Exploration for petroleum in the Cook Inlet subprovince started more than 70 years ago, but little petroleum was found until 1957, when Richfield Oil Corporation discovered oil at Swanson River field.
About 60,000 ft (18,290 m) of Mesozoic and Cenozoic sedimentary rock is present in the Cook Inlet basin. The Mesozoic rocks of marine origin crop out around the rim of the basin and do not appear to be favorable as reservoirs. Recent subsurface data indicate that they may have reservoir characteristics in the deeper part of the basin.
All Tertiary strata younger than the Chickaloon Formation are included in the Kenai Group. They are divided into the West Foreland, Hemlock, Tyonek, Beluga River, and Sterling Formations. These apparently nonmarine rocks include interbedded sandstone, claystone, siltstone, and coal. Reservoir rocks are present throughout the sedimentary section.
In structural and tectonic history, the subprovince is related closely to the Alaskan orocline, which dominates the southern half of Alaska. Four north-trending anticlines containing oil and gas have been mapped in the upper Cook Inlet.
As of December 1968, the Cook Inlet had produced 160 million bbl of oil at an average rate of 1,213 bbl per day per well from five fields, and 162 billion cu ft of gas from 15 fields. Estimated proved in-place reserves are 2.6 billion bbl of oil and 5 trillion cu ft of gas. The industry expects to recover a little more than 1 billion bbl.
Additional reserves should be found by drilling in unexplored areas, by deeper exploratory drilling in the Mesozoic section, and by searching for accumulations in stratigraphic traps. Exploratory activity has proceeded more slowly than elsewhere because of the high cost of operation, severe weather, and logistic problems.
The oil industry ultimately can be expected to discover additional estimated potential in-place reserves of 7.9 billion bbl of oil and 14.6 trillion cu ft of gas in the Cook Inlet subprovince.
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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.