Carbonate sequence stratigraphy and reservoir development; the Middle Silurian Racine Formation in the Sangamon Arch, west-central Illinois

The Racine Formation, the upper unit of the unconformity bounded Niagaran succession, in western Illinois is the only petroleum producer in the southern flank of the Sangamon Arch. It is equivalent to Moccasin Springs Formation of southern Illinois and consists of interbedded limestone, dolomite, silty argillaceous dolomitic limestone or dolomite and calcareous shale. The Racine Formation unconformably underlies the Upper Devonian-lowermost Mississippian New Albany Shale Group and sharply overlies the lower Niagaran Joliet Formation. Detailed sequence stratigraphic analysis indicates that the middle part of the Racine Formation is erosionally truncated and subsequently overlapped by the younger Niagaran deposits throughout the study area, signifying a pronounced unconformable boundary. This prominent intra-Racine unconformity subdivides the Racine Formation into two depositional sequences comprising several carbonate reservoir units within the highstand systems tracts. The reservoir units include dolomitized skeletal mudstonegrainstone, coral patch reef, or reef rudstone facies. They were deposited along a southwest trending ramp margin that was roughly parallel to the Sangamon Arch trend and graded basinward into muddy open marine carbonates below wave base. The reservoir facies grade laterally and vertically into laterally extensive impermeable and generally carbonate facies and constitute the upper part of meter-scale, shallowing-upward depositional cycles showing aggradational and progradational stacking patterns in the highstand systems tracts. Depositional and sequence stratigraphic models developed in this study will help to better understand reservoir development and predict occurrences of productive Silurian rocks in other areas of the Sangamon Arch.