Deep Porosity Preservation in the Norphlet Formation, Mobile Bay, Alabama

AJDUKIEWICZ, J. M., S. T. PAXTON, and J. O. SZABO, Exxon Production Research Company, Houston, TX

Compaction and pressure solution have commonly been assumed to destroy primary intergranular porosity in deeply buried sandstones. However, primary porosities of up to 20% are preserved at depths greater than 20,000 ft in the Norphlet Formation of Mobile Bay. Previous workers have called upon a number of mechanisms to preserve these high porosities in the Norphlet, specifically chlorite rim cements, gas emplacement, overpressuring, and decementation. In contrast, our study of data from 23 Norphlet wells, including 450 thin sections, indicates that these suggested mechanisms are not the primary cause of porosity preservation in the Norphlet. We propose an alternative interpretation: that in the Norphlet, as in other well-sorted, ductile-grain-poor sandstones, porosity loss from compact on did not go to completion under reservoir (premetamorphic) conditions, but stabilized at depths of about 5000-8000 ft and porosity values of about 26%. Porosity loss below these values is due to cementation. For cementation to occur, both an adequate source of cement and geochemical conditions favoring cement precipitation must be present. Computer simulations of Norphlet burial history, including post-depositional fluid-flow patterns, suggest that conditions favorable to quartz cementation never occurred in the bulk of the Norphlet because of the formation's stratigraphic position and isolation from a basinward source of silica-saturated fluids.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)