Representatives of the Illinois State Geological Survey, Indiana Geological Survey, Kentucky Geological Survey, Tennessee Division of Geology, and Marathon Oil Company studied the petroleum geology and prospects for future production in NPC Region 9. Region 9 includes the Illinois basin (108,000 cu mi or 450,000 cu km of sedimentary rocks), the Cincinnati arch (57,000 cu mi or 237,500 cu km), and the northern part of the Mississippi embayment (39,000 cu mi or 162,500 cu km).

The best prospects for new oil production are: (1) sandstones of the Chesteran Series (Upper Mississippian) south of the Rough Creek fault zone in southern Illinois and western Kentucky; (2) bioclastic carbonate rocks of the Mammoth Cave-Knobs (Middle and Lower Mississippian and Upper Devonian), especially south of the Rough Creek fault zone; (3) reefs and local structural traps developed in carbonate rocks of the Hunton (Middle and Lower Devonian and Silurian) in the Illinois basin and porous zones beneath unconformities at the top of and within the Hunton in much of the region; (4) fracture-controlled dolomitized streaks in the Ottawa (Middle Ordovician) and sandstones near its base; (5) porous dolomite associated with an unconformity at the top of the Knox (Lower Ordovician, Upper ambrian), and sandstone lenses and solution openings within the unit; and (6) sandstones of the Potsdam (Cambrian) in the southern part of the region, especially in the Rome trough.

Thickness maps, structure maps, and geologic cross sections incorporating new data show that about half of the sedimentary-rock volume is in the deeper, relatively untested part of the sedimentary column.