Hydrocarbon
Migration and Thermal Degradation of
Saenz, Joseph M.1, Frank E.
Denison2, Craig K. Ogawa3, Thomas J. O'Neil4,
Peter J. Fischer5 (1) Naval Facilities Engineering Service Center, Port
Hueneme, CA (2) Frank Denison Consultant, Westlake
Village, CA (3) Minerals Management Service, Camarillo, CA (4) Oxnard College,
Oxnard, CA (5) California State University, Northridge, Northridge, CA
High heat flow in the offshore
Monterey Formation sourced oil ranges in
API gravities from 5° to 10° off Point Conception, south of the Amberjack high.
This correlates with shallow gas-charged sediments, lower concentrations of
C1-C4, and with seafloor expressions interpreted from seismic data as 350+ tar
mounds and 150+ gas-vent craters. Variability of the API oil gravity off Point Arguello and Point Pedernales
ranges 15° to 35°, and ranges from 3° to 15° between Purisima
and Point Sal. Both areas are associated with near-surface to shallow
gas-charged sediments with elevated concentrations of C1-C4, and 1300+ gas-vent
craters. Gas plumes migrate upward along faults, anticlinal
folds, and steeply dipping beds into shallow sediment from depth. These are
linked to high geothermal gradients and controlled by active tectonics. Gas,
deeply sourced in the Monterey Formation migrates vertically into near-surface
sediment, and escapes through the seafloor, forming gas-vent craters.
Geochemical trends, core samples, SNIFFER and high-resolution geophysical
surveys provide evidence that the migrating and seeping gas is thermogenic in origin.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California