Abstract: Factors Influencing Porosity and Permeability of Lower Wilcox (Eocene) Sandstone, Karnes County, Texas

George D. Stanton, Earle F. McBride

Petrographic and petrophysical data from 57 sandstone samples were used to analyze a 2,500-ft (762 m) section of core from -5,094 to -7,474 ft (1,553 to 2,278 m) through a succession of stacked shoreface sandstones separated by interdeltaic and tidal-flat shale. Porosity(^phgr) and permeability(k) of sandstone, determined petrophysically, have a positive (straight line) correlation when ^phgr is plotted arithmetically versus the log of k. Petrophysical ^phgr (6 to 31 percent) has a positive correlation with thin section ^phgr (0 to 20 percent) but averages 11 percent higher. Petrographic limitations prevent the identification in thin section of microporosity in clay clasts, matrix, and kaolinite cement and accounts for part of the discrepancy. Sandstone is chiefly very fine to fine-gr ined and has an average framework composition of Q₅₉F₁₆R₂₅; matrix ranges from 0 to 46 percent.

Initial porosity of sandstone was reduced sequentially by (1) bioturbation, (2) compactional rotation of framework grains, (3) deformation of ductile grains, (4) pressure solution of framework grains, and (5) cementation by quartz, kaolinite and calcite. A few sands lost most ^phgr prior to burial by bioturbation, but most lost 10 to 15 percent by grain rotation and ductile-grain deformation during compaction, followed by up to 30 percent loss by cementation. The volume of quartz cement exceeds that lost at sutured grain contacts by an order of magnitude. Although kaolinite is present chiefly as cement, in some samples it also is present as an alteration product of plagioclase.

As much as 20 percent secondary porosity developed by dissolution of calcite cement plus dissolution of calcitized-framework grains, chiefly plagioclase and VRF's. As much as 5 percent of the secondary porosity developed either by direct dissolution of plagioclase or dissolution of calcitized-plagioclase grains.

Although dissolution of calcite undoubtedly increased the k of sandstones significantly, one minor deleterious effect was to produce silt-size quartz grains free to migrate to pore throats and slightly reduce k. The silt-size quartz grains are the residue of polycrystalline quartz grains that had been replaced partly by calcite.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA