NUMERICAL REACTIVE TRANSPORT MODELING OF THE EFFECTS OF LEAKAGE OF METHANE-RICH FLUIDS ALONG THE REFUGIO-CARNEROS FAULT IN THE SANTA BARBARA BASIN

The Santa Barbara basin is a 100 km-long sedimentary trough that originated from transpression of the southern California continental margin beginning in the late Oligocene.  The northern flank of the basin is being exposed by uplift in the Santa Ynez Mountains, while the center of the basin is undergoing subsidence, burial, diagenesis, and hydrocarbon generation.  A total resource endowment of about 3 billion barrels of oil and 7 trillion cubic feet of natural gas is distributed in sediments ranging from Cretaceous to Pleistocene in age.  Leakage of hydrocarbons to the surface has occurred at numerous locations in the northern part of the basin.  One of the more prominent examples of hydrocarbon leakage occurs in the form of extensive calcite cementation along the western part of the Refugio-Carneros fault.  Light d¹³C values of about -40 to -30‰ PDB suggest that methane-rich fluids ascended the fault and contributed carbon for calcite mineralization.  Fluid inclusion homogenization temperatures in the calcite of 80-120° C indicate that the fluids also transported significant quantities of heat.  Fluid inclusion salinities ranging from fresh water to seawater values and the proximity of the Refugio-Carneros fault to a zone of groundwater recharge in the Santa Ynez Mountains suggests that calcite precipitation in the fault may have been induced by the oxidation of methane-rich basinal fluids by infiltrating meteoric fluids descending steeply dipping sedimentary layers on the northern basin flank.  The present study sought to test this hypothesis using numerical reactive transport modeling.  The models showed that in order for the observed fluid inclusion temperatures to be reached within 200 meters of the surface, mild overpressures and high permeabilities were required in the fault zone.  Overpressures were regarded as having been generated during episodes of fault compression.  Sudden release of overpressure was caused by earthquakes that led to transient pulses of accelerated fluid flow and heat transport along the fault, most likely on the order of 100's to 1000's of years in duration.  The models also showed that meteoric groundwater recharge was an effective mechanism for methane oxidation, leading to large concentrations of calcite along the intersection of the Refugio-Carneros fault with the Vaqueros Sandstone.