The National Methane Hydrates R&D Program
All About Hydrates - Geology of Methane Hydrates
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Vein of hydrate in sediment.
Courtesy of USGS. |
Methane hydrate can form in rocks or sediments of any type given suitable
pressures, temperatures, and supplies of water and methane (please see our
discussion on Necessary Conditions for Methane
Hydrate Formation). Although natural methane hydrate has been most commonly observed occurring as disseminated grains, other forms are known, including massive layers of pure hydrate up to 4 meters thick, nodules that grow and displace surrounding sediments, veins filling small fractures, thin layers along bedding planes, and as a cement binding sedimentary grains together. Similarly, hydrate is typically found with uniform distribution, showing clear, but subtle vertical trends of increasing or decreasing abundance. However, examples of heterogeneous distribution with zones of sparse or no hydrate interspersed with zones of high concentration are also common. Although the factors that control the ultimate type, distribution, and amount of hydrate are still poorly understood, perhaps the primary controls are 1) porosity and permeability and 2) the degree of lithification of the enclosing medium. The geologic environment in which the sediments/rock exist largely determines these aspects
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| Occurrences of Methane Hydrate
in Arctic and Marine Environments
(more) |
Hydrates are commonly described as occurring in two distinct classes based primarily on geography: “marine” hydrates and “permafrost” hydrates. This distinction is based largely on the vast differences noted in the two primary field laboratories, the terrestrial arctic “Mallik” site, where hydrates occur in great concentration within sandstone reservoirs, and the marine Blake Ridge site, where dispersed, low-concentration hydrates occur encased in shale. However, recent work has indicated that perhaps the more important distinction in controlling the nature of a hydrate deposit is the enclosing media. It now appears that significant reservoir permeability is a pre-requisite for formation of concentrated hydrates.
In general, there are two primary geologic/geographic environments for hydrate accumulation: 1) areas with deep water in close proximity to land, and 2) continents in polar regions. Deep-water settings can be further classified as: 1) inland deep-water seas and lakes, 2) stable passive continental margins, 3) unstable passive continental margins, and 4) active tectonic boundaries.
Stable passive margins exist along the margins of continents where active tectonism is absent. These margins are typified by wide coastal plains and continental shelves that serve to place deep water far from the eroding land surfaces that are the source of sediment. The result of this physical separation is the efficient sorting of sediment. While sands are trapped in rivers and along the shore, the only grains capable of staying suspended in the water long enough to reach the deep ocean are very fine-grained silts and clays. As a result, the sediment in deep water passive margin settings is commonly homogenous mud. Although this mud typically possesses high porosity, its fine-grained nature commonly results in very low permeability—a hindrance to hydrate formation and a major cause of the low hydrate concentrations (typically 2% to 6%) observed on passive margin shelves. Please see the discussion on the
Blake Ridge for more information on the world's best documented passive margin hydrate accumulation.
The prospects for passive margin accumulation of methane hydrate can be augmented, however, if the margin is unstable due to the presence of deeply-buried, low-density rocks such as salt. Along the passive margin of the Gulf Coast south of Louisiana and Texas, the shelf and slope sediments have been disrupted by the formation of large salt diapirs (bodies of salt that move upward due to buoyancy forces, causing deformation) that provide fracture and fault pathways for the upward migration of deeper thermogenic methane into the hydrate stability zone. However, the presence of salt diapirs can significantly reduce the prospects of hydrate occurrence by increasing the salinity of pore waters and enabling elevated gas thermal gradients. The result is likely to be a very heterogeneous distribution of hydrate within a discontinuous, segmented hydrate stability zone. Please see the discussion on the
Gulf of Mexico hydrates for more detail on hydrate accumulations in an unstable passive margin.
Active tectonic regions may provide improved prospects for marine methane hydrate accumulation. These regions occur along plate boundaries in which one plate composed of oceanic crust is subducted beneath another plate along a deep ocean trench. When the overriding plate is also oceanic crust, a volcanic island arc forms. When the overriding plate is continental crust, a string of volcanic mountains forms on the continent. In either case, the result is the location of eroding highland areas in relatively close proximity to deep-water environments. As compared to the sediments of passive margins, this proximity may result in coarser-grained and more variable sediment. As a result, the sediments of tectonically-active regions may contain zones of high porosity together with moderate permeability. Furthermore, the sediment pile is actively deformed as it is caught between the descending oceanic plate and the edge of the continent. This deformation may result in enhanced pathways for fluid and methane migration, and increased recycling of methane from hydrate dissociation. Examples of ocean-ocean subduction include the Nankai Trough off Japan, where exploratory drilling in 1999 discovered a 16-meter-thick layer of enhanced porosity that was found to be 80% saturated with hydrate. Examples of ocean-continent subduction zones include the Mid-America Trench offshore Guatemala, where a 1 meter-thick core of solid hydrate was recovered, and "Hydrate Ridge" off the coast of Oregon, which has been the subject of two recent research cruises by the Integrated Ocean Drilling Program. Both IODP cruises shows highly-heterogeneous hydrate distribution, and a strong correlation between hydrate occurrence and reservoir grain size/permeability.
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Arctic Permafrost Region |
Arctic hydrate accumulations provide the best prospects for near term recoverability. Although the hydrate resources of such deposits are often described as insignificant relative to the marine occurrences, the proportion of in-place resources that may be suitable for economic production is probably much greater. Most significant is the existence of continental deposits such as river and beach sands interbedded with shales and coals that provide excellent reservoirs for both migrating methane and water. The presence of faults and heterogeneous sediment sequences creates traps and seals that concentrate the flow of methane and water, and therefore, the occurrence of hydrate, to specific high-permeability conduits. As in active margins, this can result in numerous isolated zones of high hydrate concentration.
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