Changes

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 (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 (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 (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 (Figure 3b).

[[file:evaluating-tight-gas-reservoirs_fig3.png|thumb|{{figure number|3}}Histograms showing (a) average porosity values and (b) average permeability values for cored tight gas reservoir facies. (From <ref name=Moslow and Tillman_1986>Moslow, T. F., and R. W. Tillman, 1986, Sedimentary facies and reservoir characteristics of Frontier Formation sandstones, southwestern Wyoming, ''in'' C. W. Spencer and R. F. Mast, eds., Low permeability sandstone reservoirs: AAPG Studies in Geology 24, p. 271-295.</ref>.)]]

[[file:evaluating-tight-gas-reservoirs_fig3.png|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 />.)]]

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.