The Smackover Formation in the southeastern Mississippi salt basin has been buried from 3.03 to 6.06 km and subjected to temperatures of 100 to 200 °C. This formation exhibits two distinct phases of burial diagenesis, one pre-hydrocarbon migration, and the other posthydrocarbon migration. Pre-hydrocarbon migration burial diagenesis is characterized by precipitation of calcite cement followed by saddle dolomite and late anhydrite.These phases formed at temperatures of less than 100 °C. The oxygen isotopic composition of calcite ranges from -5‰ to -8.4‰ PDB, and carbon isotopic composition ranges from +3.5‰ to +6.0‰ PDB. The sulfur isotopic composition of late anhydrites ranges from +18.1‰ to +19.1‰ CDT. Post-hydrocarbon-migration burial diagenesis is dominated by replacement of previously formed late anhydrite by calcite or calcite plus elemental sulfur, and precipitation of post-bitumen calcite cement. These diagenetic phases formed at temperatures above 150 °C and are present only in the deep part of the basin. The oxygen isotopic compositions of calcites that replace anhydrite range from -3.0‰ to -4.0‰ PDB, and carbon isotopic compositions range from -2.0‰ to +4.2‰ PDB. The oxygen isotopic compositions of post-bitumen calcites range from -3.4‰ to -5.6‰ PDB, and carbon isotopic compositions range from -1.6‰ to -16.3‰ PDB. Two samples of elemental sulfur have sulfur isotopic compositions of +15.8‰ and +16.1‰ CDT.
The light carbon isotopic compositions of post-bitumen calcite cement indicate that carbon incorporated in this cement was partly derived from thermal oxidation of hydrocarbon gases. The relatively heavy oxygen isotopic compositions of these cements suggest precipitation from pore waters with a heavy oxygen isotopic composition.
The high-temperature replacement of anhydrite by calcite, the presence of post-bitumen calcite cement, the sulfur isotopic composition of sulfates and elemental sulfur, and high concentration of hydrogen sulfide indicate that rapid thermochemical reduction of solid sulfates is taking place in the deep part of the Mississippi salt basin at temperatures as low as 150 °C.