Sulphur and oxygen isotopic signatures of anhydrite as well as the sulphur isotopic signature of hydrogen sulphide gas (H2S) are used to identify the source(s) of this toxic gas in unconventional gas play of the Triassic Montney Formation, Canada. The presence of hydrogen sulphide even if only in trace amounts have a significant impact on well economics. Hydrogen sulphide is highly corrosive and thus expensive infrastructure is needed to prevent well and midstream damage. Operators need to predict areas that are at high risk of encountering hydrogen sulphide and thus pre-plan to mitigate the associated risks. In this study anhydrite was chemically separated from drill cuttings and sulphur and oxygen isotopic ratios measured from representative wells. Sulphur isotopic ratios were also determined for hydrogen sulphide gas from producing wells by operators participated in our research consortium. Sulphate and oxygen isotopic ratios from sulphate minerals within the Triassic rocks range between 9.0 to 18.0 VCDT and -5.0 to 19.0 V-SMOW, respectively. Sulphate and oxygen isotopic ratios differ from the sulphate minerals in Devonian rocks which vary between 18.0 to 30.0 V-CDT and 12.0 to 30 V-SMOW, respectively. The sulphur isotopic ratio measured from H2S gas in the Montney reservoirs varies between 9.3 and 20.9 V-CDT. Isotopic analyses suggest that the H2S produced in the Montney Formation is from either Triassic sulphates or a mixture of Triassic and Devonian sources and is not solely from Devonian rocks as previously predicted. Our research will be expanded to include other reservoirs in the world (i.e., Bass Strait fields) with the goal to reduce the risk of environmental contamination. Sulphur isotopic data coupled with structural data can provide a robust geological model to help mitigate this risk.

Open-Access Online Publication: May 22, 2023