Sedimentology and Reservoir Characterization of the Silurian Carbonates in the Mt

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Sedimentology and Reservoir Characterization of the Silurian Carbonates in the Mt. Auburn Trend of the Sangamon Arch, West-Central Illinois

Yaghoob Lasemi, Beverly Seyler, Zakaria Lasemi, and Zohreh Askari Khorasgani

MI IA

SANGAMON ARCH Mt. Auburn Trend CR

ILLINOIS BASIN THB IN CR MO SI THB

VINCENNES BASIN

RR RCG CR MO KY

0 30 mi N 0 48 km

CR d 0.5 mm

Circular 577 2010

Institute of Natural Resource Sustainability ILLINOIS STATE GEOLOGICAL SURVEY Front Cover: (a) Location of the Illinois Basin and the Sangamon Arch in west-central Illinois. (b) Photo- micrograph of porous dolomitized grainstone facies under polarized light showing an irregular dolomitization front (arrows) on crinoid fragments (CR), dolomite rhombs, and void spaces (black) formed as a result of dolomitization. (c) Generalized depositional model depicting the distally steepened ramp of the Illinois Basin during Niagaran time.

© 2010 University of Illinois Board of Trustees. All rights reserved. For permissions information, contact the Illinois State Geological Survey. Sedimentology and Reservoir Characterization of the Silurian Carbonates in the Mt. Auburn Trend of the Sangamon Arch, West-Central Illinois

Yaghoob Lasemi, Beverly Seyler, Zakaria Lasemi, and Zohreh Askari Khorasgani

Circular 577 2010

Institute of Natural Resource Sustainability William W. Shilts, Executive Director ILLINOIS STATE GEOLOGICAL SURVEY E. Donald McKay III, Director 615 East Peabody Drive Champaign, Illinois 61820-6964 217-333-4747 www.isgs.illinois.edu

CONTENTS

Abstract 1 Introduction 1 Geologic Setting 1 Stratigraphic Setting 4 Stratigraphy of the Racine Formation 4 Unconformable Boundaries and Lateral Distribution 4 Cyclostratigraphy 7 Petroleum Reservoirs and Seals 9 Non-reef Reservoir Units A, B, and C 13 Reservoir Unit A 14 Reservoir Unit B 21 Reservoir Unit C 21 Patch Reef Reservoir Unit D 27 Discussion 27 Depositional Environment and Carbonate Platform 27 Patch Reef Versus Pinnacle Reef 30 Dolomitization and Reservoir Development 31 Conclusions 31 Acknowledgments 31 References 31 Figures 1 Location of the Illinois Basin and the Sangamon Arch in west-central Illinois 2 2 Map of the Mt. Auburn trend of the Sangamon Arch 3 3 General stratigraphic column, sequences, and megagroups of the Illinois Basin 5 4 General stratigraphic column of the Upper Ordovician to lowermost Mississippian strata in the Sangamon Arch area 6 5 Subsurface gamma-ray and electric log stratigraphic reference section for the Mt. Auburn trend 7 6 Map of the Mt. Auburn trend area showing the lines of stratigraphic cross sections A–A through H–H 8 7 Northwest-southeast stratigraphic cross section A–A across the Mt. Auburn trend showing the Racine Formation sequences and the lateral variations in their thickness and lithology as a result of post-depositional erosion 9 8 Stratigraphic cross section B–B across the Mt. Auburn trend showing the lateral distribution of the Racine Formation 10 9 Stratigraphic cross section C–C along the Mt. Auburn trend showing rather uniform thickness and lithology for the Racine sequences 11 10 Cross section D–D across the Mt. Auburn trend showing erosional truncation and uneven topography in the upper boundary of the lower and upper Racine sequences 12 11 Thickness map of the New Albany Shale Group from the top of the Silurian to the base of the Chouteau Limestone 13 12 Structure contour map of the base of the New Albany Shale Group showing the direction of the regional dip toward the southeast 14 13 Digitized log and core photographs of the reservoir interval (reservoir unit B/C) in the Bernard Podolsky McMillen 4-B well in the Mt. Auburn Consolidated Field 15 14 Digitized log of the Pawnee Oil and Gas Inc. Garver No. 1 well in the Harristown Field 16 15 Digitized log of the Pawnee Oil and Gas Inc. Rothwell No. 1 well in the Blackland North Field 17 16 (a) Core photograph of a bioturbated lime mudstone to wackestone facies in the Pawnee Oil Corp. Elder No. 1 well that caps a dolomitized patch reef reservoir. (b) Photomicrograph of a coarsening-upward lime mudstone to bryozoan crinoid wackestone facies from an impermeable capping limestone. (c) Polished core slab photograph of a porous dolomitized grainstone facies. (d) Photomicrograph of a part of c under polarized light showing an irregular dolomitization front on crinoid fragments, dolomite rhombs, and void spaces 18 17 Photomicrographs of (a) a porous dolomitized non-reef reservoir under plane light in which the original texture has been destroyed by pervasive dolomitization; (b) a non-reef wackestone reservoir showing partially preserved crinoids and molds of bivalves and other unrecognizable grains in a finely crystalline dolomite groundmass; (c, d) dolomitized reef facies composed mainly of coral skeletons; (e) a thin section of d showing vuggy porosity as a result of dissolution of the skeletons; (f) an enlarged portion of e 19 18 Thickness map of the dolomite reservoir unit A in the Blackland North Field showing a very limited lateral extent of the reservoir due to lateral facies changes and post-Silurian erosion 20 19 Thickness map of the dolomite reservoir unit B in the Blackland North Field showing the lenticular nature of the reservoir 21 20 Thickness map showing the lateral distribution of the dolomite reservoir unit C along the Mt. Auburn trend 22 21 Northwest-southeast cross section E–E across the Mt. Auburn trend in the Harristown Field 23 22 West-east cross section F–F across the Mt. Auburn trend showing the development of reservoir units A, B, and C 24 23 Northwest-southeast cross section G–G across the Mt. Auburn trend in the Mt. Auburn Consolidated Field showing the development of reservoir units A and C 25 24 Cross section H–H parallel to the Mt. Auburn trend showing the development of reef and non-reef reservoirs in the upper part of the sequences of the Racine Formation 26 25 Cross-section H–H showing the occurrence of patch reef reservoirs within the upper part of the lower sequence of the Racine Formation 28 26 Enlarged portion of part of the regional structural contour map shown in Figure 12 29 27 Generalized depositional model depicting the distally steepened ramp of the Illinois Basin during Niagaran time 30 Abstract in the study area. Petrographic stud- carbonate reservoirs that occur in the ies indicate that the Racine Formation upper part of the Silurian succession. The Silurian succession of the San- was deposited within the high-energy However, there are conflicting views gamon Arch, a broad southwest-trend- depositional environment of the ramp on reservoir development, petroleum ing structure in west-central Illinois, margin (roughly parallel to the Mt. entrapment, and their controls. To is composed of hydrocarbon-bearing Auburn trend) and in the adjacent date there have been no documented reservoirs that have produced mainly deeper subtidal setting. This gently studies regarding the reservoir facies from dolomitized carbonate units in sloping ramp was a part of a distally types, porosity, occurrence, and petro- the upper part of the Niagaran Series. steepened ramp (the Vincennes Basin) leum entrapment of these Silurian To determine the reservoir facies and located in the distal deep area of the reservoirs. The objectives of this study to evaluate the occurrence, geometry, Illinois Basin. Because the depositional are to determine reservoir facies and distribution, porosity development, wave energy was not strong enough assess the occurrence, geometry, dis- and petroleum entrapment mecha- in the homoclinal ramp setting of the tribution, porosity development, and nism, the present study focuses on the Sangamon Arch, only bioclast grain- petroleum entrapment mechanism in Silurian deposits along the Mt. Auburn stone shoal facies and small patch reefs the Niagaran deposits along the Mt. trend of the Sangamon Arch. In the were developed—unlike the large pin- Auburn trend of the Sangamon Arch. Mt. Auburn trend along the southern nacle reefs that developed along the The intent is to generate renewed flank of the arch, the only petroleum steeper slope of the deep Illinois Basin interest in the unexplored areas and reservoir is the uppermost Niagaran margin. encourage secondary recovery of its Racine Formation (equivalent to the producing horizons. The depositional Moccasin Springs Formation of south- Hydrocarbon production along the Mt. model developed in this study should ern Illinois). The Racine Formation is Auburn trend has been chiefly from the help producers understand reservoir characterized mostly by layers of lime- non-reef carbonate reservoirs in the development and predict occurrences stone, dolomite, and silty argillaceous uppermost part of the Silurian succes- of productive Silurian rocks in other dolomitic limestone/dolomite that sion. Most wells drilled thus far have areas of the Illinois Basin. vary in thickness and lithology from only tested the uppermost part of the place to place, making well-to-well cor- Niagaran deposits; only a few wells Available subsurface data, including relation difficult. The Racine Forma- have tested the lower reservoirs that well cuttings, cores, and geophysical tion underlies the Upper Devonian to include the newly recognized patch logs from more than 950 drill holes, Lower Mississippian New Albany Shale reefs. The results of this study sug- were studied in detail to establish and Group with a distinct unconformity gest that there are good possibilities outline facies distribution and res- and sharply overlies the lower Niagaran of finding other productive areas with ervoir characteristics of the Silurian Joliet Formation (equivalent to the St. dolomitized patch reefs and non-reef deposits in the study area. Maps and Clair Formation of southern Illinois). reservoirs in the Mt. Auburn trend of stratigraphic cross sections from differ- The variable thickness and lithology in the Sangamon Arch area. ent areas along the Mt. Auburn trend the lower part of the Racine Formation were constructed using GeoGraphix are interpreted to be the consequence and Adobe Illustrator software. The res- of sea level fall and differential post- Introduction ervoir facies are classified on the basis depositional erosion, thus indicating The upper part of the Silurian succes- of the textural classification schemes of the presence of a prominent erosional sion (Niagaran Series) in the Sangamon Dunham (1962) and Embry and Klovan unconformity within the Racine For- Arch of west-central Illinois contains (1971). mation. This unconformity subdivides prolific petroleum reservoirs that have the Racine Formation into two third- been producing for over 80 years. More order cycles (sequences) that include than 12 million barrels of oil have been Geologic Setting several producing dolomite reservoirs. produced from Silurian rocks in the The intracratonic Illinois Basin (Figure Mt. Auburn trend located along the 1) began subsiding during the Late The reservoirs include dolomitized southern flank of the Sangamon Arch Cambrian Period over the northeast skeletal wackestone-grainstone facies (Figures 1 and 2). The first commercial extension of the Late Precambrian to in the upper sequence and coral patch production from the area was in 1925 Middle Cambrian Reelfoot Rift system reef/reef rudstone facies in the lower from the Decatur Field, but major (Kolata and Nelson 1991). This subsid- sequence. The reservoirs commonly exploration activity did not begin until ence was associated with the breakup constitute the upper part of fourth- 1954 after Sun Oil Company com- of the Rodinia supercontinent during order shallowing-upward cycles and pleted the Damery No. 1 well in Macon Late Precambrian to Early Cambrian are characterized by lenticular bodies County in December 1953. That well time (Bond et al. 1984; Piper 2000, of limited lateral extent. They normally produced more than 700 barrels of oil 2004; Meert and Torsvik 2003). During grade laterally and vertically into later- per day after fracture treatment of the the Silurian Period, the rift system ally extensive impermeable limestone Silurian reservoir (Whiting 1956). formed the axis of a southward plung- facies, suggesting that fluctuations ing trough that opened to the adjacent in sea level and seawater chemistry The Mt. Auburn trend includes a Iapetus Ocean (Kolata and Nelson were the primary controls for early number of oil fields (Figure 2) that have 1991). By the Middle to Late Silurian dolomitization of Silurian carbonates produced chiefly from the dolomitized

