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 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.

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==

===Stratigraphic cross sections===

===Stratigraphic cross sections===

Lateral variability in facies relationships, and thus reservoir continuity and heterogeneity, are best determined from the construction of stratigraphic [[cross section]]s. An example of a cross section through part of a tight gas reservoir is shown in [[:file:evaluating-tight-gas-reservoirs_fig2.png|Figure 2]]. Facies interpretations are based on [[core description]]s and extrapolation of log signatures for each cored facies to adjacent uncored wells. Distributary channel sandstones form the reservoirs, and bay, marsh, and crevasse splay mudstones form the seal. The lack of production in the two wells to the east is attributed to the pinching out of these mudstone facies and substantiates its importance as a stratigraphic seal. Note the laterally discontinuous nature of individual reservoir sandstone beds as depicted in the cross section.

[[Lateral]] variability in facies relationships, and thus reservoir continuity and heterogeneity, are best determined from the construction of stratigraphic [[cross section]]s. An example of a cross section through part of a tight gas reservoir is shown in [[:file:evaluating-tight-gas-reservoirs_fig2.png|Figure 2]]. Facies interpretations are based on [[core description]]s and extrapolation of log signatures for each cored facies to adjacent uncored wells. [[Distributary channel]] sandstones form the reservoirs, and bay, marsh, and crevasse splay [[mudstones]] form the seal. The lack of production in the two wells to the east is attributed to the pinching out of these mudstone facies and substantiates its importance as a stratigraphic seal. Note the laterally discontinuous nature of individual reservoir sandstone beds as depicted in the cross section.

===Petrophysical properties of reservoir facies===

===Petrophysical properties of reservoir facies===

[[Category:Geological methods]]

[[Category:Geological methods]]

[[Category:Play type]]

[[Category:Play type]]