ABNORMAL FLUID PRESSURES AND FLUID FLOW AT ODP SITE 1251, SOUTHERN HYDRATE RIDGE, OREGON

A continuous down-hole record of fluid pressure was acquired at Site 1251 of the Ocean Drilling Program (ODP) Leg 204 to southern Hydrate Ridge, Oregon, based on consolidation tests of whole round core samples and geophysical logging–while-drilling (LWD) data. Void ratio vs. effective stress relationships from the consolidation studies show underconsolidation of the samples in the upper 140 mbsf of the section, corresponding to a zone of consistently elevated fluid pressure that reaches ~90% of the lithostatic overburden stress. Models of one dimensional sedimentation and consolidation show that the observed degree of overpressuring can be accounted for by the combined effect of rapid sedimentation (60 to 160 cm/k.y.) and low permeability (1x10^-16 to 1x10^-17 m²). Between 140 and 300 mbsf the fluid pressure varies from 90% of lithostatic to sub-hydrostatic in 5 distinct horizons. This variation may be linked to mechanical consolidation of the sediment related to pore fluid drainage along more permeable horizons, or changes in sedimentation rate related to either glacial/ interglacial cycles or the uplift history of the ridge. The pressure gradient and permeability measurements predict Darcy flow rates on the order of 1 mm/yr at this site, but a lack of laterally continuous permeable conduits likely prevents the flow generated by basin overpressures from contributing to flow at the crest of the ridge. Using an average methane concentration of 100 mM and the determined flow rate, the methane flux through the system is calculated to be ~5 x 10^-11 kg s^-1 m^-2. These data can be used to refine models of hydrate distribution and occurrence in environments subject to primarily 1-d fluid flow.