Diagenetic heterogeneity strongly influences porosity and permeability distribution in sandstone reservoirs of the lower Vicksburg Formation. Enhanced-resolution bulk density logs, which use a 3-cm (1.2-in.) rather than the standard 15-cm (6-in.) sampling increment, offer important new information in understanding diagenetically complex sandstones in the subsurface. Detailed petrographic mapping and sedimentologic description of core from the Oligocene lower Vicksburg Formation of McAllen Ranch field, south Texas, have identified cement variations that result in diagenetic zones ranging in thickness from 0.1 cm (0.04 in.) to 0.9 m (3 ft). The petrographic and petrophysical properties of these diagenetic zones define three diagenetic facies that can be correlated with log response from the enhanced-resolution bulk density log: (1) the quartz-cemented facies, averaging 13% porosity and 0.342 md permeability, and having a recorded density greater than or equal to 2.43 g/cm3 but less than 2.48 g/cm3, (2) the chlorite-cemented facies, averaging 16.7% porosity and 0.193 md permeability, and having a recorded density less than 2.43 g/cm3, and (3) the calcite- or transitional-cemented facies, averaging 9.4% porosity and 0.04 md permeability, and having a recorded density greater than or equal to 2.48 g/cm3.
Intervals cemented with different minerals are identified by the enhanced-resolution bulk density log based on the ranges in density that define each diagenetic facies. This identification of diagenetic facies not only influences the accurate determination of the net sandstone thickness and porosity of reservoirs within the lower Vicksburg Formation, but also directly controls permeability calculations. For example, classification of cement intervals from lower Vicksburg gas reservoirs using enhanced-resolution bulk density logs and porosity and permeability regressions for each of the diagenetic facies results in estimated permeability thicknesses as much as 50% greater than those calculated using standard log analysis. Because standard log analysis ignores the effects of diagenetic heterogeneity, the use of the enhanced-resolution bulk density log provides significant information that leads to more accurate reservoir and field evaluations in diagenetically complex sandstones.