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