MULTICOMPONENT SEISMIC TECHNOLOGY FOR ASSESSMENT OF FLUID-GAS EXPULSION GEOLOGY AND GAS HYDRATES: GULF OF MEXICO

Bob A. Hardage¹ and Harry H. Roberts^2
1 Bureau of Economic Geology, University of Texas at Austin, Austin, TX, 78713
² Coastal Studies Institute, Louisiana State University, Baton Rouge, LA 70803

Analysis of multicomponent seismic data (four-component ocean-bottom-cable [4-C OBC] data) represents an unparalleled research opportunity to utilize a unique data set for gas-hydrate evaluation. This data set is currently being collected across much of the northern Gulf of Mexico’s upper continental slope to a water depth of 1,000 m. These data are being acquired to provide a new seismic imaging capability for oil and gas operators. WesternGeco is collecting long-offset 4-C OBC data from which both shear wave and conventional compressional wave (P-wave) data can be derived. Not only can these long-offset 4-C OBC data allow for imaging deeper within the sediment column, but the C-wave data (upgoing SV shear wave created by P-to-SV mode conversion) made possible with this acquisition strategy will image through gas chimneys and image different stratal surfaces and boundaries than standard P-wave data. In addition, lithofacies data as well as shear moduli and bulk moduli can be calculated from 4-C OBC data. Results of this ongoing research have produced images of fluid-gas migration pathways from the deep subsurface and provide stratal information that can help define the base of the hydrate stability zone as well as properties of sediments invaded by gas hydrate. Analysis of 4-C OBC data also provides us with a means of assessing C-wave data with a focus on cross-over information that may be important to interpreting P-wave data of various frequency contents (conventional 3-D data volumes to high-frequency deep-towed profiles). Two test sites in the Green Canyon area (GC237 and GC204) were selected for study on the basis of seafloor evidence of fluid-gas expulsion and availability of 4-C OBC data. Work to date has focused on processing raw field data along two east-west transects of GC237 and one north-south transect across GC204. Because of complex structure and heat flow in the northern Gulf of Mexico, bottom-simulating reflectors (BSRs), which are considered evidence of the base of the gas hydrate stability zone, have generally been difficult to identify in industry standard P-wave seismic data. The P-SV images constructed in this study, in addition to providing different and more detailed stratigraphic character than P-P images, reveal a pronounced variation of reflector amplitudes in the zone predicted to contain gas hydrate as calculated from water depth, geothermal gradient, and gas type. Also, P-SV data image fault architectures and vertical fluid-gas migration pathways much better than P-wave data. Upward-migrating gas deteriorates P-wave reflectivity, resulting in a strong genetic link between acoustic wipe-out “chimneys.” Analysis of 4-C OBC data holds great promise for improving gas-hydrate resource evaluations and for imaging fault systems, a necessary input for constructing realistic numerical models of fluid and gas migration from the deep subsurface.