Changes

Gas reservoirs with estimated ''in situ'' gas permeabilities of 0.1 md (millidarcy) or less are officially recognized by the U.S. Federal Energy Regulatory Commission (FERC) as “tight gas reservoirs.” This absolute value for classification as a tight gas reservoir was critically important during the late 1970s and early 1980s to qualify for federally allowed enhanced prices of tight gas. Since that time, however, and for all practical purposes, a tight gas reservoir is generally recognized as any low [[permeability]] formation in which special well completion techniques are required to stimulate production (Table 1). The most commonly used recovery technique is hydraulic fracturing, without which many tight gas reservoirs would not be economical (see [[Stimulation]]). Thus, most low permeability gas reservoirs are considered “unconventional.”

Gas reservoirs with estimated ''in situ'' gas permeabilities of 0.1 md (millidarcy) or less are officially recognized by the U.S. Federal Energy Regulatory Commission (FERC) as “tight gas reservoirs.” This absolute value for classification as a tight gas reservoir was critically important during the late 1970s and early 1980s to qualify for federally allowed enhanced prices of tight gas. Since that time, however, and for all practical purposes, a tight gas reservoir is generally recognized as any low [[permeability]] formation in which special well completion techniques are required to stimulate production (Table 1). The most commonly used recovery technique is hydraulic fracturing, without which many tight gas reservoirs would not be economical (see [[Stimulation]]). Thus, most low permeability gas reservoirs are considered “unconventional.”

Low permeability gas-bearing formations occur in almost all gas-producing sedimentary basins worldwide. In North America, the vast majority of tight gas reservoirs can be grouped into two main geological categories: (1) Devonian shales from eastern United States and Canada, and (2) low permeability sandstones from throughout the United States and from the Western Canada Sedimentary basin.<ref name=pt06r133>Spencer, C. W., Mast, R. F., 1986, [http://archives.datapages.com/data/specpubs/resmi1/data/a066/a066/0001/0000/iv.htm Introduction], ''in'' Spencer, C. W., Mast, R. W., eds., Low [[Permeability]] Sandstone Reservoirs: AAPG Studies in Geology Series, n. 24, p. iv–vi.</ref> It has been estimated that in the United States alone, tight sandstone formations are likely to have recoverable reserves ranging from 100 to 400 tcf, and Devonian shales have recoverable reserves of up to 100 tcf;<ref name=pt06r97>Office of Technology Assessment, 1985, U., S. natural gas availability—gas supply through the year 2000: U. S. Congress Office of Technology Assessment, OTA-E-245, 252 p.</ref> cited in <ref name=pt06r133 />. The successful exploitation of tight gas resources in the future will depend in large part on advancements made in the proper geological evaluation of low permeability reservoirs.

Low permeability gas-bearing formations occur in almost all gas-producing sedimentary basins worldwide. In North America, the vast majority of tight gas reservoirs can be grouped into two main geological categories: (1) Devonian shales from eastern United States and Canada, and (2) low permeability sandstones from throughout the United States and from the Western Canada Sedimentary basin.<ref name=pt06r133>Spencer, C. W., Mast, R. F., 1986, [http://archives.datapages.com/data/specpubs/resmi1/data/a066/a066/0001/0000/iv.htm Introduction], ''in'' Spencer, C. W., Mast, R. W., eds., Low [[Permeability]] Sandstone Reservoirs: AAPG Studies in Geology Series, n. 24, p. iv–vi.</ref> It has been estimated that in the United States alone, tight sandstone formations are likely to have recoverable reserves ranging from 100 to 400 tcf, and Devonian shales have recoverable reserves of up to 100 tcf;<ref name=pt06r97>Office of Technology Assessment, 1985, U., S. natural gas availability—gas supply through the year 2000: U. S. Congress Office of Technology Assessment, OTA-E-245, 252 p.</ref> cited in Spencer and Mast.<ref name=pt06r133 /> The successful exploitation of tight gas resources in the future will depend in large part on advancements made in the proper geological evaluation of low permeability reservoirs.

==Tools and methods==

==Tools and methods==