--> ABSTRACT: Middle and Upper Miocene Natural Gas Sands in Onshore and Offshore Alabama, by Robert M. Mink, Ernest A. Mancini, Bennett L. Bearden, and Charles C. Smith; #91036 (2010)

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Middle and Upper Miocene Natural Gas Sands in Onshore and Offshore Alabama

Robert M. Mink, Ernest A. Mancini, Bennett L. Bearden, Charles C. Smith

Thirty Miocene natural gas fields have been established in onshore and offshore Alabama since the discovery of Miocene gas in this area in 1979. These fields have produced over 16 bcf of natural gas from the middle Miocene Amos sand (24 fields) and upper Miocene Luce (3 fields), Escambia (1 field), and Meyer (3 fields) sands. Production from the Amos transgressive sands represents over 92% of the cumulative shallow Miocene natural gas produced in onshore and offshore Alabama. In addition, over 127 bcf of natural gas has been produced from upper Miocene sands in the Chandeleur area.

The productive Miocene section in onshore and coastal Alabama is interpreted to represent transgressive marine shelf and regressive shoreface sands. Reservoir porosity is primary intergranular and ranges from 21 to 35% with permeabilities that may exceed 2,000 md. In onshore and offshore Alabama, Miocene marginal marine clays have petroleum source rock potential, but the more organic-rich and more thermally mature source rocks are basinal Miocene clays. Petroleum traps are principally sandstone porosity pinch-outs against regional dip. Subtle closure and anticlinal nosing are a secondary factor in many of the traps.

The middle Miocene Amos sand bars are the most productive reservoirs of natural gas in onshore and coastal Alabama, principally due to the porous and permeable nature of these transgressive sands and their stratigraphic relationship to the underlying basinal clays in this area. However, these middle Miocene sands thin and grade basinward into finer grained lithofacies in offshore Alabama.

In offshore Alabama the upper Miocene sands become thicker and are generally more porous and permeable than their onshore equivalents. Because of their deeper burial depth in offshore Alabama, these upper Miocene sands are associated with marine clays that are thermally more mature. The combination of reservoir grade lithologies associated with moderately mature petroleum source rocks enhances the natural gas potential of the upper Miocene sands in offshore Alabama.

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