Geochemical analysis is essential for characterization of fluids in hydrocarbon reservoirs and has been widely applied in evaluation of conventional petroleum systems; however limited studies exist on application in unconventional plays. In this study we analyse over three hundred samples of gas from Devonian Horn River Group Formations with an aim to characterize the gases, reservoir and provide insight into the unconventional shale gas system. The Horn River Basin, NE British Columbia contains significant shale gas reserves trapped within organic-rich marine sedimentary rock formations of ultra-low permeability. We determine chemical and stable isotope compositions (δ13Cmethaneδ13Cethane, δ13Cpropane) of gases from the Muskwa, Otter Park and Evie Formations of the Horn River Group using GC IR-MS. Mineralogical compositions of the shales were obtained by XRF along the wellbore paths from wells completed in the Muskwa and Otter Park Formations. During the production of shale gases, we monitor compositional changes for several wells (NTS 94-O-08) completed in Horn River Group Formations as time series over short term (25-50 days) and long term (~1250 days) periods. Carbon isotope lateral profiles along horizontal well legs placed in the Muskwa and Otter Park Formations map the intra and interformation isotope composition variations within these shale gas target formations. Carbon isotope lateral profiles created for the Muskwa Formation showed greater uniformity than in the Otter Park Formation which may be indicative of enhanced flow connectivity within the Muskwa shale. The phenomenon of isotope reversal was frequently observed among produced gases from Horn River Group formations and include cases of partial isotope reversal as well as full isotope reversal where δ13Cmethane > δ13Cethane, > δ13Cpropane. In general, during early phases of gas production 12Cmethane enrichment of gases occurred while subsequent to periods of well ‘shut in’ 13Cmethane enrichment was observed. δ13Cmethane values of Horn River Group gases varied during production and ranged from approximately −38%o to −28%o over the long term. Carbon isotope signatures of produced gases from the Muskwa, Otter Park and Evie Formations suggest isotope fractionation occurs during gas transport through the ultra-low permeability shale formations.
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