Large reserves of oil and some gas have been discovered and produced in the Southern Oklahoma folded belt and future reserves remain to be found, although they will not approximate those of the past. As of January 1, 1968, approximately 2.25 billion bbl of the known recoverable oil reserves had been produced. Approximately 90 percent (2.05 billion bbl) was produced from Pennsylvanian, Ches- teran, and Simpson sandstone reservoirs, and much (1.18 billion bbl) of this was from Pennsylvanian rocks.
Future significant reserves probably will be found in these same three sandstone reservoirs; the Simpson should be the most important and the Pennsylvanian probably the least important. Production from carbonate rocks in the past has not been great, and it is anticipated that future reserves will be small.
In the Southern Oklahoma folded belt, most of the oil is trapped in anticlines or fault traps, although sandstone pinchout and truncation contribute an important stratigraphic factor. Past exploration has been essentially along the margins of the basins and on the uplifts. These areas have been tested extensively and, though additional reserves will continue to be found, the total probably will be small.
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.