ABSTRACT: Paleohydrology of the Illinois Basin: Oil Migration, Ore Genesis, and Sediment Diagenesis
BETHKE, CRAIG, MING-KUO LEE, and KURT LARSON, Department of Geology, University of Illinois, Urbana, IL
Developing an understanding of past groundwater flow within the Illinois basin provides a basis for interpreting the distribution of petroleum reservoirs, genesis of metallic ores, and origin of late diagenetic cements. Oil found in reservoirs in central Illinois probably migrated about 150 km northward from the Devonian-Mississippian source beds that produced it. Source beds became mature at the end of the Paleozoic or later, probably after the basin had infilled and its sediments compacted. Oil migrated along an unconformity beneath the New Albany shale in response not only to the oil's buoyancy, but the hydrologic drive set up by tectonic uplift of the southern basin during the Mesozoic or latest Paleozoic.
Uplift in the south likely drove the flow regime that formed the hydrothermal ores of the Upper Mississippi Valley mineral district. Numerical modeling suggests that groundwaters, which migrated slowly through the strata that hosted oil migration, moved rapidly through underlying Cambrian and Ordovician aquifers. Brines in deep strata carried heat and dissolved salts and metals as they migrated toward the basin's northern margin. Successful models account for heat conduction as well as salt diffusion into the basin from the underlying continental crust.
Groundwater migration further helps explain the origin of dolomite and potassium feldspar that occur as late authigenic cements. Reaction modeling shows that brines from the deep basin precipitated these minerals when they migrated toward basin margins. The volume of late dolomite observed in basin strata, assuming flow rates predicted by numerical modeling, suggest that hot brines probably invaded northern strata for an interval of less than a million years.
AAPG Search and Discovery Article #91013©1992 AAPG Eastern Section Meeting, Champaign, Illinois, September 20-22, 1992 (2009)