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Evolution of a hydrocarbon migration pathway along basin-bounding faults: Evidence from fault cement

¹ Department of Geological Sciences, University of California, Santa Barbara, California, 93106; boles{at}geol.ucsb.edu
² Department of Geological and Environmental Sciences, Stanford University, Stanford, California, 94305-2115
³ Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland, 21218-2687
⁴ Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, 91125

James Boles is a professor of geology at the University of California at Santa Barbara. His B.S. degree is from Purdue, his M.S. degree is from the University of Wyoming, and his Ph.D. is from the University of Otago (New Zealand). He has conducted research on various aspects of clastic diagenesis for more than 35 years. His current research is on fault diagenesis, pressure solution, and geochemical tools for interpreting diagenetic processes.Peter Eichhubl (Ph.D., University of California, Santa Barbara, 1997) is a research associate affiliated with the Stanford Rock Fracture Project. His research interests include fault and fracture mechanics, deformation processes in sediment and sedimentary rock, fluid flow in sedimentary basins, and the chemical interaction of rock and pore fluid.

Grant Garven is a professor of hydrogeology at Johns Hopkins University and founding coeditor of the new journal Geofluids. His research focuses on the paleohydrology of sedimentary basins and the application of reactive fluid-flow modeling for understanding processes of diagenesis, hydrothermal ore formation, and poroelastic deformation.

James Chen (Ph.D., University of California, Santa Barbara, 1977) is a research scientist affiliated with the Jet Propulsion Laboratory, Caltech. His research interests include investigating various aspects of isotope geochemistry, geochronology, and cosmochemistry using both long-lived and short-lived radioactive nuclei.

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 (15-mi)-long fault zone, which has less than 150 m (490 ft) 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 greater than 500 ka, indicating that fluid migration was intermittently active during the recent uplift history of the basin flank. Stable carbon isotopic values of the calcite are ¹³C_PDB = –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- to 3-km (1.2- to 1.9-mi) 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 yr.

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-yr timescale, 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.

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