A seafloor observatory to monitor gas hydrates in the Gulf of Mexico

Abstract

Background Gas hydrates are solid structures composed of gas molecules encased in cages of water molecules. Most interesting to the energy community are hydrates that contain hydrocarbon gases. In the northern Gulf of Mexico, hydrates of this type form in water depths greater than 450m and often occur in mounds located where faults intersect the sea floor. Typically, these hydrates consist of sea water from which the salt has been excluded and gases that have migrated up faults from buried hydrocarbon reservoirs. In addition to the hydrates, the mounds contain large amounts of calcium carbonate and various other minerals precipitated by microbes that extract energy from the hydrocarbon fluids. It has been observed that microbial activity is an order of magnitude greater in the vicinity of mounds containing hydrate outcrops than elsewhere on the sea floor. The proliferation of microbes around gas hydrate sites is not coincidental; it is the result of a synergistic relationship between hydrocarbon gas hydrates and microbes, i.e. the carbonrich gases within hydrates provide sustenance for the microbes and biosurfactants produced by the microbes enhance the formation of hydrates. Recent laboratory experiments have shown that small concentrations of biosurfactants in the water wetting porous mineral surfaces have the effect of increasing the formation rates and decreasing the induction times of hydrates substantially. Moreover, biosurfactants exhibit surface specificities for particular mineral surfaces; that is, biosurfactant adsorption on a specific mineral surface results in hydrates nucleating and emanating from that surface. Surfaces of smectite clay, common in the northern Gulf, serve this purpose particularly well. Hydrates outcropping at the sea floor are in direct contact with large volumes of sea water. They are stable only marginally because salt, in sufficient concentration, impedes hydrate formation. For this reason hydrate outcrops can be ephemeral; significant changes may occur within rather short periods of time. In the absence of pressure fluctuations, whether the outcrops accumulate or dissociate is determined by variations in water temperature and rate of gas flow. Water temperature in the northern Gulf is influenced greatly by warm eddies shed from the Loop Current which enters the Gulf between the Yucatan Peninsula and Cuba (fig.1). These eddies drift slowly westward along the continental slope and can raise bottom-water temperatures by several degrees Celsius. Observations (Roberts et al., 1998) confirm that gas activity correlates with water temperature; gas flow increasing a few hours after the temperature increases (fig.2). Where hydrates occur within sediments, they are stable under the proper conditions of pressure, temperature, gas composition and salinity of pore fluid. If gas migrating up a fault encounters appropriate conditions as well as sediments of sufficient permeability, hydrates can form within the pore spaces and act to cement the sediment grains. This increases the shear modulus...

Time integrated variation of sources of fluids and seepage dynamics archived in authigenic carbonates from Gulf of Mexico Gas Hydrate Seafloor Observatory

“…1) gas hydrate site is located on the eastern side of Mississippi Canyon along the continental slope of the northern GOM. It is home to a seafloor observatory where gas hydrate dynamics are monitored (McGee, 2006;Lapham et al, 2008;Macelloni et al, 2013). The site was discovered during dives of the Johnson Sea Link (JSL) research submersible in 2002 (dives 4414 and 4415).…”

“…McGee, 2006;Lapham et al, 2008;Ingram et al, 2010). The physical and biogeochemical processes associated with gas hydrates and cold seep systems are increasingly understood (e.g.…”

Heat-flow regimes and the hydrate stability zone of a transient, thermogenic, fault-controlled hydrate system (Woolsey Mound northern Gulf of Mexico)

“…Woolsey Mound, MC118, is the site of the Gulf of Mexico Hydrates Research Consortium's (GOMHRC) Seafloor Observatory [McGee, 2006], where the HSZ has been the subject of intense and continuing investigation since 2005 [Lutken et al, 2011a]. The onekilometer-diameter carbonate/hydrate mound, located on the continental slope, in~890 m water depth ( Fig.…”

“…Pizzi et al, (2012) and Simonetti et al, (2013) have conducted very accurate analyses of fault movements detectable on high resolution chirp data. They have both found that all master faults have been active in very recent geologic time.…”

Optimal filtering high-resolution seismic reflection data using a weighted-mode empirical mode decomposition operator

“…The main goal of the Consortium is to install and operate the first permanent sea-floor monitoring station which will monitor more or less continuously the Gas Hydrates Stability Zone (GHSZ) via geochemistry, geophysical and biological measurements (McGee, 2006). In order to facilitate the best observatory site selection, over the last few years the GOMHRC has been carrying out several seismo-acoustic surveys around the Northern Gulf of Mexico continental Shelf using the Surface Source Deep Receiver (SSDR) technique (McGee, 2000).…”