Variation in Discharge from Marine Hydrocarbon Seeps at Coal Oil Point, CA: Implications for Offshore Oil Production

Bruce P. Luyendyk (UCSB Geological Sciences) and E. Thor Egland

Abundant natural marine hydrocarbon seeps occur along the northern margin of the Santa Barbara Channel, west of the city of Santa Barbara. The seeps comprise hydrocarbon gas bubble plumes along with crude oil. The bubble plumes can be identified by strong sonar backscatter signals. The most intense seepage is located in the Coal Oil Point seep field area offshore of Coal Oil and Campus (Goleta) Points. Repeat 3.5 kHz reflection sonar surveys have been completed over the seep field in 1996, 1998, and 1999. We analyzed these sonar data with the aim of detecting time variations in seep locations and seep gas discharge. Sonar system failures and repairs in 1998 affected the sonar output and noise level for part of this survey and the 1999 survey. The result was that the sonar outgoing pulse ratio method of Quigley (1997) that also accounts for ambient noise level could not be used to estimate gas flux for 1998 or 1999 or to compare the three surveys. Instead, sonar backscatter data were normalized to the sea floor bottom bounce sonar return amplitude from specific locations. This allowed us to create maps of seep locations and relative backscatter intensity for each survey. This revealed that the main locations of intense seepage did not vary in the three-year period between 1996 and 1999. The major seep areas include 1) the Coal Oil Point seep inshore and one kilometer south of Coal Oil Point sensu stricto, 2) at the sea floor Seep Tents 1.5 km east of Platform Holly in 60 meters depth, 3) at the La Goleta Seeps three kilometers east-southeast of the tents, and 4), at another concentration 1.5 km further east along the trend. From 31 (1996) to 44 (1999) individual seeps were detected by the survey coverage. However higher resolution 100 kHz side scan sonar surveys in the same period detected over 200 individual seeps. Gas capture with a surface spar buoy was conducted under a separate CES grant to L. Washburn. Capture surveys were conducted in parallel with the 1999 sonar surveys. The combined sonar and capture data analyses suggest that the spatial distribution of seeps follows a lognormal model and further that the gas discharge from the surveyed area (18 km2) is on the order of 100,000 m3/day. The area surveyed by sonar does not include a significant shallow water seep immediately west of Coal Oil Point in 20 meters depth (Shane Seep), so this number represents a minimum estimate for gas flux from the Coal Oil Point field.

Luyendyk, Bruce P. and Egland, E. Thor, "Variation in Discharge from Marine Hydrocarbon Seeps at Coal Oil Point, CA: Implications for Offshore Oil Production", Final report for University of California Energy Institute CES grant FY 1999-2000, Contribution of the Institute for Crustal Studies Number 418-137TC, July 20, 2001. 42 pages.