25 YEARS OF HYDROCARBON SEEP STUDIES IN THE SANTA MARIA BASIN AND ALONG THE NORTHWESTERN BOUNDARY OF THE SANTA BARBARA BASIN, OFFSHORE CALIFORNIA USING SEISMIC AND STRATIGRAPHIC DATA (1995 THROUGH 2020)

Historic seismic and stratigraphic records were used to understand the relative effects of active tectonics on hydrocarbon seeps in the Santa Maria Basin (SMB), offshore California. The study confirms that hydrocarbon seeps are associated with the Hosgri-Purisuma-Lompoc fault zones tapping into a single major reservoir, the Monterey Formation. In the northern and central areas, these Monterey reservoirs occur in growing anticlinal folds that are faulted and fractured by the Hosgri Fault zone, acting as a major conduit for gas and oil seeps. In the southern area, heavy oil migrating up-dip in the Monterey formation is found near (or at) the seafloor in the hanging–wall of the Santa Barbara Basin along the North Channel fault zone.

We find an abundance of evidence to suggest continuous or episodic upward movement of fluids as migrating gas plumes from deeper sediments into surface sediments. The analysis shows that bright spots on the seismic reflection profiles are gas-plumes, linked to the highest geothermal gradients and controlled by active tectonics. Gas, deeply sourced in the Monterey Formation migrates upward along faults, anticlinal folds, and steeply dipping beds into shallow sediment. Gas chromatographic analysis from mud logs samples in wells near gas plumes show the highest concentrations of total continuous gas, methane, ethane, propane, and butane.

We believe that temperature changes induced by geothermal heating during burial caused in-reservoir 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, and variable geothermal gradients ranging from 1.7°F/100 ft to 3°F/100 ft, with downhole temperatures ranging from 118°F to 248°F.

Within close proximity to the Hosgri Fault zone, lithologic analysis revealed three areas where siliceous Monterey rocks have been diagenetically altered to glassy cherts related to high geothermal gradients and reservoir pressures (2115 psig to 3385 psig). Active tectonics has fractured these brittle reservoir rocks, forming migration pathways that serve as conduits for seafloor hydrocarbon seeps.