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]]). |