HR: 08:00h
AN: OS31D-01 INVITED     [Abstracts]
TI: Geologic and Site Survey Setting for JIP Gulf of Mexico Gas Hydrate Drilling
AU: * Hutchinson, D R
EM: dhutchinson@usgs.gov
AF: USGS, 384 Woods Hole Rd., Woods Hole, MA 02543
AU: Snyder, F
OS31D-01 AF: Western Geco/Schlumberger, 3600 Briar Park Dr., Houston, TX 77042
AU: Hart, P E
OS31D-01 AF: USGS, 345 Middlefield Rd., Menlo Park, CA 94025
AU: Ruppel, C D
OS31D-01 AF: Georgia Tech, School of Earth and Atmos. Sci., Atlanta, GA 30332
AU: Pohlman, J
OS31D-01 AF: NRL, 4555 Overlook Ave. SW, Washington, DC 20375
AU: Wood, W T
OS31D-01 AF: NRL, Code 7432, Stennis Space Center, MS 39529
AU: Coffin, R B
OS31D-01 AF: NRL, 4555 Overlook Ave. SW, Washington, DC 20375
AU: Edwards, K M
OS31D-01 AF: Integrated Statistics, 384 Woods Hole Rd., Woods Hole, MA 02543
AB: The JIP Gulf of Mexico drilling program targeted two contrasting geologic settings to understand natural gas hydrates: a salt-withdrawal minibasin and a mound/seep site, both at mid-slope water depths of about 1300 m. The minibasin site (lease block Keathley Canyon 151) contains a Bottom Simulating Reflection (BSR) that deepens from 260 m below the sea floor near the edge of the basin to 500 mbsf towards the center of the basin. Drilling was conducted at a site in which the BSR is about 415 mbsf. Seismic stratigraphy of the minibasin consists of continuous laminated sequences of variable thicknesses alternating with more massive units of discontinuous reflections. These sequences represent uniform hemipelagic deposition, which drapes the basin, and turbidite deposition, which pinches out along the basin edges. The BSR crosses several of these sequences. A map of amplitude values at the BSR shows a strong banding pattern indicative of the layering, with the highest amplitudes interpreted to be trapped gas in the coarser-grained units. Prior to drilling, piston-core data indicated extensive shallow mass wasting near the edges of the minibasin. Heat flow data indicated thermal gradients that in general predicted a BSR deeper than observed in the seismic data. Full-waveform inversion of 3D multichannel data indicated a probable thick zone of low-saturation hydrate immediately above the BSR. There is little coherent seismic stratigraphy at the mound/seep site in the Mississippi Canyon (lease blocks Atwater Valley 13/14), as the canyon fill is dominated by a complex mix of turbidite and mass-wasting deposits. Hints of a possible BSR that is warped upwards beneath the mound can be seen in both 3D and 2D multichannel seismic data, but it cannot be traced laterally away from the mound with any certainty. A seismic pull-down pseudo-structure beneath the mound suggests the presence of a free-gas low-velocity zone at shallow depths. Pore-water analyses from shallow piston cores and closely-spaced heat-flow data indicate the mound is a site of probable fluid venting. A transect of bottom photographs crosses a possible mud flow and numerous bacterial mats, consistent with features seen in fluid venting at other sites in the Gulf. Prestack inversion of the multichannel data did not predict significant gas hydrate at the site on the edge of the mound. However, at the control site off the mound, predictions were more favorable for low hydrate saturations in the deeper part of the drill hole.
DE: 3002 Continental shelf and slope processes (4219)
DE: 3004 Gas and hydrate systems
DE: 3021 Marine hydrogeology
DE: 8169 Sedimentary basin processes
SC: Ocean Sciences [OS]
MN: Fall Meeting 2005