Geometry And Porosity Trends in Subsurface Ooid Shoals of the Mississippian Ste. Genevieve Formation,

Illinois Basin, USA

By

GIBSON, ANTHONY C., and FOUKE, BRUCE W.

Department of Geology, University of Illinois Urbana-Champaign, 1301 West Green Street, Urbana, IL 61801,

LEETARU, HANNES E.

Illinois State Geological Survey, 615 East Peabody Dr., Champaign, IL 61820

     Genevieve Formation, an economically important Illinois Basin reservoir, requires better constraints on depositional geometry and trends of subsurface oolitic limestone. Three-dimensional modeling of subsurface core data has identified NE-SW trending ooid shoal bars that indicate strong NW-SE tidal currents were active at the time of deposition. Terrigenous siliciclastics extend for 80 km across the shoal complex from Lawrence County to the Clay City Consolidated field. The distribution of porosity is directly controlled by the orientation and position of these ooid bars. High porosity (> 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 contributed to early 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 deeper-burial diagenesis of the surrounding facies. Entrapment of hydrocarbons in these lithologies is a result of structural closure, coincident with lateral and vertical changes low-permeability mudstones into wackestones. Future exploration and development strategies in the Ste. Genevieve Formation should incorporate the regional northeast trend of the ooid shoals. Future plays will target high porosity dolomitic reservoirs in the overlying oolite sand bodies as well as along the interbar flanks of the ooid shoals.