Illinois Petroleum 158 2003 Rod R

IP 158 The Origin of Prolific Reservoirs in the Geneva Dolomite (Middle Devonian), West-Central Illinois Basin

Beverly Seyler, John P. Grube, and Zakaria Lasemi

Illinois Petroleum 158 2003 Rod R. Blagojevich, Governor Department of Natural Resources Joel Brunsvold, Director ILLINOIS STATE GEOLOGICAL SURVEY William W. Shilts, Chief Equal opportunity to participate in programs of the Illinois Department of Natural Resources (IDNR) and those funded by the U.S. Fish and Wildlife Service and other agencies is available to all individuals regardless of race, sex, national origin, disability, age, religion, or other non- merit factors. If you believe you have been discriminated against, contact the funding source’s civil rights office and/or the Equal Employment Opportunity Officer, IDNR, One Natural Resources Way, Springfield, IL 62702-1271; 217/785-0067; TTY 217/782-9175.

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Cover photo: Tank battery and flare for the Deep Rock Energy Corporation Warren No.1-SDU well in Marion County, Illinois (photo by David G. Morse).

Editorial Board Jonathan H. Goodwin, Chair Michael L. Barnhardt David R. Larson B. Brandon Curry John H. McBride Anne L. Erdmann Donald G. Mikulic William R. Roy

Printed by authority of the State of Illinois PRT 3295114 - 0.8M-4/03 v Printed with soybean ink on recycled paper The Origin of Prolific Reservoirs in the Geneva Dolomite (Middle Devonian), West-Central Illinois Basin

Beverly Seyler, John P. Grube, and Zakaria Lasemi

Illinois Petroleum 158 2003 Rod R. Blagojevich, Governor Department of Natural Resources Joel Brunsvold, Director ILLINOIS STATE GEOLOGICAL SURVEY William W. Shilts, Chief Natural Resources Building 615 E. Peabody Drive Champaign, Illinois 61820-6964 Home page: http://www.isgs.uiuc.edu/

Contents Abstract 1 Introduction and Purpose 1 Geologic Setting 1 Stratigraphy and Facies Relationships 1 Structure 8 Geneva Dolomite Quarries and Outcrops 9 Meshberger Quarry 11 North Vernon Quarry 11 Scott Quarry, Southern Indiana 11 Regional Setting of Geneva Dolomite Reservoirs in Illinois 12 Historical Production in the Geneva Dolomite 14 Indiana Geneva Production 14 Illinois Geneva Production 15 Racoon Lake Field 16 Sandoval Field 16 Patoka Field 17 Tonti Field 17 Log, Core, and Rock Descriptions of Geneva Dolomite in Historical Fields 19 Recent Geneva Dolomite Discoveries 23 St. James Field 23 Lillyville Field 25 Miletus Field 27 Discovery at Stephen A. Forbes State Park 29 Petroleum Entrapment in the Geneva Dolomite 29 Dolomitization 31 Conclusions 33 Acknowledgments 34 References 34 Figures 1 The Miletus Oil Field and the Stephen A. Forbes State Park 2 2 Structure map contoured on the base of the New Albany Shale 3 3 Stratigraphic column showing major unconformities in Paleozoic rocks 4 4 Correlation and nomenclature of the Devonian and Silurian sections 5 5 The Middle Devonian Geneva Dolomite Member of the Grand Tower Limestone unconformably overlies Lower Devonian to Middle Silurian strata increasing in age from south to north in Illinois 6 6 Cross section X–X' in Marion County, Illinois 7 7 Facies map showing the distribution of the Geneva Dolomite and Dutch Creek Sandstone Members of the Grand Tower Limestone 7 8 Distribution of the Geneva Dolomite in Illinois and Indiana 9 9 Meshberger Quarry near Elizabethtown, Indiana 10 10 North Vernon Quarry near North Vernon, Indiana 10 11 The biostromal facies of the Jeffersonville Limestone 11 12 Trapping mechanisms for the Geneva Dolomite reservoirs 12 13 West-east stratigraphic cross section (A–A') of the study area in Marion County 12 14 Stratigraphic cross section B–B' from the Raccoon Lake Field to the Patoka Field in Marion County 13 15 Structure map of the Raccoon Lake Field contoured on the base of the New Albany Shale 14 16 Structure map of the Sandoval Field contoured on the base of the New Albany Shale 15 17 (a) Graph of annual production from the Raccoon Lake Field. (b) Cumulative production curve 16 18 (a) Graph of annual production from the Sandoval Field. (b) Cumulative production curve 16 19 Structure map of the Patoka Field contoured on the base of the New Albany Shale 17 20 (a) Graph of annual production from the Patoka Field. (b) Cumulative production curve 18 21 (a) Graph of annual production from the Tonti Field. (b) Cumulative production curve 18 22 Old-style geophysical log from the Sandoval Field 19 23 (a) Core of Geneva Dolomite from the Sandoval Field at 2,993 feet. (b) Thin section 20 24 (a) Core of Geneva Dolomite from 3,003 feet. (b) Thin section 20 25 Geophysical log from the Raccoon Lake Field 21 26 Geophysical log from the Patoka Field 21 27 (a) Brown sucrosic Geneva Dolomite with moldic porosity. (b) Bottom view of same core sample. (c) Photomicrograph of collapsed breccia 22 28 (a) Brown, sucrosic, sandy Geneva Dolomite. (b) Thin section from core sample 22 29 Geophysical log from Plains, Illinois, Smail No. 25 in the St. James Field 23 30 Porosity (a) and permeability (b) data for the cored interval from the Plains, Illinois, Smail No. 25 in the St. James Field 23 31 Core from the Plains, Illinois, Smail No. 25 in the St. James Field 24 32 Structural cross section C–C' of the Geneva Dolomite reservoir at the St. James Field 24 33 (a) Close-up of core from the Geneva Dolomite in the Plains, Illinois, Smail No. 25 in the St. James Field. (b) Thin section 25 34 Geophysical log from the Dart Energy Corporation Breer-Heuerman No. 3-1 well in the Lillyville Field 26 35 Combined graphs of the monthly production and cumulative production for the Dart Energy Corporation Breer-Heuerman No. 3-1 well and the offset Breer No. 2-1 well 26 36 Core from the Dart Energy Corporation Breer-Heuerman No. 3-1 well in the Lillyville Field 26 37 Seismic line over a Silurian reef at the Lillyville North Field 27 38 Combined graphs of the monthly production and cumulative production from the Geneva Dolomite reservoir in Ceja Corporation wells in the Miletus Field 27 39 Combined graphs of the monthly production and cumulative production from the Ceja Corporation wells in the Miletus Field 28 40 Structure map on the top of the Geneva Dolomite porosity at the Miletus Field 29 41 Thickness map of the Middle Devonian carbonates overlying the Geneva Dolomite at the Miletus Field 29 42 Geophysical log of the Ceja Corporation Hogan No. 2 well in the Miletus Field 30 43 Map showing the structural grain at the base of the New Albany Shale in Marion County 31 44 Cross sections of vertical seals, thickness of Middle Devonian carbonates, and their relationship to hydrocarbon entrapment in the Geneva Dolomite 32 45 Geographic distribution of Geneva Dolomite coinciding with thin New Albany Shale 32 Abstract of closure at Sandoval and Raccoon the pronounced closure needed for Lake Fields. Patoka Field, which petroleum entrapment. The Geneva Dolomite, commonly overlies a larger, deeper seated the basal member of the Middle structure, also produces from the Three-dimensional (3-D) seismic Devonian Grand Tower Limestone Ordovician Trenton Limestone. The technology was used at Tonti Field in much of the Illinois Basin, is an St. James Field is an anticline that to accurately delineate the subtle- exploration target that has recently initially produced from shallower ties of reef structures in the field, es- generated much interest. A new dis- Mississippian strata prior to the tablishing the presence of multiple covery in the Geneva was completed discovery of Devonian reservoirs. Geneva high areas that likely mimic for up to 3,000 barrels of oil per day Examination of core from Geneva an undulatory surface of the under- at a depth of 4,000 feet in the west- Dolomite reservoirs shows the rock lying Silurian pinnacle reef. Details central part of the Illinois Basin. A to be a brown, vuggy, and sucrosic of the topography of a pinnacle reef study of reservoirs in the Geneva dolomite. We suggest that post-dep- show clustered high areas that are Dolomite at the Raccoon Lake, San- ositional dolomitization combined key to exploration and development doval, Patoka, Miletus, and St. James with dissolution of fossil material of Geneva Dolomite reservoirs. oil fields shows that pronounced of Geneva carbonates is a viable Implementation of existing 3-D seis- structural closure, fracturing, and mechanism to explain the enhanced mic technology should significantly formation of secondary porosity porosity, permeability, and brec- improve the drilling success rate of through dolomitization and dissolu- ciation found in Geneva Dolomite these reservoirs. This study shows tion are associated with reservoir reservoirs. Mapping suggests that the successful application of newer, development and entrapment of Devonian age structures, commonly yet readily available, technologies to petroleum. associated with underlying Silurian re-examine old fields and trends to discover and develop economically The draping of younger Middle De- reefs, are an element that enhances reservoir porosity, and more recent significant quantities of oil in the vonian strata over Silurian reefs has mature Illinois Basin. resulted in approximately 100 feet structural movement has created

Introduction and tions. These technologies have been fields that have similarities to the implemented in other mature pro- newly discovered reservoir under- Purpose ducing provinces but had not been lying Stephen A. Forbes State Park Prolific reservoirs are associated proved and refined for application (fig. 2). These older fields were also with the Geneva Dolomite in the in the Illinois Basin until recently. highly productive, and a review Illinois Basin. In March 2002, a new of the data from these fields may This study was undertaken to pro- field discovery was completed im- assist in the future exploration and vide information on the geologic mediately south of the Miletus Field development of Geneva Dolomite factors controlling the origin of in Marion County, Illinois, flowing reservoirs. Currently accepted theo- highly prolific petroleum reservoirs oil at a rate of up to 3,000 barrels a ries that explain the dolomitization in the Geneva Dolomite. Geologic day from a horizontal well bore. This process on a regional scale are also factors include structural, strati- prolific discovery in the Geneva Do- discussed. graphic, diagenetic, and sedimen- lomite follows several earlier discov- tologic relationships. The Geneva eries in the late 1990s that sparked Dolomite is widely exposed in quar- the drilling of an assortment of Geologic Setting ries located near the Geneva outcrop exploratory wells in the region. The belt in Indiana (Perkins 1963, Leon- Stratigraphy and Facies earlier wells were vertical tests and ard 1996). The description of the Relationships produced in excess of 300 barrels of Geneva Dolomite in these quarries oil per day. Production from these The stratigraphic column in figure and examination of continuous core wells shows a low rate of decline. 3 shows the major unconformities from two recently drilled Geneva The recent horizontal discovery and found in the predominantly Paleo- wells and core biscuits from wells in offset horizontal development wells zoic bedrock strata of the Illinois several neighboring Geneva reser- extend under the Stephen A. Forbes Basin. The sub-Kaskaskia uncon- voirs aided in (1) the interpretation State Park (fig. 1). Three-dimen- formity separates Upper Silurian of facies, (2) the determination of sional (3-D) seismic technology was through Lower Devonian strata stratigraphic relationships within employed to establish the prospect. from Middle Devonian strata in Il- the Grand Tower and Jeffersonville Further, the combined application linois (Buschbach and Kolata 1991). Limestones (fig. 4), and (3) the inter- of 3-D seismic and horizontal drill- The Geneva Dolomite and Dutch pretation of diagenetic alterations ing technologies in the Illinois Basin Creek Sandstone are the basal mem- that occurred to create the highly should enhance the exploration and bers of the Grand Tower Limestone porous and permeable sucrosic development opportunities in the (fig. 4); (Meents and Swann 1965, dolomite of the Geneva. Included Middle Devonian Geneva Dolomite North 1969) and were deposited on is a discussion of Geneva Dolomite as well as structural and strati- the eroded, exposed surface of the reservoirs from several neighboring graphic prospects in other forma- Upper Silurian (Devera and Fraun-

Illinois State Geological Survey Illinois Petroleum 158 1 �� 18 17 16 15 14 ��

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Figure 1 The Miletus Oil Field and the Stephen A. Forbes State Park.

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Figure 2 Structure map contoured on the base of the New Albany Shale. Contour interval is 100 feet. Cross section locations and Geneva Dolomite reservoirs are shaded in gray. (Modified from Cluff et al. 1981.)

