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Recent exploratory and development drilling in Mobile Bay, southwest Alabama, has proven prolific gas production from the Norphlet sandstone at depths greater than 20,000 ft with individual well tests of 10-27 MMCFGD. Excellent reservoir qualities are a function of preserved primary porosity and permeability developed in an eolian setting.
In Mobile Bay, thick eolian sediments (200-600 ft) lie directly on Pine Hill or Louann evaporites. Three facies of the Norphlet have been recognized: (1) a thin (20-30 ft) basal wet sand flat or sabkha facies, (2) a massive dune facies, and (3) a thin (30-40 ft) upper marine reworked facies.
The wet sand flat or sabkha facies is characterized by irregular to wavy horizontally bedded sandstone associated with adhesion ripples. It is probably sporadically developed in response to localized wet lows during earliest Norphlet deposition.
The majority of the Norphlet section is characterized by massive wedge-planar and tabular-planar cross-stratified sandstone, interpreted to be stacked dune and dry interdune deposits. Individual dune sets range in height from a few feet to 90 ft. Cross-bed sets exhibit internal stratification patterns similar to large- and small-scale dunes described by G. Kocurek and R. Dott, Jr.
The marine reworked facies is characterized by structureless to diffuse or wavy laminated sandstone that reflects a reworking of the dune deposits by the ensuing Smackover transgression.
Reservoir quality is affected by textural properties determined by depositional processes associated with these various facies. Diagenetic patterns further reducing reservoir quality occur in the depositionally less-porous sediments. Dune facies sediments exhibit the best reservoir qualities. Variations of reservoir quality within the dune facies are related to dune height and dune versus interdune accumulations.
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