NATURAL HYDROCARBON SEEPS ON THE INNER SHELF OFF PT. CONCEPTION AND WESTERN SANTA BARBARA CHANNEL, CALIFORNIA

A study of natural oil and gas seeps on the inner shelf of the western Santa Barbara Channel and at the southern end of the Santa Maria Basin, offshore California, was conducted in collaboration with the Minerals Management Service (MMS). The goal was to establish the geologic framework for, and to document the locations of, active seeps. The area surveyed forms a broad V-shaped swath 3 to 5-km wide both north and east of Pt. Conception. Each side of the V is about 15-km long, covering water depths between 40-150 m. Sidescan sonar with a resolution of 50-cm along-track, and 17-cm across-track, was collected throughout. For most of the survey, a chirp sonar system provided high-definition profiles of the underlying geology and seep structures. While seeps are found in areas of bare rock outcrop, eroded during the last sea-level transgression, many active seeps occur on mounds, which previous MMS work indicates are formed by tar residue and sand. These mounds range from a few meters across to large accumulations that can exceed a kilometer in width and 5 m height. We recognize them primarily just west of Pt. Conception, where they cover an irregular area of approximately 5 km². Overall, a total of more than 100 likely active seeps were identified that are the targets for ongoing collection of gas and tar samples. This mapping activity is in support of an effort to establish chemical correlations between offshore active seeps and coastal oil residues as well as to measure the rate of natural seepage at individual sites that can be used to assess regional rates.