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Figure 3 Stratigraphic column showing major unconformities in Paleozoic rocks. The sub-Kas- kaskia unconformity separates Upper Silurian through Lower Devonian strata from Middle Devonian strata in Illinois. Quat., Quaternary; Cret., Cretaceous; Perm., Permian. (From Buschbach and Kolata 1991.)

4 Illinois Petroleum 158 Illinois State Geological Survey Illinois State Geological Survey Illinois Petroleum 158 5 ��

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Figure 4. Correlation and nomenclature of the Devonian and Silurian sections of the stratigraphic column in Illinois and Indiana. (Modified from Droste and Shaver 1987, Droste et al. 1975. )

felter 1988, Devera and Hasenmuel- common in the Geneva Dolomite. 1975, Devera and Fraunfelter 1988). ler 1991) through Lower Devonian A facies map showing distribution The limestone facies primarily strata in Illinois (fig. 5). Figure 6 of the Geneva Dolomite and Dutch occurs in the southern portion of shows the relationship of the Middle Creek Sandstone Members of the the Basin, including its extension Devonian Geneva Dolomite Member Grand Tower Limestone is shown into west-central Indiana, and the of the Grand Tower Limestone with in figure 7 (Meents and Swann dolomite facies primarily occurs in underlying strata in Marion County, 1965). Prolific wells also have been the central to northern portion of Illinois, the site of the new Geneva completed in the Dutch Creek Sand- the Basin (Meents and Swann 1965, discovery. stone, a highly porous and perme- North 1969). The dark brown, sandy able dolomitic sandstone (Meents dolomite of the Geneva occurs at The Geneva Dolomite has been con- and Swann 1965). the base of the dolomite facies of sidered to be a facies of the Grand the Grand Tower and Jeffersonville Tower Limestone in Illinois and the Both the Grand Tower and Jeffer- Limestones. Jeffersonville Limestone in Indiana sonville consist of a pure, fossil- (Meents and Swann 1965, Droste iferous southern limestone facies The Geneva Dolomite is buff to and Shaver 1975). Thin Dutch Creek (commonly biostromal and/or dark brown but oxidizes into pale Sandstone locally underlies the biohermal) and a northern dolomite tan, cream, and even white in Geneva Dolomite in the study area, facies (Schwalb 1955, Meents and near-surface exposures (Droste and and sandy dolomite intervals are Swann 1965, Droste and Shaver Shaver 1975). The distinctive brown

4 Illinois Petroleum 158 Illinois State Geological Survey Illinois State Geological Survey Illinois Petroleum 158 5 Subsurface distribution of the �� Geneva Dolomite forms an arcu- �� ate belt that curves northwest from the outcrop to west-central Indiana and bends to the southwest from Clark and Edgar Counties in Illinois and extends as far as Montgomery, Bond, and Clinton Counties (fig. 8). The Geneva ranges from 0 to 50 feet thick in central Illinois but locally is thicker in some areas (Schwalb �� 1955). Studies of well cuttings from recently drilled wells show that the Geneva locally may be up to 90 feet thick. In central and west-central �� Indiana, the Geneva forms a semi- circular body ranging in thickness �� from 0 to 60 feet. The distribution of the Geneva was attributed by Perkins (1963) to pre-Jeffersonville �� erosion (Geneva erosion). However, �� no sedimentologic or petrographic evidence was given. Droste and Shaver (1975) suggested that the Geneva distribution pattern was Bailey �� most likely related to the underly- Subcrop �� ing Silurian reefs rather than to �� post-Geneva erosion. In Indiana, the Clear Creek thickest Geneva Dolomite appar- Subcrop ently correlates with the thickened Backbone part of the underlying Silurian reef Subcrop system and the narrow area of Jef- fersonville thinning.

� �� The relationship between the Geneva and the Grand Tower or Jef- �� fersonville has been controversial because of the poor preservation of fossil constituents and correlation Figure 5 The Middle Devonian Geneva Dolomite Member of the Grand Tower problems. A few earlier studies have Limestone unconformably overlies Lower Devonian to Middle Silurian strata suggested that the Geneva Dolomite increasing in age from south to north in Illinois. The underlying strata in much of is older than the Jeffersonville in the study area is the Lower Devonian Clear Creek Chert. (Modified from North Indiana and its lateral equivalent 1965.) Grand Tower in Illinois (Patton and Dawson 1955, Perkins 1963). Conversely, floating quartz sand color has been attributed to organic interpretation that the Geneva is es- grains, invertebrate fossils, and di- material disseminated within the sentially nonfossiliferous (Collinson agnostic chitinozoans of the Geneva rock (Schwalb 1955). When samples 1967), the Geneva has a bioclastic are also common in the southern are dissolved in hydrochloric acid, and pelletoidal texture (packstone bioclastic facies of the Grand Tower this organic material floats to the to grainstone) with common molds (Warthin and Cooper 1944, Devera surface. In most places, the color and casts of branching and solitary and Fraunfelter 1988). The Geneva darkens toward the base, although corals, stromatoporoids, and some Dolomite is now generally consid- in some areas the reverse is true brachiopods, bivalves, gastropods, ered to be a member of the Grand (Schwalb 1955). Quartz sand grains and ostracodes (Droste and Shaver Tower Limestone in Illinois (Meents “floating” in the carbonate are pres- 1975). The Geneva appears to have and Swann 1965, p. 7; Devera and ent throughout the Geneva but are been deposited as skeletal shoals or Frauntfelter 1988) and the Jefferson- especially common near the base. banks, particularly on preexisting ville Limestone in Indiana (Becker The dolomite is massive to thin bed- topographic highs created by the 1974, Droste and Shaver 1975, Leon- ded and granular and vuggy for the underlying Silurian reefs (Droste ard 1996). According to Meents and most part. In contrast to an earlier and Shaver 1975). Swann (1965), the Geneva Dolomite

6 Illinois Petroleum 158 Illinois State Geological Survey Illinois State Geological Survey Illinois Petroleum 158 7 �� NW SE � ��

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Figure 6 (above) Cross section X–X' in Marion County, Illinois. Silurian through Devonian strata show the �� sub-Kaskaskia unconformity sepa- � �� rating Upper Silurian and Lower Devonian strata from Middle De- �� vonian strata. Truncation of Upper �� Silurian through Lower Devonian ��

strata formed an eroded surface on �� which the Middle Devonian Geneva �' Dolomite and Dutch Creek Sandstone �� were deposited. The contact between �� the Geneva Dolomite and overlying �� carbonates in the Grand Tower Lime- stone may define a parasequence (at arrow) boundary. (Modified from Meents and Swann 1965.)

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�� Figure 7 (right) Facies map show- �� miles ing the distribution of the Geneva 0 5 10 �� Dolomite and Dutch Creek Sand- �� stone Members of the Grand Tower Limestone. Both members are at the base of the Grand Tower. Thick- nesses shown are of the Grand Tower Limestone. (Modified from Meents and Swann 1965.)

6 Illinois Petroleum 158 Illinois State Geological Survey Illinois State Geological Survey Illinois Petroleum 158 7 in the Illinois subsurface is a facies 1975) have been presented by Per- Illinois, however, appear to have that grades laterally into the lower kins (1963) and for the Vernon Fork been deposited in shelf lagoon and part of the Grand Tower Limestone equivalent in Illinois by Lasemi tidal flat environments that were in southern Illinois. Droste and (2001). The Vernon Fork ranges periodically subjected to subaerial Shaver (1975) reached the same from 0 to 80 feet thick in Indiana, exposure, as indicated by the pres- conclusion regarding the relation- but its equivalent in central Illinois ence of mud cracks (Perkins 1963, ship between the Geneva and the can reach up to 105 feet thick. The Lasemi 2001). In such restricted Jeffersonville in Indiana. Vernon Fork extends farther north shallow-water settings, evapora- and south than does the Geneva tion of seawater could have formed The Geneva conformably overlies Dolomite. In west-central Indiana, the Mg-rich solutions that may the Dutch Creek Sandstone where it the Vernon Fork Member is primar- have been responsible for forma- is present at the base of the Grand ily a microcrystalline dolomite, but tion of the dolomites in the Grand Tower. However, in many places, it contains laminated lithographic Tower and Jeffersonville Limestones the Geneva unconformably overlies to sublithographic limestone and (Droste and Shaver 1975, Lasemi Lower Devonian to Middle Silurian dolomitic limestone in the eastern 2001). The boundary between the strata that increase in age from outcrop belt. The Vernon Fork is normal marine Geneva Dolomite south to north in Illinois (Meents slightly fossiliferous and contains and the overlying restricted ma- and Swann 1965) (figs. 4 to 6). The sporadic sand grains, birds-eye rine Vernon Fork Member marks a stratum underlying the Middle De- structures, brecciated laminae, and transgressive-regressive sequence. vonian Grand Tower Limestone in mud cracks. No gypsum or anhy- However, biostratigraphically, there the southern part of Marion County drite has been found, but possible is no significant time break along is the Lower Devonian Clear Creek calcite pseudomorphs after gypsum this boundary (Norby 1991; Norby, Chert (figs. 5 and 6). In the northern are present (Perkins 1963). personal communication 2002), part of Marion County, the Upper and there has been no unequivocal Silurian-Lower Devonian Bailey The petrography of the Vernon Fork evidence thus far indicating major Limestone (fig. 5) directly underlies Member equivalent in east-cen- subaerial erosion. the Grand Tower Limestone. tral Illinois has only recently been documented in cores and from a The sub-Kaskaskia unconformity The Geneva Member is overlain quarry exposure in Douglas County, separating the Upper Silurian and by a somewhat thicker section of Illinois (Lasemi 2001). Here, the unit Lower Devonian strata from the lighter-colored dolomite. The do- is up to 105 feet thick and consists Middle Devonian strata is shown in lomite above the Geneva has been of a light yellowish gray to yellow- the cross section in figure 6. Trunca- referred to as the “laminated beds,” ish tan microcrystalline dolomite. tion of the Upper Silurian through “laminated zone,” “chalk beds,” or It is slightly to moderately sandy Lower Devonian strata formed an “fine-grained dolomite of the Jeffer- throughout, and several dolomitic eroded surface on which the Middle sonville” in Indiana (see Droste and sandstone lenses are present in the Devonian Geneva Dolomite and Shaver 1975) and the “unnamed,” lower part. Fenestral fabric is com- Dutch Creek Sandstone Members “light-colored dolomite above the mon, and mud cracks have been of the Grand Tower Formation Geneva,” or “the northward fa- observed in both quarry exposures were deposited (fig. 5) (Devera and cies of the Grand Tower” in Illinois and cores. The upper part of the Fraunfelter 1988, Devera and Hasen- (Schwalb 1955, Meents and Swann unit contains well-developed stro- mueller 1991). 1965, Collinson 1967, North 1969). matolitic laminations formed by the Droste and Shaver (1975) named the trapping of sediments by blue-green unit above the Geneva Dolomite the algae. Some of the stromatolites Structure Vernon Fork Member for the central form mound-shaped structures ap- The Upper Devonian New Albany and west-central Indiana Jefferson- proximately 10 feet high. The Tioga Shale (fig. 4) is a widespread and ville facies. Subsurface data suggest Bentonite Bed generally occurs consistent formation that is use- that the Geneva interfingers with about 20 to 30 feet below the top ful for structural mapping in the the Vernon Fork dolomite in Indiana (Lasemi 2001). Illinois Basin. A structure map (Droste and Shaver 1975) and its contoured on the base of the New equivalent unit in Illinois (Schwalb The environment changed gradu- Albany Shale reflects structural 1955, Meents and Swann 1965). The ally from normal marine condi- relationships of the Geneva Do- Geneva is correlated with the lower tions during the Geneva deposi- lomite (fig. 2). Pronounced struc- part of the Middle Devonian Vernon tion to shallow, restricted marine tural closure, as great as 100 feet, is Fork and, therefore, with the lower conditions during Vernon Fork associated with Geneva Dolomite part of the Grand Tower Limestone deposition (Perkins 1963, Lasemi production. Much of this structure of southern Illinois (Schwalb 1955, 2001). Abundant and diverse fossil is tectonic, but in some cases the Meents and Swann 1965, North allochems in the Geneva suggest closure is caused or enhanced by the 1969, Devera and Fraunfelter 1988). deposition within a normal marine drape of younger Middle Devonian environment. The overlying Vernon strata over Silurian reefs (Bristol Petrographic details for the Vernon Fork Member and its equivalent in 1974). Some of the most prolific Fork Member (Droste and Shaver

