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, 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, 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.

==Tools and methods==

==Tools and methods==

Since most tight gas reservoirs in North America are of detrital origin (shale, siltstone, and sandstone), primary processes of deposition, inferred from the examination of sedimentary characteristics in core, can have a strong impact on preserved porosity and permeability trends. An example of a sedimentological description and environmental interpretation of cored facies from a tight gas reservoir is shown in Figure 1.

Since most tight gas reservoirs in North America are of detrital origin (shale, siltstone, and sandstone), primary processes of deposition, inferred from the examination of sedimentary characteristics in core, can have a strong impact on preserved porosity and permeability trends. An example of a sedimentological description and environmental interpretation of cored facies from a tight gas reservoir is shown in Figure 1.

[[file:evaluating-tight-gas-reservoirs_fig1.png|thumb|{{figure number|1}}A cored sequence of tight gas reservoir facies and correlations to electric log responses of the Frontier Formation, Green River basin, Wyoming. Lithologies and sedimentary characteristics are summarized in this kind of description; facies and environments of deposition are shown on the right. (From <ref name=pt06r93>Moslow, T. F., Tillman, R. W., 1986, Sedimentary facies and reservoir characteristics of Frontier Formation sandstones, southwestern Wyoming, in Spencer, C. W., Mast, R. F., eds., Low Permeability Sandstone Reservoirs: Studies in Geology Series 24, p. 271–295.</ref>.)]]

[[file:evaluating-tight-gas-reservoirs_fig1.png|thumb|{{figure number|1}}A cored sequence of tight gas reservoir facies and correlations to electric log responses of the Frontier Formation, Green River basin, Wyoming. Lithologies and sedimentary characteristics are summarized in this kind of description; facies and environments of deposition are shown on the right. (From <ref name=pt06r93>Moslow, T. F., Tillman, R. W., 1986, Sedimentary facies and reservoir characteristics of Frontier Formation sandstones, southwestern Wyoming, ''in'' Spencer, C. W., Mast, R. F., eds., Low Permeability Sandstone Reservoirs: Studies in Geology Series 24, p. 271–295.</ref>.)]]

===Core to log correlations===

===Core to log correlations===