Reduction in Natural Hydrocarbon Seepage from the Offshore South Ellwood Field near Coal Oil Point, California, Due to Oil Production

J. S. Hornafius, B. P. Luyendyk, D. Quigley, A. Trial, L. Washburn, R. D. Francis, P. Fischer, G. Ouellette, and M. Geneau

Prolific natural gas seepage, a significant air pollution source in Santa Barbara County, occurs offshore from Coal Oil Point, near Santa Barbara. Seepage rates are quantified by measuring the acoustic return of sonar sources from the gas bubbles rising through the water column, and by measuring the dissolved concentrations of hydrocarbons downcurrent from the gas seep vents. In 1995 we digitally records 3.5 kHz data in the same location as a 1972-1973 survey. Comparison of the data suggests a substantial reduction in natural seepage within one mile of Platform Holly since oil production began in 1972 from the Monterey Formation reservoir, which is the source of the gas. Dissolved concentrations of hydrocarbons were measured in the ocean 2-6 miles downcurrent from Platfor Holly in an area where concentrations were previously measured in a 1981 SNIFFER survey. The dissolved propane concentrations in 1995 had a peak value of 7 ppm, whereas in 1981 the peak value was 65 ppm. A reduction in natural gas seepage rate is also indicated by the 60% decline in the amount of seep gas captured by a seep containment device located one mile east of Platform Holly. The reduction in gas seepage rate may be attributed to a 40% reduction in subsurface reservoir pressure under Platform Holly that resulted from withdrawal of 43 MMBO and a similar volume of water from the Monterey Formation since 1972.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California