8 Illinois Petroleum 158 Illinois State Geological Survey Illinois State Geological Survey Illinois Petroleum 158 9 Geneva wells are associated with the postulated underlying Silurian reefs. This postulate is related to enhanced structural closure in the strata overlying the reef, a function of differential compaction. Frac- tures within the Geneva beds may have resulted from differential compaction. Reef-induced paleostruc- ture (Droste and Shaver 1975) may �� have influenced Geneva deposition by offering sites suitable for growth of bioherms and the ensuing diagenetic alteration of these carbonates, resulting in improved reservoir porosity and permeability. Structure caused by differential compaction of the fine-grained sediments flank- � ing the rigid core of a Silurian reef � has been documented throughout � the entire overlying stratigraphic � section and can even be detected � in structural highs in the overlying �� Pennsylvanian coals (e. g., Whita- �� ker 1988). Many of these reefs also have topographic expression at the surface that may be located by present-day drainage patterns (Whitaker 1988). �� Geneva Dolomite production is also associated with structures induced by tectonism. Production from Figure 8 Distribution of the Geneva Dolomite in Illinois and Indiana. The Ge- anticlinal closure at the St. James, neva reaches a maximum thickness of about 90 feet in east-central Illinois; thins Salem, and Centralia Fields indi- to zero in southern Marion County, Illinois; and outcrops in a series of quarries cates that pronounced structural (numbered 1 through 5) in southeastern Indiana, where it is 20 to 30 feet thick. closure from tectonic deformation (Modified from Perkins 1963, Schwalb 1955.) is sufficient to trap petroleum in the Geneva Dolomite. However, the most prolific production is from highly porous and permeable Ge- fore likely to have migrated from the aggregate. The fossil content, crystal neva Dolomite draped over pinnacle black shales in the New Albany, lo- size, zonation of dolomite crystals, reefs. A number of uplifted fault cated in deeper portions of the Illi- color, staining, porosity, and perme- blocks in the Basin have coinciden- nois Basin, into the updip, although ability of the Geneva observed in tal reef structures, indicating that stratigraphically lower, Geneva quarries in Indiana are all similar to Silurian reefs may have preferen- Dolomite. Some of the petroleum the prolific reservoirs in Illinois. The tially developed atop pre-existing migration now associated with Ge- Geneva Dolomite shows widespread structural highs (Davis 1991). neva Dolomite reservoirs in Marion porosity in both cores and quarries. County likely occurred along the Geophysical logs from Indiana and The close vertical proximity of the DuQuoin Monocline (fig. 2). Illinois show that porous zones in New Albany Shale to the Geneva the Geneva are widespread through- Dolomite is significant because the out the subsurface. The distribution black shales in the New Albany are Geneva Dolomite of similar facies and porous zones considered to be the primary source Quarries and Outcrops from central Indiana to south-cen- rock for petroleum in the Illinois Ba- tral Illinois suggests possible con- A series of quarries was examined sin (Barrows and Cluff 1984). Black nectivity across the entire region. in the Geneva outcrop belt in east- shales in the New Albany reach their The outcrop region of Indiana may central Indiana that extends for 70 greatest maturity in the deepest serve as a recharge area for the ac- miles in a north-south direction (fig. portion of the basin (Barrows and tive water drive that is characteristic 8). The Geneva is 20 to 30 feet thick Cluff 1984). The hydrocarbon charge of the Geneva Dolomite reservoirs here and is quarried for construction for the Geneva reservoirs is there- throughout the Geneva fairway.

8 Illinois Petroleum 158 Illinois State Geological Survey Illinois State Geological Survey Illinois Petroleum 158 9 Figure 9 (a) Meshberger Quarry near Elizabethtown, Figure 10 (a) North Vernon Quarry near North Vernon, Indiana (number 2, fig. 8), showing the Geneva Dolomite Indiana (number 4, fig. 8). Highly porous, sucrosic Geneva and the unconformable lower contact with the Silurian Dolomite is over 20 feet thick and is overlain by the Jeffer- Louisville Formation limestone. (b) Brown Geneva Dolo- sonville Limestone. (b) Row (at arrow) of large, empty vugs mite unconformably overlies the gray Louisville Formation in the Geneva Dolomite. The highly porous dolomite con- limestone. The undulating upper contact (arrow) with the tains many vugs created by dissolution of marine fossils, Jeffersonville Limestone (Middle Devonian) may be a para- including colonial corals and stromatoporoids. (c) Close-up sequence boundary. (c) Vugs filled with late-stage dog- of vug showing detail of smaller branching corals and other tooth spar calcite (white mineral) in the Geneva Dolomite marine fossils. from the Meshberger Quarry. Vugs were likely created by dissolution of large coral heads or stromatoporoids.

10 Illinois Petroleum 158 Illinois State Geological Survey Illinois State Geological Survey Illinois Petroleum 158 11 Figure 11 The biostromal facies of the Jeffersonville Lime- stone are shown on these slabbed rock samples, which were collected from the Scott Quarry near Jeffersonville, Indiana. The Jeffersonville Limestone, considered to be equivalent to the Geneva Dolomite, has not been altered by dolomitization and dissolution.

Meshberger Quarry that they were probably created by sils also were observed in the quarry dissolution of large coral heads or walls. The matrix surrounding the Meshberger Quarry (number 2, stromatoporoids. vugs is composed of highly porous, fig. 8) is located near Elizabethtown, brown sucrosic dolomite (fig. 10c). Indiana. The Geneva Dolomite Dissolution of the larger coral heads unconformably overlies the Silurian North Vernon Quarry is more complete than that of the Louisville Formation limestone (fig. The North Vernon Quarry is located smaller branching corals and other 9). The Louisville is a gray, tightly near North Vernon, Indiana (num- marine fossils found in the matrix. cemented grainstone, and the ber 4 in fig. 8). The highly porous, The Geneva Dolomite in the North overlying Middle Devonian Ge- sucrosic Geneva Dolomite Member Vernon Quarry very closely resem- neva Dolomite is a brown, sucrosic, is over 20 feet thick and is overlain bles the Geneva Dolomite in cores porous dolomite (fig. 9). Although by the Vernon Fork Member of the from reservoirs in Marion County, there is an abrupt and distinctive Jeffersonville Limestone (fig. 10). Illinois. change between the rock units The late-stage mineralization by above and below the sub-Kaskaskia white, sparry, calcite, filling vugs unconformity exposed here, there that is prevalent in the Meshberger Scott Quarry, is no evidence of scouring or other and other quarries is not com- Southern Indiana erosion at the contact. mon in this quarry. Most Geneva The Scott Quarry is the southern- Figure 9b shows the undulating vugs in the North Vernon Quarry most exposure studied (number upper contact of the Geneva with are not affected by the late-stage 5 in fig. 8). The Geneva Dolomite the overlying Middle Devonian calcite mineralization and remain equivalent strata in this quarry are age Vernon Fork Member of the open. Although dolomitization has richer in fossils than those in the Jeffersonville Limestone at the obscured the direct fossil evidence, other quarries (fig. 11), and large Meshberger Quarry. The upper remnants of large coral heads and unaltered coral heads, stromato- contact of the Geneva is likely a stromatoporoids are common in poroids, and other fossils are very parasequence boundary (Leonard this quarry, suggesting potential abundant. The matrix surrounding 1996). In the Meshberger Quarry, biohermal or biostromal buildups. the intact fossils is a brown, slightly many vugs in the Geneva Dolomite An example of one such buildup is dolomitic limestone (fig. 11). The were filled with white, late-stage shown in figure 10b where a row of Geneva Dolomite equivalent in this dogtooth spar calcite cement, and empty vugs created by the dissolu- quarry is 15 feet thick and lacks the the matrix surrounding the vugs tion of large (40 cm × 25 cm) coral visible porosity that is present in the consists of porous brown, sucrosic heads and stromatoporoids can be quarries to the north. This strata dolomite (fig. 9c). The shape and seen. Partially dissolved smaller co- most closely resembles the Geneva size of some large vugs suggest lonial corals and other marine fos- Dolomite observed in core samples

10 Illinois Petroleum 158 Illinois State Geological Survey Illinois State Geological Survey Illinois Petroleum 158 11 �� Figure 12 Trapping mechanisms for the Geneva Dolomite �� reservoirs including drape over Silurian reefs and closure over deep-seated structures. Reefs preferentially grew on structural highs. Reactivation along regional faults likely �� created structural highs prior to accumulation of Middle �� Devonian carbonates. Thinning of Middle Devonian units occurs over Silurian reefs and major structures. (Modified from Davis 1991.)

�� ' ��

��

Figure 13 West-east stratigraphic cross section (A–A' , fig. 2) of the study area in Marion County. Lower Mississippian Chouteau Limestone thickens eastward drocarbon-trapping mechanism in of the DuQuoin Monocline (fig. 2). The Upper Devonian New Albany Shale and the Geneva Dolomite reservoirs (fig. Middle Devonian carbonates also thicken eastward. Sandoval field produces 12). In many instances, deep-seated from porous Geneva Dolomite draped over a Silurian reef. structures likely formed the founda- tion for the growth of pinnacle reefs during the Silurian (Davis 1991) because the corals and other reef- from the Sandoval Field in Marion Regional Setting of building organisms thrived in the County, Illinois. The change from shallower water found at structural- the visibly porous and permeable Geneva Dolomite ly high locations (fig. 12). Structural dolomite observed in the North Ver- Reservoirs in Illinois noses along major anticlines are non Quarry to the highly fossilifer- A substantial amount of the petro- potential sites for structural closure ous limestone in the Scott Quarry leum production from the Geneva and hydrocarbon entrapment in is due to less dissolution of calcite Dolomite comes from the northern the Geneva Dolomite. The St. James fossils and less dolomitization in the half of Marion County, Illinois. The Field, located along the southern more southern extent of the Geneva locations of many of these fields, nose of the Louden Anticline, is Dolomite equivalent strata. including the most recent discovery an example of recently discovered near Miletus Field, is shown in fig- production from closure along a ure 2. Anticlinal closure combined structural nose (fig. 2). Portions with drape over underlying pinnacle of these structural noses may also reefs in many instances is the hy- have been potential sites for growth

12 Illinois Petroleum 158 Illinois State Geological Survey Illinois State Geological Survey Illinois Petroleum 158 13 ��

��

Figure 14 Stratigraphic cross section B–B' from the Raccoon Lake Field to the Patoka Field in Marion County. Pinnacle reefs underlie the Raccoon Lake and Sandoval Fields. The Grand Tower Limestone thickens from the south to north.

of Silurian pinnacle reefs if the same A west-east stratigraphic cross sec- a Silurian reef. Eastward along the feature was structurally high during tion (A–A' in fig. 2) crosses Marion section, the Salem Field produces the Silurian. Increasing evidence County, Illinois, where several pro- from two different horizons in the suggests that many major structural lific Geneva Dolomite oil reservoirs Middle Devonian. The cross section features in the Illinois Basin have have been found (fig. 13). This sec- also shows that the lower Missis- a history of episodic movement tion crosses Sandoval Field, which sippian Chouteau Limestone, a through Paleozoic time (Davis 1991, produces from the highly porous marker horizon in some areas of the McBride and Kolata 2000). Geneva Dolomite that is draped over Illinois Basin, thickens to the east

12 Illinois Petroleum 158 Illinois State Geological Survey Illinois State Geological Survey Illinois Petroleum 158 13 ��

��

�� 1 ��

1 �� 2 1 7 2 �� 2 1 �� 6 3 1 �� 3 �� 6 1 1 �� 1 �� 1 9 10 5 �� 3 5 ��

��

9 ��

� � ��

Figure 15 Structure map of the Raccoon Lake Field contoured on the base of the New Albany Shale showing pronounced closure in circular patterns indicating drape over an underlying Silurian reef complex. Logs penetrating Silurian strata also show reef buildups. (Modified from Bristol 1974.)

of the DuQuoin Monocline (fig. 2). Lake Field is located near the south- Historical Production in The Upper Devonian New Albany ern limit of the Geneva Dolomite. Shale and Middle Devonian carbon- The cross section (fig. 14) also shows the Geneva Dolomite ates also thicken to the east of the an increase in thickness of Middle Indiana Geneva Production Duquoin Monocline, which suggests Devonian carbonates above the Ge- that the DuQuoin Monocline was a neva Dolomite from south to north. The Geneva Dolomite together with structurally high feature during de- This increase may be due to the other Middle Devonian dolomites position of these strata and a likely Lower Devonian Clear Creek Chert are productive from a number of site for accumulation of biohermal unconformity, which underlies the fields that lie within the Indiana carbonates. Middle Devonian Geneva Dolo- portion of the dolomitic fairway. mite in the southern half of Marion Hydrocarbons in these fields are A north-south cross section, B–B' County, Illinois. commonly trapped in structures (located on fig. 2), from Raccoon formed by drape over Silurian reefs, Lake through Sandoval, South particularly along the southeast- Patoka, and Patoka Fields shows the northwest–trending Terre Haute Geneva Dolomite thickening from Reef Bank (Droste and Shaver 1975) south to north (fig. 14). The Raccoon in the western Indiana counties of

14 Illinois Petroleum 158 Illinois State Geological Survey Illinois State Geological Survey Illinois Petroleum 158 15 ��

��

1 2 �� 3 �� 1 2 �� 1 2 2 �� 16 15 12 13 3 �� 14 �� 1 3 �� 1 1 2 4 2 �� 27 22 �� 1 �� 1 23 21 �� 2 24 1 �� 28 ��

�� 3 25 �� 26 ��

��

�� 1 2 �� 1 1 1 1 � 2 � ��

9 ��

� � ��

Figure 16 Structure map of the Sandoval Field contoured on the base of the New Albany Shale showing pronounced closure in circular patterns indicating drape over an underlying Silurian reef complex. Logs penetrating Silurian strata also show reef buildups. (Modified from Bristol 1974.)

