Four-component ocean-bottom-cable (4-C OBC) seismic data acquired in deep water across the Gulf of Mexico were used to study near-sea-floor geologic characteristics of fluid-gas expulsion systems. Although these 4-C OBC data were acquired to evaluate oil and gas prospects far below the sea floor, the data have great value for studying near-sea-floor geology. The research results summarized here stress the importance of the converted-shear-wave (P-SV) mode extracted from 4-C OBC data. In deep water, the P-SV mode creates an image of near-sea-floor strata that has a spatial resolution an order of magnitude better than the resolution of compressional wave (P-P) data regardless of whether the P-P data are acquired with OBC technology or with conventional towed-cable seismic technology. This increased resolution allows the P-SV mode to define seismic sequences, seismic facies, small-throw faults, and small-scale structures that cannot be detected with P-P seismic data.

The U.S. Minerals Management Service has completed a preliminary assessment of in-place gas hydrate resources in the Gulf of Mexico. A probabilistic model built on a mass balance approach to assessment provides a high degree of spatial resolution and supports detailed mapping. The model produces a Monte Carlo distribution of in-place resources that ranges from 314 trillion to 974 trillion cubic meters (TCM) with a mean value of 607 TCM. Additional work on development of a technically recoverable model component is under way.

The Minerals Management Service (MMS) is division of the United States (U.S.) Department of the Interior. Its mandate is to manage natural gas, oil, and other mineral resources on the U.S. outer continental shelf (OCS). The MMS launched a project in order to provide an assessment of the natural gas hydrate resource potential across the entire OCS, including the Alaskan, Atlantic, Gulf of Mexico, and Pacific margins. The purpose of this ongoing project is to provide a probabilistic evaluation of in-place, technically recoverable, and economically recoverable gas hydrate resources. This paper provided an overview of the project, including a preliminary assessment of in-place gas hydrate resources in the Gulf of Mexico. The paper described the probabilistic model that was built on a mass balance approach to assessment. The model provided a high degree of spatial resolution and supported detailed mapping. The model produced a Monte Carlo distribution of in-place resources that ranged from 314 trillion to 974 trillion cubic meters (TCM) with a mean value of 607 TCM. The paper also provided a link to the full report which included the model methodology, underlying assumptions, and input datasets. Additional work on the development of a technically recoverable model component is currently underway. 1 fig.

Abstract The Gulf of Mexico Methane Hydrate Joint Industry Project (JIP) has been performing research on marine gas hydrates since 2001 and is sponsored by both the JIP members and the U.S. Department of Energy. In 2005, the JIP drilled the Atwater Valley and Keathley Canyon exploration blocks in the Gulf of Mexico to acquire downhole logs and recover cores in silt- and clay-dominated sediments interpreted to contain gas hydrate based on analysis of existing 3-D seismic data prior to drilling. The new 2007-2009 phase of logging and coring, which is described in this paper, will concentrate on gas hydrate-bearing sands in the Alaminos Canyon, Green Canyon, and Walker Ridge protraction areas. Locations were selected to target higher permeability, coarser-grained lithologies (e.g., sands) that have the potential for hosting high saturations of gas hydrate and to assist the U.S. Minerals Management Service with its assessment of gas hydrate resources in the Gulf of Mexico. This paper discusses the scientific objectives for drilling during the upcoming campaign and presents the results from analyzing existing seismic and well log data as part of the site selection process. Alaminos Canyon 818 has the most complete data set of the selected blocks, with both seismic data and comprehensive downhole log data consistent with the occurrence of gas hydrate-bearing sands. Preliminary analyses suggest that the Frio sandstone just above the base of the gas hydrate stability zone may have up to 80% of the available sediment pore space occupied by gas hydrate. The proposed sites in the Green Canyon and Walker Ridge areas are also interpreted to have gas hydrate-bearing sands near the base of the gas hydrate stability zone, but the choice of specific drill sites is not yet complete. The Green Canyon site coincides with a 4-way closure within a Pleistocene sand unit in an area of strong gas flux just south of the Sigsbee Escarpment. The Walker Ridge site is characterized by a sand-prone sedimentary section that rises stratigraphically across the base of the gas hydrate stability zone and that has seismic indicators of gas hydrate. Introduction The Gulf of Mexico Methane Hydrate JIP is a consortium of energy and service companies, as well as government organizations, that began collecting data and performing research on marine gas hydrates in the Gulf of Mexico (GOM) in 2001. The project is sponsored by both the JIP members and the US Department of Energy (DOE). The last few decades have seen considerable interest in gas hydrates from both a resource perspective and the standpoint of potential seafloor stability concerns for conventional deepwater operations. Addressing either of these issues requires obtaining fundamental data on the properties of gas hydrate-bearing sediments, the formulation of predictive models for gas hydrate distribution and concentration, an understanding of wellbore and formation stability in gas hydrate-bearing sediments, and the development of methods to analyze existing and new data to infer gas hydrate concentrations.

