--> ABSTRACT: Depositional and Diagenetic Controls on Reservoir Quality of the Norphlet Sandstone, Mary Ann Field, Offshore Alabama, by Glenn M. Pense; #91036 (2010)

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Depositional and Diagenetic Controls on Reservoir Quality of the Norphlet Sandstone, Mary Ann Field, Offshore Alabama

Glenn M. Pense

The Norphlet Sandstone is a gas-prone reservoir in the subsurface of the Mobile basin. A nine-stage diagenetic history is proposed that accounts for anomalously high porosity (20%) and permeability (100 md) measured in cores taken from depths as great as 20,000 ft in the block 76-2 well of Mobil's Mary Ann field.

Major diagenetic stages were early cementation (stage 2), mechanical compaction (stage 3), decementation (stage 4), chlorite cementation (stage 5), oil migration (stage 7), and two late cementation events (stage 8). Volumetrically insignificant stages include mechanical and chemical preburial adjustment (stage 1), quartz overgrowths (stage 6), and sulfate reduction reactions (pyrite) (stage 9).

The Norphlet Sandstone was episodically deposited by wind-blown processes as a sand sea on an evaporite (Louann) shelf against the Norphlet-Smackover paleocoastline. Major reservoir lithofacies are dominated by dune and sandsheet stratification. These were the most porous and permeable and were cemented early by calcite and anhydrite, which buttressed pores against significant compaction to a depth of approximately 12,000 ft where decementation presumably resulted from reactions with maturing hydrocarbons in adjacent marine source rock. In contrast, interdune (wet and dry) stratification received no early cements, yielded to compaction, and formed relatively impermeable horizontal barriers. Chlorite, dead oil (migration stage), and late potassium feldspar and quartz cements followed a d together formed an impermeable diagenetic seal at the top of the Norphlet reservoir.

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