|Evolution of a hydrocarbon migration pathway along basin-bounding faults: Evidence from fault cement.
James R. Boles
Department of Geological Sciences
University of California
Santa Barbara, California 93106
Department of Geological and Environmental Sciences
Stanford, CA 94305-2115
Department of Earth and Planetary Sciences
Johns Hopkins University
Baltimore, MD 21218-2687
Division of Geological and Planetary Sciences
California Institute of Technology
Pasadena, CA 91125
Extensive calcite fault cement has resulted from leakage of Santa Barbara basin fluids and hydrocarbons into the Refugio-Carneros fault, a north bounding structure to the basin. Calcite cements are only found at the end segments of the 24 km long fault zone, which has less than 150 m of maximum normal offset. The calcite is contemporaneous with fault movement as evidenced by pervasive crystal twinning and brecciation as well as textures indicating repeated episodes of rapid fluid flow and calcite cementation. Based on U-Th dates of the calcite fluid flow along the fault occurred between 110 and >500 ka, indicating that fluid migration was intermittently active over extended periods of the uplift history of the basin flanks. Stable carbon isotopic values of the calcite are δ13CPDB = -35 to -41‰, which means that the carbon source is predominantly thermogenic methane. The composition of fluid inclusions in calcite is consistent with mixing of meteoric and saline water in the presence of liquid and gaseous hydrocarbons. Fluid inclusion homogenization temperatures of about 80-95°C suggest that hot water leaked from 2-3 km depths in the basin and moved up faults on the basin flank at rates rapid enough to transport substantial heat to shallow depths. Finite element models show that, in this case, this process requires faulting of an overpressured basin and that a single flow event would have lasted for at least 103 years.
Subsurface fluid pressures at comparable depths in the offshore section today are close to hydrostatic, and therefore only slow hydrocarbon seepage occurs. When combined with the U/Th age data, this suggests that over a 105 year time scale, basin fluid flow has evolved from the rapid expulsion of hot water and gas being carried up along active bounding faults derived from overpressured strata, to present hydrostatic conditions of slow buoyancy-driven seepage of hydrocarbons.