Introduction

Gas hydrates were first identified in 1810 by Sir Humphrey Davy (Davy, 1811) and their compositions were later established by Faraday (Faraday 1823). Gas hydrates are naturally occurring crystalline inclusion compounds (clathrates) characterized by strictly determined structures for different gases (Makogon, 1997). Clathrates form when water establishes, due to hydrogen bonding, a cage-like structure around small guest molecules (e.g., light alkanes, carbon dioxide, hydrogen sulphide, nitrogen and oxygen) at low temperature and high-pressure conditions. Their formation, stable existence and decomposition depend upon the pressure, temperature and composition of the system. Gas hydrates can exist in one of the three most-common structures, I, II (Claussen, 1951; von Stackelberg and Müller, 1954; Jeffrey and McMullan, 1967) and H (Ripmeester et al., 1987) (Figure 1). Both CH4 and CO2 form structure I hydrate (Davidson, 1973), as do the mixtures of these gases (Adisasmito et al., 1991).

Figure 1. Different forms of clathrate hydrate structures. The numbers over the arrows indicate the number of cage types. (Modified from Sloan, 2003).

Very large sedimentary deposits of natural gas hydrates (14x1012 t.o.e.), whose existence was proven 30 years ago (Makogon, 1997), are inferred to occur in two types of geologic environments: permafrost regions (where cold temperatures dominate) and beneath the sea in sediments off the outer continental margins (where high subsurface pressures dominate). Both deposit types are regarded as potential energy storehouses. While methane, propane, and other gases are included in the hydrate structure, methane hydrates are the most common, and thus so are the structure-I hydrates. The methane source for clathrate hydrates formation may be either biogenic or thermogenic. Biogenic CH4, resulting from the microbial breakdown of organic matter, form hydrates in shallow sediments. About 98% of all gas hydrate deposits come from biogenic methane sources, accumulated offshore in upper sedimentary layers. Thermogenic methane, coming from the thermal alteration of organic matter in sediment at depth, may form hydrates where deep gas migration pathways exist (Kvenvolden and Lorenson, 2001).

Figure 2. Worldwide occurrences of natural gas hydrate deposits (Modified from Kvenvolden and Lorenson, 2001).

Figure 2. Worldwide occurrences of natural gas hydrate deposits (Modified from Kvenvolden and Lorenson, 2001).

The formation and distribution of gas hydrates in subsurface sediments is controlled by two opposing factors: temperature increases associated with increasing depth (the geothermal gradient) and pressure gradients. Since hydrates are generally stable under relatively low temperatures, the geothermal gradient (2-3oC/100m) limits hydrate formation and existence to relatively shallow, cooler regions of the Earth's crust. Considering seabed temperature and water salinity, hydrates are generally stable at water depths greater than 300 m in continental areas and 500 m in offshore areas. These two factors restrict the existence of clathrate hydrates in subsurface sediments to a region called the Hydrate Stability Zone (Figure 3).

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