The Idaho-Wyoming Overthrust belt is a part of the Rocky Mountain orogenic complex. The belt is an area of severely deformed Paleozoic and Mesozoic strata characterized by westward-dipping thrust faults. Eastward movement on these faults has resulted in considerable shortening of the sedimentary blanket by the placement of older rocks over younger rocks. Four or five major thrust faults, lesser thrusts, and folds, tear faults, and "post-thrusting" normal faults have resulted in the complex structure of the orogenic belt. Many untested surface anticlines are present within the area, and some of them may reflect closure in lower thrust plates.
The present width of the Overthrust belt is roughly coincident with the average eastern hinge line of a Paleozoic and early Mesozoic miogeosyncline whose depocenter was in south-central and southeastern Idaho. The transition from a thick marine section to a thinner shelf section in western Wyoming provides both source and reservoir beds.
Because of shows and evidence of porosity in test wells and/or surface exposures, the Mississippian Brazer-Madison, Pennsylvanian Weber-Wells, Triassic Thaynes and Nugget, and Jurassic Twin Creek strata are considered to be primary potential reservoirs.
Approximately 78 significant test wells have been drilled within the Overthrust belt, but only 16 of 66 surface anticlinal trends have been tested. Because asymmetry and greater complexity with depth have been the rule, most of these tests probably have not been on crestal positions at depth. Fewer still have been based on seismic data.
Although not now productive, the Idaho-Wyoming Overthrust belt meets the classic requirements of a petroleum province, i.e., the presence of reservoir source rock and caprock under structural deformation. A volumetric comparison of this area with a maturely explored area of Wyoming indicates that a "speculative" 3.3 billion bbl of recoverable reserves might be present in the area. Despite rugged topography and complex structure, an integrated exploration program could develop substantial reserves within the Overthrust belt.
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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.