3-D Geometries and Porosity Trends of Subsurface Ooid Shoal Reservoirs in the Mississippian Ste. Genevieve Formation of the Illinois Basin, USA

A. C. Gibson
University of Illinois at Urbana- Champaign, Dept. of Geology, Urbana, IL

The Ste. Genevieve Formation is an economically important hydrocarbon reservoir in the Illinois Basin, whose further exploration depends upon knowing the geometries and regional trends of subsurface oolitic limestones. The main lithologic compositions of these reservoirs are oolitic grainstones to packstones and microcrystalline dolomites. Entrapment of hydrocarbons in these lithologies is a result of structural closure, coincident with changes in depositional facies both laterally and vertically into a low-permeability mudstones and wackestones. Three-dimensional modeling of the Ste. Genevieve has revealed northeast-southwest trending ooid shoal bars. This trend is perpendicular to the regional strike, indicating the influence of strong tidal currents flowing northeast-southwest. These tidal currents also distributed terrigenous clastics from a drainage system to the northeast across a large paleoshoal complex. This paleoshoal complex is interpreted to extend approximately 80 km from Lawrence County to the Clay City Consolidated field in Clay County, Illinois.

The distribution of porosity is directly controlled by the orientation and location of these ooid bars. Most of the higher porosities (>7%) occur as either residual primary interparticle porosity in the oolites or microintercrystalline porosity in microcrystalline dolomites. Periodic subaerial exposure trapped freshwater lenses within the ooid bars causing initial dolomitization at the base of the ooid bars and in the lime-mud-rich sediments directly underlying the ooid bars. Large interbar lenticular layers of microcrystalline dolomite also occur along the flanks of these ooid bars, possibly indicating a deeperburial diagenesis of the surrounding facies.

AAPG Search and Discovery Article #90902©2001 AAPG Foundation Grants-in-Aid