Illinois State Geological Survey Circular 577 1 MI IA

SANGAMON ARCH Mt. Auburn Trend

ILLINOIS BASIN THB

IN MO

THB

VINCENNES BASIN

RR RCG MO KY

0 30 mi N 0 48 km

Figure 1 Location of the Illinois Basin (Buschbach and Kolata 1991) and the Sangamon Arch in west-central Illinois. The Sangamon Arch is defined by the zero isopach contour of the Lower to Middle Devonian deposits (Whiting and Stevenson 1965) in the northwestern portion of the Illinois Basin. The Mt. Auburn trend is located along the southeastern flank of the Sangamon Arch and includes several oil fields (Figure 2). Line abbreviations: RR, the northwest limit of the Reelfoot Rift; RCG, the northern limit of the Rough Creek Graben (Kolata and Nelson 1991); VB, the northern limit of the Vincennes Basin; THB, the limit of the known pinnacle reefs in the Terre Haute reef bank (Droste and Shaver 1980, 1987). Both the Vincennes Basin and the Terre Haute reef bank may have marked a slope break in southern Illinois and southwestern Indiana with deeper water to the south.

2 Circular 577 Illinois State Geological Survey R 2 W R 1 W R 1 E R 2 E

T LOGAN 17 SANGAMON MACON N

Forsyth

Decatur Harristown

T 16 N Blackland North

CHRISTIAN

Harristown South Blackland

T 15 Blackland N South

Mt. Auburn Consolidated

T 14 N

SHELBY

0 3 mi Oil well Abandoned oil well Dry hole County boundary Township boundary N 0 4.8 km

Figure 2 Map of the Mt. Auburn trend of the Sangamon Arch. The trend is delineated by a number of oil fields that have produced from the Silurian reservoirs. Reservoirs are differentiated from one another by color.

Period, a platform margin (the Terre formed as a result of upward warping of oil fields in parts of Macon and Haute reef bank) marked a slope break during Silurian and Devonian times Christian Counties in west-central Illi- in southern Illinois and southwestern (Whiting and Stevenson 1965). The nois (Figure 2). Indiana that faced the deep proto- Sangamon Arch formed a ramp area Illinois (Vincennes) Basin (Droste and on the northwestern part of the Illinois Subsidence coincident with the long- Shaver 1980, 1987). Basin, where the general direction of term Silurian global (eustatic) sea level the regional dip is toward the south- rise that culminated during the Middle The Sangamon Arch is a broad northeast. The Mt. Auburn trend is located Silurian (Golonka and Kiessling 2002, east-southwest–trending structure in along the southern flank of the arch Haq and Schutter 2008) resulted in west-central Illinois (Figure 1) that was (Figure 1) and encompasses a number deposition of over 400 feet (120 m) of

Illinois State Geological Survey Circular 577 3 mainly shallow marine carbonates in stratigraphic nomenclature scheme of chronostratigraphic events and may the Sangamon Arch area. Carbonate Willman and Atherton (1975) for west- coincide with the boundaries of deposition was terminated as a result ern Illinois (Figure 4) has been adopted lithostratigraphic units. The surfaces of the worldwide pre-Middle Devonian to identify the Silurian deposits of the represent (a) the top of the Joliet For- sea level fall (Sloss 1963, Vail et al. 1977, Sangamon Arch area. Based on this mation, (b) the base of a thin radioac- Haq and Schutter 2008) and upwarp- classification, the Silurian deposits of tive shale, (c) the top of a radioactive ing of the arch during Late Silurian the Mt. Auburn trend comprise, from calcareous shale or argillaceous lime- through Middle Devonian times. In base to top, the Alexandrian Edgewood stone that may reach a thickness of 30 the deep basinal area to the south, and Kankakee Formations and the feet (9 m) above marker b, (d) the top however, deposition was continuous Niagaran Joliet and Racine Forma- of the lower Racine sequence, (e) the across the Silurian-Devonian bound- tions. In the Mt. Auburn trend, the base of a resistive cherty limestone ary (Droste and Shaver 1987, Mikulic uppermost Niagaran Racine Formation marker near the Silurian-Devonian 1991). includes several hydrocarbon-pro- boundary, (f) the top of the upper ducing dolomite horizons. The Racine Racine sequence, and (g) the top of the Formation (Hall 1861 in Willman and New Albany Shale Group. Stratigraphic Setting Atherton 1975) takes its name from The Silurian succession of the Illinois the quarry exposures at Racine, Wis- The lower part of the Racine Forma- Basin is a part of the Silurian-Middle consin. It is up to 300 feet (90 m) thick tion displays characteristic and widely Devonian Hunton Megagroup (Swann and consists of pure limestone and/or traceable geophysical log signatures, and Willman 1961), which constitutes dolomite reef facies and silty argilla- which commonly represent alternat- the upper part of the Tippecanoe ceous dolomite or limestone inter-reef ing gray fossiliferous limestone, silty sequence of Sloss (1963) and overlies facies (Willman and Atherton 1975). argillaceous dolomitic limestone/ the Upper Ordovician (Cincinnatian) The Racine Formation is equivalent dolomite, and calcareous shale (Figure deposits with the distinct unconfor- to the Moccasin Springs Formation of 5). The log signatures exhibit an over- mity below the Tippecanoe II sub- southern Illinois that displays a charac- all upward decrease in resistivity and sequence (Figure 3). In the platform teristic “two-kick, three-kick” interval an upward increase in radioactivity in areas (inner to mid ramp) of the Illinois (resistive signatures best recognized on the lower part of the formation (e.g., Basin, the Silurian rocks unconform- old electric logs) in its lower part, just depths 2,100 to 2,030 feet in Figure 5). ably underlie the Middle Devonian above the Niagaran St. Clair Formation This interval is similar to the lower part deposits with the pronounced sub- (equivalent to the Joliet Formation) of the Moccasin Springs Formation in Kaskaskia erosional unconformity, (Willman and Atherton 1975). southern Illinois, but the characteristic but, in the deep part of the Basin, the “two-kick, three-kick” resistivity signa- Silurian-Devonian boundary is con- tures (Willman and Atherton 1975) are formable (Willman and Atherton 1975, Stratigraphy of the not fully discernible in the Mt. Auburn Droste and Shaver 1987, Mikulic 1991). Racine Formation trend area. In the Sangamon Arch area, Lower and In the study area, the Racine Forma- The thickness of the Racine Formation Middle Devonian deposits are absent tion underlies the Upper Devonian on the Sangamon Arch is variable due (Whiting and Stevenson 1965), and through the lowermost Mississippian to post-depositional erosion, as is dis- Silurian carbonates are overlain by the New Albany Shale Group with an cussed more fully in the following sec- Upper Devonian to lowermost Mis- interregional unconformity (Figure 4). tion. Deposition of the Racine Forma- sissippian New Albany Shale Group It overlies, with a sharp contact, the tion on the Sangamon Arch was coeval (Figure 4). In the Sangamon Arch area, thick-bedded fossiliferous limestone of with the deposition of the Moccasin the Silurian encompasses the Alexan- the Joliet Formation (equivalent to the Springs Formation of southern Illi- drian and Niagaran Series (Willman St. Clair Limestone of southern Illinois) nois and the lower part of the Wabash and Atherton 1975), which consist (Figure 5). A prominent unconformity Formation of Indiana (Willman and mainly of limestone and dolomite. subdivides the Racine Formation Atherton 1975, Droste and Shaver 1987, The Alexandrian-Niagaran boundary into two sequences (Y. Lasemi 2009a, Mikulic 1991). is marked by an unconformity (Will- 2009b). man and Atherton 1975, Mikulic 1991, Kluessendorf and Mikulic 1996). The A subsurface reference section, Unconformable Boundaries unconformity corresponds with the the Equitable Resources Explora- and Lateral Distribution major phase of the Caledonian Orog- tion IL-3032 well (southwest of the Examination of the geophysical log sig- eny during the Early Silurian through Mt. Auburn Consolidated Field) in natures and stratigraphic correlation Late Devonian (Golonka and Kiessling Christian County, has been chosen in the study area (Figures 6 through 10) 2002). to illustrate significant and laterally indicates fairly uniform thickness and continuous geophysical markers in the The Silurian System of the Illinois lithology for the lower Niagaran Joliet area (Figure 5). The reference section Formation (equivalent to the St. Clair Basin has been the subject of several surfaces are designated in ascending lithostratigraphic classifications (Will- Formation of southern Illinois) and the order, from base to top, by letters a to underlying Alexandrian Edgewood- man and Atherton 1975, Droste and g, which correspond to important Shaver 1987). In the present study, the Kankakee Formations (Figure 7), which

4 Circular 577 Illinois State Geological Survey SERIES SYSTEM SEQUENCE 37˚N 42˚N Pleistocene Pliocene Tejas 42˚N Eocene Paleocene Upper Zuni fault block in western Kentucky Wolfcampian sub-Zuni unconformity Virgilian 37˚N Missourian

Desmoinesian Absaroka

Atokan Morrowan sub-Absaroka unconformity Chesterian Pope

Valmeyeran Mammoth Cave Kinderhookian Upper Kaskaskia Knobs erosion from uplift of Pascola Arch Middle Lower reef sub-Kaskaskia unconformity Cayugan Hunton Niagaran sub-Tippecanoe II unconformity Alexandrian Cincinnatian Ottawa Champlainian Tippecanoe I Tippecanoe II sub-Tippecanoe unconformity

OrdovicianCanadian Silurian Devonian Mississippian Pennsylvanian Perm. Cret. Tertiary Quat.

Knox

Upper Potsdam Sauk

Cambrian Middle sub-Sauk unconformity New Madrid Rift Complex Lower Reelfoot Rift

conglomerate sandstone limestone shaley dolomite shale limestone

coal chert

Figure 3 General stratigraphic column, sequences, and megagroups of the Illinois Basin (Swann and Willman 1961, Sloss 1963, Buschbach and Kolata 1991, Kolata 2005). Abbreviations: Cret., Cretaceous; Perm., Permian; Quat., Quaternary.

