ABSTRACT: Preservation of Primary Porosity in Deeply Buried Sandstones: A New Play Concept from the Cretaceous Tuscaloosa Sandstone of Louisiana
S. T. Paxton, J. O. Szabo, C. S. Calvert, J. M. Ajdukiewicz
Primary intergranular porosity commonly is assumed to decline continuously with burial depth. However, in the past decade, producible reservoirs with high porosities have been found at depths in excess of 15,000 ft. Striking examples of this deep porosity are provided by the Tuscaloosa trend of Louisiana in which porosities in excess of 20% are common at depths greater than 20,000 ft.
The following observations and conclusions appear to be applicable to the Tuscaloosa Formation and other deeply buried porous sandstones. (1) Textural and geochemical evidence suggest that the bulk of deep porosity is primary and not due to cement dissolution or leaching of framework grains. (2) Intergranular volume (IGV), a key parameter for assessing sandstone compaction, stabilizes at about 26% between 5,000-8,000 ft burial depth. Porosity loss below this depth is due to cementation rather than compaction. (3) Overpressuring is not a cause of porosity preservation because IGV is similar for both normal and overpressured rocks. Burial history for the Tuscaloosa suggests the sandstones were fully compacted prior to the onset of abnormal pressure. (4) Chlorite clay rims may inhibit bu do not prohibit silica cementation. (5) Intergranular pressure solution and concomitant silica cementation are rare in these deeply buried sandstones and is not a cause of localized porosity reduction. (6) In the Tuscaloosa, porosity has been preserved because cement volumes generated during burial were inadequate to fill the stabilized IGV. Given a limited cement supply, facies characterized by thicker sand packages have maximum porosity preservation.
AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990