ABSTRACT: Development of Geopressure by Ductile Shear Deformation
NUNN, JEFFREY A.
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. Pore fluid pressures in the well are >93% of lithostatic below 2 km. Compaction disequilibrium accounts for only about 75% of the excess fluid pressure. 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 fractures. Drill stem tests indicate a fluid pressure-dependent permeability (100 mD to 0.1 mD depending on drawdown pressure). Similar observations of highly overpressured sediments, pore pressure-dependent permeabilities, and unusual sediment rheology have been documented in shear zones in other tectonic environments including the sub-salt play in the Gulf of Mexico. 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 increases the aspect ratio of pores which decrease 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 are long term barriers to fluid flow.
AAPG Search and Discovery Article #90941©1997 GCAGS 47th Annual Meeting, New Orleans, Louisiana