Minerals Report - International Seabed Authority

presence of higher molecular weight gases (ethane or propane) will cause
the phase boundary to shift right, allowing hydrate to form at lower
pressure (i.e. at shallower water) or at higher temperature. The presence
of salts in the pore water shifts the phase boundary to the left, causing a
decrease in the hydrate stability zone 26-28 . The intersection of the
hydrothermal profile with the gas hydrate phase boundary curve
corresponds to the minimum water depth beneath which hydrates will
be stable. This minimum water depth will be less, if the water is colder,
and greater if the water is warmer. Approximately, this minimum depth
is about 300 m in the Arctic area, and 600 m in sub-tropical regions 27 . In
deep-sea sediments, temperature normally increases with depth and
eventually reaches a point at which hydrate is unstable, despite the
continued increase of pressure with depth. Thus a zone within the
sediments exists in which gas hydrate is potentially stable from the
seafloor down to a depth, commonly from a few hundred to a thousand
meters below the seafloor. The thickness from the seafloor down to the
base of the gas hydrate stability is considered to be the thickness of the
gas hydrate stability zone (GHSZ). If gas (methane) saturation exists
within this zone, gas hydrate will form.
Figure 4. Phase diagram showing boundary between free methane gas and methane
hydrate for a pure water and pure methane system (after Kvenvolden 2; Miles 26)
INTERNATIONAL SEABED AUTHORITY 525