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Hydraulic fracturing is a reservoir stimulation technique that involves the injection of high-pressure fluids to enhance recovery from unconventional hydrocarbon reservoirs. Often this involves the injection of surface waters (along with additives such as biocides) into formational fluids significantly different isotopic and geochemical compositions facilitating geochemical fingerprinting of these fluid sources. In some instances, the produced fluids experience an increase in hydrogen sulfide (H2S) concentration over the course of production resulting in an increased risk to health and safety, the environment, and infrastructure due to the toxic and corrosive nature of H2S. However, questions remain as to the origin and processes leading to H2S formation following hydraulic fracturing. In this study, we analyzed a series of produced waters following hydraulic fracturing of a horizontal well completed in the Montney Formation, Western Canada to evaluate variations in geochemical and microbiological composition over time and characterize potential sulfur species involved in the production of H2S. Initially, sulfur isotope ratios (d34S, VCDT) of dissolved sulfate in produced water had a baseline value of 27per mil similar to the d34S value of 25per mil for solid anhydrite derived from core material. Subsequently, d34S values of sulfate in produced fluids sequentially increased to 35per mil coincident with the appearance of sulfides in produced waters with a d34SH2S value of 18per mil. Oxygen isotope values of dissolved sulfate exhibited a synchronous increase from 13.2per mil to 15.8per mil VSMOW suggesting sulfate reduction commenced in the subsurface following hydraulic fracturing. Formation temperatures are |
