Measurement report: Isotopic composition of CH₄ emitted from gas exploration sites in the Transylvanian Basin, Romania

Abstract. Isotope measurements are increasingly used to constrain the methane budget on various scales, from global to regional. The success of isotope-based source attribution depends to a large degree on the knowledge of the isotope signatures of the various source categories at the point of emission, but this information is in many cases lacking. Here we report the isotopic composition of CH₄ emitted from 48 installations in the gas production region of Transylvania, Romania. The isotopic source signatures are quite homogeneous across the basin with average values of δ¹³C = (-65.6 ± 0.5 ‰) and δD = (-184 ± 1 ‰) confirming the biogenic origin of the Transylvanian gas, produced by hydrogenotrophic CO₂ reduction. This is similar to values reported previously from natural seeps in Transylvania, to the natural gas exploited in the Dolj region in Southwestern Romania, and to the natural gas in the distribution grid in Cluj-Napoca. However, is more depleted in heavy isotopes than the oil-associated gas emitted in the Southern Romanian Plain, and gas leakages in the city of Bucharest. In addition, we present a step-by-step derivation of the underlying "Keeling plot" mass balance approach that is used to derive isotope source signatures.