HR: 10:35h
AN: OS22A-02    [Abstracts]
TI: Formation of Regional Bottom-Simulating Seismic Reflectors (BSRs) and Their Use in Quantifying Upward Fluid Flow
AU: * Haacke, R R
EM: rhaacke@nrcan.gc.ca
AF: Pacific Geoscience Centre, Geological Survey of Canada, 9860 W. Saanich Rd, Sidney, BC V8L 4B2, Canada
AU: Westbrook, G K
EM: g.k.westbrook@bham.ac.uk
AF: School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, United Kingdom
AU: Hyndman, R D
EM: rhyndman@nrcan.gc.ca
AF: Pacific Geoscience Centre, Geological Survey of Canada, 9860 W. Saanich Rd, Sidney, BC V8L 4B2, Canada
AB: It is becoming apparent that the character of regional hydrate and free-gas distributions in passive-margin environments is different to the equivalent distributions in accretionary wedges at convergent margins. Accretionary wedges typically have widespread BSRs with underlying gas zones of a few tens of metres in thickness; passive margins have comparatively rare BSRs with gas zones typically a few hundred metres thick. In both cases, the sub-BSR free gas typically occupies a few percent, or less, of pore space. Hydrate recycling caused by seabed uplift, sedimentation, bottom-water warming etc. (where free gas comes from dissociating hydrate) is one mechanism for producing sub-BSR free gas that works well in accretionary wedges. However, hydrate recycling does not work well in passive margins where recycling mechanisms are slower than in accretionary wedges (there is no seabed uplift), but the sub-BSR free-gas zones are thicker: we expect the opposite if hydrate recycling is the primary mechanism producing sub-BSR free gas. Here we present a mechanism for the production of gas beneath hydrate-bearing sediments that is complementary to hydrate recycling in all environments, but which becomes the dominant mechanism in passive-margin settings. This mechanism produces a downward decreasing distribution of gas in low concentrations (few percent of pore space) across thick zones (hundreds of metres) when the upward fluid flow is low (less than a few tenths of mm/yr) and when the gas--water solubility curve is downward decreasing beneath the gas hydrate stability zone. The latter requires moderate to high geothermal gradients and pressures. We find that the sub-BSR free-gas zone produced by the solubility-curvature mechanism achieves a steady-state thickness that is controlled by the rate of upward fluid flow and the concentration of methane dissolved within it. Using the seismically derived free- gas zone thickness for a test case offshore Svalbard (Norwegian Arctic) as the steady-state thickness, we show that the solubility curvature mechanism can be used to forward model the observed gas distribution and, consequently, to constrain the rate of upward fluid flow.
DE: 3004 Gas and hydrate systems
DE: 3021 Marine hydrogeology
DE: 3653 Fluid flow
SC: Ocean Sciences [OS]
MN: 2007 Fall Meeting