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
Abstract: Decrease in Natural Hydrocarbon Seepage Offshore Coal Oil Point, California, Associated with Oil Production
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 1x and 2x105 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.
AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California