Abstract: Reservoir Property Changes Caused by Thermochemical Sulfate Reduction in the Smackover

John A. Rhodes

Thermochemical sulfate reduction (TSR) destroys methane, causes contamination of reservoir fluids with H₂S and other gasses, and results in significant changes to the reservoir-rock composition. Smackover reservoirs are affected by this process and exhibit significant variation in gas composition. Wells in the Wiggins arch area of southern Mississippi and Alabama indicate it is an area where the reaction has gone to completion, and demonstrate the effect of different initial conditions on ultimate reservoir conditions. The significant controls are the ratio of gas-filled porosity to anhydrite, and the availability of iron. In a deep reservoir, anhydrite (10% by volume) can react with methane (12% by volume) to form H₂S (10% by volume) and a 20% increase in the re ervoir pressure. If iron is available, the reaction will form pyrite (3% by volume), resulting in a "wet" reservoir.

AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994