Miocene Marine Shelf-Bar and Deltaic Petroleum Reservoirs of Coastal Alabama and Mississippi/Alabama Shelf

Robert M. Mink, Ernest A. Mancini, Bennett L. Bearden

Middle and upper Miocene gas reservoirs found in coastal Alabama and the Mississippi/Alabama shelf are predominantly inner to middle neritic shelf-bar or deltaic sands. A Miocene delta system prograded from the west-northwest in offshore Louisiana-Mississippi into coastal Alabama and the Mississippi/Alabama shelf. Deltaic sedimentation primarily affected the western portion of the Mississippi/Alabama shelf, while neritic sedimentation predominated in the northeastern portion of the region in coastal Alabama. Basinal clays are prevalent in the southeastern portion of the Mississippi/Alabama shelf.

The productive Miocene reservoir sands occur between depths of 1,100 and 5,100 ft and generally are from 10 to 200 ft thick. The quartz-rich sands range from fine to coarse grained and have subangular to rounded and moderately to well-sorted quartz grains. Reservoir porosity is primary intergranular and generally ranges from 21% to 35%, with permeabilities that may exceed 2,000 md. The natural gas in these shallow reservoirs is primarily biogenic in origin. Productivity of the reservoirs is highly variable and is often characterized by high water saturation. Reservoir pressures, which generally range from 550 to 2,300 psi, are also a major factor controlling productivity of these reservoirs. Middle Miocene reservoirs are most common in coastal Alabama, and upper Miocene reservoirs are most common in the Mississippi/Alabama shelf.

Petroleum traps are principally sandstone porosity and permeability pinch-outs against regional dip with subtle closure and anticlinal nosing as secondary factors in many of the traps. These middle and upper Miocene gas sands are best delineated with relative amplitude seismic reflection data on which gas-charged sands are apparent as bright spots.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.