Many of the world's productive deepwater hydrocarbon basins experience significant and ongoing vertical migration of fluids and gases to the modern seafloor. These products, which are composed of hydrocarbon gases, crude oil, formation fluids, and fluidized sediment, dramatically change the geologic character of the ocean floor, and they create sites where chemosynthetic communities supported by sulfide and hydrocarbons flourish.Unique fauna inhabit these sites, and the chemosynthetic primary production results in communities with biomass much greater than that of the surrounding seafloor.

Abstract The northern Gulf of Mexico continental slope is the most thoroughly imaged and studies continental slope in today's oceans. The entire slope is covered with 3D-seismic and much of the area has been imaged with higher resolution acoustic systems. These data describe a complex seafloor impacted by fluid-gas expulsion. Alvin dives in 2006 and ROV Jason dives in 2007 have produced seabed observations and samples that have greatly improved our understanding of cross-slope and along-slope distribution and variability in biologic communities and their hydrocarbon seep related habitats. Site selection for the 2006 Alvin dives was accomplished though surface reflectivity analysis of the MMS slope-wide 3-D seismic database followed by photo reconnaissance. From 80 potential sites, 20 were subjected to photo reconnaissance from which 10 sites were selected for Alvin dives. Four sites, found in AC 818, AC 601, GC 852 and AT 340 had impressive and diverse chemosynthetic communities as well as well-defined fluid-gas expulsion geology. In addition to chemosynthetic communities, GC 852 had abundant hard and soft corals seated on substrates of authigenic carbonate boulders. At AC 601 (WD ~ 2340 m) a brine lake (4 m deep and 180 m wide, salinity ~ 90 ‰) was investigated and sampled. White " flocs?? floating in the brine and concentrated at the " shoreline?? were found to be barite. No visible animal life was observed in the brine. Isolated living communities of mussels and urchins were found on the lake margins. Geochemically, the water column methane concentration above the lake exceeded all other Alvin dive sites by an order of magnitude. Methane was supersaturated all the way to the surface, suggesting the site could be a source of methane to the atmosphere. At the four key sample locations Autonomous Ultrawater Vehicle (AUV) data were acquired prior to the 2007 ROV Jason cruise. Multibeam bathymetry from the AUV was used as a navigation underlay for detailed sampling by Jason. Depth and geographic ranges for key components of chemosynthetic communities were improved and our understanding of the geologic aspects of hydrocarbon seep habitats was significantly advanced by data from the 2006 and 2007 dives.

A bottom-simulating reflection (BSR) is a seismic reflectivity phenomenon that is widely accepted as indicating the base of the gas-hydrate stability zone. The acoustic impedance difference between sediments invaded with gas hydrate above the BSR and sediments without gas hydrate, but commonly with free gas below, are accepted as the conditions that create this reflection. The relationship between BSRs and marine gas hydrate has become so well known since the 1970s that investigators, when asked to define the most important seismic attribute of marine gas-hydrate systems, usually reply, “a BSR event.” Research conducted over the last decade has focused on calibrating seafloor seismic reflectivity across the geology of the northern Gulf of Mexico (GoM) continental slope surface to the seafloor. This research indicates that the presence and character of seafloor bright spots (SBS) can be indicators of gas hydrates in surface and near-surface sediments (Figure 1). It has become apparent that SBSs on the cont...

Questions as to the role of modern carbon in methanogenesis and the maximum depth of methane sources in the Gulf of Mexico continental slope remain unanswered. A research submersible was used to sample mixed bacterial and thermal gas (δ13C of methane=−62.8‰, δD=−176‰) venting to the water column from the Gulf slope in Green Canyon (GC) 286. The Δ14C value of the methane (−998‰) is consistent with fossil carbon. Another gas vent on GC 185 is 100% methane (δ13C=−62.9‰, δD=−155‰) and may be from a bacterial source. The Δ14C (−997‰) of this bacterial methane is also consistent with fossil carbon. Fossil bacterial methane and thermal hydrocarbons are present in Pliocene to Pleistocene reservoirs (∼3509–4184 m) of Genesis Field (GC 205, 161, 160). Oil in these reservoirs is biodegraded but gas is not, suggesting that gas charge to reservoirs continues presently at 3–4 km depth. Mixed thermal and bacterial methane may charge the deep reservoirs, and fossil methane from depth may ultimately vent on the sea floor at GC 286 and GC 185. Results of this study of Green Canyon suggest that bacterial methane in gas vents and in reservoirs is from deep fossil sources.

Abstract 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.