Provenance of Oil in Southern Cuyama Basin, California

Alan S. Kornacki

Five oil fields have been discovered in the southern Cuyama basin, California. Approximately 280 million bbl of high-gravity oil ( ^approx 30°-40° API) has been produced from Dibblee and Colgrove sandstones of the lower Miocene Vaqueros Formation in the South Cuyama and Russell Ranch fields. A much smaller amount of lower gravity oil (< 30° API) also has been produced from younger sandstone reservoirs in these fields. Two oil-prone Miocene source rocks exist in the southern Cuyama basin. The Saltos Shale Member of the middle Miocene Monterey Formation contains as much as 3.9 wt.% HC yield; the Soda Lake Shale Member of the Vaqueros Formation contains as much as 1.4 wt.% HC yield. Eocene and Cretaceous shales are nonsource rocks in this basin. Corrected b ttom-hole temperature data indicate the present geothermal gradient in the southern Cuyama basin is ^approx 1.5°F/100 ft.

Source rock maturity modeling, which was calibrated using the maturity measured in four key wells, indicates that the Saltos Shale Member (Monterey) is only marginally mature (VR ^approx 0.4-0.6%) in the Wells Ranch syncline. However, the deeper Soda Lake Shale (Vaqueros) is at a high enough level of maturity (VR ^approx 0.8-1.2%) to have generated the high-gravity oils recovered in the basin. Geochemical evidence supports the conclusion that the Soda Lake Shale is the principal source of oil in the southern Cuyama basin. For example, Soda Lake bitumen and topped oils from the South Cuyama and Russell Ranch fields are isotopically heavier [^dgr¹³C (PDB) ^approx -21.5%] than Monterey bitumen [^dgr¹³C (PDB) ^approx -23.5%]. The Monterey Formation also contains much higher concentrations of the diagnostic 28,30-bisnorhopane biomarker than does the Soda Lake Shale or an oil sample from the Russell Ranch field.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.