Biodegradaton
of Hydrocarbons Associated with Subsurface Reservoirs and Natural Seepage
Valentine, David L.1, George
D. Wardlaw2, Franklin S. Kinnaman2, Susan Mau2,
Christopher M. Reddy3, Robert K. Nelson3, J. Samuel Arey4
(1) University of California, Santa Barbara, CA (2) University of California,
(3) Woods Hole Oceanographic Institution, (4) Swiss Ferderal Institute of
Technology,
Microbes are capable of consuming
hydrocarbons under favorable environmental conditions which extend from the
ocean to the deep subsurface. However, the rates, patterns and variability of
biodegradation remain poorly characterized for most environments. The Coal Oil
Point (COP) seep field and associated subsurface reservoirs, located offshore Goleta, CA, serve as an ideal natural
laboratory to investigate the biodegradation of hydrocarbons. This paper
considers the biodegradation of hydrocarbons in sediments, waters and
underlying reservoirs of the COP seep field. Subsurface biodegradation is
observed to impact oil composition dramatically, as observed using
comprehensive two-dimensional gas chromatography. Rapid physical weathering
processes become important at the sea surface, where the role of biodegradation
remains unknown. Spatial variability in oil composition indicates differential
biodegradation levels between nearby reservoirs and seeps. Hydrocarbon gases
appear to be biodegraded in the subsurface, though only in select areas.
Oxidation of gases in the sediment and water column occurs rapidly, but only
impacts the dissolved fraction, and with a strong preference for propane and
butane over methane and ethane. The apparent ubiquity of hydrocarbon
degradation in the COP seep field underscores the relevance of microbial
activity to petroleum systems.