Illinois State Geological Survey Circular 577 5 suggests no differential erosion prior to the deposition of the upper Niagaran Racine Formation. The Racine Forma- LITHOLOGY tion, however, is characterized by vari- GROUP/ SERIES able thickness and lithology of strata, SYSTEM making well-to-well correlation diffi- FORMATION cult (Figures 7 through 10). - The variable thickness and litholo- gies in the lower part of the Racine Devonian

Formation (see Figures 7 through 10) Group are interpreted to be the consequence Kinderhookian of global sea level fall and differential Upper New Albany Shale Albany New post-depositional erosion, thus indicating the presence of an erosional Mississippian unconformity within the Racine For- mation (Y. Lasemi 2009a, 2009b). The Middle

presence of an unconformity within Devonian- the Niagaran deposits was first suggested by Whiting and Oros (1957) in the vicinity of the study area. The Lower differential erosion along this unconformity is as much as 70 feet (21 m) in parts of the study area (e.g., compare the Texas Co. Dipper No. 1 with Pawnee URS Oil & Gas, Inc. Beatty 5 in Figure 8). The intra-Racine unconformity records a major tectono-eustatic sea level shale fall during Middle Silurian time and Racine Formation LRS divides the Racine Formation into two depositional sequences (Y. Lasemi dolomite 2009a, 2009b). Niagaran The interregional unconformity at limestone the top of the Racine Formation is the Silurian consequence of a major global sea level argillaceous limestone/ fall at the Silurian-Devonian boundary calcareous shale (Sloss 1963, Vail et al. 1977, Haq and Joliet Formation Schutter 2008) and could be partly the argillaceous dolomite/ result of Late Silurian through Middle calcareous shale Devonian upwarping of the Sangamon Arch (Y. Lasemi 2009a, 2009b). Pro- upper Racine reservoir longed exposure of the Sangamon Arch resulted in partial erosion of the Kankakee Formation Edgewood-

Silurian deposits and formation of Alexandrian lower Racine reservoir an uneven topography that was later buried by the Upper Devonian to lowermost Mississippian New Albany hiatus Shale Group

Shale. The uneven topography is well Upper Maquoketa displayed on the isopach map of the Ordovician New Albany Shale Group (Figure 11) and the structural contour map on the Figure 4 General stratigraphic column of the Upper Ordovician to low- top of the Silurian succession (Figure ermost Mississippian strata in the Sangamon Arch area. Note that the 12), which shows several noses and a Lower and Middle Devonian deposits are absent in the Sangamon Arch few minor closures. The irregular ero- area because of the pronounced sub-Kaskaskia unconformity and that sional topography is also illustrated the New Albany Shale Group rests on the Silurian strata. An unconformity by the stratigraphic cross sections that subdivides the Racine Formation into lower and upper sequences. Note are hung on the base or top of marker that non-reef reservoirs occur in the upper part of the upper sequence, horizons within the Silurian deposits but coral patch reefs exist in the upper part of the lower sequence. (Figures 7 through 10). The New Albany Abbreviations: LRS, lower Racine sequence; URS, upper Racine Shale Group generally becomes thin- sequence.

6 Circular 577 Illinois State Geological Survey ner toward the east and southeast on API No. 120212443100 the Mt. Auburn trend, a positive Silu- rian remnant, but thickens toward the west and northwest along the crest of the arch, where the Silurian rocks are Equitable Resources Exploration IL-3032 deeply eroded (Figures 10 and 11). The T14N, R2W, Sec. 19 thickening of the New Albany could be Christian County explained by differential compaction GAMMA-RAY RESISTIVITY of the shale over the Silurian erosional remnant and by increased accom- 0 API 150 0.2 ILD (Ohm-m) 2,000 modation space for shale deposition, SP (mV)

SERIES -200 0 0.2 ILM (Ohm-m) 2,000 GROUP/ SYSTEM 0.2 FOC (Ohm-m) 2,000 probably as a consequence of the FORMATION downwarping of the Sangamon Arch Cht Ls area during Late Devonian time. g The thickness of the Racine Forma- tion is fairly uniform along the Mt. Auburn trend (cross section C–C in Figure 9). However, the thickness of the 1,800 Racine Formation generally decreases toward the west and northwest and increases toward the northeast and east of the study area (Figures 7, 9, and 10). The Racine Formation reaches a

Group maximum thickness of 235 feet (70.5 m) in the Forsyth Field (Triple G Oil

New Albany Shale Albany New Co. Schwarze-Pense Community No.

Devonian-Mississippian 2 well in Figure 9) in Macon County. About 50 miles (80 km) to the east of Upper Devonian-Kinderhookian

1,900 the study area in Douglas County (Villa Grove Quadrangle, T16N, R9E, Sec. f 30), Mikulic and Kluessendorf (2001) assigned over 400 feet (120 m) of the e upper part of the Silurian succession to the Moccasin Springs Formation (equivalent to the Racine Formation of the Sangamon Arch). In Grafton Quarry d (Jersey County) in western Illinois,

LRS URS about 75 miles (120 km) to the south-

2,000 west of the study area, the uppermost Silurian interval of about 45 feet (14 m) that overlies the Joliet Formation c was assigned to the Racine Formation Silurian Niagaran by Willman and Atherton (1975). This

Racine Formation youngest Silurian interval was recently b correlated with the Sugar Run Forma- tion, a lithostratigraphic unit underlying the Racine Formation in northeast- ern Illinois (Mikulic and Kluessendorf a 2000). 2,100 Fm Joliet Cyclostratigraphy TD = 2,200 feet The erosional unconformity within the Racine Formation divides the Racine Figure 5 Subsurface gamma-ray and electric log stratigraphic reference section Formation into two third-order cycles for the Mt. Auburn trend. Significant and laterally continuous geophysical hori- (Y. Lasemi 2009a, 2009b) or deposi- zons are designated with letters a through g. Abbreviations: Cht Ls, Chouteau tional sequences (in the sense of Vail et Limestone; Fm, Formation; FOC, focused; ILD, induction log deep; ILM, induction al. 1977 and Van Wagoner et al. 1988). log medium; LRS, lower Racine sequence; SP, self-potential; TD, total depth; URS, Theses cycles are designated as lower upper Racine sequence.

Illinois State Geological Survey Circular 577 7 LOGAN CЈ T17N SANGAMON MACON

E R 2E R2W R1W R1E HЈ E’ A B

T16N

F FЈ

CHRISTIAN BЈ D

GЈ DЈ

H T15N

AЈ

T14N

C SHELBY

0 3 mi Oil well Abandoned oil well Dry hole County boundary Township boundary N 0 4.8 km

Figure 6 Map of the Mt. Auburn trend area showing the lines of stratigraphic cross sections A–A through H–H.

Racine sequence and upper Racine upward and attains a maximum value Smaller-scale high-frequency fifth- to sequence to facilitate descriptions of at the maximum flooding surface (e.g., sixth-order cycles are also distinguish- the petroleum reservoirs of the Mt. depths 1,962 feet and 1,884 feet in able (see Figure 13). The petroleum Auburn trend. Figure 13). In the third-order regressive reservoirs in the Racine Formation tract, however, radioactivity gener- commonly constitute the upper part Each third-order cycle consists of ally decreases upward (Figures 13). of high-frequency fourth-order trans- smaller-scale fourth-order cycles The regressive tracts are incomplete gressive-regressive cycles (Figures 13 (Figure 13). In the third-order trans- due to post-depositional erosion. through 15). gressive tract, radioactivity increases

8 Circular 577 Illinois State Geological Survey 121672506900 120212433300 120210150100 31,555 feet 11,213 feet

Equitable Resources Exploration Podolsky, Bernard Jarvis, S. D. IL-3025 McMillen 4-B Butcher, Owen E. 1 T16N, R1W, Sec. 7 T15N, R1W, Sec. 9 T15N, R1W, Sec. 21 Sangamon County Christian County Christian County

GAMMA-RAY RESISTIVITY SELF- RESISTIVITY POTENTIAL API ILD (Ohm-m) 0 200 0.2 2,000 SP (mV) ILS (Ohm-m) API ILM (Ohm-m) 200 400 0.2 2,000 -200 0 0 100 SERIES GROUP/ SP (mV) ILD (Ohm-m) SYSTEM GAMMA-RAY RESISTIVITY 0 100

-200 0 FORMATION API ILD (Ohm-m) 0 200 0.2 2,000

API ILM (Ohm-m) 1,800 200 400 0.2 2,000 Shale

SP (mV) FOC (Ohm-m) Group -200 0 0.2 2,000 Upper Devonian New Albany 1,900 1,700 URS 1,900 URS 2,000 A AЈ 1,800 Datum LRS Line Racine Formation LRS 2,000 Niagaran Silurian 2,100 TD = 2,049 1,900

LOGAN SANGAMON MACON A

2,200 ? Joliet Formation 2,000 Ј

A Edgewood-Kankakee- CHRISTIAN Alexandrian SHELBY 0 6 mi N 0 9.6 km TD = 2,525 feet

TD = 2,126 feet Upper Ordovician Maquoketa

Figure 7 Northwest-southeast stratigraphic cross section A–A across the Mt. Auburn trend showing the Racine Formation sequences and the lateral variations in their thickness and lithology as a result of post-depositional erosion. Note that the Silurian rocks underlying the Racine Formation (mainly resistive limestone) display fairly uniform thickness. Datum (labeled A–A) is horizon c (see Figure 5) within the lower sequence. Abbreviations: FOC, focused; ILD, induction log deep; ILM, induction log medium; ILS, induction log shallow; LRS, lower Racine sequence; Ls, Limestone; SP, self-potential; TD, total depth; URS, upper Racine sequence.

Petroleum Reservoirs ent horizons. These zones generally impermeable units that have formed include dolomitized wackestone to flow barriers dividing the Racine and Seals grainstone facies in the upper Racine Formation into stratigraphic cycles The Racine Formation along the Mt. sequence and dolomitized coral patch of impervious and porous horizons Auburn trend comprises several per- reef/reef rudstone facies in the lower (Figures 13, 14, and 15). The reservoir meability pinch-out zones (herein Racine sequence (Figure 4). The zones units are light yellowish brown, due to referred to as reservoir units) at differ- are interlayered with laterally extensive oil staining, instead of the light gray

Illinois State Geological Survey Circular 577 9 - B Ј 100 100

ILS (Ohm-m)

ILD (Ohm-m)

RESISTIVITY 2,100 0 2,000

0 SELF- SP (mV) TD = 2,162 feet Macon County POTENTIAL Dipper, D. 1 121150069000

-200 0 T16N, R1E, Sec. 28 The Texas Company 3 -0.1 -0.1 3,267 ft 3 g/cm Density Por. Corr. Neutron Por. 0 25

0.3 0.3 2

1,900 , 150 TD = 2,021 feet Beatty 5 API Macon County Inches Caliper 121152170600 GAMMA-RAY BULK DENSITY 15 7 16 T16N, R1E, Sec. 21 T15N T16N Pawnee Oil & Gas, Inc.

