Dugan, Brandon¹, Jack Germaine², William Winters¹, Peter Flemings³
(1) U.S. Geological Survey, Woods Hole, MA
(2) MIT, Cambridge, MA
(3) Pennstate University, University Park, PA

ABSTRACT: Laboratory Constraints and Models of Pressure, Hydrate, and Stability in Shallow Mississippi Canyon Sediments (MC 855), Deepwater Gulf of Mexico

Fluid pressure and stress history influence hydrate and slope stability. Shallow water flows (SWF) document shallow (<1000 m below seafloor, mbsf) overpressure in the Gulf of Mexico, but pore pressure observations in shallow silt/clay are rare. We use constant rate of strain (CRS; 0.5%/hr) consolidation experiments to interpret overpressure equaling 50% of the hydrostatic effective stress at 6.8 and 19.2 mbsf in 1318 m water (Marion Dufresne giant piston core MD02-2567, Mississippi Canyon Block 855). Experimentally determined permeability of these sediments is 1x10^-16m² at 6.8 mbsf and decreases to 1x10^-17m² at 19.2 mbsf is sufficiently low to prevent drainage to hydrostatic conditions. Specimens deformed along the virgin compression curve for void ratio < 1.1, indicating high in situ porosity and low effective stress. Grain size variation between 6.8 and 19.2 mbsf has minor effects on consolidation behavior but large effects on permeability. Higher overpressure in the past may have driven failure along a regional detachment surface at 125 mbsf. Overpressure also increases the theoretical thickness of the hydrate stability zone.
We hypothesize that pressure dissipation and flow in permeable conduits from a SWF sand at 250 mbsf at this location contributes to near-seafloor overpressure. Ongoing experiments from nearby locations will characterize how depth to the sand influences fluid pressure and strength of overlying sediments. Depositional models simulate fluid pressure evolution relative to the SWF sand, track heat and chemical transport during deposition, and provide estimates of hydrate formation/dissociation and slope instability, which are active processes in this region.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.