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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 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 IL SANGAMON ARCH Mt. Auburn Trend CR ILLINOIS BASIN THB IN CR MO SI THB VB VINCENNES BASIN RR RCG CR MO KY AR TN 0 30 mi N 0 48 km CR 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 dolomi- tization 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. a b c © 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.
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