Using in situ measurements of δ13C in methane to investigate methane emissions from the western Canada sedimentary basin

Abstract

During COP26, Canada joined the Global Methane Pledge aiming to reduce global methane (CH₄) emissions by 30% below 2020 levels by 2030. Rapid reduction of anthropogenic CH₄ emissions in the atmosphere is considered one of the most effective strategies to slow global warming in concert with carbon dioxide mitigation measures. In Canada, a large part of anthropogenic CH₄ emissions can be attributed to the oil and gas industry in the Western Canada Sedimentary Basin (WCSB), which is the fourth largest reserve of fossil fuel in the world. Recent studies highlighted big discrepancies between CH₄ emissions reported in Canada's National Inventory and emissions estimated with approaches using atmospheric measurements for the oil and gas sector. Measuring the isotopic signature of CH₄ (δ¹³CH₄) at different atmospheric monitoring stations and comparing these measurements to detailed archives of δ¹³CH₄ from oil and gas wells and associated CH₄ leaks in the WCSB could help better characterize the emissions from different CH₄ sources over the region. In this study, we compare two independent sets of data: (1) thousands of δ¹³CH₄ of samples from oil and gas production wells and their associated leaks (surface casing vents and ground migration) collected across the WCSB, and (2) atmospheric mixing ratios of CH₄ and their δ¹³CH₄ measured successively at three locations across this region between 2016 and 2020 combined with their atmospheric footprints modeled with HYSPLIT-STILT. We observed a gradient in the isotopic signatures of the oil and gas samples within the WCSB with δ¹³CH₄ being more depleted in the Southeast than in the Northwest. The analysis of these samples showed that the isotopic signature of CH₄ emitted by the production of fossil fuel depends on the geological formations from which it is extracted. Also, CH₄ isotopic signatures vary more depending on the region where CH₄ comes from than on the type of fossil fuel extracted (natural gas, oil, heavy oil, shale gas). The atmospheric measurements showed a strong seasonality of δ¹³CH₄ at one of the atmospheric monitoring sites with isotopic signatures associated with thermogenic sources in winter and isotopic signatures associated with a mix of biogenic and thermogenic sources in summer. Using the δ¹³CH₄ archive database from oil and gas samples, we estimated that 28–39 % of the CH₄ emitted near this site in summer was coming from a biogenic source (most likely wetlands). Ultimately, atmospheric measurements collected during winter combined with their associated HYSPLIT-STILT footprints presented a spatial distribution of δ¹³CH₄ over the WCSB comparable to the one observed in the archive database from the oil and gas sector, confirming that the oil and gas sector is the main source of CH₄ in this region.