Print this pageDiagenesis and Secondary Oil Migration in Mississippian Reservoir Sandstones, Illinois Basin: Influence of Faults and Fractures
PITMAN, J. K., and M. D. LEWAN
The Illinois Basin is a prolific, mature hydrocarbon province that generated and expelled large amounts of oil in the late Paleozoic when major tectonic and ore-forming events were taking place in the Mid-continent Region. A significant fraction of the oil distributed in sandstone reservoirs is associated with major folded and faulted structures, suggesting that vertical migration corridors may have predominated. The hydrocarbon carrier systems, migration extent, and entrapment sites, however, are poorly understood. Organic-geochemical (Rock-Eval) data indicate that Devonian source rocks are mature and expelled oil throughout the central and southern part of the basin, thus lateral migration distances may be shorter than previously thought. Basin-scale diagenetic patterns combined with regional porosity and permeability trends are being studied and related to hydrocarbon migration and entrapment. Throughout the basin, carbonate precipitation and quartz cementation took place after deposition and later during deep burial, significantly reducing the quality of many sandstone reservoirs. Saddle ferroan dolospar localized in some sandstones in southern Indiana suggests the influence of high temperature (ore-forming?) fluids during precipitation. Episodes of carbonate dissolution, some possibly linked with hydrocarbon generation followed by illite-chlorite-kaolinite crystallization, also occurred in the basin. Well-cemented sandstones favor migration along faults and fractures whereas porosity-enhanced sandstones are compatible with porous-media migration.