On Tectonic and Hydro Meteorological Conditions of Methane Genesis and Migration in the Offshore Waters of East Vietnam

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors would like to report subsurface methane distributions and hydrological controls on the western side of the South China Sea (SCS) proper. The data are interesting while the explanations are questionable.

1. At the end of Section 5, the authors suggested that 'Typhoon NAKRI had a significant impact on methane transport into the water column of the Phu Khanh basin'. It is speculative. The authors did NOT show any evidence supporting the typhoon may have impact on subsurface waters beneath the 50-m depth. In my knowledge, it is difficult for a SCS-born typhoon to cause this effect.

2. The authors also suggested that the existence of a thermohaline barrier prevented the spread of dissolved methane into the surface water layer (the third to last paragraph of Section 6). To me, this is sudden and oversimplified. The roles of methanotrophs MUST be considered. Refer to: Kessler, J.D., Valentine, D.L., Redmond, M.C., et al., 2011. A persistent oxygen anomaly reveals the fate of spilled methane in the deep Gulf of Mexico. Science 331, 312–315. https://doi.org/10.1126/science.1199697

3. Please unify diverse units of nM/L, nl/l, ml/l and nM/dm³. They should be nmol/L (for dissolved CH₄) or μmol/L (for dissolved CO₂).

4. There are some typos. For example, at the final paragraph of Section 4, what do you mean with 'The was...'?

5. Figures 3 and 4 could be merged into one. How to connect the subsurface methane hotspot in the water colume (Fig. 4b) with the potential sediment source area (Fig. 3b)? Similar question should be answered in the Phu Khanh Basin (Fig. 6 versus Fig. 8).

Author Response

Thank you for taking the time and providing thoughtful suggestions which we feel have improved the paper. Our response to the reviewer suggestions follows. We have made some major changes to the manuscript as a result of the suggestions. We marked our revision in yellow. 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Dear authors,
Your manuscript is very interesting because it introduces and integrates approaches that consider tectonic, hydrocarbon seepages and meteorological data.
Seeing all data used and the interesting research idea, I encourage to improve the manuscript because it needs a deep rewriting before it can be published.

 Structural framework

The tectonic led the hydrocarbon migration and, in the considered area,  the structural setting is not so clear. Also, how the faults influence the hydrocarbon migration. Add figures and cross-sections (e.g. seismic line ?)

Could you explain, adding more data?

 Petroleum system
The origin of the methane is thermogenic or biogenic and in the manuscript the relationship from these different gases and the location of the gas field is not clear. What are the relationships between biogenic and thermogenic gas fields? Are they linked? Are The faults cutting the hydrocarbon fields?

Could you explain, adding more data?

 The relationship from structural setting and hydrocarbon pathways
In the manuscript the behavior of the gases in the seabed is unclear, taking into account if the gas is biogenic or thermogenic. Could you explain, adding more data?

 Conclusions
The hydrocarbon seepages are always linked with a fault system that lead and generate the pathways. The faults can deeply cut hydrocarbon filed and less deep gas fields.
Also distribution of the seepages cab be helped to evaluate the presence of gas in the seabed and can confirm the tectonic system. 

Author Response

Thank you for taking the time and providing thoughtful suggestions which we feel have improved the paper. Our response to the reviewer suggestions follows. We have made some major changes to the manuscript as a result of the suggestions.  Revised text marked in yellow. 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

There are still many issues. Please make thoroughly double checks before resubmission, not only response reviewers' comments. 

1. In the Conclusion section, you said that 'The depth of the degassing sources is confirmed by the presence of high concentrations of carbon dioxide in the water'. Where are the 'concentrations of carbon dioxide in the water'? Only 'Depth sources of degassing are confirmed by the presence of high concentrations of carbon dioxide here (0.40-0.50 mL/L or 18-20 μmol/L)' in subsection 'Red River Basin' is not enough. 

2. In Section 4. Material and Methods, 'The research included the determination of the methane, carbon dioxide, helium, and hydrogen content in bottom sediments'. However, you only presented results of methane. If you don't want to report data of the other gases in this manuscript, please rewritten this Section. In addition, free CO₂ in seawater is different from other dissolved gases. It is  a species of dissolved inorganic carbon (the sum of [CO₂], [CO₃^-] and [CO₃^2-]), subject to quick re-equilibration with the other species. So, the correction of headspace concentration of CO2 MUST be fully explained with details.

3. What do you mean with 'the vertical distribution of temperature, salinity, density (T, S, P)'? The abbreviation of density should NOT be P. Usually the abbreviation P means pressure or depth.

4. I still remind the authors to unify the unit. For example, in Figure 3 only, two units of CH₄ (nM/dm³ and nM/L) were used. This can NOT be accepted for a publication. Moreover, usually nM = nmol/L, and μM = μmol/L. Neithor nM/dm³ nor nM/L is right.

5. Please provide a clean version.

6. Ref. [35] is in Russian.

 

Author Response

The authors are grateful to the referee for the attention to our work and comments. We agree with the reviewer and tried to take all comments into account. The following changes have been made to the text.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Thank you for reviewing the manuscript in some of the chapters.

I don’t agree about the structural information added. Considering the aim of the manuscript, it can be published. I invite you in the future to use all the data available, even if the data comes from an oil company. You can cancel time or depth scales or use a line drawing method. 

Author Response

Thank you for reviewing the manuscript and for your constructive suggestions. We agree with the reviewer that providing graphics would improve the data presentation and consider this for our future research in the area.

Round 3

Reviewer 1 Report

Comments and Suggestions for Authors

Most of my earlier concerns have been addressed, although I can't see the final version at this stage. Note that the final sentence of Section 4 MUST be removed, since no helium and hydrogen results were reported.

Author Response

Thank you for your detailed comments, they have significantly helped us to improve the text. I apologize that the file was not final. I am attaching the final version without marks. The last sentence on helium and hydrogen in section 4 has been deleted.

My co-authors and I wish you a happy New Year and Merry Christmas!