Based on stable carbon isotope and porphyrin data, most oil, oil seep, and oil-stained samples examined from the west flank of the San Joaquin basin and east flank of the Cuyama and Salinas basins, all adjacent to the San Andreas fault, were generated from source rocks of Cretaceous (group IV), Eocene (group I), or Miocene (groups II and III) age. Based on similar analysis of source rock samples from wells and outcrops in the same area, the Eocene and Miocene source rocks compare favorably with oil samples. These oil-oil and oil-source rock correlations identify at least two unnamed petroleum systems.

Oil, oil seep, and oil-stained samples show stable carbon isotope values ranging from about -27 to -29^pmil for the Eocene and -22 to -26^pmil for the Miocene petroleum systems. Wide variations in the Ni/(Ni + V) porphyrin ratio among these samples indicate oil mixing or considerable variations in the organic facies of the source rocks. For example, group III represents a single family of petroleums derived from different organic facies of the Miocene source rock. These oil samples show similar stable carbon isotope compositions but different Ni/(Ni + V) porphyrin ratios.

In general, oils from the northern study area can be tied to Eocene Kreyenhagen source rock (group I). Oil stains on the east side of the San Andreas fault can be tied to low-maturity Miocene McDonald shales (group III), a lateral equivalent to the Monterey Formation. Samples on the west side of the San Andreas fault can be tied to the Soda Lake Shale Member of the Vaqueros Formation (group II). Oil (group IV) from the Oil City field is suspected to originate from the Moreno Formation.