Devonian gas shales, a sequence of brown to black, low-permeability laminated rocks that contain 2 to 16% by volume of organic matter, underlie more than 170,000 mi² (440,000 km²) of the Appalachian basin, mainly under the Appalachian plateaus. Their volume exceeds 12,600 mi³ (52,500 km³), and they contain more than 3.3 trillion tons of gas-producing organic matter. Their permeability ranges from 0.1 to 10 microdarcys and their porosity from 1 to 4%. Although their total gas production has been only about 3 tcf during the past 160 yr (since 1821), mainly from the Big Sandy area of eastern Kentucky, their gas-in-place resource has been estimated in the range from 200 to 1,860 tcf. Shale gas is of low thermal maturity near the wes ern outcrops, whereas dry gas deep in the basin to the east is of high thermal maturity. Because most of the shale-generated gas has been adsorbed by the organic components in the rock, gas shales must be broken by an extensive natural fracture system before they will yield gas in commercially exploitable volume. In the western part of the basin, gas shales are both source and reservoir for gas. To the east, dark gas shales interfinger with an eastward thickening sequence of low-permeability siltstone and sandstone turbidites. Where fractured, these more brittle beds are also reservoirs for gas migrating from adjacent gas shales. Successful exploitation of shale gas requires careful evaluation of geologic and engineering factors.

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