Gas Hydrate Stability Zone

Fig. 14.4 Left Stability field of pure methane hydrate at normal seawater salinity, as defined by temperature and pressure expressed as water depth. Intersections of the temperature profiles (stippled lines) with the phase boundary (heavy line) define the area of the gas hydrate stability zone (GHSZ). Right Inferred thickness of the gas hydrate zone in sediments at a schematic continental margin assuming a typical geothermal gradient of 28°C km. Typical bottom water temperatures are marked, and range from 18°C on shallow shelf regions to 2°C at the bottom of the continental rise (after Kvenvolden and McMenamin 1980).

Fig. 14.9 Distribution of gas hydrate (after Egorov et al. 1999) superimposed on a schematic vertical model of the temperature field (after Ginsburg et al. 1999) in the Hakon Mosby Mud Volcano. The gas hydrate stability zone (GHSZ, shown by bold lines) is determined by pressure and temperature conditions the zone of gas hydrate (GH) accumulation depends on both the thermal gradient and the flux rate of methane.

In their analyses of past and future state of the hydrate reservoir, Buffett and Archer (2004b), suggest that if elevated gas pressures do occur as a transient response to warming, a rapid release of methane may be triggered by the development of critical pressures in the gas phase. Critical gas pressure below the base of the gas hydrate stability zone can trigger vertical migration of free gas to the seafloor. 

BSR An abbreviation for the so-called bottom simulating reflection. A reflection recorded in seismic reflection profiles that results from an acoustic velocity contrast produced by the decrease in sound speed caused primarily by the presence of gas trapped beneath the gas hydrate stability zone. BSRs provide a remotely sensed indication of the presence of gas hydrate. 

Gas hydrate stability zone The region within deep ocean... 

FIGURE 2 (A) Phase boundary of methane hydrate in the ocean (solid line). The pressure axis has been converted to depth into the ocean, so pressure increases downward. (B) The same phase boundary as shown in (A) with a seafloor inserted at 2 km depth and a typical temperature curve (dashed line). Hatched region shows the vertical extent of the gas hydrate stability zone under these assumed conditions. 

The precise location of the base of the gas hydrate stability zone under known pressure/temperature conditions varies somewhat depending on several factors, most important of which is gas chemistry. In places where the gas is not pure methane, for example, in the Gulf of Mexico, at a pressure equivalent to 2.5 km water depth, the base of the gas hydrate stability zone will occur at about 21 °C for pure methane, but at 23" C for a typical mixture of approximately 93% methane, 4% ethane, 1% propane, and some smaller amounts of higher hydrocarbons. At the same pressure (2.5 km water depth) but for a possible mixture of about 62% methane, 9% ethane, 23% propane, plus some higher hydrocarbons, the phase limit will be at 28°C. These differences will cause major shifts in depth to the base of the gas hydrate stability zone as would be implied by Fig. 2B. Such mixtures of gases essentially make the formation of gas hydrate easier and therefore can result in the formation of gas hydrate near the seafloor at shallower depths (lower pressures) than for methane hydrate... 

The geothermal gradient tends to be quite uniform across broad regions where sediments do not vary. Thus, for a given water depth, the sub-bottom depth to the base of the gas hydrate stability zone will be quite constant. However, because a change in water depth causes change in pressure, we anticipate that the base of the gas hydrate stability zone will extend further below the seafloor as water depth increases (Fig. 3). 

Fortunately, the base of the the gas hydrate stability zone is often easy to detect in seismic reflection profiles. Free... 

FIGURE 5 Seismic refiection profile near the crest of the Blake Ridge off South Carolina in the southeastern United States. Note the strong reflection marked BSR that defines the base of the gas hydrate stability zone. The vertical axis is in two-way travel time of sound, which varies with sound velocity in the medium thus, this axis is a variable scale with respect to distance. 

FIGURE 10 Seismic reflection profile across a salt diapir beneath the continental slope off North Carolina in the southeastern United States. GHSZ indicates gas hydrate stability zone within the sediments. The BSR, denoting the base of the gas hydrate stability zone, rises markedly over the diapir because of the thermal and chemical effects created by the diapir. This doming of the base of gas hydrate stability forms a trap for gas. Compare this to the middle diagram of Figure 9. 

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