Hydrocarbon Migration and Thermal Degradation of Monterey Formation Oil in Santa Maria Basin, Offshore California

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 Santa Maria Basin released hydrocarbon gases from Monterey Formation source rocks. Temperature changes induced by geothermal heating during burial caused thermal cracking of the oil to lighter-end hydrocarbon gases that migrate as gas plumes into shallow burial depths. Drill stem test records indicate that Monterey Formation API oil gravities range from 3° to 35°. Oil gravities are related to zones of shallow gas-charged sediments, variable geothermal gradients ranging 1.7°F/100 ft to 3°F/100 ft, and temperatures up to 250°F in the Monterey Formation.

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.