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