Surficial Gas Hydrates of the Louisiana Continental Slope-Initial Results of Direct Observations and In Situ Data Collection

Abstract

Abstract Naturally occurring gas hydrates have recently become one of the most interesting and rapidly expanding research topics in the geologic oceanographic and engineering sciences. These frozen mixtures of hydrocarbon gas (mostly methane) and water occur over vast areas of the ocean floor in special temperature and pressure regimes. They are important because they:represent an enormous potential energy source,have been implicated as drivers of global climate change by releasing large volumes of methane (a greenhouse gas) into the atmosphere, andpresent potential engineering geology problems for man's operations in deep water and may be placed in the category of a geohazard. However, the identification of gas hydrates in the shallow subsurface from remotely sensed acoustic data is not straight-forward in complex geological provinces like the continental slope of the northern Gulf of Mexico and essentially no definitive data on seabed methane flux from dissociation of gas hydrates occurring at or near the seabed exists in the scientific literature. Repeated direct observations (manned submersible) of sites in the deepwater northern Gulf of Mexico where gas hydrates have been identified as exposures on the seafloor indicate that hydrates can form and decompose within the return observation period of 1-year. A 1-year in situ data collection experiment reported on in this document was designed to produce data for developing a better understanding of surficial gas hydrate formation and decomposition cycles and their possible links to oceanographic forcing. Current meter data and records of gas emission from a site of exposed gas hydrate suggest a coupling of temperature excursions of < 2°C and pulses of gas emission (gas hydrate decomposition). Increases in water temperature are strongly coherent with north-south pulses in the current field. Higher frequency cycles of gas emission also occur at the tidal time scale. However, these small variations are superimposed on longer-term events of 1–2 weeks. These in situ collected data indicate that intrusions of Loop Current water are not needed to force dissociation of surface and near-surface gas hydrates. Surface sediments associated with areas characterized by exposures of gas hydrate have extremely variable geotechnical properties. Much of this variability relates to shell content and products of early diagenesis. Introduction At present, knowledge of gas hydrates in natural settings outside of permafrost areas is just starting to be acquired. The most sitespecific data have been collected from a gas hydrate field on the Blake Ridge off the coast of North Carolina, ODP Leg 164 (Holbrook et al., 1996; Dickens et al., 1997). By using a special pressurized sampling device, gas hydrates and host sediments were kept at their in situ pressures and temperatures until they could be studied in the laboratory onboard ship. It was found that a vertical profile of samples through the hydrate zone indicated that 0-9% of the pore volume in the hydrate zone is occupied with gas hydrate and that gas comprises up to 12% of the pore volume in the underlying free-gas zone. Gas hydrate was found to occur as finely disseminated pieces in pore spaces to large nodular masses (up to 30 cm diameter). The methane in these hydrate deposits is of biogenic origin and estimates of gas in the Blake Ridge hydrate field suggest that this area alone contains enough methane to supply the United States needs for over 100 years (Dickens et al., 1997).