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 Cambrian), 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.
Figures & Tables
The geology of the entire United States, including the continental shelf and slope, was studied by petroleum geologists to determine its petroleum potential. Prospective areas of the 11 regions were assessed qualitatively and, usually, quantitatively.
The prospective basinal area covers approximately 3.2 million sq mi (statute; 8.3 million sq km) and contains approximately 6 million cu mi (25 million cu km) of sedimentary rock above basement or 30,000 ft (9,144 m). Other less prospective areas are, in the aggregate, large.
The prospective area has not been explored adequately. Many high-potential areas are indicated by the geology and extent of exploration, particularly in parts of Alaska, California, Colorado, Louisiana, Mississippi, Montana, New Mexico, North Dakota, Oklahoma, Texas, Utah, and Wyoming, and in parts of the offshore of Alaska, California, Louisiana, and Texas. The prospective Atlantic, Florida, and Alaska continental shelves, and the entire continental slope, barely have been touched by drilling, and other prospective areas and depths on land and the continental shelf remain largely unexplored.
Estimates of potential crude oil reserves of the basinal area only, exclusive of known reserves, range from 227 to 436 billion bbl of original oil in place. The potential probably exceeds the mean of 332 billion bbl. Approximately 32 percent of the oil in place would be recoverable at known rates of recovery. Ultimately, the rate of recovery may reach 60 percent.
Estimates of potential natural gas reserves exclusive of known reserves range from 595 to 1,227 trillion cu ft of recoverable natural gas. The gas potential also probably exceeds the mean of 911 trillion cu ft.
The ultimate petroleum potential of the United States, including known reserves, may exceed 432 billion bbl of crude oil, 1,543 trillion cu ft of natural gas, and 49 billion bbl of natural gas liquids.
Finding and developing the large petroleum potential will require a great amount of drilling because a significant percentage of the visualized undiscovered crude oil and natural gas is in stratigraphic traps, combination stratigraphic and structural traps, reefs, and complex structural situations. Estimates of future domestic demand call for accelerated exploration. To the extent that policies of industry and government militate against accelerated exploration, particularly drilling, a high percentage of the petroleum resources of the United States will not be reduced to possession.