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

摘要

Currently, the complex continental slope opposite Louisiana isrncovered with a high quality database for interpreting seafloorrngeology. This database consists of large, adjacent, andrnoverlapping tracks of 3D-seismic data. Linking seismic datarnwith field verification data derived from manned submersiblernobservations and samples has produced a qualitativernunderstanding of seafloor response to a spectrum of fluid andrngas expulsion rates. Slow flux rates tend to produce a seafloorrncharacterized by hard bottoms (mounds, hardgrounds, andrnnodular masses in unconsolidated sediment) created byrnprecipitation of 13C-depleted Ca-Mg carbonates. Otherrnprecipitates such as barite have also been observed in slow-tomoderaternflux settings.rnAt the other end of the expulsion spectrum are responsernfeatures derived from rapid delivery of fluids (includingrnfluidized sediment) and gases to the seafloor. Mud-pronernfeatures such as mud volcanoes of various dimensions andrnthin, but widespread mud flows characterize the rapid flux partrnof the expulsion spectrum. Considerable heat and nonbiodegradedrnhydrocarbons frequently accompany rapid flux ofrnfluidized sediment.rnBelow water depths of approximately 500 m, intermediaternflux settings seem best exemplified by areas where gasrnhydrates occur at or very near the seafloor. Thesernenvironments display considerable variability with regard tornsurficial geology and on a local scale have elements of bothrnrapid and slow flux. However, this dynamic setting apparentlyrnhas a constant supply of hydrocarbons to promote gas hydraternformation at the seafloor even though oceanic temperaturernvariations cause periodic hydrate decomposition. Thernpresence of these deposits provides the unique set ofrnconditions necessary to sustain dense and diversernchemosynthetic communities.rnThe cross-slope variability of seafloor response to fluidrnand gas expulsion is not well known. However, present datarnindicate that the expulsion process is highly influenced byrnmigration pathways dictated by salt geometries that changerndownslope from isolated salt masses to canopy structuresrnto nappes.