KVENVOLDEN, KEITH A., FRANCES D. HOSTETTLER, ROBERT J. ROSENBAUER, THOMAS D. LORENSON, and PAUL R. CARLSON, U.S. Geological Survey, Menlo Park, CA; and AUGUSTA WARDEN, U.S. Geological Survey, Denver, CO

Abstract: Oil Seeps as Sources of Some Coastal Bitumen at the California Shoreline

An investigation of the shoreline of the Monterey Bay National Marine Sanctuary has shown the common occurrence of residues of coastal bitumen (tarballs washed ashore), indicating that the Sanctuary has been previously contaminated with crude oil. Organic geochemical analyses, utilizing biomarkers and carbon isotopes, demonstrate that most of these residues have a common primary origin, namely, crude oils and/or crude-oil derivatives from the Miocene Monterey Formation source rocks of California. The question is, are these residues from human activities or from natural oil seeps? North Slope crude oil, transported from Alaska, would be the most likely oil to be spilled, yet no tarballs derived from North Slope oil have been found in the Sanctuary. The widespread distibution, similarity in chemical composition, and correlation with California Miocene oils suggest that the residues are from natural oil seeps. However, study of the few available crude oils and oil seeps in the immediate vicinity of the Sanctuary has failed, as yet, to provide a strong specific correlation with the coastal residues. Therefore, the search for possible origin of the residues has been extended. One California coastal region of known onshore and offshore natural oil seeps and abundant occurrences of coastal residues is the Santa Barbara Channel. Here local correlations are being investigated to determine the relationships between oil seeps and coastal bitumen. This information may help in understanding the source of the coastal residues in the Sanctuary.

AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California