Future Petroleum Resources in Post-Mississippian Strata of North, Central, and West Texas and Eastern New Mexico
J. K. Hartman, Lee R. Woodard, 1971. "Future Petroleum Resources in Post-Mississippian Strata of North, Central, and West Texas and Eastern New Mexico", Future Petroleum Provinces of the United States—Their Geology and Potential, Volumes 1 & 2, Ira H. Cram
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Most of the Pennsylvanian Period in Region 5 was characterized by expanding seas between emergent landmasses, some of which had been raised at the beginning of the period and others of which were uplifted during Early and mid-Pennsylvanian time. Foreland positive areas supplied clastic detritus to parts of the intervening basins, and outlying geosynclines received most of the debris from nearby mountain ranges. Carbonate layers were deposited on marginal shelves, and organic limestone reefs were built at various localities within and adjacent to the shelf areas. After a major tectonic episode in earliest Permian time that affected many areas of previous crustal weakness, the deep basins began to fill with fine clastic sediment as carbonate sediment accumulated along marginal platforms. As the seas became progressively more restricted, evaporite deposition encroached over the rest of the Permian basin. A relatively thin veneer of Cretaceous, Tertiary, and Quaternary sediments completed the sequence.
Crude oil and natural gas are found in structural and stratigraphic traps in sandstone and limestone reservoirs, including reefs. Total petroleum resources yet to be discovered in Pennsylvanian and Permian reservoir rocks are estimated to be 41.9 billion bbl of in-place crude oil and 43.8 trillion cu ft of in-place natural gas. Addition of these amounts to the known Pennsylvanian and Permian quantities that remain in place after production to year-end 1967 gives 116 billion bbl of crude oil and 83.2 trillion cu ft of natural gas.
Much of the undiscovered petroleum is expected to be contained in stratigraphic traps in the less densely drilled areas, and may be present as major accumulations. With all phases of exploratory activity declining, it will take more creative thinking and coordinated effort to find the remaining petroleum.
In addition to the oil and gas normally considered potentially recoverable, other petroleum hydrocarbons are believed to be disseminated in sedimentary rocks and dissolved in subsurface waters. Although the volumes of the disseminated hydrocarbons and the dissolved gaseous hydrocarbons are very large, these hydrocarbons are categorized as very speculative and unobtainable from an economic standpoint.
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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.