a limestone marker that directly overlies the Racine reservoir near the top of the Silurian deposits. This transgressive limestone marker rests on the reservoir with a sharp contact (Figures 10 and 11),

f

1,863 ft

a b e f2.5 cm 2.5 cm 1 mm

1 mm 0.5 mmdc

Voidspace

Figure 17 (a, b) Core samples and (c, d) photomicrographs of the reservoir showing porous, partially dolomitized crinoid fragments (CR) and (d) medium to coarsely crystalline dolomite. Note the intercrystalline and vugular secondary porosities in panels c and d. (e, f) Core sample and photomicrograph of bioturbated dense caprock facies. Samples are from a core in Podolsky Oil Company McMillen B-4, Mt. Auburn Consolidated Field.

reflecting a synchronous, relatively rapid sea level rise event. According to the reservoir isopach map (Figure 12), the reservoir at Forsyth is lenticular, appears to pinch out laterally, and has an overall northeast–southwest trend. It lies along the southeastern flank of the Sangamon Arch (Figure 2) on a regional dip that slopes to the southeast (Figure 5). Except for southeast-plunging noses (the promi-nent one being the nose at the southwest of the map), no structural closure exists. Therefore, the main control on petroleum entrapment is stratigraphic. Combined depositional and diagenetic processes and an updip pinch-out of the reservoir against the Sangamon Arch were respon-

sible for petroleum entrapment (Lasemi 2010, 2014; Lasemi et al. 2010).

POTENTIAL FOR IMPROVING RECOVERYAt Forsyth Field, initial oil production was relatively low and mainly less than 100 barrels of oil. The average cumulative primary production per well is less than 10,000 barrels, which is below average for the Mt. Auburn trend area. Solu-tion gas drive was the primary recovery mechanism. A drill stem test report from a couple of wells in the Forsyth Field area indicated low shut-in pressure (less than

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Impe

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Por

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are

a

Por

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/do

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Una

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tone

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dsp

ace

Fig

ure

18

Dep

ositi

onal

and

dia

gene

tic m

odel

for

the

deve

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ent

of a

Silu

rian

dolo

mite

res

ervo

ir in

the

Mt.

Aub

urn

tren

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ea o

f th

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Arc

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3). S

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m a

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ly d

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itiza

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100 psi) and minimal fluid recovery (for an example, see the Scout Check report in Mazzarino Schwartz No. 1, API No. 121150005100). Low reservoir pressure and insignificant fluid recovery, low ini-tial oil production, and below-average cumulative primary production per well suggest poor permeability of the dolomite reservoir interval at Forsyth. Therefore, large-volume hydraulic fracturing may be necessary (e.g., Cluff 2015) in any future development and for EOR.

Great potential exists for improv-ing petroleum recovery in the Forsyth Field. A large portion of the field is not completely developed, and several loca-tions in the field are undrilled (Figure 3). Development of the undrilled areas, infill drilling, and large-volume hydrau-lic fracturing will undoubtedly improve recovery in the field. Horizontal drilling and high-volume multistage fractur-ing is another possibility for improved recovery. The wells drilled previously are all vertical holes, and the reservoir was stimulated with relatively small-volume (about 20,000 gal, [75,708 L]) hydraulic fracturing. The application of a horizon-tal drilling technique in the porous and poorly permeable Racine reservoir at Forsyth could result in better communi-cation between the undrained isolated pore spaces and laterally discontinuous compartments that may exist, leading to a significant increase in oil production from the field.

Forsyth Field has never been water-flooded and has great potential for CO

2-EOR, which could result in both

increased oil production and storage of CO

2 produced by anthropogenic activ-

ity. The calculated original oil in place (OOIP) of the reservoir, based on the acre-feet of the reservoir region (about 14,000), a porosity of 16%, a formation volume factor of 1.05, and oil saturation of 50%, is more than 9 million barrels. The Racine at Forsyth has produced approxi-mately 8% of its OOIP. Preliminary assess-ment of the field (Damico et al. 2014) indicated significant quantities of recov-erable oil through CO

2-EOR. Reservoir

simulation results of CO2-EOR indicated

an estimated oil recovery of 26% and 42% of OOIP for near-miscible and miscible

conditions, respectively (Damico et al. 2014). Assuming a repressurized reservoir and full deployment of the field (with sev-eral new producing and injection wells drilled), more than 3,000,000 barrels of oil could be recovered through CO

2-EOR.

The field is approximately 3 mi (4.8 km) from Archer Daniels Midland Company, which could serve as a source for com-mercial CO

2.

CONCLUSIONSThis study focused on geological charac-terization of the Racine Formation at For-syth Field, which is vital in evaluating the remaining recoverable oil through pri-mary and EOR methods. The main results relevant to geological characterization of the Racine Formation are as follows:

1. The Silurian reservoir interval at Forsyth consists of porous, low-permeability, heterogeneous, and compartmentalized lenticular dolomite bodies in the regressive part of the upper Racine sequence. Deposited in the nearshore areas of a carbonate ramp margin, these bodies display a northeast–southwest trend paralleling the Silurian paleoshoreline.

2. The main reservoir constitutes the upper part of small-scale, shallowing-upward cycles capped, with a sharp contact, by a transgressive limestone facies; it reaches a net thickness of nearly 12 ft (3.66 m) and has an average porosity of 16%. Low recorded shut-in pressure and minimal fluid recovery in drill stem tests, insignificant initial oil production, and below-average cumulative primary production per well suggest poor permeability of the reservoir interval at Forsyth.

3. The wells previously drilled at For- syth Field are vertical and were stimulated with low-volume hydraulic fracturing. Infill drilling and development of the undrilled areas, horizontal drilling, and large-volume hydraulic fracturing will significantly improve recovery from the field.

4. The reservoir has produced less than 8% of its OOIP of more than 9 million barrels and has never been

water-flooded; therefore, it has great potential for retrieval of its recoverable residual oil through EOR methods.

5. The proximity of the field to a commercial source of CO

2 makes the

Racine reservoir a potential target for CO

2-EOR, which could result in

storage of CO2 produced by anthro-

pogenic activity and significant oil recovery.

ACKNOWLEDGMENTSThis article benefited from reviews by Z. Lasemi and D.C. Willette of the Illinois State Geo logical Survey. S.G. Whittaker and H.E. Leetaru reviewed the final ver-sion of the manuscript, which resulted in significant improvement. The author thanks S. Krusemark for editing the final version of the manuscript and M.W. Knapp for graphic suggestions and layout. Z. Askari prepared the early drafts of contour maps and cross sections. Maps and cross sections were prepared using IHS PETRA software through the Univer-sity Grants Program.

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