Consideration of the geology and hydrocarbon potential of the Klamath Mountains and northern and north-central Coast Ranges provinces leads to the conclusion that the area probably lacks significant reserves. It is remotely possible that highly speculative reserves of indeterminate quantity and type could be found by explbration beneath the Franciscan assemblage for unaltered Paleozoic rocks similar to those originally present in the Klamath Mountains.
The geologically complex area is divided by the South Fork Mountain fault into the Klamath Mountains province in the northeast and the northern and north- central Coast Ranges province in the west.
In the Klamath Mountains province, a section of Paleozoic and Mesozoic metasedimentary and volcanic rocks as thick as 50,000 ft (15,240 m) is broadly intruded by granitoid plutons and mafic and ultramafic rocks. The regional structure of the province is believed to be an antiform with its western flank overturned and thrust westward over partially coeval sedimentary rocks along the South Fork Mountain and related thrust faults. All the sedimentary rocks have been severely altered by local and regional metamorphism, and, as a result, no oil or gas reserves are envisioned in the Klamath Mountains.
The northern Coast Ranges province consists predominantly of Franciscan-assemblage graywacke, volcanic rocks, and metamorphic rocks. It is divided into two Tertiary subprovinces. One is in the northwestern part of the province and the other is in the southeastern sector. The regional structure of the northern Coast Ranges province is a NW-SE-trending antiform with oblique faults and minor folds subparallel with the South Fork Mountain fault. This fault crosses the province in the north but curves southward and forms the eastern boundary of the province in the south.
Since 1864 small seepages of heavy oil have been known and a little oil has been produced from the youngest (probably Cretaceous) and the least-metamorphosed (zeolite) facies of the Franciscan assemblage within the area south and southeast of the Humboldt basin. Similar small accumulations probably remain untested in this general area.
The Humboldt Tertiary subprovince in the northwest is probably the terminal onshore part of a much larger offshore Tertiary basin. ft contains about 12,000 ft (3,660 m) of Tertiary sedimentary rocks, from which two small gas fields have been developed and a third can reasonably be expected. The total prospective reserves of this Tertiary subprovince are estimated to be 75-100 billion cu ft of gas.
The Sonoma-Orinda-Livermore Tertiary subprovince is roughly rectangular, trends northwest-southeast athwart the Carquinez Strait, and is laterally fault bounded. The subprovince is bisected diagonally by a major fault zone from its southwest to its northeast corner. The Sonoma and Orinda basins in the northwestern part of the subprovince are separated by the Carquinez Strait. The Livermore basin, south of the Carquinez Strait, occupies the southeastern part of the subprovince.
Tertiary and Upper Cretaceous miogeosynclinal sedimentary rocks are found in all three basins. Oil and gas shows and one minor oil and gas field are known in the Sonoma basin. Minor production has been discovered in the Livermore basin, but the Orinda basin has failed to yield production or substantial oil or gas shows. A reasonable estimate of the in-place undiscovered reserves within the two Tertiary subprovinces is on the order of 100 billion cu ft of gas and 13 million bbl of oil.
Figures & Tables
Future Petroleum Provinces of the United States—Their Geology and Potential, Volumes 1 & 2
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.