The offshore area of southern California from south of the Channel Islands to the Mexico boundary and from the shoreline to the continental margin, but excluding the present offshore oil fields that have been extended from onshore fields, is estimated to contain 75 billion bbl of oil in place. This oil is contained in 17,780 cu mi (74,090 cu km) of rock above the basement, primarily in 5,323 cu mi (22,180 cu km) of basinal sedimentary racks of Pliocene and late Miocene age.

This area is in the Peninsular Ranges province, which also includes the prolifically oil-productive Los Angeles basin. The offshore area is similar in structural grain, stratigraphic sequence, and geologic history to the Los Angeles basin; therefore, the well-known geologic conditions and oil-content values of that basin are used as norms in evaluating the offshore area by application of appropriate discounting factors to each offshore basin. Eleven of the factors responsible for the oil accumulations onshore are rated, depending on less favorability or less certainty of effectiveness, for each offshore area, and the reduced ratings are applied to the volume of rock as compared to the Los Angeles basin volume of similar rock.