Diagenesis of Upper Jurassic Norphlet Formation, Mobile and Baldwin Counties and Offshore Alabama

R. L. Vaughan, Jr., D. J. Benson

The Upper Jurassic Norphlet Formation is an important deep gas reservoir in Mobile and Baldwin Counties and offshore Alabama. The producing reservoir consists of a well-sorted fine-grained subarkose to arkose. Sedimentological studies indicate this unit was deposited on a broad desert plain in environments ranging from eolian dune and interdune to wadi and beach-shoreface.

The Norphlet contains a suite of authigenic minerals which includes hematite, anhydrite, ferroan and nonferroan dolomite and calcite, quartz, K-feldspar, illite, chlorite, and pyrite. These diagenetic minerals comprise from 5 to 20% of the bulk volume of the sandstone. Porosity ranges from less than 3% to more than 25% and averages around 10%. Most of the porosity consists of hybrid solution-enlarged intergranular and intragranular pores resulting from the dissolution of cements, framework grains, and grain replacements.

Arid depositional conditions and later marine transgression placed an early diagenetic imprint on the sandstone. Near-surface cementation by carbonate and sulfate minerals, quartz, K-feldspar, and hematite occurred concurrently with mechanical compaction. Continued burial resulted in the formation of chlorite grain coatings and additional grain overgrowths. The thermomaturation of overlying Smackover source rocks caused the migration of carbonic and carboxylic acids and hydrogen sulfide, which resulted in the widespread dissolution of framework grains, grain replacements, and early cements and the reduction of hematite to pyrite. Pressure solution, saturation of the acidic fluids, and Louann brine invasion resulted in the late-stage precipitation of carbonate, illite, anhydrite, quart , and K-feldspar cements.

Geographic and stratigraphic variations in the sequence and intensity of these diagenetic events, particularly in regard to clay distribution, determined present reservoir quality. Reservoir quality porosity can be expected throughout the area, especially in northern Baldwin County and offshore.

AAPG Search and Discovery Article #91036©1988 GCAGS and SEPM Gulf Coast Section Meeting; New Orleans, Louisiana, 19-21 October 1988.