FROZEN HEAT | Volume 1

per cent of the organic carbon buried in ocean sediment is found beneath relatively shallow water near the continents (Hedges and Keil 1995; Buffett and Archer 2004). In peri- ods of much lower sea levels, however, organic carbon was deposited farther from the continents’ current edges, on what is now the continental slope (Muller and Suess 1979; Jasper and Gagosian 1990). Gas hydrate volume estimates rely on two basic parame- ters: the amount of pore space, or porosity, available for gas hydrates in the stability zone (Kvenvolden, 1988a; Collett, 1995; Dickens 2001; Klauda and Sandler 2005), and the per- centage of that space occupied by gas hydrates, called the gas hydrate saturation. The gas hydrate saturation is related to the amount of methane that can be formed from available organic matter and transported into the GHSZ (Harvey and Huang 1995; Archer et al. 2009). Gas hydrates tend to be distributed quite unevenly because the porosity, the perme- able paths for liquid and gas flow, and the conditions con- trolling the conversion of organic material into methane gas can all vary dramatically over short distances (Expert Panel on Gas Hydrates 2008; Frye 2008; Solomon et al. 2008). The Earth’s heterogeneous gas-hydrate distribution and uncertainties in porosity and gas hydrate saturation have led to widely varying global estimates of the methane con- tained in hydrates (Fig. 1.6). Even the lowest estimates, however, are so large they are given in terms of gigatonnes of carbon (GtC). A gigatonne equals 10 9 tonnes, equivalent to 1 petagram or 10 15 g. A petagram of water, for example, takes up 1 cubic kilometre. For a sense of scale, it is esti- mated that approximately 1.8 Gt of methane carbon was consumed globally as natural gas in 2011 (U.S. Energy In- formation Administration 2010). The earliest global estimates of methane content in gas hy- drates were made prior to the first recovery of gas hydrates from marine sediment (green region in Fig. 1.6). These esti- mates assumed gas hydrates existed wherever pressure and temperature conditions for gas hydrate stability were satis- fied. This was equivalent to assuming gas hydrates were pre- sent in sediments beneath about 93 per cent of the world’s oceans (Milkov 2004).

Estimates of the methane held in hydrates worldwide

Gigatonnes carbon

10 7

First observation of marine hydrate Methane in marine hydrates (based only on the pressure and temperature requirements for hydrate stability)

10 6

10 5

Methane in marine gas hydrates

10 4

10 3

10 2

Methane in permafrost gas hydrates

Global natural gas reserves (conventional)

10

1

1970 1975 1980 1985 1990 1995 2000 2005 2010

Figure 1.6: Estimates of the methane held in hydrates worldwide. Early estimates for marine hydrates (encompassed by the green region), made before hydrate had been recovered in the marine environment, are high because they assume gas hydrates exist in essentially all the world’s oceanic sediments. Subsequent estimates are lower, but remain widely scattered (encompassed by the blue region) because of continued uncertainty in the non-uniform, heterogeneous distribution of organic carbon from which the methane in hydrate is generated, as well as uncertainties in the efficiency with which that methane is produced and then captured in gas hydrate. Nonetheless, marine hydrates are expected to contain one to two orders of magnitude more methane than exists in natural gas reserves worldwide (brown square) (U.S. Energy Information Administration 2010). Continental hydrate mass estimates (encompassed by the pink region) tend to be about 1 per cent of the marine estimates (Figure modified from Boswell and Collett (2011)). Estimates are given in Gigatonnes of carbon (GtC) for comparison with other organic hydrocarbon reservoirs (see Figure 1.7). At standard temperature and pressure, 1 GtC (Gigatonnes of carbon) represents 1.9 Tcm (trillion cubic meters) of methane which has an energy equivalent of approximately 74 EJ (exajoules).

A GLOBAL OUTLOOK ON METHANE GAS HYDRATES 19

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