Abstract

In the Waha and West Waha area of Reeves and Pecos Counties, gas and condensate are produced from seven wells in the Mississippian limestones. The electric logs and production data from these seven Mississippian producing wells indicate vast differences in production per well reserves. The difference is because the best wells have vuggy porosity. This reservoir type can be defined using detailed log analysis.

The most common logging suite in the study area is a SP - ILD - SN - sonic log combination; however, a small number of SNP logs and two CNL - Lithodensity logs are available. Sonic porosity was calculated using a matrix travel time of 47.5 μsec/ft (limestone). The limestone lithology was based on lithodensity lithology plots. The SP was used to determine the formation water resistivity (Rw=0.039 ohm-m).

The upper 150-200 ft of the Mississippian reservoir has a cleaner gamma ray response, greater SP deflection, and higher resistivity character than the lower part of the Mississippian. Plots of bulk volume water (BVW), and Rt/Rw versus Rsn/Rz (Dew), indicate that the better reservoir rocks are present in the high resistivity zones in the upper Mississippian. In the high resistivity zones, BVW values are lower and ratios of Rt/Rw are higher than the lower portions. The best well in the field (>110 billion cubic feet gas), when compared to a poor well and a non-producing well, contained lower BVW values, higher Rt/Rw ratios and higher porosities. These data indicate that the best reservoirs have more porosity and larger pores. Net pay summaries, based on cut-offs of Vcl <= 10%, Φ>= 6% and Sw <= 20%, reveal that the best well has over four times the net pay and hydrocarbon pore-feet (HCPFT) compared to the other two wells. Thus, the petrophysics supports the production data: BVW, Rt/Rw versus Rsn/Rz, and net pay summaries can be used to identify the better Mississippian reservoirs.