URS LRS 3 -0.1 -0.1 925 ft

R1E BЈ 3 MACON g/cm Density Por. Neutron Por. Corr. 0 25

0.3 2 0.3

1,900 2,000

B 150 Beatty 3 TD = 2,050 feet R1W API 12115216880 Macon County Inches Caliper 15 7 16 GAMMA-RAY DENSITY BULK T16N, R1E, Sec. 21 28 CHRISTIAN Pawnee Oil & Gas, Inc.

URS LRS 3 -0.1 -0.1 1,474 ft 3  ) is below the shown thin as layer radioactive correc horizon Corr., b in Abbreviations: Figure 5. g/cm Corr. Density Por. Neutron Por. BULK DENSITY 0 25

2 0.3 0.3

2,000 1,900 150 Scherer 2 TD = 2,100 feet API Macon County 121152165400 Inches Caliper GAMMA-RAY T16N, R1E, Sec. 21 15 7 16 Triple G Oil Company Ltd. Line  across Auburn trend the showing Mt. the lateral distribution of the Note Racine the Formation. erosional truncation and Datum

2,000

2,000 1,6138 ft

ILD (Ohm-m)

ILM (Ohm-m)

RESISTIVITY

FOC (Ohm-m)

0.2

1,800 1,900 2,000 2,100

200

400 IL-3026 TD = 2,200 feet

API

API SP (mV)

121152232600

GAMMA-RAY 200

-200

LRS Joliet Formations Joliet FORMATION

Sangamon County Group Shale

T16N, R1W, Sec. 13

Maquoketa B Edgewood-Kankakee- GROUP/ Racine Formation Racine New Albany New

SERIES Upper Niagaran Alexandrian Equitable Resources Exploration Upper

Str ?

atigraphic cross section B–B

SYSTEM Silurian Devonian Ordovician tion; FOC, focused; ILD, induction log deep; ILM, induction log medium; ILS induction log shallow; LRS, lower Racine sequence; Por., porosity; SP, self-potential; self-potential; induction log ILM, ILD, deep; induction SP, porosity; log focused; FOC, ILS tion; medium; induction LRS, lower log Racine Por., shallow; sequence; total URS, upper depth; TD, Racine sequence. uneven topography in topography the uneven upper boundary of the lower and upper Note Racine also sequences. that the lower Racine sequence and thickens the upper sequence pinches out the toward Datum northwest. horizon (labeled B–B Figure 8

10 Circular 577 Illinois State Geological Survey Ј C 100 100

ILS (Ohm-m)

ILD (Ohm-m)

RESISTIVITY

0 2,100 2,200 2,300

0 TD = 2,892 feet SELF- SP (mV) Macon County 121152144400 POTENTIAL

-200 T17N, R2E, Sec. 24

Triple G Oil Company Ltd. Ј

Schwarze-Pense Community 2 URS LRS 33,400 feet MACON 2.5 0.1 0.1 3 g/cm Neut. Por. Neut. Corr. Dens. Por. Dens. 0 25

0.3 0.3 2,000 2,100 16 150 300 TD = 2,175 feet Garver 1 API 2 API Macon County CHRISTIAN 121152170300 Inches Caliper T16N, R1E, Sec. 2 GAMMA-RAY BULK DENSITY LOGAN 150 15 6

Pawnee Oil & Gas, Inc. C SANGAMON 21,535 feet 2.5 0.1 0.1 3 Corr. Neut. Por. Neut. Dens. Por. Dens. 0 2.5

0.3 0.3

1,900 2,000 2,100 16 150API2 g/cm TD = 2,100 feet Scherer 2 Macon County 121152165400 Inches Caliper T16N, R1E, Sec. 2 GAMMA-RAY DENSITY BULK 15 6 Triple G Oil Company Ltd.

URS LRS Line Datum 2,000 2,000 2,000 33,720 feet ILD (Ohm-m) ILM (Ohm-m) FOC (Ohm-m)

0.2 0.2 0.2 1,800 1,900 2,000 0 150 300 TD = 2,049 feet API API McMillen 4-B SP (mV) 120212433300 Christian County GAMMA-RAY RESISTIVITY Podolsky, Bernard 0 -200 150 T15N, R1W, Sec. 9  along Auburn trend the showing Mt. rather uniform thickness and lithology the for Datum Racine horizon sequences. 2,000 2,000 2,000 60,555 feet

ILD (Ohm-m) ILM (Ohm-m) RESISTIVITY FOC (Ohm-m)

2,000 2,100 0.2 1,900 0.2 0.2 0 150 300 IL-3032 20212443100 API API TD = 2,200 feet Christian County SP (mV)

GAMMA-RAY

T14N, R2W, Sec. 19

0 -200 150 FORMATION Shale Group Shale R URS LRS

GROUP/ Racine Formation Racine New Albany New C Fm

Joliet

Equitable Resources Exploration Str

Upper Devonian Upper

SERIES Niagaran atigraphic cross section C–C

SYSTEM Devonian Silurian (labeled C–C  ) is below the within thin layer the radioactive lower sequence Fm, shown as horizon focused; FOC, Dens., b density; in Abbreviations: Figure 5. self- induction log ILM, ILD, SP, deep; induction Formation; porosity; log IRS, induction medium; LRS, lower log Racine Neut., Por., shallow; sequence; neutron; total URS, upper depth; TD, Racine sequence. potential; Figure 9

Illinois State Geological Survey Circular 577 11 Ј D

2,000

2,000 2,000 -

ILD (Ohm-m)

ILM (Ohm-m)

RESISTIVITY

FOC (Ohm-m)

2,000 2,100 2,200 0.2 2,300 0.2

0.2

0 TD = 2,620 feet

150

300 Barnard, N. 1 Macon County 121152250000

API

API SP (mV) T15N, R1E, Sec. 10

GAMMA-RAY

150

-200 Eastern American Energy Corp. 13,467 feet

) 100 100

Ohm-m

ILS (Ohm-m)

ILD (

RESISTIVITY

0 1,800 1,900 2,000 2,100

0 TD = 3,780 feet Macon County 121150010300 SELF- SP (mV) Sun Oil Company T15N, R1E, Sec. 5 POTENTIAL Damery, Joseph F. 1

-200

URS LRS

2,000

2,000 2,000

21,769 feet

Ј SHELBY D

ILD (Ohm-m)

ILM (Ohm-m)

RESISTIVITY

FOC (Ohm-m)

MACON

1,800 1,900 0.2 1,700 0.2

0.2

0 LOGAN TD = 2,049 feet

150

300

CHRISTIAN D

API

API McMillen 4-B 120212433300 SP (mV) Christian County Podolsky, Bernard SANGAMON T15N, R1W, Sec. 9

GAMMA-RAY

150

-200 Line Datum  ) is the same as datum Figure for 9 (shown as horizon Cht b Ls, in Abbreviations: Figure 5).

2,000

2,000 2,000 42,911 feet

ILD (Ohm-m)

ILM (Ohm-m)

RESISTIVITY

FOC (Ohm-m)

 across Auburn trend the showing Mt. erosional truncation in and topography the uneven upper boundary

1,600 1,700 1,900 0.2 1,800 0.2 0.2

150 300 Darnall 2

API

API 121672484400 SP (mV) Podolsky, Bernard Sangamon County

TD = 1,955 feet T16N, R2W, Sec. 32

GAMMA-RAY

150

-200

FORMATION Fm Shale Group Shale LRS

New Albany New GROUP/ D Racine Formation Racine Joliet Cht Ls

Cross Niagaran SERIES Devonian-Kinderhookian Upper

section D–D

SYSTEM Devonian-Mississippian Silurian of the lower and upper Note Racine that sequences. the New Albany Shale Group but the thickens, Racine Formation becomes thin ner Datum the toward (labeled west. D–D induction log ILM, ILD, deep; induction Chouteau log focused; Fm, FOC, ILS, induction Limestone; medium; Formation; LRS, log shallow; total depth. TD, self-potential; lower Racine URS, upper sequence; Racine SP, sequence; Figure 10

12 Circular 577 Illinois State Geological Survey LOGAN 150 SANGAMON MACON T17N

200

R2E R1E R2W 190 R1W

180 170

140

160 T16N 150

CHRISTIAN

170 180 140

T15N

150

T14N

220 198 175 153 130 0 3 mi N 0 4.8 km Thickness (feet)

Oil well Abandoned oil well Dry hole County boundary Township boundary 10-foot contour intervals

Figure 11 Thickness map of the New Albany Shale Group from the top of the Silurian to the base of the Chouteau Limestone. The New Albany Shale Group thickens toward the northwest, away from the Mt. Auburn trend and toward the cen- ter of the Sangamon Arch (see Figure 1 for the location of the Sangamon Arch), suggesting that the arch area was subsiding at the onset of the Upper Devonian deposition. The closures in the Mt. Auburn trend area are due to pre-Devonian differential erosion. that is typical of non-reservoir facies. No structural closures are present in were deposited in quiet water environ- The reservoir units are defined here the study area, and minor closures in ments following the flooding stages on the basis of facies and their strati- some areas of the Sangamon Arch are of high-frequency sea level cycles (Y. graphic position and are designated, the consequence of post-depositional Lasemi 2009c). from top to base, as non-reef reservoir erosion and the formation of uneven units A, B, and C and patch reef/reef topography at the sequence boundar- Non-reef Reservoir rudstone reservoir unit D. Individual ies (Y. Lasemi 2009c). A combination reservoirs may show internal vertical of depositional and diagenetic strati- Units A, B, and C porosity variations (e.g., the high- graphic traps controlled petroleum Non-reef reservoir units A, B, and C frequency fifth-order cycles and core entrapment in the Mt. Auburn trend, occur in the upper part of the upper photographs from the Bernard Podol- and the reservoirs are normally sealed sequence of the Racine Formation sky McMillen 4-B well in Figure 13). by lime mudstones to packstones that (Figure 4). They are characterized by

Illinois State Geological Survey Circular 577 13 -1375

LOGAN T17N SANGAMON MACON

-1250 -1250

R2E R2W R1W R1E

-1125

-1375

T16N

-1500 -1125

CHRISTIAN

-1250

T15N -1625

-1500

-1375

T14N -1750 -938 -1,130 -1,322 -1,514 -1,706 0 3 mi N 0 4.8 km Subsea elevation (feet)

Oil well Abandoned oil well Dry hole County boundary Township boundary 25-foot contour intervals

Figure 12 Structure contour map of the base of the New Albany Shale Group showing the direction of the regional dip toward the southeast. Note minor closures and noses that are due to uneven pre-New Albany Shale Group topography.

