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===

Average core analysis values for porosity, permeability, oil, gas, and water saturation should be determined for each facies recognized to identify those facies of greater and lesser reservoir quality ([[:file:evaluating-tight-gas-reservoirs_fig3.png|Figure 3a]]). In gas-bearing sandstones, very low values of porosity and permeability are acceptable and expected. While the average air permeability values rarely exceed 1.0 md (millidarcy) for tight gas reservoirs, a significant difference in permeability values often occurs between facies ([[:file:evaluating-tight-gas-reservoirs_fig3.png|Figure 3b]]).

Average core analysis values for porosity, permeability, oil, gas, and water saturation should be determined for each facies recognized to identify those facies of greater and lesser reservoir quality ([[:file:evaluating-tight-gas-reservoirs_fig3.png|Figure 3a]]). In gas-bearing sandstones, very low values of porosity and permeability are acceptable and expected. While the average air permeability values rarely exceed 1.0 md (millidarcy) for tight gas reservoirs, a significant difference in permeability values often occurs between facies ([[:file:evaluating-tight-gas-reservoirs_fig3.png|Figure 3b]]).

Anomalously high values from core analysis measurements may also identify zones of fracture porosity and permeability in tight gas reservoirs (see [[Evaluating fractured reservoirs]]). However, one must be careful in interpreting such results because erroneously high measurements can also be produced by bypassing or artificial fracturing of core samples during analysis. Checks should be made to ensure that a sufficient number of samples have been analyzed for each facies or unit and that permeability and porosity values correspond to observed lithologies in core.

Anomalously high values from core analysis measurements may also identify zones of [[fracture]] [[porosity]] and [[permeability]] in tight gas reservoirs (see [[Evaluating fractured reservoirs]]). However, one must be careful in interpreting such results because erroneously high measurements can also be produced by bypassing or artificial fracturing of core samples during analysis. Checks should be made to ensure that a sufficient number of samples have been analyzed for each facies or unit and that permeability and porosity values correspond to observed lithologies in core.

===Petrological and mineralogical assessment===

===Petrological and mineralogical assessment===

* [[Introduction to geological methods]]

* [[Introduction to geological methods]]

* [[Lithofacies and environmental analysis of clastic depositional systems]]

* [[Lithofacies and environmental analysis of clastic depositional systems]]

* [[Subsurface maps]]

* [[Subsurface maps]]

* [[Flow units for reservoir characterization]]

* [[Flow units for reservoir characterization]]

* [[Effective pay determination]]

* [[Effective pay determination]]

* [[Multivariate data analysis]]

* [[Cross section]]

* [[Geological cross sections]]

* [[Evaluating structurally complex reservoirs]]

* [[Evaluating structurally complex reservoirs]]

* [[Conversion of well log data to subsurface stratigraphic and structural information]]

* [[Conversion of well log data to subsurface stratigraphic and structural information]]

* [[Reservoir quality]]

* [[Reservoir quality]]

* [[Carbonate reservoir models: facies, diagenesis, and flow characterization]]

* [[Carbonate reservoir models: facies, diagenesis, and flow characterization]]

* [[Evaluating stratigraphically complex fields]]

* [[Evaluating stratigraphically complex fields]]

* [[Evaluating diagenetically complex reservoirs]]

* [[Evaluating diagenetically complex reservoirs]]

==References==

==References==

[[Category:Geological methods]]

[[Category:Geological methods]]

[[Category:Play type]]

[[Category:Play type]]