Fluid inclusion and stable isotopic studies of thermochemical sulphate reduction from Burnt Timber and Crossfield East gas fields in Alberta, Canada

Thermochemical sulphate reduction (TSR) is generally considered to be the s ource of high H2S content in deep Devonian and Mississippian gas fields. In creasing concentration of both H2S and CO2 is observed to correlate in gene ral with increasing depth, and therefore temperature. The TSR reaction betw een anhydrite and natural gas to produce calcite and HS 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 Fo othills and adjacent Plains of Southwestern Alberta, reveal details of the TSR reaction. Petrographic, stable isotopic, fluid inclusion microthermomet ry and gas compositional data have been gathered to calibrate the temperatu re range of TSR and to obtain semi-quantitative compositional data on the v ariation and evolution of natural gas composition and reaction products as TSR proceeds. Fluid inclusions in late-stage calcite have higher homogeniza tion temperature (Th: 110-160 degreesC) and lower salinity (3.9 to 9.9wt.% NaCl equivalent) than anhydrite cement (Th: 80-105 degreesC; 11.6-17.8wt.% NaCl equivalent). They also have distinctively lower salinity (5-20wt.% NaC l equivalent) and heavier oxygen isotopic composition (delta O-18(water) = 7.8-11.7% V-SMOW) than formation water in the area (delta 18O(water) = -5- +7%c 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 be tween 110 and 160 degreesC and less saline (fresh?) water was produced with in the reservoirs during TSR. Water from the TSR reaction has locally dilut ed the formation water and, potentially, may have increased the TSR reactio n rate because hydrocarbon solubility is greater in less saline water. The delta S-34 values of H2S gas increase with increasing H2S% in the gas f ields 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 d elta C-13 values of calcite (-23.9 to and -12.0% V-PDB) reflect incorporati on of organic carbon from the isotopically light hydrocarbons. The occurenc e of light hydrocarbon gases (e.g. CH4, C2H6 and C3H8) in fluid inclusions in calcite suggests their participation in TSR.