Surficial Gas Hydrates, Part of the Fluid and Gas Expulsion Response Spectrum: Identification From 3D Seismic Data
Authors: Roberts Harry H. , Hunt Jesse L. , Shedd William W. , Sassen Roger 
Published: October 2008

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 13C-depleted Ca-Mg carbonates. Other precipitates such as barite have also been observed in slow-tomoderate 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 nonbiodegraded 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.

Introduction

Much of what we know about the geology of the northern Gulf continental slope comes from data collected in search of hydrocarbons. The northern Gulf slope is the most mature deep-water oil and gas province in the world. With 3Dseismic and higher resolution acoustic data sets used for geohazards assessment, a new vision of the slope seafloor is being established. Even though the highest resolution data sets are exceptionally revealing regarding the seafloor and probable processes impacting it, they typically cover small areas. These high resolution data sets are site specific and usually separated by considerable distances. Therefore, the wide variety of bottom features and their links to formative processes have been difficult to determine. Recently, multibeam bathymetry and 3D-seismic have provided a detailed and yet regional view of the slope surface. These two data types have emphasized the complexity of the slope's surface geology. Mapping surface amplitude anomalies from 3D-seismic data (Trabant, 1996; Roberts et al., 1992a, Roberts, 1996; Hill, 1996) has helped identify fluid and gas expulsion features. Coupled with polarity analysis of thesurface reflector, these methodologies can identify hard bottom from soft bottom areas as well as sediments charged with gas.