Decrease in Natural Hydrocarbon Seepage Offshore Coal Oil Point, California, Associated with Oil Production

QUIGLEY, DEREK C., BRUCE P. LUYENDYK*, Department of Geological Sciences, and Institute for Crustal Studies, University of California Santa Barbara, J. SCOTT HORNAFIUS, Mobil Oil, London, England, ROBERT D. FRANCIS, Department of Geological Sciences, California State University, Long Beach, JORDAN CLARK, Department of Geological Sciences, University of California Santa Barbara, LIBE WASHBURN, Department of Geography, University of California Santa Barbara

Natural seepage of hydrocarbons from submarine vents offshore of Coal Oil Point in the Santa Barbara Channel, California contributes significant quantities of hydrocarbons to the local ocean and atmosphere. Seep emissions here have previously been used to extrapolate levels of natural marine petroleum pollution and global methane flux from the world's continental shelves.  Hydrocarbon bubble plumes within the water column above submarine seep vents act as acoustic scattering targets, allowing seep distribution offshore Coal Oil Point to be mapped from 3.5 kHz sonar data acquired in August, 1996, July 1995, and July 1973. Calibration of the sonar against a known source allowed emission rates to be estimated.  The entire field today emits between 1 x and 2 x 10^5 m3/day of natural gas. Comparison of the seep distribution reveals a 50% decrease in the areal extent of seepage, accompanied by a decline in the volume of emissions in an area above a producing oil reservoir in the Monterey Formation. Declines in reservoir pressure and depletion of seep hydrocarbon sources associated with oil production at nearby Platform Holly are the inferred mechanisms that explain the observed declines in seep area and emission volume. Our study demonstrates the potential impact of oil production to reduce seepage rates in some areas and perhaps alter the natural influx of hydrocarbons into the environment.

  Quigley, Derek C. et al. "Decrease in natural hydrocarbon seepage offshore Coal Oil Point, California, associated with oil production". 1999. (1999 AAPG PACIFIC SECTION MEETING; ABSTRACTS; AAPG Bulletin; Vol. 83, no. 4; p. 699)