Changes

==Tools and methods==

==Tools and methods==

[[file:evaluating-tight-gas-reservoirs_fig1.png|left|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, [http://archives.datapages.com/data/specpubs/resmi1/data/a066/a066/0001/0250/0271.htm 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|left|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 Moslow and Tillman.<ref name=pt06r93>Moslow, T. F., Tillman, R. W., 1986, [http://archives.datapages.com/data/specpubs/resmi1/data/a066/a066/0001/0250/0271.htm Sedimentary facies and reservoir characteristics of Frontier Formation sandstones, southwestern Wyoming], ''in'' Spencer, C. W., Mast, R. F., eds., Low Permeability Sandstone Reservoirs: [http://archives.datapages.com/data/alt-browse/aapg-special-volumes/sg24.htm Studies in Geology Series 24], p. 271–295.</ref>)]]

The extremely low permeability of tight gas reservoirs severely restricts the ability of gas to migrate appreciable distances. Consequently, the most important geological characteristic of this type of reservoir is the nature and distribution of [[porosity]] and permeability (Table 1). The most common reason for the minimal permeabilities is the occlusion of interstitial pore throats by detrital or authigenic clays or cement (see [[Rock-Water interactions: formation damage]]). Thus, a proper geological evaluation of tight gas reservoirs requires a multidisciplinary approach to assess the depositional and diagenetic controls on [[reservoir quality]] and heterogeneity.

The extremely low permeability of tight gas reservoirs severely restricts the ability of gas to migrate appreciable distances. Consequently, the most important geological characteristic of this type of reservoir is the nature and distribution of [[porosity]] and permeability (Table 1). The most common reason for the minimal permeabilities is the occlusion of interstitial pore throats by detrital or authigenic clays or cement (see [[Rock-Water interactions: formation damage]]). Thus, a proper geological evaluation of tight gas reservoirs requires a multidisciplinary approach to assess the depositional and diagenetic controls on [[reservoir quality]] and heterogeneity.

===Core to log correlations===

===Core to log correlations===

[[file:evaluating-tight-gas-reservoirs_fig2.png|thumb|{{figure number|2}}The depositional dip-oriented cross section through the Frontier Formation, Moxa arch area, Wyoming, showing facies relationships and inferred geometries. (Modified from <ref name=pt06r93 />.)]]

[[file:evaluating-tight-gas-reservoirs_fig2.png|thumb|{{figure number|2}}The depositional dip-oriented cross section through the Frontier Formation, Moxa arch area, Wyoming, showing facies relationships and inferred geometries. (Modified from Moslow and Tillman.<ref name=pt06r93 />)]]

Documenting characteristic log signatures for reservoir facies can provide a valuable tool for constructing regional cross sections, determining facies relationships, and extrapolating reservoir geometries in areas of minimal or nonexistent core control (see [[Quick-look lithology from logs]]). Commonly, the gamma ray log provides the most distinctive log signature for individual facies ([[:file:evaluating-tight-gas-reservoirs_fig1.png|Figure 1]]). For low permeability gas reservoirs, crossover of the compensated neutron-formation density logs is the most reliable well log for indicating gas-saturated and porous intervals and for determining which intervals in the reservoir should be perforated and/or stimulated by hydraulic fracturing.

Documenting characteristic log signatures for reservoir facies can provide a valuable tool for constructing regional cross sections, determining facies relationships, and extrapolating reservoir geometries in areas of minimal or nonexistent core control (see [[Quick-look lithology from logs]]). Commonly, the gamma ray log provides the most distinctive log signature for individual facies ([[:file:evaluating-tight-gas-reservoirs_fig1.png|Figure 1]]). For low permeability gas reservoirs, crossover of the compensated neutron-formation density logs is the most reliable well log for indicating gas-saturated and porous intervals and for determining which intervals in the reservoir should be perforated and/or stimulated by hydraulic fracturing.

===Petrophysical properties of reservoir facies===

===Petrophysical properties of reservoir facies===

[[file:evaluating-tight-gas-reservoirs_fig3.png|left|thumb|{{figure number|3}}Histograms showing (a) average porosity values and (b) average permeability values for cored tight gas reservoir facies. (From <ref name=pt06r93 />.)]]

[[file:evaluating-tight-gas-reservoirs_fig3.png|left|thumb|{{figure number|3}}Histograms showing (a) average porosity values and (b) average permeability values for cored tight gas reservoir facies. (From Moslow and Tillman.<ref name=pt06r93 />)]]

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]]).

===Petrological and mineralogical assessment===

===Petrological and mineralogical assessment===

[[file:evaluating-tight-gas-reservoirs_fig4.png|thumb|{{figure number|4}}Correlation of sedimentary facies and lithologies to petrographic reservoir quality. Distribution of reservoir facies in the subsurface Is compiled from observations of cores, well logs, and cross sections. (From <ref name=pt06r93 />.)]]

[[file:evaluating-tight-gas-reservoirs_fig4.png|thumb|{{figure number|4}}Correlation of sedimentary facies and lithologies to petrographic reservoir quality. Distribution of reservoir facies in the subsurface Is compiled from observations of cores, well logs, and cross sections. (From Moslow and Tillman.<ref name=pt06r93 />)]]

A petrological thin section, SEM, and X-ray diffraction analysis of core samples from each sedimentary facies is highly recommended in any geological evaluation of tight gas reservoirs (see [[Thin section analysis]] and [[SEM, XRD, CL, and XF methods]]). Analyses of several tight gas sandstones have attributed the low average permeabilities, and thus poor reservoir quality, to the presence of authigenic or detrital clays or cements.<ref name=pt06r82>Masters, J. A., 1984, Elm worth—Case Study of a Deep Basin Gas Field: [http://store.aapg.org/detail.aspx?id=67 AAPG Memoir 38], 316 p.</ref><ref name=pt06r133 /> Since the occurrence of these constituents can be quite variable within a depositional system and can be facies dependent, a broad range of porosities, permeabilities, and gas saturation values often exists in any reservoir ([:file:evaluating-tight-gas-reservoirs_fig4.png|Figure 4]]). Identifying and mapping those units of greatest reservoir potential are key to a successful evaluation.

A petrological thin section, SEM, and X-ray diffraction analysis of core samples from each sedimentary facies is highly recommended in any geological evaluation of tight gas reservoirs (see [[Thin section analysis]] and [[SEM, XRD, CL, and XF methods]]). Analyses of several tight gas sandstones have attributed the low average permeabilities, and thus poor reservoir quality, to the presence of authigenic or detrital clays or cements.<ref name=pt06r82>Masters, J. A., 1984, Elm worth—Case Study of a Deep Basin Gas Field: [http://store.aapg.org/detail.aspx?id=67 AAPG Memoir 38], 316 p.</ref><ref name=pt06r133 /> Since the occurrence of these constituents can be quite variable within a depositional system and can be facies dependent, a broad range of porosities, permeabilities, and gas saturation values often exists in any reservoir ([:file:evaluating-tight-gas-reservoirs_fig4.png|Figure 4]]). Identifying and mapping those units of greatest reservoir potential are key to a successful evaluation.