ABSTRACT
Solid, ice-like mixtures of natural gas and water have been found immobilized in rocks beneath the permafrost in Arctic basins, and in muds under deep water along the continental margins of the Americas. While these unusual compounds have been known and studied by chemists for over a century and a half, and by gas transmission engineers for the past fifty years, the discovery of natural occurrences came as a surprise to earth scientists.
Hydrates of methane gas are stable at temperatures above the freezing point of water if the pressures exceed about 400 psi (2.8 MPa). Therefore, natural gas at near-saturation volumes would combine with water to form a solid in areas of low temperature and moderate-to-high pressures, eg., in sediments immediately beneath thick permafrost, or immediately beneath deep (ca 300 m) water. The structure of hydrates -- a crystalline network of interconnected water molecules, the interstices of which contain single molecules of gas -- is such that there are almost six water molecules for each gas molecule combined into the crystal.
Hydrates are solid; therefore they have the potential of immobilizing large quantities of both water and gas in favorable environments. These solid masses will act as barriers to normal fluid movement through the host sediments, and may even form seals and traps that will trap free gas where the structure and stratigraphy of the rocks themselves would not. Moreover, at about 300 meters, a unit volume of reservoir under permafrost can contain six times as much gas in the hydrate form as in the free gas form. This enrichment factor decreases with depth, reaching parity at about 2000 meters, but is an important consideration in calculating possible gas reserves in Arctic regions.
Because such large areas of sedimentary basins of the globe are underlain by permafrost or are covered by water (along the continental margins), the volumes of gas locked in hydrate "reservoirs" may be enormous. For example, based on Soviet estimates of hydrated reserves in Siberia, the shallow reservoirs under the permafrost of North America would contain 200 trillion cubic feet (TCF), or 5.7 × 1012m3, of such gas. The muds under the deep water along the continental margins of North America could contain almost 20,000 TCF, or 5.7 × 1014m3, of gas, but probably only a small fraction, eg., 200 TCF (5.7 × 1012mi, in rocks porous enough to be considered reservoir rocks.
None of this gas is recoverable with present technology. However, the very magnitude of the resource is so large that naturally occurring hydrates should be the object of continuing study and research.
