Why Does Gas Hydrate Occur In A Particular Setting

A review of previous gas hydrate studies reveal that the formation and occurrence of gas hydrate is controlled by numerous factors (Collett, 1995); two of the most important appear to be formation temperatures and the composition of the gas molecule included in the gas hydrate clathrate. Other factors, including the availability of gas, also exhibit important controls on the occurrence of gas hydrate.

Gas hydrate exists under a limited range of temperature and pressure conditions such that the depth to the top of the zone and the thickness of the potential gas-hydrate stability zone can be calculated (Fig. 2) and shown

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Figure 2. Graph showing the depth-temperature zone in which gas hydrate is stable in a permafrost region [assuming a 9.795 kPa/m pore-pressure gradient] (modified from Holder etal., 1987)

as a series of subsurface temperature profiles from an onshore permafrost area and two laboratory-derived gas-hydrate stability curves for different natural gases (modified from Holder et al., 1987). This gas-hydrate phase-diagram (Figure 2) illustrates how variations in formation-temperature and gas composition can affect the thickness of the gas-hydrate stability zone.

The zone of potential gas-hydrate stability in Figure 2 lies in the area between the intersections of the geothermal gradients and the gas-hydrate stability curves.

It is well documented that the formation of gas hydrate requires a large source of natural gas. In most gas hydrate resource assessment studies; the characterization of gas sources has been based on assessing a set of minimum source-rock criteria that includes organic richness (total organic carbon), sediment thickness, and thermal maturity. It has been shown that the availability of large quantities of hydrocarbon gas from both microbial and thermogenic sources are an important factor controlling the formation and distribution of natural gas hydrate (Kvenvolden, 1988; Collett, 1993). Carbon isotope analyses indicate that the methane in most oceanic hydrate is derived from microbial sources. However, molecular and isotopic analyses indicate a thermal origin for the methane in several offshore Gulf of Mexico and onshore Arctic gas-hydrate occurrences.

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