Abstract: Ductile Shear Deformation in Fault Zones
NUNN, JEFFREY A., Louisiana State University
In 1993, the A-20ST Pathfinder well drilled through a growth fault zone in Eugene Island 330 minibasin, offshore Louisiana. Pore fluid pressures reach ~93% of lithostatic pressure below 2 km depth. Below the A fault, core plugs have a measured porosity of 30% indicating undercompaction. A surprising result is the very weak theology of the sediments. Possion's ratio is above 0.4 and shear modulus is less than 1 GPa. While a ductile rheology for the fault zone is indicated by the sonic log, examination of the core plus a FMI log showed numerous faults and fractures. Many of the faults and fractures contain oils with similar chemistry to oils in overlying reservoir sands. Formation waters from these overlying Pleistocene sands have been isotopically dated as Oligocene or older. I suggest that the development of highly overpressured sediments within the fault zone in Eugene Island is related to ductile shear deformation. During shearing as the fault zone moves in response to salt evacuation, intergranular pressure solution, grain rotation, and fracture sealing reduces the volume and increase the aspect ratio of pores. These processes reduce permeability and increase pore fluid pressure. Thus, during periods of ductile deformation, the shear zone is an effective seal against fluid flow. As deformation continues, pore fluid pressures exceed the fracture strength of the sediments and a short period of brittle deformation occurs. Hydrofracture within the shear zone enhances its 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.