Sullivan, Vigo, Clay, and Greene. Illinois Geneva Production sure in the overlying strata at these Two fields with significant Middle two fields is caused by differential Structure maps contoured on Devonian production are Siosi and compaction over the rigid reef core the base of the New Albany Shale Fairbanks, both of which are in Sul- relative to the finer-grained com- show pronounced circular closure, livan County. The Siosi was discov- pactable sediments flanking the suggesting drape over underlying ered in 1926 and has produced 5.2 reef core. There are over 150 feet of Silurian reefs at Raccoon Lake and million barrels of oil through 1999. closure at Raccoon Lake and 140 Sandoval Fields (figs. 2, 15, and The Fairbanks was discovered in feet of closure at Sandoval Field, and 16) (Bristol 1974). Logs penetrat- 1950 and has produced 3.2 million each field is less than one mile wide. ing Silurian strata also show reef barrels of oil through 1999 (Cazee Thirteen wells in Raccoon Lake and buildups with a thickened section 2000). There have been no signifi- 32 wells in the Sandoval Field were of Silurian carbonates during the cant new Geneva field discoveries completed in the Geneva Dolomite, reef growth period (fig. 14). Clo- since the late 1960s in Indiana. which is the most prolific reservoir

14 Illinois Petroleum 158 Illinois State Geological Survey Illinois State Geological Survey Illinois Petroleum 158 15 a Raccoon Lake Field Annual Production a Sandoval Field Annual Production 700,000 900,000

800,000 600,000 700,000 500,000 600,000

400,000 500,000

400,000 Barrels oil

300,000 Barrels of Oil 300,000 Geneva 200,000 200,000 Dolomite Geneva Dolomite Discovery: 1938 Discovery: Oct. 1951 100,000 100,000 0 0 4 8 58 64 193 1936 193 1940 1942 1944 1946 1948 1950 1952 1954 1956 1958 1960 1962 1964 1966 1968 1970 1972 1949 1950 1951 1952 1953 1954 1955 1956 1957 19 1959 1960 1961 1962 1963 19 1965 1966 1967 1968 1969 1970 1971 1972

b Sandoval Field Cumulative Production b Raccoon Lake Field Cumulative Production 7,000,000

4,000,000 6,000,000

3,500,000 5,000,000 3,000,000 4,000,000 2,500,000 Geneva Dolomite Discovery: 1938 2,000,000 3,000,000 Barrels of Oil

Barrels of Oil 1,500,000 2,000,000 Geneva Dolomite Discovery: Oct. 1951 1,000,000 1,000,000 500,000

0 0 2 2

1949 1950 1951 1952 1953 1954 1955 1956 1957 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 197 1934 1936 1938 1940 1942 1944 1946 1948 1950 1952 1954 1956 1958 1960 196 1964 1966 1968 1970 1972

Figure 17 (a) Graph of annual production from the Figure 18 (a) Graph of annual production from the San- Raccoon Lake Field showing the rapid increase from doval Field showing the large increase following discov- 220,000 barrels of oil to 660,000 barrels of oil with dis- ery and development of the Geneva Dolomite reservoir. covery of the Geneva Dolomite reservoir. (b) Cumulative (b) Cumulative production curve in Sandoval Field also production curve shows that much of the production in the shows a significant increase in production following the Raccoon Lake Field occurred in the five years following the discovery and development of the Geneva Dolomite discovery of the Geneva Dolomite reservoir. reservoir. in each of these fields. Many of these including the Cypress, Benoist, produced over 2.5 million barrels of reefs are less than one mile in size and Rosiclare Sandstones and the oil from an area of only 230 acres. and have approximately 100 feet or Ohara and McClosky Limestones. more of closure. It is likely that 3-D The Geneva Dolomite reservoir was Sandoval Field seismic techniques can be used suc- discovered with the drilling of the cessfully to explore for additional Texas No. 10 C. Langenfeld in Sec. 3, The Geneva Dolomite reservoir at reef and reef-drape production. A T1N, R1E, in 1951 at a depth of 3,385 the Sandoval Field was first exploit- recent 3-D seismic survey was suc- feet. Initial daily production from ed in 1938 with the drilling of the cessfully used to locate and drill a the well was 109 barrels of oil and Southwest Oil and Gas No. 21 Ben- Geneva well on a local high point on 101 barrels of water. The discovery oist in Sec. 8, T2N, R1E, at a depth the Tonti Field reef (Helpingstine et and development of the Geneva Do- of 2,926 feet. The initial production al. 2001). lomite reservoir rapidly increased of this well was 319 barrels of oil annual production from 220,000 to flowing in 19 hours with a natural completion. The Sandoval Field was Raccoon Lake Field 660,000 barrels of oil (fig. 17). The cumulative production curve (fig. initially discovered in 1908 and has Raccoon Lake Field was discovered 17b) shows that much of the pro- produced over 6 million barrels of in 1949 using seismic technnol- duction in the Raccoon Lake Field oil from the middle Mississippian ogy. Shallow reservoirs at Raccoon occurred in the 5 years following the Cypress and Benoist Sandstones Lake are found in middle Missis- discovery of the Geneva Dolomite and the Middle Devonian Geneva sippian sandstones and carbonates reservoir. The Geneva Dolomite has Dolomite. Over 2.5 million barrels

16 Illinois Petroleum 158 Illinois State Geological Survey Illinois State Geological Survey Illinois Petroleum 158 17 Silurian reef has been reported at the northeast end of the field (Bristol 1974) over which four wells are pro- �� ducing from the Geneva Dolomite. � � �� Most of the wells in the area of the � � �� Geneva reservoir were drilled to the Ordovician Trenton carbonates, but �� records indicate that most produc- �� tion is from the Geneva. Initial �� production in the field was from � �� the middle Mississippian Benoist �� 9 �� sandstone. Discovery of the Geneva �� Dolomite reservoir occurred in 1943 �� at a depth of 2,908 feet in the Adams �� Oil and Gas No. 1 D. Pugh well, Sec. �� 29, T4N, R1E, Marion County. The �� Patoka Field has produced in excess �� of 14.9 million barrels of oil. The

�� field’s largest increase in annual ��

�� production came after the 1943 �� discovery of the Geneva Dolomite �� reservoir when annual production �� increased from 298,000 barrels of �� oil in 1943 to 630,000 in 1944 and �� 1,644,000 barrels of oil in 1946 (fig. �� 20a). Cumulative production in �� the field (fig. 20b) increased from �� 2,841,000 barrels of oil in 1943 to �� over 10 million barrels of oil by 1950.

�� Approximately 10 million barrels of

�� the total production from the Patoka � � Field can be attributed to the four Geneva Dolomite wells. ��

�� Tonti Field �� Another highly productive Ge- neva Dolomite reservoir in Marion County, Illinois, is the Tonti Field. The Geneva Dolomite produces from 15 feet of brown, porous sucro- Figure 19 Structure map of the Patoka Field contoured on the base of the sic dolomite draped over a Silurian New Albany Shale. The structural feature at Patoka is larger than the structures reef (Bristol 1974). Approximately induced by pinnacle reefs at Raccoon Lake and Sandoval Fields. There is evi- 80 feet of closure is apparent at dence of deep-seated tectonic control of structure. (Modified from Bristol 1974.) the base of the New Albany Shale. The Tonti Field was discovered in 1939 and initially produced from middle Mississippian sandstone and were produced from a relatively Patoka Field small area of 280 acres in the 5 years carbonate reservoirs in the Benoist, The Patoka Field was discovered in immediately following the initial Aux Vases, Spar Mountain, and 1937 using structure mapping on the Geneva Dolomite discovery. The McClosky. The Geneva Dolomite Herrin (No. 6) Coal Member and was rapid increase in annual production reservoir was discovered in 1940 at confirmed by a seismic survey. The followed development of the Geneva a depth of 3,414 to 3,430 feet in the structure map of the Patoka Field il- Dolomite reservoir in the field (fig. Harvey No. 6 “B”, J. K. Kagy well, lustrates a pronounced closure of 80 18a). The cumulative production Sec. 33, T3N, R2E. This well flowed feet (fig. 19), most of which is likely curve (fig. 18b) in the Sandoval Field 4,200 barrels of oil per day from the from tectonic deformation. The also shows the significant increase Geneva Dolomite. Although Tonti four-mile-long and one-mile-wide in production following the discov- is a small field, with just 80 acres anticline at Patoka is a larger feature ery and development of the Geneva of proved reserves, it has produced than the reef drape structures found Dolomite reservoir. more than 13.5 million barrels of oil at Sandoval and Raccoon Lake. A from eight wells. Annual produc-

16 Illinois Petroleum 158 Illinois State Geological Survey Illinois State Geological Survey Illinois Petroleum 158 17 a Patoka Field Annual Production a Tonti Field Annual Production

�� 3,000,000

�� 2,500,000 ��

�� 2,000,000 �� 1,500,000 Geneva Dolomite Discovery: Jan. 1940 Barrels of Oil �� Barrels of Oil �� 1,000,000 ��

�� 500,000

�

�� 0 1939 1940 1941 1942 1943 1944 1945 1946 1947 1948 1949 1950 1951 1952 1953 1954 1955 1956 1957 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972

b Tonti Field Cumulative Production

b Patoka Field Cumulative Production ��

16,000,000 �� 14,000,000 �� 12,000,000 �� 10,000,000

�� 8,000,000 Barrels of Oil

Barrels of Oil 6,000,000 ��

4,000,000 �� 2,000,000 �� 0 8 0 � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 1936 1938 1940 1942 1944 1946 1948 1950 1952 1954 1956 195 1960 1962 1964 1966 1968 197 1972 ��

Figure 20 (a) Graph of annual production from the Patoka Figure 21 (a) Graph of annual production from the Tonti Field showing a large increase between 1943 and 1944 Field showing outstanding production after the discovery that coincides with discovery of the Geneva Dolomite and development of the Geneva Dolomite reservoir. Annual reservoir in the field. (b) Cumulative production graph in production increased from 900,000 barrels of oil in 1939 to Patoka Field shows a large increase in production starting 2,560,000 barrels of oil in 1940. (b) Cumulative production in 1943. The increase in cumulative production can be at- graph shows an increase in production from 900,000 bar- tributed to production from the Geneva Dolomite reservoir. rels of oil in 1939 to over 8 million barrels by 1947.

