ABSTRACT

Thermochemical sulfate reduction (TSR) destroys hydrocarbons, causes contamination of reservoir fluids with H₂S and other gases, and can significantly change reservoir-rock composition. Smackover reservoirs are affected by this process, and associated gases exhibit significant compositional variation. Data from wells in the Wiggins Arch area of southern Mississippi and Alabama indicate that the TSR reaction has gone to completion and demonstrate the impact of different initial lithologies on ultimate reservoir conditions following TSR. 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). This reaction will increase reservoir pressure by 20%. If iron is available, H₂S will react to form pyrite (3% by volume), resulting in a "wet" underpressured reservoir.