Pre-Mississippian Hydrocarbon Potential of Illinois Basin
The Illinois basin is primarily a Paleozoic epeirogenic basin located in the east-central United States. Taken at its broadest possible definition, this basin contains a maximum of 20,000 ft of sedimentary rocks. These represent every Phanerozoic system except the Triassic and Jurassic.
Seven important tectonic episodes are recognized. These begin with the establishment of Eocambrian basement rift faults, followed by six rejuvenation events of varying magnitude.
More than 3.5 billion bbl of oil have been produced from the Illinois basin, mainly from Pennsylvanian and Mississippian rocks. These rocks represent only 20% of the total basin sedimentary volume. Source rock maturation studies suggest that none of this oil is indigenous to the Pennsylvanian n or Mississippian, but all has migrated upward from at least three pre-Mississippian sources. If basin sedimentary volume is taken to be roughly proportional to hydrocarbon reserves, there may be as much as 12 billion BOE remaining to be found in the largely untested pre-Mississippian of the Illinois basin.
A thermal history model and Lopatin analysis suggest that oil generation began in Ordovician time and continued through the Jurassic in the deepest part of the basin.
At the present stage of exploration, the Hunton Megagroup (Silurian-Devonian) is recommended as the primary pre-Mississippian drilling target. However, understanding the interplay of the pre-Middle Devonian unconformity with contemporaneous paleotopographic-paleobathymetric expression of prospective features is critical to successful Hunton porosity prediction. This interplay is demonstrated at Centralia and Sandoval fields, Clinton and Marion counties, Illinois.
AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.