tion (fig. 21a) was outstanding after (Helpingstine et al. 2001). A Vibro- a time map to depth, (4) a top-of- the discovery and development of seis source was used in a design that Devonian structure map, and (5) a the Geneva Dolomite reservoir in included six north-south receiver time-slice map taken at 569 mil- the Tonti Field. Annual production lines and seven east-west shot lines liseconds (Helpingstine et al. 2001). increased from 900,000 barrels of spaced 110 feet apart. A sequence Based on these company maps, the oil in 1939 to 2,560,000 barrels of of four maps interpreted from the original interpretation of structural oil in 1940. Cumulative production 3-D seismic data was used to alter closure in the field was revised and increased from 900,000 barrels of oil the original interpretation of a used to define a new well location in 1939 to over 8 million barrels by single closure over a single Silurian that encountered the Geneva pay 1947 (fig. 21b). pinnacle reef. The data revealed zone 12 feet higher than the exist- two separate areas of closure over ing well scheduled for replacement. A 3-D seismic survey limited to 0.37 multiple Geneva high areas that Such detailed 3-D seismic has the square miles was completed at the mimic the undulating surface of the potential for redefining reef struc- Tonti Field to redefine the structural underlying Silurian pinnacle reef. tures in older fields and delineating interpretation of the Geneva Dolo- Maps used in the interpretation new reef structures in the Geneva mite reservoir in order to replace included (1) a two-way time map to Dolomite. a damaged production well and to the top of the Devonian, (2) a map The previously mentioned Marion identify potential new well loca- of the range and distribution of the County oil fields all experienced tions for incremental production average velocity to the Devonian, (3)

18 Illinois Petroleum 158 Illinois State Geological Survey Illinois State Geological Survey Illinois Petroleum 158 19 rapid increases in production when Geneva Dolomite reservoirs were �� discovered and developed. Graphs �� of annual production (figs. 17, 18, �� 20, and 21) from each of these fields �� show approximately six years of �� significantly increased annual �� production from relatively few wells. The increased production attributed to the development of the Geneva Dolomite reservoir substantially contributed to the total cumulative production in each field. In most of these fields, the Geneva Dolomite is the most prolific reservoir. Log, Core, and Rock Descriptions of Geneva Dolomite in Historical Fields �� Examination of core biscuits from the Geneva Dolomite in Sandoval, Raccoon Lake, and Patoka Fields indicate that, prior to dolomitization, �� the rock was originally composed of �� shallow marine, bioclastic pack- stones to grainstones. Correlations �� and similarities to quarries and outcrops in Indiana support this �� interpretation (Leonard 1996). Dis- �� solution of fossils and dolomitization of the surrounding matrix carbonate have enhanced porosity and permeability. The amount of moldic porosity appears to be dependent on the initial quantity of fossil material in the sediment and the extent of the dissolution of those fossils. Poros- �� ity and permeability are least where the quantity of coarse fossil material was initially small and dissolution of the fossils was incomplete. Al- though dolomitization has obscured the original fossil content of the �� Geneva sediments, the porous and �� moldic characteristics of the highly dolomitized Geneva sediments Figure 22 Old-style geophysical log from the Sandoval Field for the Kingwood strongly indicate that the visible and Oil Company No. 1 Konrad in Sec. 8, T2N, R1E, Marion County, Illinois. Forma- measured porosity and permeability tion depths, cored interval, and thin section depths are shown on log. This well are now greatest in sediments where was drilled in 1939 with an initial production of 600 barrels of oil per day. large amounts of fossil material have been completely dissolved and the original carbonate matrix is at a depth of 2,993 feet show numer- with alizarine red, is from the core completely dolomitized. ous branching corals surrounded by sample in figure 23a and illustrates a matrix of brown sucrosic dolomite intact branching corals composed The geophysical log in figure 22 is (fig. 23a). The branching corals are of red-stained primary calcite. The from the Kingwood Oil Company composed of calcite, remain intact, tan, unstained mineral in the thin No. 1 Konrad, in the Sandoval Field and show little evidence of dissolu- section is dolomite, and the white, in Sec. 8, T2N, R1E. Core samples of tion. The photomicrograph in figure silt-sized grains are quartz. Porosity the Geneva Dolomite from this well 23b, under white light and stained is shown as blue in the thin section.

18 Illinois Petroleum 158 Illinois State Geological Survey Illinois State Geological Survey Illinois Petroleum 158 19 Figure 23 (a) Core of Geneva Dolomite from the Sandoval Figure 24 (a) Core of Geneva Dolomite from 3,003 feet. Field at 2,993 feet. Branching corals are common. (b) Thin Few fossils remain. Numerous moldic pores have been section shows porous dolomite with shallow marine fossil created by the dissolution of fossils. (b) Thin section shows fragments composed of primary calcite stained red. Scale fine- to medium-crystalline, porous, sucrosic, dolomite. bar is 0.5 mm. Moldic pores are common; no primary calcite is seen. Scale bar is 0.5 mm.

The dolomitized matrix surround- a porous, fine- to medium-grained this well shows the New Albany ing the red-stained branching coral sucrosic dolomite. Porosity is high- Shale, the Geneva Dolomite, and a is porous. lighted by blue-stained epoxy. Mol- portion of the Silurian section. The dic pores created by dissolution of cored interval includes samples In contrast, the core sample of the fossils are common in this sample, from the Geneva Dolomite and the Geneva Dolomite from 10 feet below and calcite is missing, as indicated underlying Silurian rocks. Thin sec- the previous sample, at a depth of by the absence of alizarine red tions of the Geneva Dolomite from 3,003 feet, contains little to no intact staining. The porosity and perme- this core contain small amounts of fossil material (fig. 24a) and shows ability in this sample are greater undissolved coral fragments. Al- numerous moldic pores created by than those found in the sample from though the original fossil fragments dissolution of the original marine 2,993 feet. remain in the rock, the moldic po- fossils (fig. 24b). These two samples rosity created by dissolution of coral from the Sandoval Field show the A geophysical log (fig. 25) from the and other marine fossil fragments influence that the dissolution of ma- Eddie Self Dunbar No. 1 well in the has enhanced the overall porosity rine fossils has on the development Raccoon Lake Field, Sec. 2, T1N, of these samples. Fine- to medium- of porosity and permeability in the R1E, shows the character of the grained, zoned dolomite crystals Geneva Dolomite. A thin section of Geneva Dolomite reservoir. This with scattered sand grains are the this core sample (fig. 24b), photo- well initially flowed at the rate of 960 most common features observed in graphed under white light, shows barrels of oil per day. The log from

20 Illinois Petroleum 158 Illinois State Geological Survey Illinois State Geological Survey Illinois Petroleum 158 21 ��

��

�� CORE Figure 25 Geophysical log from the Raccoon Lake Field for the Eddie Self Dunbar No. 1 in Sec. 2, T1N, R1E, Mari- on County, Illinois. Formation depths and cored interval are shown on the log. ��

Figure 26 Geophysical log from the Patoka Field for the thin sections of samples from the Sohio Petroleum Company Howell Community–Patoka Raccoon Lake Field. No. 1-D well, Sec. 28, T4N, R1E, Marion County, Illinois. Evidence of fracturing following Formation depths and cored interval are shown. Perme- dolomitization is found in cores of ability is graphed on the left side of the log. the Geneva Dolomite in the region. Some fractures are open, but oth- ers are filled with late-stage sparry from 2,921 to 2,922 feet in the How- stained red by alizarine red dye, calcite cement. Late-stage fluorite ell Community–Patoka No. 1-D well filled the fracture after fragments of mineralization has also been ob- is composed of highly porous and the dolomite had collapsed into it, served along some fracture planes. permeable, brown, sucrosic Geneva indicating that dissolution occurred An example of this fracturing was Dolomite that displays a consider- prior to the calcite cementation and found in the Sohio Petroleum able amount of moldic porosity. Evi- showing that there were at least two Company Howell Community–Pa- dence of dissolution along a fracture stages of carbonate dissolution that toka No. 1-D well, Sec. 28, T4N, R1E, can be seen in the solution collapse enhanced porosity and permeabil- Patoka Field (fig. 26). Initial produc- breccia located along the fracture ity. The first stage involved dissolution from this Marion County well plane shown in figure 27. Fragments tion of fossils composed of calcite; was 240 barrels of oil per day and of dolomite have collapsed into a the second stage involved dissolu- 340 barrels of water per day from the fracture enlarged by dissolution tion of some dolomite along fracture Geneva Dolomite. and were then cemented within the planes. fracture by late-stage calcite. The Figure 26 shows the Mississippian late-stage calcite cement appears The Geneva Dolomite in the Howell Chouteau Limestone, Upper Devo- as a white mineral in both views of Community–Patoka No. 1-D well is nian New Albany Shale, and Middle the core sample (fig. 27, a and b). highly permeable. The permeability Devonian Grand Tower Limestone The thin-section photomicrograph data graphed on the left side of the and the cored interval between of the collapse breccia (fig. 27c) geophysical log (fig. 26) show per- 2,897 to 2,930 feet. The core sample shows that late-stage calcite cement, meabilities exceeding 600 millidarcies (md) at some intervals.

20 Illinois Petroleum 158 Illinois State Geological Survey Illinois State Geological Survey Illinois Petroleum 158 21 Figure 27 (left) (a) Brown sucrosic Geneva Dolomite with moldic porosity. Solution collapse breccia is located at arrow along fracture plane in sample from the Patoka Field. (b) Bottom view of same core sample. Solution collapse breccia located along the fracture plane in sample from the Patoka Field. Fragments of dolomite have collapsed into a solution-enlarged fracture and were cemented by a late-stage calcium carbonate (white mineral at arrow in core photo). (c) Photomicrograph of collapsed breccia. Late-stage calcite cement is stained red and surrounds fragments of collapsed dolomite. Scale bar is 0.5 mm.

Figure 28 (above) (a) Brown, sucrosic, sandy Geneva Dolomite. Large amounts of moldic porosity created by the dissolution of marine fossils. Vugs were created by the dissolution of larger fossils. Sample is from 2,911 feet. Poros- ity is 13.7%, and permeability is 624 md. (b) Thin section from core sample. Porosity is highlighted by blue. Rounded quartz sand grains with overgrowths, some floating in finely crystalline dolomite. Scale bar is 0.5 mm.

22 Illinois Petroleum 158 Illinois State Geological Survey Illinois State Geological Survey Illinois Petroleum 158 23 a �� 3410 �� 3412

�� 3414 3416 �� 3418 3420 3422

eet 3424

F 3426 3428 �� 3430 3432

�� 3434 3436 �� 3438 3440 �� 8 10 12 14 16 18 20 22 24 26 �� Percent

3410 3412 �� 3414 �� 3416 �� 3418 � �� 3420 3422 �� 3424 �� eet 3426 F 3428 � �� 3430 3432 �� 3434 �� 3436 �� 3438 �� 3440 �� 0 100 200 300 400 500 � �� Millidarcies �� Figure 30 Porosity (a) and permeability (b) data for the cored interval from the Plains, Illinois, Smail No. 25 in the Figure 29 Geophysical log from Plains, Illinois, Smail St. James Field. There is a highly porous and permeable No. 25 in the St. James Field. This is well No. 1 in cross reservoir zone in the upper portion of the Geneva Dolo- section C–C' (figs. 2 and 32). Cored interval is shown. Core mite. The cherty zone starting at 3,418 (core) feet is much footage is approximately 3 feet high to log footage. less permeable than the brown, sucrosic dolomite above it.

A Geneva Dolomite core sample Recent Geneva Geneva Dolomite Member at 3,434 from a depth of 2,911 feet contains feet (fig. 31). The upper contact moldic porosity created by the dis- Dolomite Discoveries of the Geneva Dolomite with the solution of marine fossils. Measured St. James Field overlying limestone of the Grand porosity in this core sample is Tower at 3,410 feet is undulatory and 13.7%, and permeability is 624 md. A The Geneva Dolomite reservoir abrupt (fig. 31). There is no transi- thin-section photomicrograph from in the St. James Field (fig. 2) was tion zone from the white, tightly ce- this sample (fig. 28b) shows a large discovered in 1998 with the drill- mented bioclastic grainstone above amount of porosity (highlighted by ing of the Plains Illinois Smail No. and the brown, highly porous and blue-stained epoxy) and numer- 25 well in Sec. 36, T6N, R2E, Fayette permeable Geneva Dolomite below. ous rounded quartz sand grains County. A geophysical log of the The widespread and abrupt nature with overgrowths, some floating in Smail No. 25 is shown in figure 29. of the upper contact suggests a pos- finely crystalline dolomite. Sandy The cored interval in this well from sible parasequence boundary across dolomite and very thin dolomitic 3,410 to 3,434 feet includes part the St. James Field and across the re- sandstones were observed in some of the dense bioclastic limestone gion. The lower portion of the cored Geneva cores. Sand grains can be of the Grand Tower overlying the interval contains a cherty, light- to scattered or concentrated into thin Geneva Dolomite Member. Graphs medium-brown, fine-grained beds in the Geneva Dolomite. Beds of core analysis measurements of dolomite containing algal lamina- of the Dutch Creek Sandstone com- porosity and permeability (fig. 30) tions and tidal rhythmites with low monly underlie and grade sharply show porosities as high as 24% and measured porosity and low permea- into the Geneva, although interfin- permeability as high as 400 md in bility. The cherty interval correlates gering of these sands may occur, the Geneva Dolomite reservoir. well with a similar cherty interval particularly along the southern The core from the Smail No. 25 well in a Lillyville Field core from the boundary of the Geneva fairway. starts in the dense limestone of Dart Energy Corporation Breer- the Middle Devonian Grand Tower Heuerman No. 3-1, Sec. 1, T8N, R6E, Limestone at 3,393 feet and extends Effingham County, Illinois. down into the upper portion of the

22 Illinois Petroleum 158 Illinois State Geological Survey Illinois State Geological Survey Illinois Petroleum 158 23 Figure 31 Core from the Plains, Illinois, Smail No. 25 in the St. James Field, Sec. 36, T6N, R2E, Fayette County. This is well No. 1 in cross section C–C'. The cored interval is from the Middle Devonian Grand Tower Limestone through a cherty zone in the Geneva Dolomite. The upper contact (at arrow) of the Geneva Dolomite with the overlying Grand Tower Lime- stone is abrupt. There is no transition zone from the bioclastic grainstone above and the brown Geneva Dolomite below. The contact between the Geneva Dolomite of the typical brown sucrosic dolomite with the underlying cherty interval also is abrupt (arrow).

