HR: 16:15h
AN: H12K-02    [PDF]
TI: Near-Seafloor Overpressure in the Deepwater Gulf of Mexico Interpreted from Laboratory Experiments
AU: * Dugan, B
EM: bdugan@usgs.gov
AF: US Geological Survey, 384 Woods Hole Road, Woods Hole, MA 02543 United States
AU: Germaine, J
EM:
AF: MIT, Dept. of Civil and Environmental Engineering, Cambridge, MA 02139 United States
AU: Winters, W J
EM:
AF: US Geological Survey, 384 Woods Hole Road, Woods Hole, MA 02543 United States
AU: Flemings, P B
EM:
AF: Penn State University, Dept. of Geosciences, University Park, PA 16802 United States
AB: Consolidation experiments on two silty clay samples from 1318 m water depth in the Gulf of Mexico (Marion Dufresne giant piston core MD02-2567, Mississippi Canyon Block 855, 90 km offshore Louisiana, USA) show that sediment is overpressured at 6.8 and 19.2 m below the seafloor (mbsf). The transition from elastic to elastic-plastic deformation is well defined, and is used to predict normalized overpressure ($\lambda^{*}$) equal to 0.5 [$\lambda^{*}$ = (P-P$_{h}$)/(S$_{v}$-P$_{h}$)]. P is fluid pressure, P$_{h}$ is hydrostatic fluid pressure, and S$_{v}$ is overburden stress. In situ permeability, estimated from constant rate of strain experiments, is 1x10$^{-16}$m$^{2}$ at 6.8 mbsf and decreases to 1x10$^{-17}$m$^{2}$ at 19.2 mbsf. The compressibility and low permeability of the sediments (1) controls the rate at which pressure is generated during deposition, (2) influences the ability of external pressure sources to alter the sediments, and (3) hinders pressure dissipation to hydrostatic conditions. The constant rate of strain experiments were controlled at 0.5%/hr, which maintained an overpressure to total vertical stress ratio of 0.1 for a 1.8 cm drainage path. Both specimens deformed along the virgin compression curve with void ratio $<$1.1 (porosity $<$52%). Minor differences in initial laboratory void ratio and virgin consolidation trends are interpreted as a function of observed grain size variation between the samples. The grain size variations, however, appear to dramatically impact permeability. Overpressure in the shallow subsurface decreases effective stress that is already low. Higher overpressure in the past may have facilitated failure along a regional detachment surface imaged in seismic data at 125 mbsf. We hypothesize that pressure dissipation from a shallow water flow (SWF) sand at 250 mbsf at this location is a source of near-seafloor overpressure in this region of the Gulf of Mexico. Ongoing experiments from locations where the SWF sand is deeper will characterize how depth to the sand influences fluid pressure and strength of low permeability sediments that overlie the sand.
DE: 3022 Marine sediments--processes and transport
DE: 5114 Permeability and porosity
SC: Hydrology [H]
MN: 2003 Fall Meeting