Coastal erosion of permafrost. Notice the thin covering of vegetation on the surface. Below the vegetation, the thick permafrost zone consists of ice, rock, and soil.

In Arctic regions, methane hydrates occur in close association with permafrost. Permafrost layers form where groundwater in the pore spaces of rock and soil is permanently frozen. Permafrost develops because the average annual air temperature is low enough to maintain a continuous surface temperature below 0°C; thus, the depth to which groundwater freezes in winter exceeds the depth of summer thawing. Approximately twenty percent of the land area in the Northern Hemisphere is underlain by permafrost, ranging in thickness from less than one to several hundred meters.

Permafrost and gas hydrates are commonly found together; however, there is no direct causal link between them. In cold polar regions, two parallel events occur: 1) with surface temperatures hovering around -12°F, even with a normal geothermal gradient, sub-freezing temperatures can persist to depths of 600 meters or more, and 2) the very low temperatures, when combined with steadily increasing subsurface pressures, allow methane hydrate to be stable at depths ranging from 300 meters to 1 kilometer. Thus, the conditions that are favorable for the formation of permafrost are coincidentally also the conditions that are favorable for gas hydrates to form.

Location of sedimentary basins in the Northern Hemisphere that may contain gas hydrate. Source: Collett and Dallimore, 2000.

Gas hydrates are known to occur both within and below permafrost in polar areas. Several areas in the Arctic show potential for having gas hydrate accumulations. Three provinces are in North America and four are in Russia: (1) northern Alaska, (2) the Mackenzie Delta-Beaufort Sea region, (3) Sverdrup basin of Canada, (4) Western Siberia basin, (5) Lena-Tunguska province (Vilyuy basin), (6) Timan-Pechora basin, and (7) several sedimentary basins in northeastern Siberia and the Kamchatka area. Additionally, (8) the Svalbard archipelago (Norway) and (9) sedimentary basins under the ice cap of Greenland (Denmark) may have pressure and temperature conditions favorable to the formation of gas hydrates.

Data from over 1000 wells provide both direct and indirect evidence for the existence of arctic gas hydrates. Most of the information has come from drilling activity in Alaska, Canada, and Siberia. Direct evidence for the existence of gas hydrate comes from cores which recovered hydrates in wells of the Prudhoe Bay area, on the North Slope, Alaska in 1992, and the Mackenzie Delta in 1998. Indirect evidence comes from well logs, drill-stem tests, bottom-hole surveys, gas chromatographic data, and water samples. Subsurface well logs, for example, record high electrical resistivities and rapid acoustic velocities in the Mackenzie Delta-Beaufort Sea area that are thought to be indicative of hydrate presence.

Mallick Well in Canada

Geological parameters that affect gas hydrate formation include subsurface temperature, pore pressure, gas chemistry, and pore-water salinity. The geothermal gradient in Arctic regions with hydrate accumulations ranges from 1.0°C to 8.0°C/100 meters. Pore pressures can vary considerably. Abnormally high pore pressures were recorded from beneath the permafrost in offshore wells drilled on the continental shelf in the Mackenzie Delta region, and abnormally low pore pressures were found in the Lena-Tunguska region and Timan-Pechora basin. However, no abnormal pressures were recorded in northern Alaska, the Mackenzie Delta-Beaufort Sea (onshore), Sverdrup basin, and western Siberia. Methane is the primary gas and accounts for 87% to over 99% of the total gas content. In the Timan-Pechora Basin, methane is associated with coal. Pore-water salinity is typically low, ranging from 0.5-35 ppt. Using these figures, computer modeling suggests that the depth to the base of the methane-hydrate stability zone lies between 1000-2000 meters.

For suggestions on further reading on the Arctic Regions, see Arctic Regions References.