Correlation of Oils and Source Rock Characteristics Using Biological Markers, Cuyama Basin, California
Paul G. Lillis
Biological marker data obtained from gas chromatography and gas chromatography/mass spectrometry were used to correlate the oils in the Cuyama basin and to characterize the potential source facies. Biological markers provide a wealth of information about petroleum and source rocks, including information on paleoecology, depositional environment, and thermal maturity. Pristane/phytane ratios and sterane and hopanoid distributions indicate the source facies was deposited in a restricted marine basin and the organic matter was derived primarily from marine phytoplankton with a significant contribution from land plants and bacteria.
Previous studies have documented that the Miocene Monterey Formation is a major source rock for oils in several California basins. However, clear differences exist between the composition of Cuyama basin oils and typical Monterey oils. Cuyama basin oils have lower sulfur contents (< 0.5 wt. %), higher pristane/phytane ratios (1.7-1.9), and no 28,30-bisnorhopane, in comparison to Monterey oils, which have higher sulfur contents (1-6 wt. %), lower pristane/phytane ratios (< 1), and significant amounts of 28,30-bisnorhopane. The low sulfur content of the Cuyama basin oils is probably due to precipitation of microbially reduced sulfur with iron from terrigenous clay input, thus preventing sulfur incorporation into the kerogen. The higher clay content in the source facies is also ind cated by higher diasterane content.
Petroleum geochemistry studies indicate that the Cuyama basin oils share a common source. The source facies appears to be an atypical Monterey Formation deposited in an inboard basin, with significant terrigenous organic and clay mineral debris contributing to the autochthonous biogenous sediments.
AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.