ABSTRACT

In 1993, the Global Basins Research Network drilled into a growth fault zone that forms the northern boundary of the Eugene Island 330 minibasin, offshore Louisiana. Measured pore fluid pressures in the well reach approximately 93 per cent of lithostatic pressure below two kilometers depth. Compaction disequilibrium accounts for only about three-quarters of the overpressures. While a ductile rheology for the fault zone is indicated by a low shear modulus and high Poisson's ratio, examination of core and FMI log showed numerous faults and fractures. Drill stem tests indicate a highly fluid pressure-dependent permeability (100 mD to 0.1 mD depending on drawdown pressure). Similar observations of highly overpressured (> 90 per cent of lithostatic) sediments, pore pressure dependent permeabilities, unusual sediment rheology, and episodic fluid flow have been documented in shear zones in other tectonic environments including the sub-salt play in the Gulf of Mexico and mud dominated accretionary wedges. I suggest that development of highly overpressured sediments within the fault zone in Eugene Island is related to reduction in porosity and permeability associated with ductile shear deformation. During shearing as the fault zone moves in response to salt evacuation, ductile deformation reduces the volume and increase the aspect ratio of pores which reduce permeability and increase pore fluid pressure. Thus, during periods of ductile deformation, the shear zone acts as a seal. As ductile deformation continues, pore fluid pressures eventually exceed fracture strength and a short period of brittle deformation occurs. Hydrofracture within the shear zone enhances porosity and permeability. Subsequent fluid flow along the fault causes pore pressures to drop and fluid from adjacent reservoir sands to be drawn upwards along the fault zone. Rapid decline in pore fluid pressures causes the fracture network to collapse and the shear zone to reseal. Cycles of ductile and brittle deformation may be repeated as shear movement continues. Once shear deformation stops, elevated pore fluid pressures within the shear zone will diffuse down to the regional level. However, low permeability will remain. Thus, shear zones may be long term barriers to fluid flow.