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extends beneath most of the coastal plain province and has thicknesses >1000 m
in the Prudhoe Bay, Kuparuk River, and Milne Point oil fields on the North Slope
of Alaska. The estimated amount of gas within these gas hydrate accumulations
is ~37–44 Tcf, which is equivalent to twice the volume of conventional gas in the
Prudhoe Bay field [11]. More details on the locations of gas hydrate reservoirs in
Alaska are given in various USGS reports (2012, pers. comm.). As mentioned in
Refs. [3,9,17–54] (USGS, 2012, pers. comm.), besides Alaska, hydrate fields have
been discovered in other countries of the world, which include Japan, China, India,
Korea, Russia, and Canada. In the United States, hydrates have also been discovered in the Gulf of Mexico [3,9,40,47].
The size of the oceanic methane clathrate reservoir is poorly known. The
recent estimates constrained by direct sampling suggest the global inventory
occupies between 1 and 5 million cubic kilometers. This estimate corresponds to
500–2500 gigatons carbon that is substantially larger than 230 gigatons estimated
for other natural gas resources. The reservoir in Arctic permafrost has been estimated at 400 gigatons, but no estimates for Antarctic reservoirs are available. Low
concentrations at most sites imply that only small percentage of clathrate deposits
may be economically recoverable [3,9,17–54] (USGS, 2012, pers. comm.).
There are two distinct types of oceanic deposits. The most common type is one
where methane is contained in I clathrate and generally found in the depth of the
sediment. This type is derived from microbial reduction of CO 2 . These deposits are
located within a mid-depth zone around 300–500  m thick in the sediments. The
second less common type is found near the sediment surface. This type is formed
by the thermal decomposition of organic matter. Examples of this type are found in
the Gulf of Mexico and Caspian Sea. Some deposits have characteristics intermediate between the microbial and thermal source types, and they are considered to be
formed from a mixture of two.
While the sedimentary methane hydrate reservoir probably contains 2–10 times
the currently known reserves of conventional natural gas, the majority of the site’s
deposits are too dispersed to recover economically. The detection of viable sources
is also problematic. The technology for extraction of methane gas from hydrate is
also an issue. To date, Messoyakha Gas field in the Russian city of Norilsk is the
only sustained commercial operation. Japan is planning to develop a commercial
operation by 2016 [20,33,43], and China has invested $100 million over 10 years to
study hydrates [45]. A possible economic reserve in the Gulf of Mexico may contain
10 10  m 3 of gas [3,9,40,47].
Gas hydrates are of great importance for a number of reasons graphically illustrated in Figure 12.5 [54]. Naturally occurring methane gas clathrates contain an
enormous amount of strategic energy reserve [37,39,46]. In offshore hydrocarbon
drilling and production operations, gas hydrates can cause major and potentially
hazardous flow assurance problems. The recovery of gas hydrates by carbon dioxide
provides an opportunity to dispose carbon dioxide by sequestration [61–83]. Gas
hydrates also provide an increasing awareness of the relationship between hydrate
and subsea slope stability. Gas hydrates also pose a potential danger to deep-water
drilling installations, pipelines, and subsea cables [55–59,84–116] (LaBelle, 2012,
pers. comm.; Tohidi, 2012, pers. comm.). Finally, it poses a long-term concern