Abstract: The Origin of H₂S by Thermal Sulfate Reduction

Ian Hutcheon

Thorough understanding of the role of thermal sulfate reduction (TSR), the process that produces high concentrations of the toxic acid gas H₂S, is critical to the exploitation of deep natural gas occurrences. To predict H₂S concentrations it is necessary to identify the source of sulfate, the process by which sulfate is reduced to sulfide; possible sinks for H₂S; and the potential for H₂S migration. As an acid gas, H₂S may cause dissolution and enhance porosity. The reaction itself has been suggested as a source of heat.

Devonian dolomitic reservoirs of the Western Canada sedimentary basin have variable, high amounts of H₂S and CO₂ that are observed to increase as temperature increases, but temperature is not an accurate predictor of H₂S concentration. Isotopic data identify anhydrite as the source of H₂S and comparison of the volume of H₂S and anhydrite at reservoir conditions shows that only a small percentage of anhydrite is required to produce high H₂S concentrations, suggesting that local sources can produce observed concentrations. In reservoirs where TSR has taken place, coarse, late-stage, pore-filling calcite cement is observed. Examination of calcite from three reservoirs shows it has negative ^dgr¹³C (-24 to -12%), reflecting incorporation of organic carbon from hydrocarbons. Fluid inclusions in the calcite have higher homogenization temperatures and lower salinity than replaced anhydrite, and they contain CH₄, H₂S, CO₂, C₂H₆, and C₃H₈ gases with chemical compositions that have similarities to the reservoir gas. The water in calcite inclusions has distinctly lower salinity and heavier oxygen isotopic composition than formation water in Devonian rocks. The fluid inclusion data indicate that TSR occurred over a temperature ra ge between 110 and 160°C, and less saline water was produced within the reservoirs during TSR. According to the simplest reaction between anhydrite and methane to produce calcite and H₂S, water must also be generated. Water from the TSR reaction has locally diluted the formation water and, potentially, may have increased the TSR reaction rate by increasing hydrocarbon solubility.

The TSR reaction is often stated to be a source of reservoir heating, but modeling of heat generation shows that most reactions proposed for TSR require heat. The reaction between anhydrite and hydrocarbons to produce H₂S and calcite generates heat, but the amount is so small for any reasonable reaction rates that only undetectable reservoir heating could be expected.

These observations suggest that TSR is a local process, perhaps even at the pore scale. If so, porosity effects are governed by the dissolution of anhydrite, with porosity declines caused by the accompanying precipitation of calcite. There is some evidence that H₂S may migrate over distances of a few kilometers, and if this occurs, porosity increases can be more substantial.

AAPG Search and Discovery Article #90948©1996-1997 AAPG Distinguished Lecturers