Chemometrics Identifies
Families of Crude Oils, Seep Oils, and Stranded Tarballs
in Coastal California
Peters, Kenneth E.1, Frances
D. Hostettler2, Thomas D. Lorenson1, Robert J. Rosenbauer1
(1) U.S. Geological Survey, Menlo Park, CA (2) U. S. Geological Survey, Menlo
Park, CA
Biomarkers (molecular fossils) and carbon
isotope ratios were measured for >600 samples of produced oils, seep oils,
and tarballs from coastal California. Chemometrics
(multivariate statistics) of the data included principal components analysis
(PCA) to establish genetic families. A unique, multi-tiered decision tree
consisting of multiple K-Nearest Neighbor (KNN) models was constructed to
classify new samples. The decision tree assigns confidence limits to each
classification using soft independent modeling of class analogy (SIMCA). The
data were also used to infer the age, lithology,
organic matter input, and depositional environment of the source rock for each
sample. Results identify three tribes of oil samples that originated from organofacies of Miocene source rock, including the Monterey
Formation. Tribe 1 contains four families having C22/C21
and C24/C23 tricyclic terpane ratios typical of marine shale source rock. This
tribe has high Ts/Tm, low C35S/C34S, and abundant oleanane, indicating clay-rich, oxic-suboxic
source rock with higher-plant input. Examples include produced oil from
Platform A (Dos Cuadras
Field), nearby seep oils, and tarballs from Goleta and Gaviota
in southern California. Tribe 3 contains five
families that lack oleanane and have geochemical
characteristics of anoxic marine carbonate source rock. Examples include tarballs from Point Reyes and Drakes Bay in northern California. Tribe 2 contains four
families with intermediate compositions indicating marine marl source rock.
Examples include produced oils from the Harvest, Hondo, and Heritage wells,
nearby seep oils, and tarballs from Santa Rosa and San Miguel Islands. Some tarballs correlate with samples of produced oils or seep
oils, thus establishing a link between source and collection site.