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Families of Miocene Monterey Crude Oil, Seep, and Tarball Samples in Coastal California

Kenneth E. Peters, Frances D. Hostettler, Tom D. Lorenson, and Robert J. Rosenbauer
U.S. Geological Survey, 345 Middlefield Road MS 969, Menlo Park, CA 94025

Biomarker and stable carbon isotope ratios were used to infer the age, lithology, organic matter input, and depositional environment of the source rocks for 388 samples of produced crude oil, seep oil, and tarballs from coastal California. These samples were used to construct a chemometric (multivariate statistics) decision tree to classify 288 additional samples, including tarballs collected from Monterey and San Mateo County beaches after a storm in early 2007. The results identify three tribes of 13C-rich oil samples inferred to originate from thermally mature equivalents of the clayey-siliceous, carbonaceous marl, and lower calcareous-siliceous members of the Monterey Formation described at Naples Beach by Isaacs (1983). Geochemical similarities among the oil tribes and their widespread distribution support the prograding margin model (Isaacs, 2001) or the banktop-slope-basin model of Hornafius (1991) rather than the classic ridge-and-basin model (Blake, 1981) for deposition of the Monterey Formation. Tribe 1 contains four oil families having geochemical traits of clay-rich marine shale source rock deposited under suboxic conditions with substantial higher-plant input. Tribe 3 contains five oil families with traits of distal marine carbonate source rock deposited under anoxic conditions with pelagic, but little or no higher-plant input. Tribe 2 contains four oil families with intermediate traits, except for abundant 28,30-bisnorhopane, indicating suboxic to anoxic marine marl source rock with hemipelagic input. Tribes 1 and 2 occur mainly south of Point Conception in paleogeographic settings where deep burial of the Monterey source rock favored generation from all three members or their equivalents. In this area, oil from the clayey-siliceous and carbonaceous marl members (Tribes 1 and 2) may overwhelm that from the lower calcareous-siliceous member (Tribe 3) because the latter is thinner and less oil-prone than the overlying members. Tribe 3 occurs mainly north of Point Conception, where shallow burial caused preferential generation from the underlying lower calcareous-siliceous member or a similar unit.

 

AAPG Search and Discovery Article #90076©2008 AAPG Pacific Section, Bakersfield, California