Surficial Gas Hydrates, Part of the Fluid and Gas Expulsion Response Spectrum: Identification from 3D Seismic Data

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Conference Proceeding

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Currently, the complex continental slope opposite Louisiana is covered with a high quality database for interpreting seafloor geology. This database consists of large, adjacent, and overlapping tracks of 3D-seismic data. Linking seismic data with field verification data derived from manned submersible observations and samples has produced a qualitative understanding of seafloor response to a spectrum of fluid and gas expulsion rates. Slow flux rates tend to produce a seafloor characterized by hard bottoms (mounds, hardgrounds, and nodular masses in unconsolidated sediment) created by precipitation of C-depleted Ca-Mg carbonates. Other precipitates such as barite have also been observed in slow-to-moderate flux settings. At the other end of the expulsion spectrum are response features derived from rapid delivery of fluids (including fluidized sediment) and gases to the seafloor. Mud-prone features such as mud volcanoes of various dimensions and thin, but widespread mud flows characterize the rapid flux part of the expulsion spectrum. Considerable heat and non-biodegraded hydrocarbons frequently accompany rapid flux of fluidized sediment. Below water depths of approximately 500 m, intermediate flux settings seem best exemplified by areas where gas hydrates occur at or very near the seafloor. These environments display considerable variability with regard to surficial geology and on a local scale have elements of both rapid and slow flux. However, this dynamic setting apparently has a constant supply of hydrocarbons to promote gas hydrate formation at the seafloor even though oceanic temperature variations cause periodic hydrate decomposition. The presence of these deposits provides the unique set of conditions necessary to sustain dense and diverse chemosynthetic communities. The cross-slope variability of seafloor response to fluid and gas expulsion is not well known. However, present data indicate that the expulsion process is highly influenced by migration pathways dictated by salt geometries that change downslope from isolated salt masses to canopy structures to nappes. 13

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Proceedings of the Annual Offshore Technology Conference

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