METHANE SEEPAGE ALONG FAULTS IN THE SANTA BARBARA COASTAL AREA: HYDROGEOLOGIC MODELING OF FLUID FLOW AND GEOCHEMICAL MIXING

Tertiary clastic sediments of the Santa Barbara basin host significant offshore hydrocarbon reservoirs and display paleohydrologic features of seepage and mixing with meteoric groundwater within the onshore subaerial formations. Fluid inclusion and isotopic analysis of calcite-cemented faults suggest petroleum seepage from 2-3 km depths in the offshore basin with a transient thermal and hydrodynamic history. These geofluids preferentially migrated up faults along the coastal flank of the basin, and the seepage appears to have evolved from relatively hot fluid venting to cold seepage over time scales of 10⁵ to 10⁶ years.

To characterize the paleohydrology of this continental margin, we have developed finite element models of coupled fluid flow, heat transport, and geochemical mass transfer for a two-dimensional profile across the coastal mixing zone. The mathematical model solves the coupled equations for Darcian flow, heat transfer, and chemical mass transport with equilibrium-based water-rock reactions in porous, nonisothermal, multi-mineralogical, heterogeneous media. Fault zones are represented as narrow continuum elements with distinct hydraulic, thermal and geochemical properties. Numerical experiments are presented to show how east-west trending faults structurally controlled the pattern of submarine seepage and fluid mixing along the Santa Barbara coastal margin and how the paleohydrology and geothermal regime evolved during changes in sea level and uplift of the coast ranges. We propose that offshore reverse faults served as barriers to flow until they were rotated into high-angle permeable faults with normal offsets. For example, the Refugio-Carneros Fault juxtaposes Oligocene Vaqueros sandstone, with about 100 m of normal offset, against overlying Miocene Rincon Shale. Flow rates of 0.5 to 1.0 m/yr and hydrothermal venting temperatures up to 100 ^oC are predicted to occur within the Refugio Fault which acted as a conduit for migration, mixing and degassing of submarine and subaerial fluids during the uplift of the coastal Santa Ynez Mountains.