Effects of Diagenesis on Enhanced-Resolution Bulk Density Logs in Tertiary Gulf Coast Sandstones: An Example from the Lower Vicksburg Formation

Jeffry D. Grigsby, Richard P. Langford

Enhanced-resolution bulk density logs, which use a 1.2 inch (3 cm) rather than the standard 6 inch (15 cm) sampling increment, offer important new information in diagenetically complex sandstones. Detailed petrographic mapping and sedimentologic description of core from the Oligocene lower Vicksburg Formation of McAllen Ranch field, South Texas have identified centimeter-scale cement variations that strongly influence porosity and permeability distribution. These cement variations result in diagenetic bands that range in thickness from 0.04 inches (0.1 cm) to 3 ft (0.9 m). Correlation of these cement variations with log response from the enhanced-resolution bulk density log result in the identification of three classes of sandstones. These are (1) quartz-overgrowth-cement d sandstones averaging 12.9% porosity, 0.34 md permeability, and having a recorded density greater than or equal to 2.43 g/cc but less than 2.48 g/cc, (2) chlorite-cemented sandstones averaging 16.7% porosity, 0.19 md permeability, and having a recorded density less than 2.43 g/cc, and (3) calcite or transitional-cemented sandstones averaging 9.4% porosity, 0.04 md permeability, and having a recorded density greater than or equal to 2.48 g/cc.

The ranges in density defined by the sandstone classes indicate that the enhanced-resolution bulk density log can be used to differentiate intervals cemented with different minerals in the lower Vicksburg Formation. Identifying the different sandstone classes and making separate porosity and permeability regressions for the quartz-, chlorite, and calcite or transitional-cemented intervals allow for a more accurate prediction of permeability. 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 sandstone classes, results in estimated permeability thicknesses as much as 40% greater than those calculated using standard log analysis. This is significant if m re accurate reserve calculations are to be made in the future.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995