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ABSTRACT: Petroleum Geology of California's Coastal Basins

Stephen P. Thurston

Coastal California, west of the San Andreas fault, contains 13 offshore and 11 onshore basins related to Neogene transform margin tectonics. These basins are smaller and possess a more complex Cenozoic depositional and tectonic history than the large Sacramento and San Joaquin fore-arc basins to the east. Eight of the coastal basins have contributed half of California's current total production of 27.5 billion bbl (O + OEG).

The coastal basins typically contain multiple reservoirs in well-developed structures. Objective reservoirs are Eocene and Oligocene fore-arc sandstones (pretransform), Miocene and Pliocene turbidites, and fractured Miocene biogenic shales. Future exploration in the coastal basins is hindered by poor seismic imaging in structurally complex areas, increasing urbanization, and increasing political and environmental opposition.

The typical Neogene depositional sequence, 5000-20,000 ft thick, consists of thin lower Miocene transgressive marine sandstones overlain by shallow marine extensional detritus and volcaniclastics. Continued basin development during a middle Miocene sea level rise produced deep marine organic-rich shales followed by sand-rich turbidites from upper Miocene through the Pliocene. Structural development began as early as middle Miocene and peaked in most basins in the Pleistocene. Peak levels of maturity for hydrocarbon generation are from the Pliocene to present.

Only 3 of the 13 offshore basins have been adequately explored, and they account for nearly 2 billion bbl (O + OEG) of proven and undeveloped reserves. There are five underexplored offshore basins with over 3.0 billion bbl (O + OEG) undiscovered resource potential.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990