�� NW �� SE

��

� �' ��

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Figure 32 Structural cross section C–C' (fig. 2) of the Geneva Dolomite reservoir at the St. James Field in southern Fay- ette County, Illinois. Included in the cross section is the Plains, Illinois, Smail No. 25 (well No. 1). Section shows a well-developed zone of highly porous dolomite (at arrows) immediately underlying dense, bioclastic grainstone in the Grand Tower Limestone. This upper contact of the Geneva Dolomite with the Grand Tower Limestone is abrupt across the field.

24 Illinois Petroleum 158 Illinois State Geological Survey Illinois State Geological Survey Illinois Petroleum 158 25 Figure 33 (a) Close-up of core from the Geneva Dolomite in the Plains, Illinois, Smail No. 25 in the St. James Field. The core is a brown, sucrosic, highly porous dolomite with large amounts of moldic porosity caused by dissolution of fossil fragments. (b) Thin section of this core from 3,411 feet with 200 md of permeability and 19% porosity is a fine- to medium-grained crystalline dolomite. The scale bar is 0.5 mm. Dolomite rhombs are zoned, indicating several episodes of recrystallization.

well. Characteristics of the Geneva Dolomite in this well are similar to those in the Geneva Dolomite reservoir in the Smail No. 25 in St. James Field. The Geneva in this core (fig. 36) shows an abrupt upper contact with the overlying Middle Devo- nian carbonates of the Grand Tower Limestone similar to that observed in the St. James Field. This abrupt Cross section C–C' (figs. 2 and 32) crystals were enlarged by the ad- contact, from the brown sucrosic through the Geneva Dolomite dition of dolomite on crystal edges dolomite typical of the Geneva reservoir at the St. James Field in (fig. 33b). The enlargement of dolo- to the overlying dense, light gray southern Fayette County, Illinois, mite crystals occurred during sev- grainstone, suggests the presence of shows a well-developed zone of eral episodes of dolomite recrystal- a parasequence boundary. This core highly porous Geneva Dolomite lization. Porosity (indicated in the also contains a cherty, light brown immediately underlying the dense, thin section by blue-stained epoxy) dolomite interval underlying the bioclastic grainstone in the Grand and permeability appear to increase typical highly porous, dark brown Tower Limestone. with dolomite recrystallization. Geneva dolomite that is similar to the cherty interval that under- Figure 33a is a photograph of a typi- lies the high-porosity zone in the cal core sample from the Geneva Lillyville Field Geneva in the St. James Field. The Dolomite reservoir from the Smail Another recently discovered Geneva low-porosity cherty interval can No. 25 from a depth of 3,411 feet. The Dolomite reservoir was found in the be correlated on geophysical logs sample is a brown, sucrosic, highly Lillyville Field by the Dart Energy between the St. James and Lillyville porous, and permeable dolomite Corporation, Breer-Heuerman fields. with large amounts of moldic poros- No. 3-1 well, in Sec. 1, T8N, R6E, ity caused by dissolution of fossil Effingham County, Illinois (fig. 34). The Lillyville and Lillyville North fragments. This sample has 200 md This well was a deeper test drilled Fields formed along a northeast- of permeability and 19% porosity in in 1989. Combined production from trending fault block that shows a fine- to medium-grained, crys- this well and the offset Breer No. 2-1 steeply dipping beds to the south- talline dolomite. The thin section exceeds 180,000 barrels of oil (fig. east into the Illinois Basin and from 3,411 feet shows zonation of 35). The cored interval is shown on more subtle dips to the northwest. A dolomite crystals, indicating several the geophysical log (fig. 34) of the seismic line over the Lillyville North episodes of precipitation in which Field shows a small reef with a pro-

24 Illinois Petroleum 158 Illinois State Geological Survey Illinois State Geological Survey Illinois Petroleum 158 25 BREER 2-1 - LILLYVILLE

�� 1,000,000 ��

100,000

�� Production Rates 10,000

Oil Production (bbl) �� 1,000 ��

��

�� 100 �� 1990 1992 1994 1996 1998 2000 2002 2004 �� Time

Figure 34 (left) Geophysical log from the Dart Energy Cor- poration Breer Heuerman No. 3-1 well in the Lillyville Field in Sec. 1, T8N, R6E, Effingham County, Illinois. Formation depths and cored interval are shown. �� Figure 35 (above) Combined graphs of the monthly pro- �� duction (solid line) and cumulative production (dashed line) � �� for the Dart Energy Corporation Breer-Heuerman No. 3-1 �� well and the offset Breer No. 2-1 well in Sec. 1, T8N, R6E, � �� Effingham County, Illinois, showing production from the �� Geneva Dolomite reservoir exceeded 180,000 barrels of oil. (Unpublished data from the IHS Energy Group.)

Figure 36 Core from the Dart Energy Corporation Breer-Heuerman No. 3-1 well (log in fig. 34) in the Lillyville Field, Ef- fingham County, in Sec. 1, T8N, R6E. This core shows the abrupt upper contact with Middle Devonian carbonates of the Grand Tower Limestone (at arrow). This core contains a cherty interval in the Geneva Dolomite similar to that observed in the Plains Illinois No. 25 Smail at the St. James Field. The Geneva Dolomite extends north of Marion and southern Fayette Counties. Comparison of this well with the Smail No. 25 in St. James Field shows similar reservoir characteristics.

26 Illinois Petroleum 158 Illinois State Geological Survey Illinois State Geological Survey Illinois Petroleum 158 27 South �� North

��

Figure 37 Seismic line over a Silurian reef at the Lillyville North Field in Effingham County, T9N, R7E. The �� structure over the reef is obvious �� on this seismic line. (Modified from Whitaker 1988.) ��

nounced structural rollover in the Silurian through Devonian section �� (fig. 37) that is easily observed and interpreted even on older seismic data (fig. 37) (Whitaker 1988). Most �� of the early seismic data acquired in Illinois are best suited for struc- �� tural interpretation and, to a lesser �� degree, stratigraphic interpretation. The seismic line in figure 37 shows a positive amplitude waveform �� anomaly that is shaded in light gray �� in the Silurian and is unique to the �� reef facies (Whitaker 1988). Figure 38 Combined graphs of the monthly production (solid line) and cumula- A large amount of vintage 2-D seis- tive production (dashed line) from the Geneva Dolomite reservoir in Ceja Corpo- mic data from the 1970s and 1980s ration wells in the Miletus Field showing a sharp increase in annual and cumula- for the Illinois Basin is available for tive production of over 2 million barrels of oil over a period of two and one-half purchase and reprocessing. These years. (Unpublished data from the IHS Energy Group.) data can be useful for interpreting the presence of structural features and possibly defining reef location. to be non-commercial and was (fig. 38) show a sharp increase in However, recent successful uses of abandoned. In late 1996, a second annual and cumulative production 3-D seismic to redefine structures attempt was made to drill an explor- of more than 2 million barrels of oil has shown that 2-D seismic data atory well to the Geneva Dolomite. over a period of 2.5 years. Monthly have limitations and that the newer The second attempt was successful and cumulative production graphs techniques may offer key informa- during fall 1996 when the Ceja Cor- from the Ceja Corporation Church tion for locating overlooked pros- poration Basset No. 3 was deepened No. 5, Hogan No. 2, Keller No. 2, and pects with high potential. into the Middle Devonian Geneva Basset No. 3 wells show very high Dolomite and completed an open monthly rates of production, some hole for 158 barrels of oil per day. exceeding 10,000 barrels of oil (fig. Miletus Field Ceja Corporation subsequently de- 39). Each of these wells produced The Geneva Dolomite reservoir veloped the Geneva reservoir in the more than 200,000 barrels of oil in the Miletus Field (figs. 1 and 2) Miletus by drilling additional wells. between late 1996 and June 2000. was first drilled in the early 1980s, The combined monthly production These data illustrate the high pro- but the initial well was determined and cumulative production graphs

26 Illinois Petroleum 158 Illinois State Geological Survey Illinois State Geological Survey Illinois Petroleum 158 27 ��

��

Figure 39 Combined graphs of the monthly production (solid line) and cumulative production (dashed line) from the Ceja Corporation wells in the Miletus Field: (a) Church No. 5, (b) Hogan No. 2, (c) Keller No. 2, and (d) Basset No. 3 wells show very high monthly rates of production, some exceeding 10,000 barrels of oil per month. Each of these wells produced over 200,000 barrels of oil between late 1996 and June 2000. These wells produce from a highly productive Geneva Dolomite reservoir that is adjacent to the new discovery underlying the Stephen A. Forbes State Park. (Unpublished data from IHS Energy Group.)

ductivity potential of good-quality the closure on the Geneva horizon (fig. 41) shows that the pronounced Geneva Dolomite reservoirs. show an arcuate geometry that pos- thinning of these carbonates coin- sibly reflects an underlying atoll-like cides with the crest of the structure A structure map on the top of the Silurian reef. Geneva production has shown on the structure map of the Geneva Dolomite porosity zone at been established in Sec. 27, 28, and Geneva Dolomite (fig. 40). The thin- the Miletus Field is shown in figure 33, T4N, R4E. The most productive ning of the section over the struc- 40. The field lies on an anticlinal wells correlate with the maximum ture could be the result of compen- feature with 60 feet of closure on the closures on the top of the Geneva sating deposition over a paleohigh top of the Geneva. The structure has (fig. 40). that was induced tectonically, or a steep east flank and becomes more by an underlying Silurian reef, or subtle in the shallower horizons, in- An isopach map of the Middle De- by a combination of the two. The dicating recurrent movement of the vonian carbonates overlying the Ge- stratigraphic section over Silurian structure through time. Details of neva Dolomite at the Miletus Field pinnacle reefs commonly is thin-

28 Illinois Petroleum 158 Illinois State Geological Survey Illinois State Geological Survey Illinois Petroleum 158 29 ner than the section adjacent to the �� reefs in the Illinois Basin.

The density-neutron log of the �� Hogan No. 2 well in Sec. 28 is shown

in figure 42. The Middle Devonian carbonates start at a depth of 3,712 �� feet and are 65 feet thick in this well.

