REACTIVE TRANSPORT MODELING OF THE ROLE OF METHANE-RICH FLUIDS IN MINERALIZATION OF THE REFUGIO-CARNEROS FAULT, SANTA BARBARA BASIN, CALIFORNIA

The Santa Barbara basin is a 100 km-long trough originating from transpression of the southern California continental margin beginning in the late Oligocene. The basin has been a prolific source of hydrocarbons which occur in at least 70 distinct fields in Cretaceous to Holocene sediments, with the greatest concentrations occurring in Neogene sediments. Uplift of the northern margin of the basin beginning in the Pliocene created a broad zone of meteoric recharge in the Santa Ynez Mountains and several prominent east-west trending faults such as the Refugio-Carneros fault. Extensive calcite cements containing isotopically light carbon (δ¹³C=-30 to -40‰) and high fluid inclusion homogenization temperatures (85-100º C) found along some of the shallower portions of the Refugio-Carneros fault indicate that the faults may have been the foci of mixing between oxidizing meteoric water and methane-rich basinal fluids. Numerical reactive transport models from the present study of a structurally complex and geologically heterogeneous 2-D profile across the basin support this hypothesis. In the models, oxidizing meteoric water enters steeply dipping aquifers outcropping in the Santa Ynez Mountains, displacing methane-bearing formational fluids and producing weak calcite mineralization along the mixing front. The greatest concentrations of calcite are precipitated in the upper levels of the Refugio-Carneros fault due to the higher magnitude and duration of methane flux in the fault as a result of its high permeability and intersection of multiple methane-rich strata. Faults were also shown to be important conduits for heat transport. The high heat flow in the basin and rapid ascent velocities driven by buoyancy and overpressuring allow model temperatures in the Refugio-Carneros fault to exceed fluid inclusion homogenization temperatures at depths within 100 meters of the ground surface, further contributing to calcite mineralization by locally increasing temperature.