HR: 0830h
AN: OS51C-0871    [PDF]
TI: Lithostatic Gas Pressures and Venting at Southern Hydrate Ridge
AU: * Trehu, A M
EM: trehu@coas.oregonstate.edu
AF: COAS, Oregon State Un., Ocean Admin Bldg. 104, Corvallis, OR 97331 United States
AU: Flemings, P B
EM: flemings@geosc.psu.edu
AF: Dept.of Geosciences, Penn State Un., 307 Deike Bldg., University Park, PA 16802 United States
AB: Seismic reflection data suggest that vents at the summit of southern Hydrate Ridge are fed by a stratigraphic horizon, 'Horizon A,' which is imaged as a strong reflection with negative amplitude. Multiple penetrations during Leg 204 of Horizon A revealed that it is composed of multiple volcanic glass-rich turbidite layers. The amplitude of this reflection increases abruptly above ~1050 m below sea level (mbsl). It decreases where it crosses from the free gas zone (FGZ) to the gas hydrate stability zone (GHSZ) beneath the western flank of Hydrate Ridge. We interpret the amplitude increase within the FGZ to represent the change from water-saturated to gas-saturated sediments and the decrease in the GHSZ to indicate formation of gas hydrate. The in situ density and seismic velocity in the the FGZ obtained by logging during Leg 204 are very low, confirming high gas saturation. We propose that gas is trapped within Horizon A by stratigraphically adjacent, less permeable sediments. Horizon A flattens within the FGZ at a depth of ~940 mbsl beneath the southern summit. Assuming that pore pressure at 1050 mbsf is hydrostatic, we determine that the gas pressure a the crest of this 110-120 m high gas column is 10.5 MPa, similar to the lithostatic stress. We interpret this to be a critically pressured system in which lithostatically pressured gas is dilating fractures that allow migration of methane gas through the GHSZ. This is a ongoing process as long as the supply of methane in horizon A is maintained. Chemical aspects of gas hydrate formation associated with this model are discussed in a companion paper (Torres et al.). This model also implies that gas venting can be suddenly initiated if the overburden thins through slope instability.
DE: 3025 Marine seismics (0935)
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting