Geological controls of hydrocarbon seeps in Santa Maria Basin, Offshore California

The offshore Santa Maria Basin contains two types of hydrocarbon seeps in surficial marine sediments and the water column: active and passive. Active seeps (macroseeps) occur where gas bubbles, pockmarks, gas-vent craters, tar seeps, or bright spots are visible on seismic profiles or side-scan sonar records. These hydrocarbon seeps presumably occur where generation and migration of hydrocarbons from source rocks are ongoing today and where migration pathways have developed along structural conduits through the overlying sediments of Late Neogene-Quaternary age. In the northern and central areas, passive and microseeps occur where the concentration of migrated hydrocarbons is low and there are no visible geophysical anomalies. Microseeps are detected by SNIFFER or geochemical surveys that sample and analyze hydrocarbon concentrations in the water column. It is probable that passive seeps occur in areas where effective regional seals or deep-water depths limit vertical migration. The primary tectonic controls for the location and distribution of hydrocarbons in near-surface to shallow sediments are active faults and growing anticlinal folds. Near the Hosgri, Purisima, Lompoc, and North Channel faults, numerous active seeps are present on high resolution seismic profiles near the water-sediment interface or in the water column. In the northern and central areas, the strike-slip regime of the Hosgri Fault zone is associated with many active and passive or episodic gas vent craters. The southern area or the northwestern Santa Barbara Basin is dominated by compressional tectonics and North Channel Fault zone, where tar seeps and mounds are present on the seafloor. These heavy oil-tar seeps are the result of a loss of volatiles and the biodegradation of the oil to tar along the seafloor outcrop of the Monterey Formation Stratigraphic evidence shows that the thinning and absence of the Pliocene Foxen Formation is related to a paleo-high in the central portion of the study area. This high provides a shorter migration path to the Quaternary sediments resulting in gas-charged shallow/surficial sediment and numerous gas seeps. Throughout the basin, methane gas concentrations exceeding 10000 ppm migrate as plumes upward through the stratigraphic section. Sediment samples collected from coreholes at shallow depths in the Foxen and Sisquoc Formations consist of cherty shales and clay. These samples show the highest concentrations of gas and they correlate with first petroleum logged in offshore wells. Higher gravity crude oils exist in the southern portion of the area, and decrease northwestward across the offshore SMB. The decrease in API gravities may be related to the loss of the lighter-end hydrocarbons upward through the stratigraphic section. The questions that need to be answered in order to economically develop this area are not restricted to whether oil is present, but where it is located, and the quality of reservoir fluid. Hydrocarbon seeps provide evidence of the location of underlying fractured formations and reservoirs. The also help in the characterization of the reservoirs.