The continental shelf appears to have a granite basement from Point Conception to about Point Arena and a metasedimentary type of basement from Point Arena to the Oregon border. Remnants of thick marine Cretaceous and Eocene sections are present throughout the length of the shelf, but because of their erosional history, the poor potential for reservoirs, and the difficulties in mapping, they appear to merit little further exploration. Deep-marine beds deposited throughout the length of the shelf from early Miocene through early late Pliocene time appear to have by far the best potential for hydrocarbon reserves. This section generally includes thick sequences of shale and ample reservoir-quality sandstone. Hydrocarbon shows are common. The overlying Pleistocene beds are n t considered as objectives.

The six basins present on the shelf, each of which has an onshore extension, began forming in late middle Miocene time. Generally they do not contain as thick a post-Eocene section as that in the producing onshore California basins.

The southern third of the offshore Santa Maria basin is similar to the onshore part and is considered to have about the same hydrocarbon potential; the northern two thirds is only slightly explored. The Outer Santa Cruz basin lacks large, mappable structural traps and extensive reservoir-quality sandstone beds; however, oil shows are common. The offshore half of the small Santa Cruz basin is considered to have about the same hydrocarbon potential as the onshore half, where production is minor. The Bodega and Point Arena basins contain most of the geologic constituents of major producing basins, but they also have 16 structurally well-located dry holes. Eel River basin, the largest and least explored of the shelf basins, has good structural-trap potential, and limited data suggest the resence of an objective Miocene-Pliocene section, but beneath deep water.