Diagenesis and Porosity Evolution, Norphlet Formation in Mobile Bay, Alabama

Brian E. Lock, Samuel W. Broussard

Major deposits of natural gas were discovered in the Norphlet Formation beneath Mobile Bay in 1979. The reservoirs are in arkosic sandstones at depths greater than 20,000 ft, yet the productive interval has porosities up to 25%. Overlying the porous zone is a tight cap of thoroughly cemented sandstone of variable thickness, which poses problems for exploration and production. The tight zone, which together with the overlying basal Smackover forms the reservoir seal, may be so thick that the underlying productive interval is substantially reduced. The upper parts of the Norphlet, in common with many other eolian sands, were reworked during a subsequent transgression. There is not a full correspondence, however, between the tight rock and the reworked facies. The origin of he impermeable zone is better understood as a function of the diagenetic history only partially related to depositional facies.

The tight zone is cemented by syntaxial quartz and feldspar overgrowths. The porous zone has textures that clearly indicate the porosity is secondary, an interpretation consistent with the low permeabilities recorded. In some samples, chlorite grain coatings are prominent and a superficial similarity to the deep Tuscaloosa is obvious. However, many other samples are virtually devoid of chlorite and the textural evidence strongly suggests porosity generation by removal of an early cement and grain replacement minerals. Rare remnants of anhydrite suggest that evaporites may be the missing components.

It is proposed that, at an early stage of diagenesis, brines derived from the underlying Louann Salt and Werner Formation deposited anhydrite and possibly halite cements in the lower part of the Norphlet Formation. Marine working of the upper sands may have helped to disperse these brines from the upper part of the Norphlet, and the depth of reworking may even have been partially influenced by incipient cementation. The zones not already cemented by evaporites were subsequently cemented by quartz and feldspar overgrowths. At a very late stage, deep in the subsurface, the evaporite cements were flushed from the lower parts of the Norphlet, and locally abundant small feldspar crystals randomly nucleated in the pores. Gas migrated into the formation shortly afterward.

Evaporites may play another important role in the petroleum geology of the deep Norphlet: the source of the gas may have been the underlying evaporites, rather than the lower Smackover (separated from the reservoir by the tight zone), as suggested by some authorities.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.