NATURAL OFFSHORE OIL SEEPAGE AND RELATED TARBALL ACCUMULATION ON THE CALIFORNIA COASTLINE

In cooperation with the County of Santa Barbara Energy Division, and the California Office of Spill Prevention and Response, a five-year study of tarballs and oils in Southern California coastal waters and beaches has been completed as part of a joint U.S. Geological Survey-Minerals Management Service project (http://pubs.usgs.gov/of/2009/1225/) The major objectives of this study were to establish the geologic setting, sources, and ultimate fate of natural oil seeps from offshore California. Our surveys focused on areas of hydrocarbon seepage that are known to occur between Point Arguello and Ventura, California. Natural sources for these petroleum hydrocarbons include ubiquitous onshore and offshore oil seeps. We distinguish these from anthropogenic sources such as accidental oil spills, vessels and sunken wrecks, offshore drilling rigs and pipelines, and ships involved in the processing and transport of oil.

Using sidescan sonar imagery and multi-beam bathymetry as reconnaissance tools, we have identified and sampled extensive areas of active seepage. Near Point Conception, coalescing asphalt mounds as much as 18-m thick cover the seafloor over an estimated 8 km² area. We have also sampled numerous offshore oil seeps around Coal Oil Point; this oil is geochemically distinct from that near Point Conception and occurs as far west as offshore Gaviota.

We have collected and analyzed about 690 tar and oil samples. 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. These samples were used to construct a chemometric fingerprint to classify additional samples. 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. The geographic distributions of samples in each tribe reveal information about their origins and seasonal patterns of tar deposition on local beaches. Our results demonstrate that tar accumulation on California beaches is almost entirely a natural phenomenon related to oil seepage from natural sources.