�� The top of the Geneva Dolomite �� starts at 3,777 feet. For many wells, �� crossplot density-neutron porosity of the Geneva interval averages approximately 15%; porosity spikes ��

exceed 20%. �� Discovery at Stephen A. ��

Forbes State Park � � �� A new field discovery was made with �� the drilling of the Deep Rock Energy Corporation Warren No. 1-SDU, followed by a development well, the Carter No. 1-SDU. Both wells have a Figure 40 Structure map on the top of the Geneva Dolomite porosity at the horizontal leg that was drilled under Miletus Field. The field lies on a pronounced nose with closure of approximately the Stephen A. Forbes State Park in 60 feet. Marion County, Illinois (fig. 1). The Warren No. 1-SDU—completed as 4E a flowing well in March 2002—is one of the most prolific producers drilled in Illinois in the last 50 years. Production is reported at up to 3,000 70 barrels a day while choked down 75 (Shirley 2002). A significant quantity of gas also is being produced from 70 this well. The surface location for this well is 129’ FSL 186’ FWL SW NW, Sec. 33, T4N, R4E (fig. 1). Recent well locations and status of wells 4N in the vicinity of this discovery are shown in figure 1. This new field discovery was defined using 3-D 60 60 seismic technology. A third horizontal well location has been staked by 65 85 80 Deep Rock Energy Corporation as of 70 July 2002. 75 80

Petroleum Entrapment in the Geneva Dolomite Figure 41 Thickness map of the Middle Devonian carbonates overlying the Geneva Dolomite at the Miletus Field. A pronounced thinning of these carbon- Figure 43 shows the structural ates coincides with the crest of the structural nose shown on the structure map grain at the base of the New Albany in figure 40. Shale, the Marion County boundary, and the southern boundary of the Geneva Dolomite. Also shown in 60 feet. Thinning of these overly- Shale. Therefore, the trap at these shaded gray is the region where the ing carbonates is apparent in all of fields shows an inverse relationship Middle Devonian carbonates overly- the described fields, and in some between the amount of structural ing the Geneva Dolomite thin to less wells there is less than 30 feet of closure and the amount of thinning than 60 feet. The best production carbonate. of the carbonates between the Ge- from the Geneva Dolomite generally neva reservoir rock and the New Al- occurs where the overlying Devo- In these fields, the ultimate seal bany Shale seal (fig. 44). Petroleum nian carbonates are thinner than appears to be the New Albany entrapment occurs when structural

28 Illinois Petroleum 158 Illinois State Geological Survey Illinois State Geological Survey Illinois Petroleum 158 29 closure on the impermeable New �� Albany Shale encompasses the Geneva Dolomite (fig. 44); therefore, greater structural closure requires less carbonate thinning, and subtle structures require a greater amount of carbonate thinning. Thinning of these carbonates overlying the Geneva can occur both within the Middle Devonian and by truncation of the carbonate section along the unconformable contact with the overlying Upper Devonian New �� Albany Shale. Much of the thinning is attributed to recurrent movement �� on deep-seated structures and the probable differential compaction associated with underlying Silurian reefs. Thinning of sediments overlying structural elements may also result from compensating deposition associated with either recurrent structural movement or differential compaction. The geographic distribution of the Geneva Dolomite coincides with a regional thinning of the New Albany �� Shale, a relationship that is shown in figure 45. This coincidence may �� suggest that accumulation of Geneva carbonates was compensated for during Late Devonian deposition of the New Albany Shale. It is also possible that the region was high during deposition of the New Albany (Workman and Gillette 1956) but low during accumulation of the original �� Geneva Dolomite carbonates. �� Events leading to the accumulation of the carbonates in the Middle De- �� vonian Geneva Dolomite are shown in figure 12. Silurian reefs likely grew in areas that were structurally favor- able to reef growth. At least some of these faulted structures were likely reactivated through time creating structural highs prior to accumulation of Middle Devonian carbonates. A long period followed when Silurian

Figure 42 Geophysical log of the Ceja Corporation Hogan No. 2 well in the Miletus Field in Sec. 28, T4N, R4E. This lease has produced over 250,000 barrels of oil since its discovery in 1996.

30 Illinois Petroleum 158 Illinois State Geological Survey Illinois State Geological Survey Illinois Petroleum 158 31 through Lower Devonian strata were eroded, forming the sub-Kaskaskia unconformity (Devera and Hansen- T6N mueller 1991). Middle Devonian car- ST JAMES bonates accumulated on the eroded surface during a major transgression of the sea, but Silurian reefs and major structures remained as high spots -2500

T5N -3250 -3000 where the Middle Devonian carbon- -2750 ates were thinner.

Dolomitization The dolomite of the Geneva and overlying units formed from replacement of carbonate sedi- T4N MILETUS ments, as indicated by the common PATOKA presence of dolomitized bioclastic allochems in the Geneva and of -3500 dolomitized algal structures and 60 FT CONTOUR scattered dolomitized fossil allo- THIN TES chems in the overlying units. The T3N A DOLOMITE Geneva Dolomite occurs at the base REGION OF of the northern dolomite facies of ONIAN CARBONA the Grand Tower and Jeffersonville DEV YING GENEVBRUBAKER Limestones in central Illinois and SANDOVAL OVERL west-central Indiana. The northern dolomite facies of the Grand Tower and Jeffersonville also includes a T2N microcrystalline dolomite facies -2750 that overlies the Geneva. This mi- -3000 A DOLOMITE crocrystalline dolomite was named the Vernon Fork Member in Indiana RACCOON (Droste and Shaver 1975). The close LAKE EDGE OF GENEV resemblance between the distribu- T1N tion of the Geneva Dolomite and -3750

that of the Vernon Fork dolomite -2500 of the Jeffersonville Limestone in -4000 Indiana and its equivalent in Illinois suggests a genetic relationship (Per- kins 1963). Therefore, any mecha- Figure 43 Map showing the structural grain at the base of the New Albany Shale nism that dolomitized the Geneva in Marion County. Shaded areas in the region show where the Middle Devo- sediment must also explain dolomi- nian carbonates overlying the Geneva Dolomite thin to less than 60 feet. Better tization of the overlying unit. production from the Geneva Dolomite occurs where the overlying carbonates are thinner than 60 feet. These carbonates thin over all known Geneva Dolomite Droste and Shaver (1975) suggest reservoirs in the region. that dolomitization occurred in a supratidal sabkha (evaporitic) environment within a highly saline, Mg-rich pore water system. precipitation of calcium carbonates sediments, a conclusion supported Although tidal flat conditions and calcium sulfates. The amount by scanning electron microscopy prevailed during deposition of the of dolomite formed in such environ- (Lasemi et al. 1989). ments is, however, very small and Vernon Fork dolomite, the sabkha Mixing zone dolomitization, dolomitization model as envisioned is restricted only to the upper 3 to 5 feet of sediments. In addition, although extensively suggested as by Droste and Shaver (1975) cannot the mechanism for dolomitization adequately explain the formation any dolomite formed from a highly saline brine generated within the of many carbonates in the 1970s of over 150 feet of combined dolo- and 1980s, has now been largely mite of the Geneva and the overly- sabkha environments appears to be a direct precipitate (Machel and discounted (Machel and Mount- ing unit. In sabkha environments, joy 1986, Hardie 1987). The very the Mg/Ca ratio of the pore fluid Mountjoy 1986, Hardie 1987) rather than a replacement of pre-existing slow rate of dolomite precipitation increases through evaporation and (because of the ordered nature of

30 Illinois Petroleum 158 Illinois State Geological Survey Illinois State Geological Survey Illinois Petroleum 158 31 Figure 44 Cross sections of vertical a b seals, thickness of Middle Devonian New Albany Shale New Albany Shale carbonates, and their relationship Closure to hydrocarbon entrapment in the Closure Geneva Dolomite. (a) Hydrocarbon entrapment occurs when structural Reef Geneva Reef Geneva closure on the impermeable New core core Albany Shale draped over a reef encompasses the Geneva Dolomite. (b) Structural closure on the New Albany Shale does not encompass Hydrocarbon entrapment No hydrocarbon entrapment the Geneva Dolomite. (Modified from in Geneva Dolomite in Geneva Dolomite Whitaker 1988.)

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Figure 45 Geographic distribution of Geneva Dolomite coinciding with thin New Albany Shale. This coincidence may suggest that accumulation of Geneva carbonates was compensated for during late Devonian deposition of the New Albany Shale. This also may suggest that the region was high during deposition of the New Albany but low during the accumulation of the original Geneva Dolomite carbonates. (Modified from Cluff et al. 1981, Perkins 1963.)

dolomite crystals) relative to calcite ronment, the amounts of dolomite times, and, as a result, mixing-zone dissolution would mean that do- formed in mixing zones is small and dolomitization may occur only be- lomite could not precipitate in sig- texturally different from the ancient low major unconformities. However, nificant quantities in a mixing zone massive, replacement dolostones. many shallowing-upward cycles (e.g., Machel and Mountjoy 1986, At low temperatures, replacement and even major unconformities with Hardie 1987). As in the sabkha envi- dolomite requires long reaction

32 Illinois Petroleum 158 Illinois State Geological Survey Illinois State Geological Survey Illinois Petroleum 158 33 exposure surfaces lack dolomite square miles to depths of 1,000 feet Conclusions (Machel and Mountjoy 1986). or more (Simms 1984), assuming no aquicludes (such as evaporites or Recent prolific oil discoveries in the Another dolomitization model clay beds) prevent downward flow. Geneva Dolomite Member of the commonly applied to dolomite se- If long-lived, such a flow system Grand Tower Limestone have again quences associated with evaporites can produce massive replacement focused attention on this Middle is the seepage-reflux model (Adams dolomite that cuts across formation Devonian rock unit in the Illinois and Rhodes 1960). Here, extensive boundaries (Hardie 1987). Basin. The Miletus Field in Marion evaporation results in a hypersa- County, Illinois, and the March line brine with a high Mg/Ca ratio Hydrographic restriction occurred 2002 discovery of a new Geneva through precipitation of gypsum. in shallow lagoonal and tidal flat pool one mile to the south are at the The dense and hot, highly alkaline settings that developed over the center of this attention. Consider- and magnesium-rich brine is ca- Vandalia Arch. According to Work- ing that initial production from the pable of percolating through porous man and Gillette (1956), the Vanda- recently completed horizontal well and permeable underlying carbon- lia Arch was a depositional high dur- is approximately 3,000 barrels per ate sediments resulting in extensive ing deposition of the Grand Tower day, that the daily production from dolomitization. Unfortunately for and Jeffersonville Limestones that individual Miletus Field vertical this mechanism, there is no evapo- trended northeast-southwest from wells exceeds 300 barrels per day, rite deposit associated with the the Clinton County area in south- and that there is a concentration of Geneva Dolomite and the overlying western Illinois to the Indiana state nearby prolific Geneva fields, the Vernon Fork Member or its equiva- line in Edgar and Clark Counties. Geneva Dolomite has been estab- lent in Illinois. A few thin brecci- Distribution of the Geneva and the lished as having excellent reservoir ated laminae and minor calcite overlying units of the Grand Tower qualities in Marion County, Illinois. pseudomorphs after gypsum in the and Jeffersonville indicates that the Although this summary has focused dolomite above the Geneva (Perkins arch extended into west-central and on the Marion County, Illinois, area, 1963) suggest the minor deposition central Indiana. The New Albany the Geneva Dolomite’s characteris- of evaporites, but not enough to Shale thins over the position of this tics and the exploration and devel- explain the formation of thick, wide- arch. Because of a lack of evidence opment techniques used at Miletus spread dolomite. Furthermore, as in indicating tectonic influence, and the other fields are universal the sabkha model, a highly saturat- Cluff et al. (1981) rejected the term within the Geneva fairway in Illinois ed, hypersaline brine created during “Vandalia Arch” and informally and Indiana. seepage-reflux would most likely referred to the area of thin New have led to precipitation of dolomite Albany as the “central thin” (Nelson The Geneva Dolomite Member is rather than the replacement of pre- 1995). However, we find that the recognized as a highly dolomitized existing carbonate sediments. dolomitization of the Grand Tower facies of the Grand Tower Limestone Formation argues for the use of the in Illinois and the Jeffersonville A variation of the reflux model as term Vandalia Arch. Evaporation Limestone in Indiana. A fossilifer- suggested by Simms (1984) is now of seawater in restricted lagoonal ous, open marine, partly biostromal favored by many as an effective and tidal flat settings on the arch to biohermal facies containing mechanism for dolomitization in could have formed denser fluids that abundant corals and stromatopo- modern and ancient carbonate percolated through the underly- rids in the Grand Tower and the platforms that experienced hydro- ing carbonate sediment. The reflux Jeffersonville correlates with and is graphic restriction. In this model, began toward the end of the Geneva comparable with the brown, vuggy, during a long residence time, the deposition and continued through porous, and permeable zone that seawater trapped on the platform deposition of the overlying Vernon characterizes the best reservoir in- top fluids becomes progressively Fork Member of the Jeffersonville in terval of the Geneva Dolomite. The concentrated by evaporation (even Indiana and its equivalent unnamed shape, size, and distribution of fos- in humid climates) to greater than unit of the Grand Tower in Illinois. sils in the Grand Tower and Jeffer- normal salinities. The slight differ- The lack of evaporites indicates that sonville match the shape, size, and ence in the density of the water on the environment was not hyper- distribution of the moldic or vuggy the shallow platform and that of the saline. Deposition of over 100 feet porosity of the Geneva Dolomite pore water generates a long-term, of carbonates with features that Member, although dolomitization vertical fluid flow system that is indicate deposition under restricted and dissolution have partially to capable of causing large-scale do- marine conditions in the Grand totally obscured the original fossil lomitization of platform sediments. Tower and Jeffersonville over the content in the Geneva. Hypersaline conditions are not Geneva Dolomite suggests that such required, and the lack of evaporites A variation of the reflux model a flow system could have persisted as suggested by Simms (1984) is does not preclude dolomitization for a relatively long time. through reflux. This flow system proposed as the mechanism for is potentially large scale, affecting dolomitization of the Geneva Dolo- rocks over a region of thousands of mite and the overlying units of the Grand Tower and the Jeffersonville

