Effect of Methane Cracking on Carbon Isotope Reversal and the Production of Over-Mature Shale Gas
Round 1
Reviewer 1 Report
The authors have submitted a manuscript describing an interesting avenue to experimentally study the thermal decomposition / thermal cracking of methane with the concomittant formation of higher alkenes and alkanes - and solid carbon residues at very high temperatures. All these processes have been studied for decades in combustion research in hot flames without O2 and in pyrolysis processes in industry. The transfer of the ideas to the geological context is relatively new.
But the description of the experimental approach is not detailed enough to allow reproduction by other groups,
there are significant discepancies between data shown in tables and figures based on the tabulated data ( the data in the figures 2c are not corresponding to the data in the table 1 - there are significant discrepancies visible ??? e.g. C2H4 is never above 2*10-3 in the table, etc. ???)
and data already published are used and presented as "measured in this study". The data in figure 5 have been published previously by Niu et al. (2020) - see fig. 7 in that article ... http://www.nggs.ac.cn/article/2020/1672-1926/1672-1926-2020-31-9-1294.shtml ... and are here presented as new data not citing Niu et al. (2020).
Besides, the discussion is not adequate as it is not referring to the many publications in the dry pyrolysis literature at similar high temperatures, it is not considering the effect of the non-isothermal nature of the experiments on the interpretation of the temperature range, in which e.g. the formation of solid C (or "soot" in the pyrolysis literature) is occuring, no discussion on e.g. a mass balance of H is presented.
And the sloppy formatting of the citations (more than half of the citations have to be corrected, comma vs. dot, brackets on volumes of journals removed, etc.) underpins this impression of the quality of the manuscript as is.
The data from the gold tube pyrolysis experiments itself are interesting and call for more work on the manuscript, before it is resubmitted in a completely newly written version.
Author Response
The response to reviewer 1 is in attchment.
Author Response File: 
Author Response.docx
Reviewer 2 Report
Authors study an important process from the point of view of engineering applications in petroleum industry. Application of new shale oil formation investigations in China is a very interesting approach. However, in some points the obtained data are not well discussed or conclusions are not sufficiently justified by the content (see specific comments included in the manuscript; the proposal of remarks are introduced using the option “Comment” in PDF document).
In order to improve the quality of the paper I suggest following:
- Authors should rewrite numerous sentences and paragraphs with the help of an expert in English language and edit the whole text.
- Some of figures should be improved.
- Provide the responses for each specific comment in PDF document of the corrected manuscript.
- I have carefully checked the manuscript and list of references and I didn’t find any manuscript from Energies. This may give an impression to readers that the work is outside the scope of Energies journal, although journal publish routinely in this area. I ask that authors rework the list of references.
Comments for author File: 
Comments.pdf
Author Response
The response to reviewer 2 is in attachment.
Author Response File: Author Response.docx
Reviewer 3 Report
Review not fully completed
See comments in attached annotated manuscript
Comments for author File: Comments.pdf
Author Response
The response to reviewer 3 is in attachment.
Author Response File: Author Response.docx
Reviewer 4 Report
The manuscript reports the effect of methane cracking on carbon isotope reversal along with the production of over mature gas in Sichuan Basin. Detailed geochemical studies are presented providing the analyses of components and carbon isotopes of gaseous products. The results reported represent a notable advance in the understanding of mechanism of isotopic reversal of over mature natural gas. This is an important topic and one which appears to be somewhat conflicting in the literature.
The title and abstract are appropriate for the content of the text. Furthermore, the manuscript is well constructed and analyses are well performed. In abstract, the authors summarized the main research question and key findings. The description of study subject and results are detailed and correct. Data collected are interpreted accurately and the results support the conclusion. The manuscript reads well. The figures are clear and readable and support the findings.
Overall, this is a clear, concise, and well-written manuscript that has implications for the theoretical basis and development of relationship between wetness and isotope reversal change of the over mature natural gas as useful criteria of CH4 cracking. In my opinion, the manuscript is suitable for publication in Energies in present form.
Author Response
The response to reviewer 4 is in attachment.
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
The authors have replied to all the comments and suggestions - but not resolved major problems pointed out.
Most important is the quality control of the presented gas compositional data - where the authors have changed now values after reintegrations as they stated. But there are still significant unexplained pecularities in the data, (extremly good reproducibility, large discrepancies in mass balances, see attached .pdf-converted excel-spreadsheet) as stated in the reply to the reply by authors (see attached .pdf-document).
In the attached excel spreadsheet these fundamental issues are depicted - with some suggestions for improvement and resulting changes in interpretation.
They must be resolved before the mauscript can be published, therefore major revisions. But: I encourage the author to make the (not small) effort with the major revisions as the approach and the overall picture evolving is interesting and in some aspects new ... and the study should be published in the end.
Comments for author File: Comments.pdf
Author Response
The responds is in attachment.
Author Response File: Author Response.docx
Reviewer 2 Report
The article can be accepted.
Author Response
Very thanks for your positive assessment on our manuscript that “The article can be accepted”.
