Sulfate Reduction, Reservoir Properties, H₂S and Sulfur (S) in Natural Gas

Ian Hutcheon, John Cody, Graham Simpson, Scott McLellan, Hugh Abercrombie

Natural gas in Alberta contains variable amounts of H₂S, the origins of which depend on reservoir temperature, mineral composition and fluid flow history. H₂S in hotter Paleozoic dolomitic reservoirs results from thermochemical sulfate reduction, but H₂S in shallow clastic reservoirs is of biological origin, controlled by influx of meteoric water. In Alberta, common, non-hydrocarbon gases may have multiple origins, depending on the burial and fluid flow history and the mineralogy of the associated reservoir rocks.

Redox couples among SO₄-H₂S, HAc-HCO₃, and C_n-C_n-1 acids suggest that organic acids may be involved in the kinetically unfavorable process of abiotic sulfate reduction. Data that support this hypothesis are being augmented by chemical and isotopic analyses of reservoir water and gas.

Native sulfur is observed in dolomite-hosted reservoirs and may affect the properties of tight gas reservoirs. For carbonates, a reaction among calcite, anhydrite and native sulfur limits the range of H₂S and CO₂ possible in the gas before sulfur deposition must occur and there is a good correlation between gas composition and the occurrence of native sulfur.

H₂S in the gas originates from the reduction of anhydrite. Surprisingly, at reservoir conditions the volume of H₂S is similar to the volume of anhydrite, so large volumes of anhydrite are not required as a source. The overall reaction involves calcite and, possibly, dolomite. The volume created by anhydrite dissolution is partly replaced by calcite precipitation, thus porosity increases can be expected to be correspondingly small in a closed system.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995