This paper presents an important dataset from the Transylvanian Basin gas fields that will improve the quality of the global database on the isotopic signatures of methane from various geological settings. I recommend that it be published after some minor revisions.

The paper would benefit from additional background information, as it is currently difficult for readers unfamiliar with the Transylvanian Basin to fully understand the evolution of biogenic methane in the region. To better place the results in context, I had to conduct an independent literature search to learn about the Transylvanian Basin and gas development within the region. With a little extra work this can become a better self-contained paper.

For each of the 12 sample locations in Figure 1, it would be useful if the authors provided more detail on the regional geology, the nature of the gas traps at each site, the depth to the produced gas, and the current understanding of the local petroleum system. Such information would help readers unfamiliar with the Transylvanian Basin and make this a more valuable dataset, particularly since the gas is biogenic in origin and its formation is of significant scientific interest.

How do these data cluster according to the geological formations mapped by Tiliţă et al. (2013) (Figures 2 and 4 in that paper)? There may exist more detailed geological information, and if the authors are aware of such a source, it could replace or supplement Tiliţă et al. (2013).

It would also be useful to see the data from this paper combined with the dataset shown in Figure 4 of Baciu et al. (2018), along with a discussion of how these results align within that graph. This addition would help readers better understand the context and evolution of the scientific discussion regarding the origin and chemical properties of the methane emitted from this unique gas field.

Overall, this manuscript is a valuable scientific contribution that will enhance the global isotopic methane database once these contextual revisions are addressed.

Minor Concerns

Line 23 - “Isotope measurements are increasingly used to constrain the methane (CH4) budget on various scales, from global to regional.”

As these data are presented as being important for informing the global database, what is the estimated rate of methane emissions from the Transylvanian Basin, and what proportion does that represent of global fossil-fuel methane emissions?

Line 155 - “For example, the “high” outlier at the Dumbraviora gas field may be caused by an interference from combustion emissions.”

What evidence supports this statement? It would be helpful if the authors provided context regarding the proximity and relative magnitude of potential combustion sources.

There is also a typo "Dumbraviora" -> "Dumbravioara"

References

Baciu, C., Ionescu, A., and Etiope, G.: Hydrocarbon seeps in Romania: Gas origin and release to the atmosphere, Mar. Petrol. Geol., 89, 130–143, https://doi.org/10.1016/j.marpetgeo.2017.06.015

Tiliţă, M., Matenco, L., Dinu, C., Ionescu, L., & Cloetingh, S. (2013). Understanding the kinematic evolution and genesis of a back-arc continental “sag” basin: The Neogene evolution of the Transylvanian Basin. Tectonophysics, 602, 237–258. https://doi.org/10.1016/j.tecto.2012.12.029

Gas production in Romania is one of the larger methane sources in Europe (as also now shown in satellite observations). The isotopic characterisation of these emissions in this study is of interest as isotopic signatures are a good way of partitioning source types. The paper includes a good introduction to the context of this source sector and of previous studies in the region, including previous isotopic signature measurements.

A biogenic source of the methane was expected (Baciu et al., 2018).  The ethane measurements on the gas in this region from previous studies should be mentioned as this also confirms a biogenic origin. Was ethane measured by the portable analysers used in this study?

How large did the methane mole fraction increase above background need to be to calculate the source signatures using the Keeling plot technique? This should be mentioned in the methodology section where it states that samples were collected in the emission plumes. How big were these plumes? What real-time sensors were used to find the plumes?

Figure 2: what do the error bars represent? They are very small in the central region, but much larger for the low and high d13C. Why is this? Please explain in the caption how the error bars were calculated.

Line 151 – is there a typo here in the numbers for δD? Most lie between -200 and -170 ‰ (not -280).

It is interesting that the d13C isotopic signature from the gas facilities measured in this study (-65.6 ± 0.5 ‰) is much lower than the global average signature for methane from natural gas (around -44 ‰, Sherwood et al., 2017). This needs to be taken into account if incorporating isotopic measurements in regional modelling. I would have liked to have seen the implications of the findings discussed more in the discussion section.