32 Illinois Petroleum 158 Illinois State Geological Survey Illinois State Geological Survey Illinois Petroleum 158 33 in Illinois and Indiana. The model • regionally widespread and cor- a tour of the facilities adjoining the employs an increase in the salinity relative, highly porous, and per- Stephen A. Forbes State Park. of seawater over time by evapora- meable reservoir rock; tion in a shallow restricted sea. An The authors are grateful for the increase in the density of the water • relatively shallow reservoirs with comments and suggestions pro- in the shallow lagoons with respect drilling depths of less than 4,200 vided by Robert Finley, Jonathan to that of pore water generated a feet; Goodwin, Dennis Kolata, and long-term, vertical fluid flow system Rodney Norby, who reviewed the • active water drive combined with manuscript. Indiana quarry owners that was capable of causing large- gas solution drive; scale regional dolomitization of the including Meshberger Stone Com- underlying sediments, including the • common high-volume comple- pany, Berry Materials Company, Geneva. tions and cumulative per well Scott County Stone Company, and production greater than 100,000 Hanson Aggregates are acknowl- The Geneva Dolomite occurs in the barrels; edged for being accomodating and subsurface in an arcuate belt or fair- helpful with this research. way, which curves northwest from • high oil gravity (40 API and the outcrop in southeastern Indi- higher); H2S is a minor drawback; ana, through west-central Indiana, and References bending to the southwest through Adams, J.E., and M.L. Rhodes, 1960, • seismically definable, closed Clark and Edgar Counties in Illinois Dolomitization by seepage reflux- structures and reef anomalies. extending as far west as Montgom- ion: Tulsa, Oklahoma, American ery, Bond, and Clinton Counties, Il- Seismic surveys have been used Association of Petroleum Geolo- linois. The Geneva is generally up to successfully in the Illinois Basin to gists, Bulletin 44, p. 1912–1921. 50 feet thick in central Illinois and is locate Geneva Dolomite oil pools. Barrows, M.H., and R.M. Cluff, 1984, locally thicker. Cuttings from some Use of established 3-D seismic New Albany Shale Group (Devo- wells show up to 90 feet of Geneva technology should significantly nian-Mississippian) source rocks Dolomite in some areas in Illinois. improve the ability to locate new and hydrocarbon generation in The Geneva typically has a distinc- pools, and the drilling success rate the Illinois Basin, in G. Demaison tive brown color caused by dis- should increase. The capability of and R.J. Murris, eds., Petroleum seminated organic material. When the 3-D tool to accurately pinpoint geochemistry and basin evalua- the dolomite is dissolved in dilute the location of subtle reef structures tion: Tulsa, Oklahoma, American hydochloric acid, the brown organic has been established at the Tonti Association of Petroleum Geolo- residue floats on the surface of the Field. The Miletus Field Geneva gists, Memoir 35, p.111–138. acid, a characteristic that separates reservoir and the new field Geneva it from other Devonian carbonates discovery immediately to the south Becker, L.E., 1974, Silurian and De- (Schwalb 1955). were precisely delineated using 3-D vonian rocks in Indiana southwest Oil is trapped in the Geneva on technology. Forefront horizontal of the Cincinnati Arch: Blooming- pronounced, closed structures. drilling techniques utilizing 3-D ton, Indiana, Indiana Geological Much of this structure is related to data to pinpoint the location of the Survey, Bulletin 50, 81 p. tectonics, but, in many cases, the drill bit while drilling was com- Bristol, H.M., 1974, Silurian pinnacle closure also seems to be caused or bined with underbalanced drilling reefs and related oil production enhanced by the draping of younger technology to complete the well in southern Illinois: Illinois State Middle Devonian strata over Silu- successfully in the new discovery Geological Survey, Illinois Petro- rian reefs. Some of the most prolific reservoir. These examples show the leum 102, 98 p. Geneva wells are associated with successful application of newer, yet readily available, technologies to underlying Silurian reefs. Some Buschbach, T.C., and D.R. Kolata, re-examine old fields and trends to uplifted fault blocks in the Illinois 1991, Regional setting of the Il- discover and develop economically Basin have coincidental reef struc- linois Basin in M.W. Leighton, D.R. significant quantities of oil in the tures, suggesting that the Silurian Kolata, D.F. Oltz, and J.J. Eidel, mature Illinois Basin. reefs may have developed prefer- eds., Interior cratonic basins: entially on paleostructures that Tulsa, Oklahoma, American As- projected above their surroundings. Acknowledgments sociation of Petroleum Geologists, Memoir 51. The following is a list of the char- The authors are grateful to Plains acteristics that make the Geneva Illinois Inc. for donating core from Cazee, J.T., 2000, Petroleum explo- Dolomite a particularly attractive the Smail No. 25 well in the St. ration, development, and pro- target for further exploration and James Field and to Dart Energy duction in Indiana during 1999: development: Corporation for donating core from Bloomington, Indiana, Indiana the Breer-Heuerman No. 3-1 in the Geological Survey, Mineral Eco- Lillyville Field. We also thank Phil nomics Series 46, 45 p. Caserotti and Deep Rock Energy for

34 Illinois Petroleum 158 Illinois State Geological Survey Illinois State Geological Survey Illinois Petroleum 158 35 Cluff, R.M., M.L. Reinbold, and Droste, J.B., R.H. Shaver, and J.D. Meents, W.F., and D.H. Swann, 1965, J.A. Lineback, 1981, The New Lazor, 1975, Middle Devonian Grand Tower Limestone (Devo- Albany Shale Group of Illinois: paleogeography of the Wabash nian) of southern Illinois: Illinois Illinois State Geological Survey, platform, Indiana, Illinois, and State Geological Survey, Circular Circular 518, 83 p. Ohio: Geology, v. 3 p. 269–272 . 389, 34 p. Collinson, C., 1967, Devonian of Hardie, L.A., 1987, Dolomitization: Nelson, W.J., 1995, Structural the north-central region, United A critical view of some current features in Illinois: Illinois State States, in International Sympo- views: Journal of Sedimentary Geological Survey, Bulletin 100, sium on the Devonian System, Petrology, v. 57, p. 166–183. 144 p. Calgary, Alberta, Canada, 1967, v. 1, p. 933–971. (Illinois State Geo- Helpingstine, D., D. Williams, P. Norby, R.D., 1991, Biostratigraphic logical Survey, Reprint 1968-G.) Caserotte, and S. Gustison, 2001, zones in the Illinois Basin, in Small-scale 3-D seismic shoot M.W. Leighton, D.R. Kolata, D.F. Davis, H.G., 1991, Pre-Mississip- adds new oil reserves: World Oil, Oltz, and J.J. Eidel, eds., Interior pian hydrocarbon potential of the September 2001, p. 94, 95. cratonic basins: Tulsa, Okla- Illinois Basin, in M.W. Leighton, homa, American Association of D.R. Kolata, D.F. Oltz, and J.J. Ei- Lasemi, Z., 2001, Devonian and Mis- Petroleum Geologists, Memoir 51, del, eds., Interior cratonic basins: sissippian rocks: Stratigraphy and p.179–194. Tulsa, Oklahoma, American As- depositional history, in Z. Lasemi sociation of Petroleum Geologists, and R.C. Berg eds., Three-di- North, W.G., 1965, Lower Devonian Memoir 51. mensional geologic mapping—A stratigraphy of Illinois interpreted pilot program for resource and from well log data: Urbana-Cham- Devera, J.A., and G.H. Fraunfel- environmental assessment in the paign, University of Illinois, M.S. ter, 1988, Middle Devonian Villa Grove Quadrangle, Douglas thesis, 50 p. paleogeography and tectonic County, Illinois: Illinois State relationships east of the Ozark Geological Survey, Bulletin 106, p. North, W.G., 1969, The Middle De- Dome, southeastern Missouri, 35–40. vonian strata of southern Illinois: southwestern Illinois and parts Illinois State Geological Survey, of southwestern Indiana and Lasemi, Z., M.R. Boardman, and P.A. Circular 441, 47 p. Sandberg, 1989, Cement origin western Kentucky, in N.J. McMil- Patton, J.B., and T.A. Dawson, 1955, lan, A.F. Embry, and D.J. Glass, of supratidal dolomite, Andros Island, Bahamas: Journal of Stratigraphy, in H.H. Murray, ed., eds., Devonian of the world, vol. Sedimentation and stratigraphy of II: Canadian Society of Petroleum Sedimentary Petrology, v. 89, p. 249–257. the Devonian rocks of southeast- Geologists, Memoir 14, p. 181–196. ern Indiana: Bloomington, Indi- Devera, J.A., and N.R. Hasenmueller, Leonard, K.W., 1996, Sequence stra- ana, Indiana Geological Survey 1991, Kaskaskia Sequence, Middle tigraphy of the lower part of the Guidebook 8, p. 37–43. and Upper Devonian Series Muscatatuck Group (Middle De- vonian) in southeastern Indiana, Perkins, R.D., 1963, Petrology of the through Mississippian Kinder- Jeffersonville Limestone (Middle hookian Series, in M.W. Leighton, in B.J. Witzke, G.A. Ludvigson, and J. Day, eds., Paleozoic Sequence Devonian) of southeastern D.R. Kolata, D.F. Oltz, and J.J. Ei- Indiana: Bloomington, Indiana, del, eds., Interior cratonic basins: Stratigraphy—Views from the North American Craton: Boulder, Geological Society of America, Tulsa, Oklahoma, American As- Bulletin 74, p. 1335–1354. sociation of Petroleum Geologists, Colorado, Geological Society of Memoir 51, p.113–123. America, Special Paper 306, p. Schwalb, H.R., 1955, Geneva (Middle 243–257. Devonian) dolomite in Illinois: Droste, J.B., and R.H. Shaver, 1975, Machel, H.G., and E.W. Mountjoy, Illinois State Geological Survey, Jeffersonville Limestone (Middle Circular 204, 7 p. Devonian) of Indiana: Stratigra- 1986, Chemistry and environ- phy, sedimentation, and relation ments of dolomitization —A Shirley, K., 2002, Find draws Illinois to Silurian reef-bearing rocks: reappraisal: Earth Science Review Basin attention: AAPG Explorer, Tulsa, Oklahoma, American As- 23, p. 175–222. July 2002, p. 10, 12. sociation of Petroleum Geologists, McBride, J.H. and D.R. Kolata, 2000, Bulletin 59, p. 393–412. Simms, M., 1984, Dolomitization A “new” Precambrian geologic by groundwater-flow systems in Droste, J.B., and R.H. Shaver, 1987, province beneath the Illinois carbonate platforms: Gulf Coast Upper Silurian and Lower Devo- Basin, USA, Geological Society of Association of Geological Societ- nian stratigraphy of the central America, North-Central Section, ies Transactions, v. 34, p. 411–420. Illinois Basin: Bloomington, Indi- 35th Annual Meeting, 33 (4), p. 10. ana, Indiana Geological Survey, Special Report 39, 29 p.

34 Illinois Petroleum 158 Illinois State Geological Survey Illinois State Geological Survey Illinois Petroleum 158 35 Warthin, A.S., Jr., and G.A. Cooper, Whitaker, S.T., 1988, Silurian pin- Workman, LE., and T. Gillette, 1956, 1944, Middle Devonian subsur- nacle reef distribution in Illinois: Subsurface stratigraphy of the face formations in Illinois: Tulsa, Model for hydrocarbon explora- Kinderhook Series in Illinois: Oklahoma, American Association tion: Illinois State Geological Sur- Illinois State Geological Survey, of Petroleum Geologists, Bulletin vey, Illinois Petroleum 130, 32 p. Report of Investigations 189, 46 p. 28, p. 1519–1527.

36 Illinois Petroleum 158 Illinois State Geological Survey