Petroleum Field Evidence for Fluid Migration across Faults in Abnormally Pressured Siliciclastic Systems

Richard G. Gibson

In the deep portions of sedimentary basins, faults commonly form portions of boundaries between domains of contrasting datum fluid pressure. Fluid escape from high-pressure domains may be achieved by either hydraulic fracturing of an essentially impermeable seal (fault or top) or fluid-flow along or across a semi-permeable fault zone. In this study, hydrocarbon distribution patterns in petroleum fields within Tertiary siliciclastic strata of the offshore Gulf of Mexico and Trinidad areas are used to distinguish between these two possibilities.

Hydrocarbon columns in Vermilion 14 field (Louisiana) and Flambouyant and Poui fields (Trinidad) are trapped in partial sand-against-sand juxtaposition in the footwall of normal faults. Fluid pressure differences of 700-5000 psi are observed across some portions of these faults. Column heights are several times longer in reservoirs that are underpressured relative to juxtaposed strata than along fault segments that juxtapose reservoirs with equivalent datum fluid pressures (either normal or abnormal). Sandstones that are juxtaposed against lower pressure strata contain anomalously short oil or gas columns. In these cases, buoyancy pressures generated by the hydrocarbon columns are insufficient to elevate the reservoir pressures to those necessary for hydraulic fracturing of the seal.

The hydrocarbon distribution in these fields is most consistent with a hydrodynamic model in which fluid flow occurs across semi-permeable fault zones from high to low datum fluid-pressure domains. Superposition of hydrodynamic effects on the capillary properties of the fault zones creates the observed enhancement or degradation of seal capacity (inferred from column heights) relative to the hydrostatic case. This model provides a simple mechanism for fluid migration out of the abnormally pressured regime, without invoking hydraulic fracturing of the seal.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California