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

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: (a) represent an enormous potential energy source, (b) have been implicated as drivers of global climate change by releasing large volumes of methane (a greenhouse gas) into the atmosphere, and (c) present 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 1year. 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 tmeperature 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.

Transience and persistence of natural hydrocarbon seepage in Mississippi Canyon, Gulf of Mexico

“…These SZ are sometimes accompanied by mud volcanism and discharge of brines and cognate fluids , (Judd and Hovland, 2007;Whelan et al, 2005). The SZ and their associated features are of interest because they occur in areas that may contain substantial accumulations of gas hydrate, a methane ice-like substance that is both a trophic source for chemosynthetic communities and a potential economic energy source (Kvenvolden and Cooper, 2003;Roberts et al, 1999;Sassen et al, 2001). Formation of gas hydrate reduces sediment porosity and flux (Roberts et al, 1999).…”

“…The SZ and their associated features are of interest because they occur in areas that may contain substantial accumulations of gas hydrate, a methane ice-like substance that is both a trophic source for chemosynthetic communities and a potential economic energy source (Kvenvolden and Cooper, 2003;Roberts et al, 1999;Sassen et al, 2001). Formation of gas hydrate reduces sediment porosity and flux (Roberts et al, 1999). However, where flow exceeds equilibrium consumption and hydrate formation rates, a fraction of gas and oil escapes into the water column from discrete seep vents within a SZ (Garcia-Pineda et al, 2010).…”

Petrographic and geochemical characterization of seep carbonate from Bush Hill (GC 185) gas vent and hydrate site of the Gulf of Mexico

“…However, additional data from recent investigations show that the majority of sulfate reduction at the Bush Hill sites is likely fueled by the oxidation of organic matter, possibly heavier hydrocarbons (C 2þ ) like oil, rather than methane (Joye et al, 2004). Moreover, the measurement of gas and water flux and chemical analyses show large degrees of spatial and temporal variability of vent rate and fluid sources in this region (Roberts et al, 1999;Roberts, 2001;Leifer and MacDonald, 2003;Chen et al, 2004;Tryon and Brown, 2004).…”

Temporal Variation in Natural Gas Seep Rate and Influence Factors in the Lingtou Promontory Seep Field of the Northern South China Sea

“…In recent years, much attention has been paid to the long-term in-situ and on-line observations of the methane flux and the spatio-temporal variability at various hydrocarbon seep locations, e.g., Bush Hill of Mexico, Coal Oil Point seep field of the Santa Barbara channel, Hydrate Ridge, the Black Sea and Hikurangi Margin of New Zealand (Roberts et al 1999;Brown 2001, 2004;Boles et al 2001;Torres et al 2002;Tryon et al 2002;Leifer and Boles 2005a, b;MacDonald et al 2005;Vardaro et al 2006;Solomon et al 2008;Sahling et al 2009;Krabbenhoeft et al 2010;Leifer et al 2010;Linke et al 2010;Römer et al 2012a, b;Etiope et al 2013). The preliminary results indicate that the flux and material associated with hydrocarbon seeps are both spatial and temporal in nature Tryon and Brown 2004;Leifer and Boles 2005b;Solomon et al 2008).…”