Isotopic signatures of methane emission from oil and natural gas plants in southwestern China

Abstract. Current evaluation of methane (CH4) emissions to the atmosphere from global oil and gas (ONG) sector is subject to considerable uncertainty. Particularly for China, few measurements have been conducted, making it difficult to quantify emissions and conduct mitigation measures. Recently, the isotopic composition of CH4 (δ13C) has been used for evaluating the contributions of fossil fuel sources to global budgets, providing a more effective approach across both regional and global scales. Here, we present a field study of CH4 mixing ratios and δ13C based on UAV sampling and ground monitoring across 11 ONG sites located in southwestern China. We found that the values of δ13C-CH4 provide a solid basis for identifying the CH4 leakage and characterizing source distributions at the ONG site-levels, despite that the meteorological and site conditions as well as the surrounding environment could exert influence on the signal strengths. With the Keeling plot approach, we determined that the mean δ13C source isotopic signatures of CH4 emission from these ONG sites were −25.66 ‰, heavier than previously found for other sites globally. This also indicates that they were mainly thermogenic sources. Finally, by incorporating the updated source isotopic signatures determined for China, we conducted a back-of-envelope assessment to qualitatively infer the global CH4 emission from the fossil fuel sources. The result suggests an overestimation of global CH4 emission from the fossil fuel inventory by 2.86 Tg CH4 yr−1, and an underestimation from the microbial sources. This study highlights the necessity of determining CH4 isotopes on ONG site-levels, providing an indirect but important reference to quantifying CH4 inventories among global industrial sectors.