Geologic appraisal of the petroleum potential of offshore southern California; the borderland compared to onshore coastal basins

Abstract

Offshore southern California is part of a much larger Pacific continental margin, and the two areas have a similar geologic history at least as far back as middle Tertiary time. Assessment of the petroleum potential of the offshore Southern California borderland is accomplished by examining the adjacent highly explored productive coastal basins in the tectonically unstable area west of the San Andreas fault. Known oil and gas accumulations in this region can be characterized as follows: 88 percent comes from the Los Angeles and Ventura basins; 87 percent has been found in late Miocene and younger strata and only 0.2 percent has been found in Eocene strata; 80 percent has been found in thick deposits of deep-water turbidite reservoirs; and 5 percent has been found in fractured Miocene siliceous shale reservoirs. The percentage of siliceous shale reservoirs will increase as a result of recent discoveries in this rock type in the Santa Barbara Channel. Of the 212 known fields only 5 are giants (greater than 500 million barrels), and these fields account for 52 percent of all past production from the region. Most fields are faulted anticlines, and the largest fields have the highest oil recoveries per acre. Geologic knowledge of the offshore is limited by the availability of data. Data have been obtained from geophysical surveys, analyses of bedrock samples from the sea floor, and extrapolations of data from the mainland and offshore islands. Several factors have a negative effect on the assessment of the petroleum potential of the southern California borderland. They are: 1. The Neogene section is relatively thin, and the Paleogene section is thin and has a limited distribution. 2. Over large areas, Miocene sediments apparently rest directly on basement. 3. Along much of the Santa Rosa-Cortes Ridge, sediments are uplifted and truncated, exposing Paleogene rocks. 4. Organic content in Paleogene sediments is believed too low to generate large amounts of petroleum. 5. Source rocks are immature, even in sediments as old as Eocene. 6. Burial and thermal history are insufficient for the generaton of hydrocarbons over much of the borderland area. 7. Documented oil and gas seeps are unknown seaward of the Channel Islands. 8. Limited exploratory drilling nearshore, the best prospective area, has not been favorable. 9. Adequate or thick reservoirs of deep-water turbidire origin are not evident. These negative factors are partly offset by the following positive factors: 1. Miocene sediments are excellent potential source rocks. 2. Asphaltic oil in rocks of Monterey-type lithology may be generated at lower than normal temperatures. 3. Shallow-water sands of reservoir quality possibly are present in the uppermost Paleogene and lowermost Neogene section. 4. Potential structural traps may be analogous to major structural producing trends onshore. 5. Fractured shale reservoirs may be present along flanks of major banks and ridges. 6. Buried and unsampled Paleogene and Late Cretaceous sediments may be better source rocks than those sampled and measured. The southern California borderland, although a part of a locally rich petroliferous region, has no known basins with geologic histories or characteristics similar to the Los Angeles and Ventura basins. Some offshore basins are similar to less endowed basins in terms of petroleum potential such as the Santa Maria and Salinas basins. Areas with the best petroleum potential may be in deep water (greater than 500 m). Some of the past high estimates of the petroleum potential may have been overstated; the existing geologic data tend to substantiate low estimates of 0.6 to 5.8 billion barrels of oil and 0.6 to 5.8 trillion ft3 of gas for the southern California borderland at the 5 and 95 percent probability level.