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FROZEN HEAT
16
Depth (metres)
0
Depth (metres)
0
200
400
600
800
1000
1200
1400
1600
200
400
600
800
1000
1200
1400
1600
20
0
01
3
0
Temperature ºC
gnittes tsorfamreP
gnittes eniraM
-30
-20
-10
0
10
20
30
Temperature ºC
Base of permafrost
Stability zone
Stability zone
Stability conditions for gas hydrates
Ground surface
Ice freezing temperature
Sea surface
Sea oor
Given adequate supplies of gas and water, the fundamental
controls on gas-hydrate formation and stability are pressure
and temperature. In general, a combination of low tempera-
ture and high pressure is needed to form methane hydrate
(Fig. 1.3). Because of Earth’s geothermal gradient – the natural
increase of temperature with depth below the ground surface
1.3
GAS HYDRATE FORMATION,
STABILITY, AND OCCURRENCE
Figure 1.3:
Stability conditions for gas hydrates. Idealized phase diagrams illustrating where methane hydrate is stable in marine and
permafrost settings. Hydrate can exist at depths where the temperature (blue curve) is less than the maximum stability temperature
for gas hydrate (orange curve). Pressure and temperature both increase with depth in the Earth. Although hydrates can exist at warmer
temperatures when the pressure is high (orange curve), the temperature at depth (blue curve) gets too hot for hydrate to be stable, limiting
hydrate stability to the upper ~1km or less of sediment. The presence of salt, a gas hydrate inhibitor, shifts the gas hydrate stability curve
(orange) to lower temperatures, decreasing the depth range of the gas hydrate stability zone. For seawater, this decrease is approximately
1.1°C (Dickens and Quinby-Hunt, 1994) (Figure modified from Kvenvolden (1988a)).