porous dolomitized wackestone to ervoirs are lenticular (Figures 18, 19, trend indicates that most of the wells grainstone (Figures 13; 16c, d; and 17a, and 20) and are encased within imper- drilled thus far have only tested the b). The reservoirs contain partially meable host limestone strata (Figure upper part of the upper sequence of dolomitized echinoderm fragments 16a, b) displaying cyclic patterns of the Racine Formation. (Figures 16d and 17b) and molds of non-reservoir to reservoir packages bioclasts that include crinoids, bra- (Figures 4, 13, 14, 15, 21, 22, and 23). Reservoir Unit A Unit A is a lenticu- chiopods, corals, and non-recognizable Porosity of the reservoir rocks exceeds lar dolomite reservoir of limited lateral grains (Figure 17b). In some areas or in 20% in some horizons. The highest extent (Figure 18) that is recognized some horizons within a reservoir, only reported initial production was 1,360 in the uppermost part of the upper dolomite crystals are recognized, but barrels of oil per day in the Comanche Racine sequence (Figures 14, 15, 21, they contain a very faint relict shadow- Oil Co. No. 3 Tomlin well, T15N, R2W, 22, and 23). Reservoir unit A is up to ing that could represent the organic Sec. 7, Mt. Auburn Consolidated Field, 14 feet (4 m) thick and is separated residue of the original bioclasts that in Christian County. Examination of from unit B or C by up to 10 feet (3 m) they replaced (Figure 17a). The res- the numerous wells in the Mt. Auburn

14 Circular 577 Illinois State Geological Survey API No. 120212433300

Podolsky, Bernard McMillen 4-B T15N, R1W, Sec. 9 Christian County Gamma-Ray Bulk Density

API units g/cm3 0 150 2 3 Self-Potential Neutron Porosity

mV Depth(feet) % -150 50 0.3 -0.10 Caliper Time Variations 1,860 feet cyclostratigraphy

GROUP/FORMATION inches 6 16 140 ␮s/foot 40 cycle order 1,800 4th 5th 3rd New Albany Shale Group Albany New

1,900 1,863 feet LRS URS Racine Formation 2,000

1,865 feet

TD = 2,049 feet

1 inch (2.5 cm) regressive reservoir unit B/C unconformity transgressive

Figure 13 Digitized log and core photographs of the reservoir interval (reservoir unit B/C) in the Bernard Podolsky McMillen 4-B well in the Mt. Auburn Consolidated Field in Christian County. Vertical facies variations and cyclostratigraphy of the Racine Formation are shown. Note the reservoir development in the upper part of a fourth-order cycle within the regressive (highstand) deposits of the upper Racine sequence. Note also vertical porosity variations in the reservoir shown in the core photographs and fifth-order cycles. Grain size is larger, and porosity is higher, in the regressive portion of the cycles (rock at 1,863 feet is more porous than at 1,860 feet and 1,865 feet). See Figure 16d, e, and f for the petrographic detail of depth 1,863 feet). Abbreviations: LRS, lower Racine sequence; TD, total depth; URS, upper Racine sequence.

Illinois State Geological Survey Circular 577 15 API No. 121152170300

Pawnee Oil & Gas, Inc. Garver 1 T16N, R1E, Sec. 2 Macon County

Gamma-Ray Bulk Density

g/cm3 0 API units 150 2 3 Gamma-Ray Density Porosity

API units Depth (feet) % 0 450 0.3 -0.10 Neutron Porosity GROUP/FORMATION Fourth-/fifth-order cycles

0.3 % -0.10 1,900 New Albany Shale Group Albany New 2,000 LRS URS Racine Formation 2,100

TD = 2,175 feet

reservoir unit A reservoir unit D regressive

reservoir unit C unconformity transgressive

Figure 14 Digitized log of the Pawnee Oil and Gas Inc. Garver No. 1 well in the Harristown Field, Macon County, showing cyclicity and reservoir development in the Racine Formation sequences. Note the development of reservoir units A and C in the upper sequence and reservoir unit D in the lower sequence (no cores/samples are available for this well, and the differentiation of reef and non-reef reservoirs is based on stratigraphic correlation). Note also that the reservoirs occur in the upper part of fourth-order cycles within the shallowing- upward portion (highstand tract) of the third-order cycles. Abbreviations: LRS, lower Racine sequence; TD, total depth; URS, upper Racine sequence.

16 Circular 577 Illinois State Geological Survey API No. 121152155700

Pawnee Oil & Gas, Inc. Rothwell 1 T16N, R1E, Sec. 21 Macon County

Gamma-Ray Bulk Density

API units 3 0 150 2 g/cm 3 Self-Potential Density Porosity

mV % -150 50 Depth (feet) 0.3 -0.10 Neutron Porosity GROUP/FORMATION fourth-/fifth-order cycles 3.0 V/V 0- 0.3 % -0.10 Cht Ls 1,800 1,900 New Albany Shale Group Albany New 2,000 Racine Formation LRS URS TD = 2,040 feet reservoir unit A reservoir unit C regressive

reservoir unit B unconformity transgressive

Figure 15 Digitized log of the Pawnee Oil and Gas Inc. Rothwell No. 1 well in the Blackland North Field in Macon County showing the development of non-reef reservoir units A, B, and C in the upper part of the upper Racine sequence. Note the occurrence of the reservoirs in the regressive portion of fourth-order depositional cycles. Abbreviations: Cht Ls, Chouteau Limestone; LRS, lower Racine sequence; TD, total depth; URS, upper Racine sequence.

Illinois State Geological Survey Circular 577 17 1,863 feet

a 2.5 cm b 0.5 mm c 2.5 cm

CR CR Si

CR d 0.5 mm

Figure 16 (a) Core photograph of a bioturbated lime mudstone to wackestone facies in the Pawnee Oil Corp. Elder No. 1 well, T15N, R1W, Sec. 12, in the Mt. Auburn Consolidated Field in Christian County that caps a dolomitized patch reef reservoir. (b) Photomicrograph of a coarsening-upward lime mudstone to bryozoan crinoid wackestone facies from an impermeable capping limestone. The groundmass is partially replaced by very finely crystalline dolomite. (c) Polished core slab photograph of a porous dolomitized grainstone facies (also shown in Figure 13). (d) Photomicrograph of a part of c under polarized light showing an irregular dolomitization front (arrows) on crinoid fragments (CR), dolomite rhombs, and void spaces (black) formed as a result of dolomitization. Note that the crinoid fragment in the upper right is partially silicified (SI).

18 Circular 577 Illinois State Geological Survey BI

BI CR

a 0.5 mm b 0.25 mm

d 1.25 cm

c 2.5 cm

e 0.7 mm f 0.5 mm

Figure 17 Photomicrographs a and b are from reservoir unit C in Bernard Podolsky McMillen 4-B well (depth 1,859, see Figure 13), T15N, R1W, Sec. 9, in the Mt. Auburn Consolidated Field in Christian County. (a) Porous dolomitized non-reef reservoir under plane light in which the original texture has been destroyed by pervasive dolomitization. Note the partial dissolution of the dolomite rhombs and enlargement of the pore spaces (arrow). (b) Photomicrograph of a non-reef wackestone reservoir showing partially preserved crinoids (CR) and molds of bivalves (BI) and other unrecognizable grains in a finely crystalline dolomite groundmass. Core photographs c, d, e, and f are of a patch reef reservoir unit D in the Pawnee Oil Corp. Elder No. 1 well (depth 2,005 feet, see Figure 24), T15N, R1W, Sec. 12, in the Mt. Auburn Consolidated Field in Christian County. (c, d) Dolomitized reef facies composed mainly of coral skeletons. (e) Photomicrograph of a thin section of d showing vuggy porosity as a result of dissolution of the skeletons, which are hardly recognizable in thin section. (f) Photomicrograph of an enlarged portion of e. Note the organic residue of the original skeletons and the dissolution of dolomite rhombs (white arrow) to form larger pore spaces.

Illinois State Geological Survey Circular 577 19 of impermeable cherty fossiliferous 22, and 23), but, where it is removed by is well illustrated in cross section G– limestone. This tight limestone is just erosion, the New Albany Shale Group G (Figure 23). In some areas of the above marker horizon e (Figure 5) and becomes the capping facies for petro- Mt. Auburn trend, deep erosion has is a laterally persistent horizon in the leum entrapment (Bernard Podolsky removed the interval that could other- Mt. Auburn trend (Figures 7, 8, 9, 10, Vail 3-A, Figure 23). In some areas of wise include reservoir unit A (Figures 21, 22, 23, and 24). Reservoir unit A is the Mt. Auburn trend, however, the 21, 22, and 23). In some areas of the Mt. capped by impermeable limestone of absence of reservoir unit A may be the Auburn trend, reservoir unit A is sub- variable thickness (Figures 14, 15, 21, result of post-Silurian erosion, which divided into two horizons separated by

5 0

0 17 16 15 14

5 0

5 23 20 0 21 22

5 5 0

0 0 27 26 29 28

>10 feet Oil well Section boundary 0 0.5 mi N 5–10 feet Abandoned oil well 5-foot contour intervals 0 0.8 km

0–5 feet Dry hole

Figure 18 Thickness map of the dolomite reservoir unit A in the Blackland North Field showing a very limited lateral extent of the reservoir due to lateral facies changes and post-Silurian erosion.

20 Circular 577 Illinois State Geological Survey a tight impermeable limestone (e.g., and 19). Reservoir unit B is up to 5 feet B pinches out, leaving reservoir unit C Bernard Podolsky McMillen 3-A, Figure (1.5 m) thick and occurs just below the below the tight limestone as the only 23). impermeable and persistent limestone reservoir there, or the tight underlying marker below geophysical marker e limestone thins out, forming a single Reservoir Unit B Reservoir unit B (Figures 15 and 22). A tight limestone combined reservoir of units B and C is another lenticular dolomitic horizon up to 6 feet (2 m) thick separates (Figures 21 and 23). along the Mt. Auburn trend of the San- reservoir unit B from the underlying gamon Arch, and it is best developed reservoir unit C. In other fields of the Reservoir Unit C Reservoir unit C in the Blackland North Field (Figures 2 Mt. Auburn trend, either reservoir unit can attain a thickness of over 20 feet

17 16 15 14

0 0

0 23 20 21 22

4 0

27 26 29 28

>4 feet Oil well Dry hole 4-foot contour intervals 0 0.5 mi N 0–4 feet Abandoned oil well Section boundary 0 0.8 km

Figure 19 Thickness map of the dolomite reservoir unit B in the Blackland North Field showing the lenticular nature of the reservoir. Note that the reservoir lenses are aligned in a northeast-southwest direction lying roughly parallel to the Mt. Auburn trend.

Illinois State Geological Survey Circular 577 21 (6 m) of dolomite in parts of the Mt. tight limestone thins out laterally underlying the widespread imperme- Auburn trend and is more extensive beyond the resolution of the geophysi- able cherty limestone marker just than the other reservoirs (Figure 20). cal logs (Figures 21 and 23). Due to below geophysical marker e (Figures It is overlain by up to 6 feet (2 m) of lateral thinning of this tight limestone, 13, 21, and 23). Reservoir unit C is also impermeable limestone in Blackland reservoir units B and C may coalesce, lenticular (Figure 20) and pinches out North Field (Figures 15 and 22). This forming a single unit C reservoir laterally into impermeable facies (e.g.,

5 0

17 16 15 14

10 5 5 0

5 0

5 5 23 20 21 22 0

15 5 10 5 0

26 28 27 29 10 5 0

>15 feet Oil well Section boundary 0 0.5 mi N 10–15 feet Abandoned oil well 5-foot contour intervals 0 0.8 km 5–10 feet Dry hole 0–5 feet

Figure 20 Thickness map showing the lateral distribution of the dolomite reservoir unit C along the Mt. Auburn trend. This lenticular dolomite unit is more extensive than reservoir units A and B and is present in all of the oil fields along the Mt. Auburn trend.

