OIL-RESERVOIRS IN GRAINSTONE APRONS AROUND BRYOZOAN MOUNDS, UPPER HARRODSBURG LIMESTONE, MISSISSIPPIAN, ILLINOIS BASIN

Several oil pools have been discovered recently in the upper Harrodsbu rg Limestone (middle Mississippian) of the Illinois basin. A depositio nal model for bryozoan mound complexes has allowed more successful exp loration and development in this play. In the Johnsonville area of Way ne County, Illinois, three lithofacies are dominant in the upper Harro dsburg: (1) bryozoan boundstones, (2) bryozoan grainstones, and (3) fo ssiliferous wackestones. Bryozoan boundstones occur as discontinuous m ounds and have low porosity. Although bryozoan boundstones are not the main reservoir lithofacies, they are important because they influence d the distribution of bryozoan grainstones and existing structure. Bry ozoan grainstones have intergranular porosity and are the main reservo ir rock. Bryozoan fragments derived from bryozoan boundstone mounds we re concentrated in grainstones around the mounds. Fossiliferous wackes tones are not porous and form vertical and lateral seals for upper Har rodsburg grainstones. Fossiliferous wackestones were deposited in deep er water adjacent to bryozoan grainstone aprons, and above grainstones and boundstones after the mounds were drowned. Upper Harrodsburg oil reservoirs occur where grainstone aprons are structurally high. The Ha rrodsburg is a good example of a carbonate mound system where boundsto ne cores are not porous, but adjacent grainstones are porous. Primary recovery in these upper Harrodsburg reservoirs is improved by strong p ressure support from an aquifer in the lower Harrodsburg. Unfortunatel y, oil production is commonly decreased by water encroaching from that underlying aquifer.