Abstract

Thermochemical sulphate reduction (TSR) is generally considered to be the source of high H2S content in deep Devonian and Mississippian gas fields. Increasing concentration of both H2S and CO2 is observed to correlate in general with increasing depth, and therefore temperature. The TSR reaction between anhydrite and natural gas to produce calcite and H2S is also thought to generate water. Fluid inclusions in TSR calcite cement from the Burnt Timber and Crossfield East natural gas fields in the Upper Devonian Crossfield Member, in the Foothills and adjacent Plains of southwestern Alberta, reveal details of the TSR reaction. Petrographic, stable isotopic, fluid inclusion microthermometry and gas compositional data have been gathered to calibrate the temperature range of TSR and to obtain semi-quantitative compositional data on the variation and evolution of natural gas composition and reaction products as TSR proceeds.

Fluid inclusions in late-stage calcite have higher homogenization temperature (Th: 110–160°C) and lower salinity (3.9 to 9.9wt.% NaCl equivalent) than anhydrite cement (Th: 80–105°C; 11.6–17.8wt.% NaCl equivalent). They also have distinctively lower salinity (5–20wt.% NaCl equivalent) and heavier oxygen isotopic composition (δ18O water = 7.8–11.7‰ V-SMOW) than formation water in the area (δ18O water = −5–+7‰ V-SMOW and 20–25wt.% NaCl equivalent); and contain CH4, H2S, CO2, C2H6 and C3H8 gases. This indicates that TSR occurred at a temperature range between 110 and 160°C and less saline (fresh?) water was produced within the reservoirs during TSR. Water from the TSR reaction has locally diluted the formation water and, potentially, may have increased the TSR reaction rate because hydrocarbon solubility is greater in less saline water.

The δ34S values of H2S gas increase with increasing H2S% in the gas fields and they approach the values of elemental sulphur (+25.4 to +26.4‰ V-CDT), which in turn are similar to the values of anhydrite. The negative δ13C values of calcite (−23.9 to and −12.0‰ V-PDB) reflect incorporation of organic carbon from the isotopically light hydrocarbons. The occurrence of light hydrocarbon gases (e.g. CH4, C2H6 and C3H8) in fluid inclusions in calcite suggests their participation in TSR.

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