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Hutchinson, D. R., P. E. Hart, C. D. Ruppel, F. Snyder, and B. Dugan,
Seismic and Thermal Characterization of a Bottom-simulating Reflection in the Northern Gulf of Mexico
Deborah R. Hutchinson,1 Patrick E. Hart,2 Carolyn D. Ruppel,3 Fred Snyder,4 Brandon Dugan5
1U.S. Geological Survey, Woods Hole, Massachusetts, U.S.A.
2U.S. Geological Survey, Menlo Park, California, U.S.A.
3Georgia Tech, School of Earth and Atmospheric Sciences, Atlanta, Georgia, U.S.A.; Present address: U.S. Geological Survey, Woods Hole, Massachusetts, U.S.A.
4Schlumberger, Houston, Texas, U.S.A.
5Rice University, Department of Earth Sciences, Houston, Texas, U.S.A.
Support for this work was provided by the U.S. Geological Survey, the Department of Energy, and the Chevron Joint Industry Project (JIP). Myung Lee (U.S. Geological Survey) processed and migrated the 2-D multichannel seismic data. We thank WesternGeco/Schlumberger for the permission to use and publish the 3-D seismic images, derivative bathymetry, amplitude map of the BSR, and traveltimes to the top of salt. Emrys Jones provided encouragement and allowed access to unpublished JIP data. This work would not have been possible without the assistance of the crew and scientists that participated in the two RV Gyre expeditions. This manuscript was improved by reviews by William Waite and Uri ten Brink. C. Ruppe was on assignment at and fully supported by the National Science Foundation (NSF) during the completion of this project. The opinions expressed reflect those of the authors and not the NSF. Generator-injector guns (GI guns) are manufactured by Seismic Systems, Inc. The use of trade names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
High-resolution multichannel seismic reflection data, exploration industry three-dimensional (3-D) seismic data, and heat-flow measurements collected on the southeast side of a minibasin (Casey basin) in the northern Gulf of Mexico continental slope have been used to characterize a bottom-simulating reflector (BSR). The BSR, which covers a small area of about 15 km2 (6 mi2), is identified by crosscutting relationships with seismic stratigraphy. Two mounds are identified. The larger Alpha mound is structurally formed at the junction of three arms of the structural high east of the minibasin. The smaller Beta mound may be a seep site. Conventional heat-flow measurements yield higher gradients (39–49 mK/m) to the northeast of the structural high and lower values (30–38 mK/m) to the south and west along the edge of the minibasin, which is separated from the structural high by the eastern Casey fault zone. When the near-sea-floor thermal gradients are extrapolated to the depth of the BSR, the resulting temperatures are generally too low if the BSR marks the base of the hydrate stability zone in a methane-only gas-hydrate system. Plausible changes in pore-water salinity or gas composition cannot account for this disparity, and thermal perturbations caused by fluid downwelling, mass wasting, or depth-dependent thermal conductivity variations might best explain the low predicted BSR temperatures. The recognition of a BSR in the study area provides geophysical evidence that a hydrate stability zone with trapped free gas at its base exists in the northern Gulf and that minibasins can be locations for finding subsurface hydrate-associated free gas and probable gas hydrate.
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