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

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 dCPDB = 35 to 41x, 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 80–95jC suggest that hot water leaked from 2to 3-km (1.2to 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 10 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 10-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. AAPG Bulletin, v. 88, no. 7 (July 2004), pp. 947–970 947 Copyright #2004. The American Association of Petroleum Geologists. All rights reserved. Manuscript received April 24, 2003; provisional acceptance August 27, 2003; revised manuscript received December 16, 2003; final acceptance February 9, 2004.