Lenticular bodies of porous, permeable ooid grainstone in the upper Ste. Genevieve Formation (Valmeyeran, middle Mississippian) form small oil reservoirs in Willow Hill field, Jasper County, Illinois. The ooid grainstones occur in a swarm of elongate, convex-upward bodies 0.2-2.5 mi (0.32-4.0 km) long by 500-3000 ft (150-1000 m) across and up to 15 ft (5 m) thick. They are interpreted as largely marine shoal deposits that developed along a bioclastic carbonate-sand shoal of low relief and NW-SE trend. Some show evidence of exposure and residence of freshwater lenses. Lime mudstones form capping and updip seals.

Early cements, mainly meteoric in origin, prevented physical compaction and "propped" original pores. Fringing isopachous cements, interpreted as phreatic, resulted in systems of well-connected pores. Ooid grainstones with this combination of cements have porosities () of 6.0-17.2% (average 12.7%) and permeabilities (k) of <0.1-228 md (average 113 md). In other ooid grainstones, meniscuslike cements (vadose) also propped original pores, but were overgrown by later blocky burial cement that resulted in less well-connected pores and poor to non-reservoir facies with s of 5.2-17.9% (average 12.8%) but ks of <0.1-1.0 md, with an average of 0.13 md. In associated bioclastic grainstones, and k values are somewhat less than those of the reservoir-quality ooid grainstones due to syntaxial cement around uncoated crinoid grains.

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