22 Circular 577 Illinois State Geological Survey E Ј 3 -0.10 -0.10 3 g/cm Density Porosity BULK DENSITY BULK Neutron Porosity

reservoir unit A reservoir unit C reservoir unit D 0.3 0.3

1,900 2,000 TD = 2,073 feet 16 150 2 API Corr. Macon County 121152160100 Inches 0 25 CALIPER T16N, R1E, Sec. 1 GAMMA-RAY 15 6 Roby-Miller #7-C 7-C Comanche Oil (now Atlantic Energy) 3 -0.10 -0.10 2,135 feet 3 g/cm Corr. 0 25 Density Porosity Neutron Porosity

0.3 0.3

1,900 2,000 2,100 TD = 2,175 feet 16 300 150 2  is the base of the tight limestone just reservoir above unit C. Garver 1 API API Inches Macon County 121152170300 CALIPER T16N, R1E, Sec. 2 150 6 15 GAMMA-RAY DENSITY BULK Pawnee Oil & Gas, Inc. 3 -0.10 663 feet 3 g/cm Density Porosity Corr. 0 25

0.3

1,900 2,000 TD = 2,055 feet 16 150 2 API Inches Macon County 121152147500 CALIPER Parish, James 4 GAMMA-RAY BULK DENSITY T16N, R1E, Sec. 2 15 6 T15N T16N Pawnee Oil & Gas, Inc.  across Auburn trend the in Mt. the Harristown Field showing the lenticular nature of the porosity pinch-outs E Ј Line Datum 3 E -0.10 R1E 1,876 feet 3 MACON g/cm Density Porosity BULK DENSITY

0.3

1,900 2,000 16 150 2 R1W TD = 2,021 feet API Corr. Macon County Inches 121152148400 CHRISTIAN 28 Parish, Gerald 6 0 25 CALIPER

T16N, R1E, Sec. 2

GAMMA-RAY 15 6 Fm FORMATION Shale Group Shale

Racine New Albany New GROUP/ E

Nor Niagaran SERIES Upper Devonian Upper thwest-southeast cross section E–E

Comanche Oil (now Atlantic Energy)

SYSTEM Devonian Silurian Abbreviations: Corr., correction; Fm, Formation; TD, total depth. TD, Fm, correction; Formation; Corr., Abbreviations: within the Silurian impermeable carbonates (lenticular reservoir unit C is most continuous reservoir in the Note study that area). Oil the and Pawnee Gas Inc. 1 well Garverencountered the No. reef/rudstone reservoir but the unit neighboringD, wells are not deep enough to test Note the also lower sequence. the uneven of topography the Silurian upper contact below the Datum New Albany horizon Shale E–E Group. Figure 21

Illinois State Geological Survey Circular 577 23

SYSTEM Devonian Silurian

Devonian -

Niagaran SERIES

Upper

Shale Group Shale FORMATION Racine Formation Racine

New Albany New GROUP/ R URS LRS T15N T16N F Ј 3 150 -0.10 3 F Ј R1E g/cm MACON Density Porosity BULK DENSITY BULK Neutron Porosity F

0.3 15

1,900 2,000 TD = 2,040 feet 150 2 Rothwell 1 API Corr. Macon County 121152155700 Inches CALIPER 0 25 GAMMA-RAY 15 6 16 T16N, R1E, Sec. 21 R1W Pawnee Oil & Gas, Inc. 28 CHRISTIAN 3 -0.10 -0.10 1,323 feet 3  is the base of the tight limestone just below reservoir B. Corr. g/cm 0 25 Density Porosity BULK DENSITY BULK Neutron Porosity

2 0.3 0.3

1,900 2,000 TD = 2,057 feet 16 150 300 Beatty 4 API API Macon County 121152168900 Inches CALIPER GAMMA-RAY T16N, R1E, Sec. 21 15 150 6

Pawnee Oil & Gas, Inc.

URS LRS 3 -0.10 -0.10 1,467 feet 3 Corr. g/cm 0 25 Density Porosity BULK DENSITY Neutron Porosity

0.3 0.3 2

2,000 1,900 2,100 TD = 2,100 feet 16 150 Scherer 2 API Macon County 121152165400 Inches CALIPER GAMMA-RAY T16N, R1E, Sec. 21 15 6  across Auburn trend the showing Mt. the of development reservoir units A, Note B, the and persistence C. of res Triple G Oil Company Ltd. Line Datum 3 -0.10 -0.10 664 feet 3 Corr. g/cm 0 25 Density Porosity BULK DENSITY Neutron Porosity

0.3 0.3 2

1,900 1,950 W est-east cross section F–F TD = 1,966 feet reservoir unit A reservoir unit B reservoir unit C 200 400 Hill Estate #1 API API Macon County 121152168200 Inches CALIPER GAMMA-RAY 200 0 6 16 T16N, R1E, Sec. 20 F Triple G Oil Company Ltd. ervoir unit C and the westward pinch-out of reservoir unit A into a dense Note impermeable also limestone. that the upper part of the upper sequence of the Racine Formation includes reservoir units A and B and the dense impermeable carbonates (cherty limestone interbeds) that thin the toward Triple west, in G 1 Oil Hill well, Company Estate as Ltd. No. a Datum result horizon of erosion. F–F pre-Devonian total URS, upper depth; TD, Racine sequence. LRS, lower Racine correction; sequence; Corr., Abbreviations: Figure 22

24 Circular 577 Illinois State Geological Survey G Ј

2,000

ILS (Ohm-m)

RESISTIVITY

ILD (Ohm-m)

FOC (Ohm-m)

0.2

1,700 1,800 1900

0 TD = 2,049 feet

150

300 McMillen 4-B

API API 120212433300 Christian County SP (mV) Podolsky, Bernard T15N, R1W, Sec. 9

GAMMA-RAY

-200

150 T15N T16N  is the base of the impermeable limestone, 1,174 feet 100 100 R1E

ILS (Ohm-m)

ILD (Ohm-m)

RESISTIVITY

MACON

0 1,700 1,800

0 TD = 1,880 feet McMillen 3-A 120210098600 Christian County SELF- Podolsky, Bernard SP (mV) T15N, R1W, Sec. 4 POTENTIAL

-200 R1W G Ј CHRISTIAN 28 G 2,230 feet 100 100

ILS (Ohm-m)

ILD (Ohm-m)

RESISTIVITY

0 1,700 1,800

0 TD = 1,887 feet Robert Heirs 1 120210057600 Christian County SELF- SP (mV) Podolsky, Bernard T15N, R1W, Sec. 4 POTENTIAL

-200  across Auburn trend the in Mt. Auburn Consolidated the Field Mt. showing the of development reservoir Line Datum 1,322 feet 100 100

ILS (Ohm-m)

ILD (Ohm-m)

RESISTIVITY

1,700 1,800

0 Vail 3-A 120210080500 Christian County SELF- Podolsky, Bernard SP (mV) T15N, R1W, Sec. 4 POTENTIAL

-200

URS

Shale Group Shale FORMATION TD = 1,891 feet

G Formation GROUP/ New Albany New

Ls Racine Cht

SERIES Niagaran Upper Devonian Upper Nor

thwest-southeast cross section G–G reservoir unit A reservoir unit C

SYSTEM Silurian Devonian horizon (Figure 5), just above reservoir C. Abbreviations: Cht Ls, Chouteau Limestone; FOC, focused; ILD, induction log deep; ILS, induction log shallow; SP, induction log ILS, horizon ILD, induction Cht deep; SP, (Figure log Ls, Chouteau focused; 5), FOC, shallow; Abbreviations: Limestone; just reservoir above C. total URS, upper depth; TD, Racine sequence. self-potential; units A and C. units Note A the and irregular C. of topography the upper contact of the Racine Formation and the lateral pinch-out of reservoir unit A into the overlying New Albany Shale as a result of post-SilurianNote also erosion. that 3-A in well, the Vail the Bernard New Albany Shale Podolsky Group caps the reservoir as a result of erosion and of removal the dense cherty limestone that normally overlies reservoir Datum unit line A. G–G Figure 23 Figure 19 Figure

Illinois State Geological Survey Circular 577 25 H Ј

150 F’

API

NEUTRON POROSITY

1,900 2,000

150 reservoir unit A reservoir unit C reservoir unit D TD = 2,074 feet

API Macon County 121152167900 Nolan, Christina 3

GAMMA-RAY T17N, R2E, Sec. 31

0 Watters Oil & Gas Co. Line Datum 3 -0.10 -0.10 10,247 feet 3 Corr. g/cm  is the base of the impermeable limestone 0 25 Density Porosity BULK DENSITY BULK Neutron Porosity

0.3 0.3

1,900 2,000 2,100 TD = 2,175 feet 16 150 2 300 Garver 1 Macon County API API 121152170300 Inches CALIPER T16N, R1E, Sec. 2 GAMMA-RAY 15 6 150 Pawnee Oil & Gas, Inc.

URS LRS 40 23,001 feet DT (Ms/foot)

TIME VARIATION TIME

100 1,900 1,850 2,000 TD = 2,057 feet Dipper 1 SELF- SP (mV) Macon County 121150068100 Atkins and Hale POTENTIAL

-140 0 T16N, R1E, Sec. 28 Ј H T15N T16N 2.5 0.0 26,399 feet 3 25  parallel to Auburn trend the showing Mt. the of development reef and non-reef reservoirs in the upper g/cm Corr. Density Porosity.

