ABSTRACT

The Smackover "A" zone of the Buckner Formation in northern Louisiana and southern Arkansas is lithologically similar to the more thoroughly studied upper Smackover Formation. However, in contrast to the regionally occurring thick, blanket-like ooid sands of the upper Smackover, the Smackover "A" zone carbonates occur as strike-parallel, individual carbonate sand bodies isolated within the clays, muds, and evaporites of the upper Buckner. This isolation, and its potential effects on fluid movement, diagenctic processes, and ultimate porosity evolution in similar sequences across northern Louisiana and southern Arkansas emphasize the importance of a well-constrained core-based field study.

A vertical sequence through the Smackover "A" at Corney Bayou Field illustrates its gradual shallowing-upward nature, which reflects an early increase in energy as the sand body evolved from an ooid-bearing fossiliferous packstone to boundstone through a mixed rhodolitic-oolitic grainstone to an oolitic grainstone. Later, energy decreased and the oolitic grainstone was overlain by an oolitic wackestone to packstone. The uppermost muddy oolitic wackestone to ooid-bearing mudstone attests to gradual dilution of carbonate production through introduction of sand- through clay-sized siliciclastics deposited in a lagoonal setting.

The depositional sequence at Corney Bayou Field produced an isolated carbonate sand-body wedged between less porous and permeable siliciciastics; in other words a stratigraphic trap. Porosity is primary intergranular and is greatest in the higher energy lithofacies. Porosity is diminished in those strata that contain siliciclastics, due to solution-compaction effects and/or large volumes of late cement. Later structural movement along a hingeline influenced the reservoir by initiating updip hydrocarbon migration.