Paleozoic rocks range in thickness from 2,800 ft (850 m) near the crest of the Findlay arch to 13,000 ft (3,960 m) at the Ohio and West Virginia border. All systems of the Paleozoic Era are present. Regional dip on top of the monoclinal basement structure is east and southeast, increasing from 40 ft (12 m) per mile in central Ohio to 150 ft (45 m) per mile in southeastern Ohio.
Oil and gas are produced from rocks in all Paleozoic systems except the Permian. Most present oil and gas production is from stratigraphic traps in the Silurian sandstones of eastern Ohio and from Cambrian dolomite in the shallow Appalachian basin of central Ohio. Pennsylvanian, Mississippian, and Devonian rocks will continue to be explored but will not add significantly to reserves. New Silurian ''Clinton" fields will be found, as indicated by the recent (1968) discovery of the Clays- vilie field in Guernsey County.
Cambrian-Ordovician strata in the deeper part of the basin of eastern Ohio are relatively unexplored and have the greatest potential for adding to reserves. The increasing volume of geophysical and geological information should aid in discovery of the deeper stratigraphic and structural traps believed to be present in the area.
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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.