0 R1E

0.3 2

1,800 1,900 2,000 16 150 300 MACON TD = 2,007 feet Elder, Veda 1 API API 120212427200 Christian County Inches Pawnee Oil Corp. H CALIPER

T15N, R1W, Sec. 12

GAMMA-RAY BULK DENSITY 15 150 6

R URS LRS FORMATION Ls

New Albany Shale Group Shale Albany New R1W GROUP/ H Cht Racine Fm Racine

28 SERIES

Niagaran Upper Devonian Upper CHRISTIAN

SYSTEM Devonian Silurian part of the sequences of the Racine Formation in Christian and Note Macon the Counties. lenticular nature of the reservoirs that change laterally and vertically to impermeable reservoir carbonate unit facies; C thins out into dense limestone in Elder Oil the Corp. Pawnee 1 well, but changes to shale in facies Oil Watters Nolan the Note and 3 also Gas well. the Co. irregular of surface erosion at the upper contact of both the lower and upper sequences of the Datum Racine horizon Formation. H–H Figure 24 Cross section H–H in the upper part of the upper sequence, horizon e (see Cht Figure Ls, Chouteau 5), Abbreviations: Limestone; just reservoir above C. URS, upper self-potential; Racine sequence. sonic LRS, time lower Racine SP, variation; sequence; DT, correction; Corr.,

26 Circular 577 Illinois State Geological Survey Watters Oil and Gas Company Nolan thin sections, only very faint relicts The depositional model of the Racine No. 3 in Figure 24). of the organic material of the original Formation in the Sangamon Arch is skeleton are visible (Figure 17e, f). that of a homoclinal ramp portion Patch Reef Reservoir Unit D Intercrystalline and moldic porosities (ramp margin and its adjacent seaward can reach more than 25%; the highest margin) of a distally steepened ramp, Based on detailed inspection of well initial production reported was 3,120 just landward of the deep Vincennes cuttings and cores and correlation of barrels of oil per day in the Atkins and Basin (Figure 27). According to Droste geophysical logs from a few wells that Hale Dipper No. 1, T16N, R1E, Sec. 28, and Shaver (1980, 1987), during the have penetrated the lower part of the in Macon County. Middle to Late Silurian Period, a plat- Niagaran Series, a patch reef and asso- form margin (Terre Haute reef bank) ciated rudstone facies are identified in The majority of wells in the study area marked a slope break in southern the upper part of the lower sequence have not tested the lower part of the Illinois and southwestern Indiana that of the Racine Formation (Y. Lasemi Niagaran deposits that include the was facing the deep Vincennes Basin 2009c). The patch reefs (Figures 14 newly recognized patch reefs, and (Figures 1 and 27). Coburn (1986) and 17c, d, e, f) are recognized in the these potentially prolific lower hori- and Whitaker (1988) envisioned a following wells along the Mt. Auburn zons have been mostly overlooked. The gently sloping ramp for the entire Illi- trend (Figures 24, 25, and 26): highest initial production (over 3,000 nois Basin that deepened toward the barrels per day) is associated with this • Atkins and Hale Dipper No. 1, T16N, south and east during Silurian time. type of reservoir; therefore, more pro- R1E, Sec. 28, in the Blackland North Restricted lagoonal and tidal flat facies ductive Niagaran patch reefs are likely Field in Macon County (Figure 25, of the inner ramp were presumably to be found along the Sangamon Arch. depths 2,012–2,020 feet and 2,037– deposited landward of the Mt. Auburn 2,050 feet). trend, toward the Mississippi River Discussion Arch, but are absent due to post-depo- • Pawnee Oil Corp. Garver No. 1, sitional erosion. T16N, R1E, Sec. 2, in the Harristown Depositional Environment Field in Macon County (Figure 25, and Carbonate Platform Although the carbonate ramp model (Ahr 1973, 1998; Read 1985; Burchette depths 2,082–2,103 feet and 2,112– The Racine Formation is composed 2,118 feet). and Wright 1992) is appropriate for mostly of various skeletal components the Illinois Basin, reactivation of old • Watters Oil and Gas Co. Graves No. that include fragments of echino- normal faults along the northeastward 1, T16N, R2E, Sec. 6; Dalton No. 4 derms, bryozoans, brachiopods, and extension of the Reelfoot Rift system and 5, T17N, R2E, Sec. 32; Nolan corals, suggesting deposition in an during the Silurian Period possibly No. 1, T17N, R2E, Sec. 31; and Nolan open marine setting. Coral fragments resulted in the formation of a distally No. 3, T17N, R2E, Sec. 31, all in the were probably derived by storm events steepened ramp (Read 1985) and the Decatur field in Macon County (see from the reefs that had existed farther Vincennes Basin (Figures 1 and 27). Figure 25, depths 2,030–2,035 feet, offshore in the deeper part of the basin. This interpretation is supported by the 2,048–2,051 feet, and 2,069–2,074 The Racine Formation consists of high- thick Lower Devonian basinal deposits feet for the Nolan No. 3 well). energy bioclast grainstone to pack- that thin to zero toward the platform stone (Figure 16c, d), coral patch reef/ area and by the steeper slope along • Pawnee Oil Corp. Elder No. 1, T15N, reef rudstone facies (Figure 17c, d), and the reef trend than in the platform and R1W, Sec. 12, in the Mt. Auburn Con- low-energy, below wave base, bioclastic basinal settings (Droste and Shaver solidated Field in Christian County mudstone to wackestone (Figure 16a, 1980, 1987). In addition, the southeast (Figure 25, depths 1,988–1994 feet, b), silty argillaceous dolomitic lime margin of the Vincennes Basin aligns 2,000–2,007 feet and 2,012–2,017 mudstone/dolomite, and calcareous with the trend of the Reelfoot Rift, feet). shale. These facies were deposited in and the platform margin parallels the a gently sloping ramp with a ramp trend of a northeastward extension The patch reef facies is characterized margin that was roughly parallel to the by porous dolomitized coral reef/reef of the rift (Figure 1), suggesting that Mt. Auburn trend and graded basin- footwall subsidence along the rift- rudstone facies (Figure 17c, d, e, f) and ward (southeast) into muddy carbon- is the lowermost productive horizon related normal faults probably formed ates (Figure 27) similar to the numer- the relatively deep Vincennes Basin recognized in the Racine Formation ous modern and ancient examples (Figures 14, 24, and 25). The patch in southern Illinois. Lower Devonian (Wilson 1975, Tucker and Wright 1990, through Mississippian deep marine reef facies consists of coarsely crystal- Flügel 2010). During deposition of the line dolomite containing remnants deposits (including turbidites) are rec- Racine Formation, long-term third- ognized in the general area of southern and molds of reef-building organisms order sea level fluctuations resulted (mainly corals). The reef-building Illinois (Lineback 1966, 1981; Banaee in the development of two third-order 1981; Cluff et al. 1981; Lineback and skeletons are recognizable in cores cycles consisting of short-term fourth- and hand samples (Figure 17c, d), Cluff 1985; Z. Lasemi et al. 1998, 2003). to fifth-order cycles as a result of short- These deposits, which conformably but the detail of their structure is lost term sea level variation (Figures 13 due to pervasive dolomitization. In overlie the Silurian succession (Norby through 15). 1991, Mikulic 1991, Droste and Shaver

Illinois State Geological Survey Circular 577 27 H Ј

150 F’

API

NEUTRON

POROSITY

1,900 2,000

150 reservoir unit D TD = 2,074 feet

API Macon County 121152167900 Nolan, Christina 3

GAMMA-RAY T17N, R2E, Sec. 31

0 Watters Oil & Gas Co. Line Datum 3 -0.10 -0.10 10,247 feet 3 Corr. g/cm 0 25 Density Porosity BULK DENSITY BULK Neutron Porosity

0.3 0.3

1,900 2,000 2,100 TD = 2,175 feet 16 150 2 300 Garver 1 Macon County API API 121152170300 Inches CALIPER T16N, R1E, Sec. 2 GAMMA-RAY 15 150 6

Pawnee Oil & Gas, Inc.

LRS LRS URS 40 23,001 feet DT (Ms/foot)

TIME VARIATION TIME

100 1,900 1,850 2,000 TD = 2,057 feet Dipper 1 SELF- SP (mV) Macon County 121150068100 Atkins and Hale POTENTIAL

-140 0 T16N, R1E, Sec. 28 Ј H T15N T16N 26,399 feet 2.5 0.0 3 g/cm Corr.

Density Porosity.  (same as Figure 24) showing the occurrence of patch reef reservoirs within the upper part of the lower sequence of the

0 25 R1E

1,800 1,900 2,000 MACON 16 150300 2 0.3 Elder, Veda 1 TD = 2,007 feet API API 120212427200 Christian County H Inches Pawnee Oil Corp. CALIPER

T15N, R1W, Sec. 12

GAMMA-RAY BULK DENSITY

15 150 6

R URS LRS Ls

FORMATION R1W

New Albany Shale Group Shale Albany New

Racine Fm Racine Cht GROUP/ H CHRISTIAN

Cross Upper Devonian Upper Niagaran

SERIES section H–H

SYSTEM Devonian Silurian Racine Formation. Datum Racine has Formation. been changed to the top of a cycle fourth-order at transgressive-regressive the base of a dense within facies the upper Note the sequence. irregular of surface erosion at the upper Cht DT, contacts correction; Ls, Chouteau Corr., of Abbreviations: Limestone; both sequences. URS, upper self-potential; Racine sequence. sonic LRS, time lower Racine SP, variation; sequence; Figure 25

28 Circular 577 Illinois State Geological Survey 35 36 31 32 33 34

R1E -1250

-1300 6 3 1 5 4 2

-1350

7 10 11 12 8 9

-1350

-1400

-1300 18 16 15 14 13 17 T16N

-1400

19 22 23 24 20 21

-1350

-1450

-1300 30 27 26 25 29 28

-1400 -1113 -1325 -1537 -1750

0 1 mi Oil well Section boundary N Abandoned oil well 10-foot contour intervals 0 1.6 km

Dry hole Township boundary

LRS (reservoir unit D) producer

Figure 26 Enlarged portion of part of the regional structural contour map shown in Figure 12. The map portion shows the location of wells that have produced from the upper and lower Racine reservoirs. Encircled wells have produced from the lower Racine reef reservoir unit D. Abbreviation: LRS, lower Racine sequence.

Illinois State Geological Survey Circular 577 29 Tidal flat Restricted lagoon

NW Ramp margin

Outer ramp Mississippi River Arch

land area restricted facies Deep ramp (removed by erosion) (Vincennes Basin) restricted facies SE (removed by erosion) bioclast grainstone shoal facies

coral patch reef/rudstone facies bioclast lime mudstone-packstone/silty argillaceous dolomite/limestone or calcareous shale pinnacle/barrier reefs (absent in the study area)

silty argillaceous limestone/calcareous shale (absent in the study area)

Figure 27 Generalized depositional model depicting the distally steepened ramp of the Illinois Basin during Niagaran time. The depositional facies of the inner ramp (restricted lagoon and tidal flat facies) are absent due to subsequent erosion at the upper contact of the Niagaran sequences and the complete removal of the Silurian deposits toward the northwest in the Mississippi River Arch area. Small coral patch reefs developed seaward of the bioclastic ramp margin, and the large pinnacle reefs were mainly restricted to the distally steepened outer ramp margin. Seaward percolation of the denser dolomitizing fluids from the restricted lagoon, in the direction of arrows, could have been responsible for early dolomitization of the precursor limestone deposits.

