Losh, Steven L.¹, Lawrence Cathles¹, Jennifer Adams¹ 
(1) Deptartment of Earth and Atmospheric Sciences, Cornell University, Ithaca, NY

ABSTRACT: Fault Seal/Fault Conduit Behavior at South Eugene Block 330, Offshore Louisiana: Observations and Modeling Results

A long-standing paradox in petroleum geology is that faults in siliciclastic sediments behave as both conduits and barriers to fluid and gas migration. In the giant South Eugene Island Block 330 Field, offshore Louisiana, at least 7 x 10⁸ cubic meters of water, oil, and gas have charged the reservoirs from the main minibasin-bounding fault system within the past ~120,000 years, requiring average fault permeabililty to be orders of magnitude higher than values measured in the laboratory or known to be compatible with large trapped column heights or documented large fluid pressure differentials across the faults. 
We show the following: 1) core data from the Pathfinder well indicates the fault probably transmits hydrocarbon only when it is actively deforming, 2) the fault has been the locus of rapid ascent of deeply-sourced water and hydrocarbon along much but not all of its length, 3) effective stress in the fault at reservoir depth is generally too high for permeability to be enhanced at ambient fluid pressures, 4) shale gouge has fault-perpendicular permeability in the nanodarcy range, and 5) different reservoirs contain distinct oils and/or brines, indicating reservoirs have probably not leaked since they were charged. Fluid flow must be episodic, and the fault zone evidently transmitted fluid only during discrete (aseismic?) slip events, which simultaneously provided a pathway from deep, nearly lithostatically-pressured sands to the reservoirs and ruptured the low-permeability gouge to form a transient, “one-way valve” into reservoir sand. After discharge of the fluid pulse, elevated fluid pressure in the fault and low gouge permeability retain hydrocarbon columns in fault traps.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.