Core and Electrical Log Descriptions of the D Sandstone
Basic to the characterizations of the D Sandstone reservoir at the Sooner Unit are data from conventional core analysis and electrical log calculation. The permeability-porosity cross- plot from D Sandstone cores is shown in figure 3. The plot suggests a porosity cut-off of between 6 and 8 percent for determination of net pay. The average net-pay porosity of the Sooner Unit reservoir is 11.5 percent. Statistics for permeability are summarized in table 1. The tabulation is intended to show that a permeability cut-off of 0.5 md is probably appropriate and a value of 0.74 for Dykstra-Parsons coefficient of permeability variation results when a log-normal frequency plot is made as shown in figure 4.
Calculations of net-pay properties from electrical logs are summarized in table 2. Net-pay thickness averages 17 ft (+/- 8 ft) with a maximum of 34 ft. At the Sooner Unit, criteria used to identify productive intervals include 1) resistivity of greater than 30 ohm-m, 2) gamma ray of less than 30 API units, micro-resistivity and caliper log readings indicating mud-cake buildup and 3) density-log porosities of greater than 8 percent. Calculations of water saturations are qualitative but are performed with reasonable success using a standard Archie equation for sandstones. The formation water is very fresh and a value of 0.06 ohm-m is used for water resistivity (Rw) at formation temperature of 220°F. Using the medium-induction curve value for formation resistivity (Rt) produces reasonable values for water saturation. A shale content of 35 percent from gamma-ray readings and water saturation of 60 percent are used to discriminate the net-pay from non-reservoir rock as summarized in table 2. The D Sandstone did not produce formation water during primary depletion and it is assumed that the reservoir water saturation was initially at irreducible conditions. Special core analysis indicates a value for irreducible water saturation of 19 percent.
