FROZEN HEAT | Executive Summary

HOW ARE GAS HYDRATES FORMED?

Methane gas hydrates form naturally where adequate supplies of methane and water can combine in a location with both high pressure and relatively low temperature. The methane itself is created by the decomposition of organic carbon, which generally migrates upward through water-laden sediment. In the right conditions, this triggers the formation of gas hydrates. Gas hydrates can form naturally in the Gas Hydrate Stability Zone (GHSZ), the depths at which pressure and temperature are suitable for gas hydrates. Exactly where the GHSZ is found and how far it extends depend on local conditions.

In the Arctic, where cold air temperatures create thick zones of permanently frozen soils (permafrost), the top of the GHSZ typically lies about 300 to 400 metres below the land surface, often in the midst of the permafrost. In regions of relatively thick permafrost, the GHSZ often extends 500 metres or more below the base of the permafrost. In oceans or deep inland lakes, where high pressures are generated by 300 to 500 metres or more of overlying water, the top of the GHSZ occurs within the water column, and the base is some distance below the sea floor.

Stability conditions for gas hydrates

Depth (metres)

Depth (metres)

gnittes eniraM

gnittes tsorfamreP

0

0

Sea surface

Ground surface

Ice freezing temperature

200

200

Stability zone

400

400

Stability zone

Base of permafrost

600

600

800

800

Sea floor

1000

1000

1200

1200

1400

1400

1600

1600

-30

-20

-10

0

10

20

30

0

01

20

3

0

Temperature ºC

Temperature ºC

A

B

Summary Graphic 2: Phase diagrams illustrating where methane hydrate is stable in marine (A) and permafrost settings (B). Hydrate can exist at depths where the temperature (blue curve) is less than the maximum stability temperature for gas hydrate (given by the hydrate stability curve in orange). Pressure and temperature both increase with depth in the Earth, and though hydrates can exist at warmer temperatures when the pressure is high (orange curve), the temperature in the Earth (blue curve) gets too hot for hydrate to be stable, limiting hydrate stability to the upper ~1km or less of sediment.

A GLOBAL OUTLOOK ON METHANE GAS HYDRATES 9