Amount of Methane

Estimates of the methane content of natural gas hydrate all suggest that the methane quantities are very large (Kvenvolden, 1999). Cherskiy and Makogon (1970) proposed that the amount of methane in naturally occurring gas hydrate is potentially "enormous," but their estimates were highly speculative because not much was known about natural gas hydrate at that time. The Potential Gas Committee (1981) summarized early global estimates: methane in gas-hydrate deposits ranging from 3.1 x 1015 to 7600 x 1015m3 (at standard conditions) for oceanic sediments and from 0.014 x 1015 to 34 x 1015 m3 for polar regions. Because oceanic gas hydrate apparently contains significantly more methane than polar region gas hydrate, the oceanic gas hydrate is emphasized in global estimations of methane content of gas hydrate.

During the period from 1980 to 1990, a better understanding of gashydrate occurrence has generally resulted in estimates within the lower ranges of the previous estimates. Kvenvolden (1988) and MacDonald (1990), working independently, estimated the methane content of the gas hydrate worldwide to be about 21 x 1015 m3. That these estimates are equal is a coincidence, but the convergence of ideas has made this value the consensus estimate.

In the 1990s Görnitz and Fung (1994) and Harvey and Huang (1995) used plausible ranges of relevant variables to provide estimates of the methane content of gas hydrate based on considerations of various General Circulation Models. The estimates of Görnitz and Fung (1994) ranged between 26 x 1015 m3 and 139 x 1015 m3, with the most likely estimate of 26 x 1015 m3 for their in situ microbial gas generation model and 115 x 1015 m3 for their pore-fluid model. These estimates lie within the range of early values but are higher that the consensus estimate. Harvey and Huang (1995) calculated the total methane content of oceanic gas hydrate to be 23 x 1015, 46 x 1015, and 91 x 1015 m3, depending on the assumptions regarding the pore space occupied by gas hydrate. They selected the intermediate value of 46 x 1015 m3 as their best estimate.

Other global estimates of methane in gas hydrate were made during this same time period and resulted in values smaller that the consensus estimate of 21 x 1015 m3. For example, Holbrook et al. (1996) estimated about 7 x 1015 m\ whereas Dickens et al. (1997) concluded that global estimates of methane in gas hydrate in the range from about 2 x 1015 to 20 x 1015 m3 are acceptable, based on gas-hydrate concentrations determined at the Blake Ridge. Other low estimates include a revised estimate by Makogon (1997) of 15 x 1015 and 1 x 1015 m3 by Ginsburg and Soloviev (1995), who challenge all larger estimates. It is apparent that by 1990 the range of estimates has been greatly constrained from estimates available in 1980. The consensus value of 21 x 1015 m3 remains about midway between the extremes. It is quite likely that the global amount of methane in gas hydrate is considerable less than 1017 m3 but probably is greater than 1015 m3, with the actual value in the lower or intermediate part of the range.

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