--> ABSTRACT: Effects of Diagenesis on Enhanced-Resolution Bulk Density Logs in Tertiary Gulf Coast Sandstones: An Example from the Lower Vicksburg Formation, by Jeffry D. Grigsby, Richard P. Langford; #91020 (1995).
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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 Previous HitporosityNext Hit and Previous HitpermeabilityNext Hit 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% Previous HitporosityNext Hit, 0.34 md Previous HitpermeabilityNext Hit, 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% Previous HitporosityNext Hit, 0.19 md Previous HitpermeabilityNext Hit, and having a recorded density less than 2.43 g/cc, and (3) calcite or transitional-cemented sandstones averaging 9.4% Previous HitporosityNext Hit, 0.04 md Previous HitpermeabilityNext Hit, and having a recorded density greater than or equal to 2.48 g/cc.

The ranges in density defined by the Previous HitsandstoneNext Hit 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 Previous HitsandstoneNext Hit classes and making separate Previous HitporosityNext Hit and Previous HitpermeabilityNext Hit regressions for the quartz-, chlorite, and calcite or transitional-cemented intervals allow for a more accurate Previous HitpredictionNext Hit of Previous HitpermeabilityNext Hit. For example, classification of cement intervals from lower Vicksburg gas reservoirs using enhanced-resolution bulk density logs and Previous HitporosityNext Hit and Previous HitpermeabilityNext Hit regressions for each of the Previous HitsandstoneNext Hit classes, results in estimated Previous HitpermeabilityTop 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