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than 50% of the global carbon reservoir is
found in marine and continental gas
hydrates (Kvenvolden, 1988).
Figure 2: Current estimation of global
carbon reserves in gigatons (1 Gt = 10 9 t)
after Kvenvolden (1988).
Natural gas hydrate deposits are only stable
under very specific pressure and
temperature conditions and occur in
sediments where water and an adequate
amount of organically produced methane is
available (i.e., at least 0.5 wt% of the
sediment has to be organic carbon;
Wallmann et al., 2006). For pure water and
methane, the boundary between solid gas
hydrates and free methane gas can be seen
in Figure 3 as a function of pressure/depth
and temperature (pT-diagram). Because of
these requirements, gas hydrates can only
be found in the upper 2,000 m of sediments
in either active and passive continental
margins or arctic permafrost regions (e.g.,
Kvenvolden, 1999). The map in Figure 4
provides an indication of the global
distribution of methane-rich gas hydrates
based on direct sampling and inferred by
geophysical techniques (Kvenvolden &
Lorenson, 2007). The main geophysical
indicator for the existence of gas hydrates is
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the bottom simulating reflector (BSR), which
can be identified in a vertical seismic profile.
An example of such seismic profile is shown
in Figure 5, in which the seismic velocity is
higher in sediments which include gas
hydrates than in gas hydrate free
sediments. The existence of free gas below
the BSR causes a further significant increase
of seismic velocity. In the situation of
missing gas below the gas hydrate deposits,
the BSR will be absent and the geophysical
method might fail.
Figure 3: Phase diagram of gas hydrate
after Katz et al. (1959) showing boundary
between free methane gas and methane
hydrate for a pure water and pure methane
system.