Expulsion of abnormally pressured fluids along faults

Numerical simulations of fluid flow and heat transport in the South Eugene Island minibasin, offshore Louisiana, show that expulsion of geopressured fluids along faults can produce temperature and pressure anomalies similar to those observed in the area. In the simulations, abnormally pressured fluid moves along the fault through a fracture network. A thermal anomaly forms adjacent to the fault, while a larger fluid pressure anomaly extends into sediments on either side. Results from constant fault permeability simulations indicate that (1) geopressured sediments must be relatively permeable (5 x 10(-17) m(2)) for expulsion to occur, (2) the sire of thermal anomalies depend on the depth to which the fault is hydraulically open, and (3) fluid is vertically transported into shallow sediments when fault permeability is high, while lateral transport along deeper sands dominates when fault permeability is low. Excess fluid pressure in abnormally pressured sediments drops to half its original value throughout much of the minibasin after 10,000 years of expulsion; the associated thermal anomaly is also larger than observations, suggesting expulsion is not continuous. Variable fault permeability simulations, in which compaction of fault zone sediments closes the fracture network, indicate that fault permeability decreases by 1-2 orders of magnitude 1-200 years after expulsion begins. Thermal and baric anomalies from variable permeability simulations are smaller than from constant permeability simulations and are more consistent with available data. Faults must remain permeable for 20-30 years to produce thermal and baric anomalies similar to those observed in the area.