The Santa Maria province in Santa Barbara and San Luis Obispo Counties, California, has been well explored during the past 70 years. Fifteen commercial oil fields, which had produced 556.5 million bbl of oil to January 1, 1969, were discovered between 1902 and 1952. No significant fields have been discovered during the past 20 years. The area and volume of Tertiary sedimentary rocks of the province are approximately 1,700 sq mi (4,400 sq km) and 1,100 cu mi (4,600 cu km), respectively. Production is from fractured upper and middle Miocene Monterey siliceous and cherty shale and chert, and from sandstone units of middle Miocene and Pliocene-Miocene ages. The Monterey is the reservoir of 75 percent and the source of at least 80 percent of the oil that has been produced. More than 95 percent of the cumulative production has come from fields in the central part of the province which produce from either anticlinal or over- lap-truncation traps. Gravity ranges from 6° to 33° API and generally is lower than 25°. The quantity of known oil originally in place is approximately 4 billion bbl. It is estimated that from 100 million to 600 million bbl of oil in place will be found in the central area in reservoirs that are now productive, and near the northern and northeastern provincial borders in older (early Miocene to Late Cretaceous) clastic reservoirs.
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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.