Print this pageGeological and Geochemical Evaluation of Origin of Heavy Oil in Salinas Basin, California
Seven oil fields have been discovered along the east flank of the Salinas basin on or near the Gabilan shelf. Approximately 400 million bbl of heavy oil (~10-14°API) have been produced from shallow upper Miocene sandstones in the San Ardo field. Heavy oil (< 15°API) also is produced from shallow middle and upper Miocene sandstones in three much smaller fields near San Ardo. Slightly higher gravity oil (~16-19°API) occurs in deeper Miocene sandstones in three small fields in the King City area. The Miocene Monterey Formation is the only good oil-prone source rock in the Salinas basin. The siliceous Hames upper member of the Monterey is a richer source rock than the clay-rich Sandholdt lower member of that formation. Corrected bottom-hole temperature data ndicate that the present geothermal gradient in the Salinas basin is nearly 1.8-2.0°F/100 ft.
Source rock maturity modeling, which was calibrated using the maturity measured in five key wells, indicates that most of the oil in the Salinas basin was generated by the Sandholdt shale member of the Monterey. Geochemical evidence supports this conclusions and additionally demonstrates that the heavy oils in the Salinas basin are biodegraded. For example, San Ardo oils typically do not contain normal alkanes or isoprenoid isoalkanes, and these heavy oils generally contain only about 1-2 wt. % sulfur. Heavy, nonbiodegraded Monterey oils from the Santa Maria basin, in contrast, have a full complement of normal alkanes and isoprenoid isoalkanes, and they typically contain about 3-8 wt. % sulfur. Bitumen in the clay-rich Sandholdt Shale contains much less organic sulfur than does bitume in the siliceous Hames shale.
AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.