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Book Chapter

Petroleum Potential of Northern Coast Ranges, California

H. D. Hobson
H. D. Hobson
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January 01, 1971


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.

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AAPG Memoir

Future Petroleum Provinces of the United States—Their Geology and Potential, Volumes 1 & 2

Ira H. Cram
Ira H. Cram
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American Association of Petroleum Geologists
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Publication date:
January 01, 1971




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