Abstract

Data from sediments in and near a large growth fault adjacent to the giant South Eugene Island Block 330 field, offshore Louisiana, indicate that the fault has acted as a conduit for fluids whose flux has varied in space and time. Core and cuttings samples from two wells that penetrated the same fault about 300 m apart show markedly different thermal histories and evidence for mass flux. Sediments within and adjacent to the fault zone in the U.S. Department of Energy-Pennzoil Pathfinder well at about 2200 m SSTVD (subsea true vertical depth) showed little paleothermal or geochemical evidence for throughgoing fluid flow. The sediments were characterized by low vitrinite reflectances (R o ) averaging 0.3% R o , moderate to high delta 18 O and delta 13 C values, and little difference in major or trace element composition between deformed and undeformed sediments. In contrast, faulted sediments from the A6ST well, which intersects the A fault at 1993 m SSTVD, show evidence for a paleothermal anomaly (0.55% R o ) and depleted delta 18 O and delta 13 C values. Sodium is depleted and calcium is enriched in a mudstone gouge zone at the top of the fault cut in the well; this effect diminishes with distance from this gouge zone. Cuttings from other wells in South Eugene Island Block 330 show slightly elevated vitrinite reflectance in fault intercepts relative to sediments outside the fault zone. Overall, indicators of mass and heat flux indicate the main growth fault zone in South Eugene Island Block 330 has acted as a conduit for ascending fluids, although the cumulative fluxes vary along strike. This conclusion is corroborated by oil and gas distribution in downthrown sands in Blocks 330 and 331, which identify the fault system in northwestern block 330 as a major feeder. Simple modeling of coupled heat and mass flux indicates the paleothermal anomaly in the fault zone intersected by A6ST well was short-lived, having a duration less than 150 yr. The anomaly could have been produced by a 2X10 6 m 3 pulse of fluid ascending the fault at an actual velocity of over 1 km/yr (Darcy flux of 330 m/yr) from 3 km deeper in the basin. Simple Darcy law computation indicates a transient fault permeability on the order of 110 md during this flow. Pulsing of fluid up the fault was probably the norm, although most flow did not produce such strong thermal anomalies as the one detected in the A6ST well. Analysis of fluid pressures shows that the main fault is a profound lateral permeability barrier having up to 1800 psi of water pressure differential across it. The hydrocarbon sealing capacity of the fault depends on the pressure difference across the fault. Fault permeability is best understood in terms of effective stress. Under ambient conditions, the fault is at high pressure relative to downthrown reservoirs. A pulse of high-pressure fluid ascending the fault lowers effective stress in the fault zone sufficiently to produce a significant transient increase in permeability. If the fluid is in an area of the fault adjacent to downthrown, relatively low pressure reservoir sands, the fluid will discharge into them. Permeability in and adjacent to the fault then decreases, such that fluid cannot reenter the fault zone and escape from the reservoir.

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