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44476-AC8
Overpressure and Slope Stability in the Deepwater Gulf of Mexico
Overpressures measured with pore pressure penetrometers during Integrated Ocean Drilling Program (IODP) Expedition 308 reach 70 % and 60% of the hydrostatic effective stress (λ* = (u-uh)/(σ'vh)) in the first 200 meters below sea floor (mbsf) at Sites U1322 and U1324, respectively, in the deepwater Gulf of Mexico, offshore Louisiana. High overpressures are present within low permeability mudstones where there have been multiple, very large, submarine landslides during the Pleistocene. Beneath 200 mbsf at Site U1324, pore pressures drop significantly: there are no submarine landslides in this mixture of mudstone, siltstone and sandstone. The penetrometer measurements did not reach the in situ pressure at the end of the deployment. We used a soil model to determine that an extrapolation approach based on the inverse of square route of time (1/√t) requires much less decay time to achieve a desirable accuracy than an inverse time (1/t) extrapolation. Expedition 308 examined how rapid and asymmetric sedimentation above a permeable aquifer drives lateral fluid flow, extreme pore pressures, and submarine landslides. We interpret that the high overpressures observed are driven by rapid sedimentation of low permeability material from the ancestral Mississippi River. Reduced overpressure at depth at Site 1324 suggests suggest lateral flow (drainage) whereas high overpressure at Site 1322 requires inflow from below: lateral flow in the underlying permeable aquifer provides one mechanism for these observations. High overpressure near the seafloor reduces slope stability and provides a mechanism for the large submarine landslides and low regional gradient (2 degrees) offshore from the Mississippi delta.
We also conducted extensive uniaxial consolidation tests on whole core samples to obtain the consolidation properties of the Ursa mudstones. The results suggest that the compression index linearly decreases with in situ void ratio. This implies that a locally-defined virgin compression curve cannot validly be extrapolated over a large range in effective stress. This effect is particularly important at shallow depth where void ratio decreases rapidly. We have shown that the relationship of compressibility index versus void ratio can be obtained from a single consolidation test by compressing the soil over a large range in effective stress. A virgin compression curve can then be constructed based on this relationship to predict pore fluid pressure. In the Ursa Basin, this new approach successfully predicted pressures interpreted from the penetrometer measurements within the non-deformed sediments. The mass transport deposits appear to be more compacted than the non-deformed sediments. The virgin compression curve based on the assumption of uniaxial strain underpredicts the in situ pressure in the mass transport deposits.
We have also shown that seismic facies and material properties of mass transport deposits (MTDs) in the Ursa Basin are controlled by porosity, not lithology. Deformation is distributed throughout MTDs, but tends to be greatest towards the base. In seismic, MTDs are imaged as low-amplitude zones above a high-amplitude basal reflector. Within these zones we identify 1) a locally high-amplitude chaotic seismic facies, and 2) a discontinuous/locally stratified facies. In seismic cross section, locally high-amplitude chaotic facies appears as high-amplitude discontinuous reflections. These reflections are sinuous in interval amplitude maps. The basal reflector is high amplitude, continuous, and records grooves (~10 km long). The top reflector is lower amplitude than the basal reflector. The locally stratified facies contains discontinuous reflectors that abut against pyramid-shaped islands (“pinnacles”) of parallel stratified reflectors. Each pinnacle sticks above the surrounding material and is attached to the base of the MTD. The basal reflector is high-amplitude beneath the discontinuous reflectors, but relatively dim beneath the pinnacle features. The locally high-amplitude chaotic facies is denser (i.e., that is lower porosity), has higher shear strength, and higher resistivity than bounding undeformed sediment. Deformation in this facies appears as a deformed mud package with a homogeneous appearance, occasional folds, and rare mud clasts. The locally stratified facies has only slightly increased density (i.e., lower porosity), has slightly greater shear strength, and a slightly greater resistivity than bounding undeformed sediment. Deformation is subtle in this facies and recorded as tilted bedding and small-offset faults. The higher density of MTDs of both facies creates a strong impedance contrast between bounding undeformed sediment, and thus a strong reflection at the top and base. However, the amplitude of the basal reflector is much higher amplitude because the density contrast is much higher at the base. We interpret that the locally high-amplitude/chaotic facies records debris flows formed by a long runout; the locally stratified facies records slumps formed by only a short run-out with minimal deformation.
