HR: 16:30h
AN: OS34B-03    [Abstracts]
TI: Mechanism for free gas migration through South Hydrate Ridge hydrate system
AU: * Liu, X
EM: xliu@geosc.psu.edu
AF: Pennsylvania State University, Deike 320 Department of Geosciences, State College, PA 16802
AU: Flemings, P
EM: flemings@geosc.psu.edu
AF: Pennsylvania State University, Deike 320 Department of Geosciences, State College, PA 16802
AB: At southern Hydrate Ridge, offshore Oregon, free gas vents through hydrate-bearing sediments where free gas coexists with gas hydrate and brine in shallow subsurface. To establish three-phase (liquid, hydrate and gas) equilibrium within the gas hydrate stability zone (GHSZ), salinity must increase from a baseline value of 3 wt.% at the base of the GHSZ to about 12 wt.% at the seafloor. This predicted salinity profile is similar to in situ pore water salinity calculated at ODP Site 1249. We interpret that the upward increase in salinity is driven by rapid hydrate formation in the case of high methane flux from below. As hydrates form, dissolved ions are excluded and pore water salinity rises. Ultimately, salinity rises to the point where three-phase equilibrium is reached and additional methane exists in free gas phase. By assuming that there is no transport of water or salt, we can estimate the hydrate amount from change in salinity relative to the baseline. Hydrate volume faction would increase from 0 below the GHSZ to 70% at the seafloor. This predicted hydrate distribution is consistent with resistivity log- and PCS- derived hydrate occurrences at Hydrate Ridge. Thus, both observations and calculations of three-phase equilibrium suggest that the formation of marine methane hydrate is a self-equilibrating process at locations where methane supply is abundant. This provides a mechanism whereby free gas migrates through the GHSZ without being converted into hydrate.
DE: 3099 General or miscellaneous
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting