Regional Isotopic Fingerprinting of Methane and Ethane from the Fourth largest Hydrocarbon Producer Worldwide: Ethane as a Source-Specific Emissions Indicator

Abstract:

The recent increase in atmospheric methane concentration is of great concern to industry and stakeholders. Effective future methane emissions reduction involves identifying the individual sources of atmospheric methane. Modelled and measured regional atmospheric δ¹³C value of fugitive methane may not help in source-specific attribution. The Western Canada Sedimentary Basin is the fourth largest hydrocarbon producer worldwide with both conventional and non-conventional operations. We have measured, mapped and analyzed the n-alkanes and carbon dioxide isotopic composition from production, n= 1624; surface casing vents (SCV), n = 2946; ground migration (GM), n= 1094. Overall, the δ¹³C of methane from production and oil associated gas varies from –87.2 to –23.2 ‰ (VPDB) (avg. –48.7 ± 11.6 ‰); SCV varies from –87.7 to –13.8 ‰ (VPDB), averaging –56.4 ± 9.7‰; from GM, ranges from –91.5 to –14.2 ‰, (avg. –58.2 ± 18.7‰). However, in numerous oilfields, the δ¹³C of methane from production gases, being vented intentionally or unintentionally, fugitive gases from faulty wells, and natural biogenic environmental gases are similar, thus thwarting attribution and subsequent mitigation. In those cases, we find that the δ¹³C of ethane, propane and CO₂ better discriminate between the putative emission sources. We propose that despite its low abundance,δ¹³C of atmospheric ethane needs to be used to unambiguously identify and attribute greenhouse gas emissions from various components of the fossil fuel industry.