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