About This Item
Share This Item
The Lower Cretaceous Tuscaloosa sandstone is of interest because of high porosity (up to 29%) at depths as great as 21,000 ft (6,400 m). We believe that the porosity is 70% primary and 30% secondary. Diagenetic events are major controls for the high porosity but overpressure also contributes.
Primary porosity exists because quartz diagenesis was retarded by chlorite grain coats. Where grains coats are absent or thin, prismatic quartz overgrowths occur. Chlorite coats constitute up to 29% of the rock. Chlorite originates from alteration of lithic material (i.e., volcanic and basic igneous intrusives) in the sandstone. An excellent direct correlation exists between the amounts of lithic fragments and chlorite coats. Sandstones with extensive chlorite and lithic fragments are not good reservoirs because of reduced permeability resulting from rearrangement and mechanical breakage of weak grains during compaction. Commonly, authigenic titanium minerals (e.g., anatase) are associated with the altered lithic material.
Secondary porosity has formed from the dissolution of lithic fragments and feldspars, and to a lesser degree carbonates. Secondary porosity related to highly chemically susceptible lithic fragments developed early (prechlorite) and was subject to later collapse; whereas pores related to later dissolution of less soluble lithic material and feldspar were less affected by compaction because of a more stable framework. Diagenetic evidence indicates that the dissolution occurred prior to, during, and after the formation of chlorite coats.
Calcite cement occludes porosity in some intervals. However, textural evidence from leaching experiments suggests that dissolution of this calcite is not a major source of secondary porosity.
End_of_Article - Last_Page 498------------