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Gas hydrates are crystalline substances composed of water and gas, mainly methane, in which a solid-water lattice accommodates gas molecules in a cage-like structure, or clathrate. These substances commonly have been regarded as a potential unconventional source of natural gas because of their enormous gas-storage capacity. Significant quantities of naturally occurring gas hydrates have been detected in many regions of the Arctic, including Siberia, the Mackenzie River Delta, and the North Slope of Alaska. On the North Slope, the methane-hydrate stability zone is areally extensive beneath most of the coastal plain province and has thicknesses greater than 1000 m in the Prudhoe Bay area.
Gas hydrates have been inferred to occur in 50 North Slope exploratory and production wells on the basis of well-log responses calibrated to the response of an interval in a well where gas hydrates were recovered in a core by ARCO and Exxon. Most North Slope gas hydrates occur in six laterally continuous lower Tertiary sandstones and conglomerates; all these gas hydrates are geographically restricted to the area overlying the eastern part of the Kuparuk River oil field and the western part of the Prudhoe Bay oil field. The volume of gas within these gas hydrates is estimated to be about 1.0 × 1012 to 1.2 × 1012 m3 (37 to 44 tcf), or about twice the volume of conventional gas in the Prudhoe Bay field.
Geochemical analyses of well samples suggest that the inferred hydrates probably contain a mixture of deep-source thermogenic gas and shallow, microbial gas that was either directly converted to gas hydrate or first concentrated in existing traps and later converted to gas hydrate. The thermogenic gas probably migrated from deeper reservoirs along the same faults thought to have been migration pathways for the large volumes of heavy oil that occur in the shallow reservoirs of this area.
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