ABSTRACT

The Upper Jurassic Norphlet Formation is an important deep gas reservoir in Mobile and Baldwin Counties and the adjacent Alabama state waters. The reservoir is a well-sorted, fine-grained subarkose to arkose deposited in eolian dune, interdune, wadi, and beach-shoreface environments.

Arid depositional conditions and a subsequent marine transgression left an early diagenetic imprint on the sandstone. Near-surface cementation by hematite, quartz, k-feldspar, carbonate, and sulfate minerals occurred concurrently with mechanical compaction. With continued burial, illite and chlorite grain coatings and additional carbonate cements were precipitated. Thermomaturation of organic matter in the overlying Smackover produced carbonic and carboxylic acids and hydrogen sulfide. The introduction of these components into the Norphlet resulted in widespread dissolution of framework grains, grain replacements, and early cements.

Pressure solution, saturation of the acid fluids, and invasion of brines from the underlying Louann Salt resulted in the late-stage precipitation of quartz, K-feldspar, anhydrite, and carbonate cements.

Porosity in the Norphlet is a mixture of primary intergranular porosity and secondary porosity produced through dissolution of cements, detrital grains, and grain replacements prior to hydrocarbon emplacement. Porosity ranges from zero to 25 percent and averages around 10 percent. Permeabilities range from 0.1 millidarcy to almost 100 millidarcies but are typically less than 1.0 millidarcy. Porosity and permeability vary within the study area as a consequence of geographic differences in diagenetic history and authigenic mineralogy. The best reservoirs occur in northern Baldwin County and in the offshore area.