1987), further suggest that a distally Arch has recently been documented (Y. 1983). Because the depositional wave steepened ramp could have existed in Lasemi 2009c). These patch reefs are of and current energy were not strong the southern part of the Illinois Basin limited lateral and vertical extent in the enough to cause the reef builders to during the deposition of the Niagaran upper part of the lower sequence of the construct large buildups in the homo- Series. Racine Formation (Figures 4, 14, 21, clinal ramp setting of the Sangamon 24, and 25). They may occur in one to Arch, only bioclast grainstone shoal four horizons with a total thickness of facies and small patch reefs were devel- Patch Reef Versus up to 30 feet (9 m), and they constitute oped—in contrast to the large pinnacle Pinnacle Reef the upper parts of fourth- to fifth-order reefs (Lowenstam 1949, Bristol 1974) The known productive reefs in Illi- shallowing-upward cycles (Figures 14 that developed along the southern nois are pinnacle reefs up to 700 and 25). Illinois platform margin (Shaver et al. feet (210 m) thick (Lowenstam 1949, 1978; Droste and Shaver 1980, 1987). Patch reefs of the Sangamon Arch Bristol 1974) that developed almost Whitaker (1988) suggested that pin- developed on the very gently sloping entirely during the Middle to Late nacle reefs had existed in the shelf homoclinal ramp platform (ramp plat- Silurian Period along the northeast- areas of the Illinois Basin, including the form of Ahr 1973), similar to that in southwest–trending platform margin Sangamon Arch, but that much of their the present-day Persian Gulf (Purser (possibly along a distally steepened thickness had been reduced by pre- 1973). Reef-building metazoans typi- ramp margin) (Figure 27) facing the Devonian erosion. The present study, cally build laterally and vertically along deep Vincennes Basin (Shaver et al. however, does not support the devel- the platform margin, where nutrient 1978, Droste and Shaver 1980, 1987). opment of pinnacle reefs along the supply is abundant and depositional The presence of patch reef reservoirs Sangamon Arch (see Y. Lasemi 2009c energy is very high (Wilson 1975, James in the Niagaran rocks of the Sangamon for further discussion).

30 Circular 577 Illinois State Geological Survey Dolomitization and to those of the associated limestone; Acknowledgments Reservoir Development they change to dolomitic limestone and lose their porosity in both land- The early draft of this article was Detailed subsurface studies of the Silu- ward and basinward directions. reviewed by B.D. Keith at the Indiana rian succession along the Mt. Auburn Geological Survey, Indiana Univer- trend have revealed the presence of • Dolomite layers are resistant to sity, who provided much critical and several lenticular dolomitized reser- chemical compaction and the constructive advice. Reviews of the voirs (permeability pinch-outs) that resulting cementation (Brown 1997, final version of the manuscript by B. are interlayered with laterally extensive Lucia 2004, Ehrenberg et al. 2006), Huff, J.H. Goodwin, D.G. Morse, and impermeable limestone intervals. The thus preserving their original poros- D.G. Mikulic at the Illinois State Geo- reservoirs generally constitute the ity. Laterally continuous interlayered logical Survey greatly improved the upper parts of fourth-order transgres- lime mudstone to packstone facies, manuscript. The authors thank C.K. sive-regressive cycles within the Racine however, compact and lose their Nimz for editorial improvement of Formation (Figures 13, 14, and 15). The initial high porosity during burial the manuscript and M.W. Knapp for laterally discontinuous reservoirs are diagenesis. graphic assistance and layout. S. Lang, cyclic and are commonly surrounded an independent petroleum geologist, by impermeable pure limestone or It appears that dolomitization ini- provided core samples of Elder Well No. finely crystalline silty argillaceous tially affected the carbonate lime mud 1. R. R. Mumm and J. Freiburg at the dolomitic limestone/dolomite (Figures matrix of the groundmass (Figures 16b Illinois State Geological Survey Geo- 13, 14, 15, 21, 22, 23, and 24), suggest- and 17b) and the more susceptible logical Samples Library and J. Brenizer ing that sea level fluctuations and fossil fragments; crinoids were the (now with Rex Energy Corp.) provided seawater chemistry were the primary last grains to be dolomitized (Figures assistance with cores and samples; J. controls for early dolomitization of the 16d and 17b). Later reservoir porosity Dexter prepared the core photographs. compartmentalized Silurian reservoirs. enhancement likely occurred during This work was partially supported by prolonged subaerial exposure, result- U.S. Department of Energy Contract Based on comparisons with their ing in partial dissolution of the dolo- number DE-FC26-04NT15510. The use modern counterparts, the initially mite crystals along the pore walls by of GeoGraphix software was made pos- impermeable mud-rich precursors had fluids undersaturated with respect to sible through the University Grant Pro- an initial porosity of around 70% (Enos dolomite (Figure 17a). gram provided by Landmark Graphics. and Sawatsky 1981, Z. Lasemi et al. 1990), which was reduced to less than 5% during burial diagenesis. Dolomi- Conclusions References tization was probably caused by reflux The Racine Formation was deposited Ahr, W.M., 1973, The carbonate ramp: of slightly evaporated seawater (Sun in a gently sloping ramp with a ramp An alternative to the shelf model: 1995) in the inner ramp setting; the margin roughly parallel to and south- Transactions of the Gulf Coast Asso- initial grainstone facies of the ramp east of the trend of the Sangamon Arch. ciation of Geological Scientists, v. 23, margin and the adjacent open marine The Racine Formation is subdivided p. 221–225. bioclastic packstone-wackestone acted into two depositional sequences that, as a conduit for the dolomitizing sea in the Mt. Auburn trend, contain mul- Ahr, W.M., 1998, Carbonate ramps, water. As shown by the depositional tiple dolomitized reservoirs. 1973–1996: A historical review, in model (Figure 27), the slightly evapo- V.P. Wright and T.P. Burchette, eds., rated, denser water in the back barrier The reservoirs are compartmental- Carbonate ramps: London, England, inner ramp lagoon, which had higher ized and are commonly surrounded Geological Society of London, Spe- magnesium-to-calcium ratio than by impermeable limestone, suggesting cial Publication 149, p. 7–14. normal seawater, could have perco- fluctuations in sea level and seawater lated through the sediments deposited chemistry as the primary controls for Banaee, J. 1981, Microfacies and depo- during the previous sea level cycles, early dolomitization of the Silurian sitional environment of the Bailey leading to early dolomitization and reservoirs. The interlayered undolomi- Limestone (Lower Devonian), south- formation of laterally discontinuous tized limestone was compacted during western Illinois, U.S.A., a carbonate dolomite lenses. The dolomitizing fluid burial diagenesis, but the dolomite turbidite: University of Illinois at apparently could not reach the distal resisted compaction, thus retaining its Urbana-Champaign, M.S. thesis, portion and the lower part of the previ- porosity. 122 p. ously deposited limestone cycle, lead- Examination of numerous wells indi- Bond, G.D., P.A. Nickeson, and M.A. ing to lenticular dolomite bodies. This cates that most of those drilled thus far Kominz, 1984, Breakup of a super- interpretation is supported by these have tested the uppermost part of the continent between 635 Ma and 555 findings: Niagaran succession; only a few wells Ma: New evidence and implications • The lenticular dolomitized reser- have tested the lower reservoirs that for continental histories: Earth and voirs show gradational contacts include the newly recognized patch Planetary Science Letters, v. 70, p. with the underlying limestone, and reefs, so the potentially prolific lower 325–345. their skeletal remains are identical horizons have been mostly overlooked.

Illinois State Geological Survey Circular 577 31 Bristol, H.M., 1974, Silurian pinnacle Embry, A.E., and J.E. Klovan, 1971, A Lasemi, Y., 2009b, Carbonate sequence reefs and related oil production in Late Devonian reef tract on north- stratigraphy and reservoir develop- southern Illinois: Illinois State Geo- eastern Bank Island, Northwest Ter- ment: The Middle Silurian Racine logical Survey, Illinois Petroleum ritories: Bulletin of Canadian Petro- Formation in the Sangamon Arch, 102, 98 p. leum Geology 19, p. 730–781. west-central Illinois: Program and Abstracts, AAPG Eastern Section Brown, A., 1997, Porosity variation in Enos, P., and L.H. Sawatsky, 1981, Pore Thirty-eighth Annual Meeting, Sep- carbonates as a function of depth: networks in Holocene carbonate tember 20–22, 2009, Evansville, Indi- Mississippian Madison Group, sediments: Journal of Sedimentary ana, p. 42–43. Williston Basin, in J.A. Kupecz, J. Petrology, v. 51, p. 961–985. Gluyas, and S. Bloch, eds., Reservoir Lasemi, Y., 2009c, Oil potential seen quality prediction in sandstones and Flügel, E., 2010, Microfacies of carbon- in Silurian reef-related reservoirs in carbonates: Tulsa, Oklahoma, AAPG ate rocks: Berlin, Germany, Springer- Illinois Sangamon Arch: Oil and Gas Memoir 69, p. 29–46. Verlag, 984 p. Journal, v. 107, p. 36–40. Burchette, T.P., and V.P. Wright, 1992, Golonka, J., and W. Kiessling, 2002, Lasemi, Z, R.D. Norby, and J.D. Tre- Carbonate ramp depositional sys- Phanerozoic time scale and defini- worgy, 1998, Depositional facies and tems: Sedimentary Geology, v. 79, tion of time slices, in W. Kiessling, sequence stratigraphy of a Lower p. 3–57. E. Flügel, and J. Golonka, eds., Carboniferous bryozoan-crinoidal Phanerozoic reef patterns: Tulsa, carbonate ramp in the Illinois Basin, Buschbach, T.C., and D.R. Kolata, 1991, Oklahoma, Society for Sedimentary mid-continent USA, in V.P. Wright Regional setting of Illinois Basin, in Geology, Special Publication 72, and T.P. Burchette, eds., Carbonate M.W. Leighton, D.R. Kolata, D.F. Olts, p. 11–20. ramps: London, England, Geological and J.J. Eidel, eds., Interior cratonic Haq, B.U., and S.R. Schutter, 2008, A Society of London, Special Publica- basins: Tulsa, Oklahoma, AAPG tion 149, p. 369–395. Memoir 51, p. 29–55. chronology of Paleozoic sea-level changes: Science, v. 322, p. 64–68. Lasemi, Z, R.D. Norby, J.E. Utgaard, Cluff, R.M., M.L. Reinbold, and J.A. W.R. Ferry, R.J. Cuffy, and G.R. Dever Lineback, 1981, The New Albany James, N.P., 1983, Reef environment, in P.A. Scholle, D.G. Bebout, C.H. Jr, 2003, Mississippian carbonate Shale Group of Illinois: Illinois State buildups and development of cool- Geological Survey, Circular 518, 83 p. Moore, eds., Carbonate depositional environment: Tulsa, Oklahoma, water-like carbonate platforms in Coburn, G.W., 1986, Silurian of the AAPG Memoir 33, p. 345–440. the Illinois Basin, midcontinent Illinois Basin: A carbonate ramp: Oil USA, in W.M. Ahr, P.M. Harris, W.A. and Gas Journal, v. 84, p. 96–100. Kluessendorf, J., and D.G. Mikulic, Morgan, and I.D. Somerville, eds., 1996, An early Silurian sequence Permo-Carboniferous carbonate Droste, J.B., and R.H. Shaver, 1980, Rec- boundary in Illinois and Wisconsin, platforms and reefs: Tulsa, Okla- ognition of buried Silurian reefs in in B.J. Witzke, G.A. Ludigson, and homa, AAPG Memoir 83, p. 69–95. southwestern Indiana: Application J.E. Day, eds., Paleozoic sequence to the Terre Haute bank: Journal of stratigraphy: Views from the North Lasemi, Z., P.A. Sandberg, and M.R. Geology, v. 88, p. 567–587. 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