17 / 78
A GLOBAL OUTLOOK ON METHANE GAS HYDRATES
17
Prudhoe
Bay area
Mallik test site
Northern Gulf
of Mexico
Blake Ridge
Svalbard
Indian
Ocean
Ulleung Basin
Qilian
Mountains
Eastern
Nankai
Trough
Shenhu
Basin
Gumusut-
Kakap
Taiwan
Messoyahka
New Zealand
Cascadia Margin
Japan Sea
Mexico
Costa Rica
Peru
Selected gas-hydrates study areas
– gas hydrates are stable only in locations where high pres-
sures can be attained in shallower, cooler sediments. The verti-
cal extent over which these conditions occur at any location is
known as the gas hydrate stability zone (GHSZ). In this report,
unless otherwise stated, the GHSZ is for Structure I methane
hydrate, the most common gas hydrate on Earth.
The GHSZ exists in Arctic regions where cold average air
temperatures create thick zones of permanently frozen soils
(permafrost). In these regions, the top of the GHSZ typically
occurs about 200 to 300 metres below the land surface, often
within an interval of permafrost. The GHSZ can extend 500
metres or more below the base of the permafrost (Fig. 1.3).
The GHSZ also exists in oceans or deep inland lakes where
high pressures are generated by relatively deep water – typi-
cally 300 to 500 metres or more, depending on the bottom-
water temperature. The top of the GHSZ occurs within the
water column, with the base of the GHSZ some distance be-
low the sea floor (Fig. 1.3). The thickness of the GHSZ gener-
ally increases with increasing water depth. In areas of deep
water and low geothermal gradients, the GHSZ can extend 1
000 metres or so below the sea floor (Milkov 2004), with the
most deeply buried deposits being as warm as 20°C or more
(see Collett
et al.
2009). Even this maximum depth for gas
hydrates is shallow compared to many conventional hydro-
carbons, which are now being sought nearly 10 000 metres
below the sediment surface (Lewis
et al.
2007; Mason 2009).
Just because a given location satisfies the pressure and tem-
perature requirements for gas-hydrate stability, there is no
guarantee gas hydrates are present. If pressure and tem-
perature were the only determinants, gas hydrates would
be virtually ubiquitous throughout oceanic sediment. In ad-
Figure 1.4:
Selected gas hydrate study areas. The yellow squares indicate a few of the historically-significant gas hydrate research sites, along
with locations where gas hydrates have been recovered from depths greater than 50 meters beneath the sediment surface. Remote sensing
studies have inferred the presence of gas hydrate in numerous other locations. Though widespread, methane gas hydrates are restricted to
locations where adequate supplies of methane are available, which is generally on or near continents (Figure modified from Ruppel
et al
. 2011).