Review Reports - Depositional Structures and Their Reservoir Characteristics in the Wufeng–Longmaxi Shale in Southern Sichuan Basin, China

Round 1

Reviewer 1 Report

The theoretical and experimental background of MS are quite well and reported data have high scientific soundness; however, I cannot say it for the results section. The results section should be re-organized by the authors, because this section is quite long and section 5.1 report data should be reported firstly. Therefore, the authors should firstly report mineralogical compositions, TOC values and sedimentary petrography data of studied samples then report layering and related fundamental data of each layers. Secondly, in the section 5.3, the authors should report only the results of ICP-MS, and the reported data in the sections 5.3.1 and 5.3.2 could be combined with sections 6.1 and 6.2. In the scientific point of view, the porosity data has high soundness; nevertheless, the authors should consider that porosity is not only controlled by the depositional settings, especially meso- and micropores are more related with mineralogical and maceral compositions. The authors reported mineral porosity of the studied clayey and silty shales are similar, and I think this similarity could be related compositions of clayey matrixes reported in the related SEM-BSE images. Therefore, reporting brief and well-organized mineralogical compositions, XRD and SEM-EDS data is important. Because the clay matrixes of studied shale samples are illitic- and smectite composition, which have higher surface areas than other clay minerals. Therefore, the SEM-BSE images should be re-organized as like Fig. 23. More interestingly, the authors mentioned the differences on OM porosity. This again could be related with the depositional conditions; however, the reported graptolite reflectance of analyzed layers is higher than 2%, which could also effect on OM porosity. For instance, relatively higher graptolite reflectance displaying graptolite remains could display higher OM porosity, or the presence of bitumen grains could also influence on OM porosity. Additionally, the authors mentioned the cleat/fractures infilled with OM, such infillings and reported graptolite reflectance imply that the studied samples are mature enough for oil-generation or have immigrated oil in the studied beds. Therefore, I recommend to the authors try to check dispersed organic matter compositions of selected samples in order to have better understanding about OM porosity. Providing such data could also increase scientific soundness of the MS. I added several notes and some suggested studies in the revised MS. Overall, I could suggest a major correction, and re-consider the MS after the suggested corrections done.

Comments for author File: Comments.pdf

Author Response

The theoretical and experimental background of MS are quite well and reported data have high scientific soundness; however, I cannot say it for the results section.

The results section should be re-organized by the authors, because this section is quite long and section 5.1 report data should be reported firstly. Therefore, the authors should firstly report mineralogical compositions, TOC values and sedimentary petrography data of studied samples then report layering and related fundamental data of each layers.

[Answer-1-01]

Thanks. We revised the manuscript and describe mineralogical compositions, TOC values and sedimentary petrography data of studied samples firstly (See lines 253-258).

Secondly, in the section 5.3, the authors should report only the results of ICP-MS, and the reported data in the sections 5.3.1 and 5.3.2 could be combined with sections 6.1 and 6.2.

[Answer-1-02]

Thanks. We have re-organized the section 5.3 and combined the reported data in the sections 5.3.1 and 5.3.2 could be combined with sections 6.1 and 6.2 (See lines 469-530 and lines 743-831)

In the scientific point of view, the porosity data has high soundness; nevertheless, the authors should consider that porosity is not only controlled by the depositional settings, especially meso- and micropores are more related with mineralogical and maceral compositions. The authors reported mineral porosity of the studied clayey and silty shales are similar, and I think this similarity could be related compositions of clayey matrixes reported in the related SEM-BSE images. Therefore, reporting brief and well-organized mineralogical compositions, XRD and SEM-EDS data is important. Because the clay matrix of studied shale samples are illitic- and smectite composition, which have higher surface areas than other clay minerals. Therefore, the SEM-BSE images should be re-organized as like Fig. 23.

[Answer-1-03]

Thanks. We have re-organized the corresponding sections according to your suggestion (See “4 Results”) .

More interestingly, the authors mentioned the differences on OM porosity. This again could be related with the depositional conditions; however, the reported graptolite reflectance of analyzed layers is higher than 2%, which could also effect on OM porosity. For instance, relatively higher graptolite reflectance displaying graptolite remains could display higher OM porosity, or the presence of bitumen grains could also influence on OM porosity. Additionally, the authors mentioned the cleat/fractures infilled with OM, such infillings and reported graptolite reflectance imply that the studied samples are mature enough for oil-generation or have immigrated oil in the studied beds. Therefore, I recommend to the authors try to check dispersed organic matter compositions of selected samples in order to have better understanding about OM porosity.

[Answer-1-03]

Thanks for your suggestion. We have re-organized the corresponding sections. (See “4 Results”). By counting the types of organic pores, results showed that there were a large number of organic matter pores in pyrobitumen of the Wufeng- Longmaxi shale, with large diameter and strong connectivity,and solid kerogen had a little organic matter pores (Figure 18). Pores developed in organic matters contributed more to the total organic porosity. This paragraph has added to the Section 4.4.4. But the influence of shale maturity on the type of OM pore is not obvious. Relevant work is needed to carry out in the future.

Reviewer 2 Report

The manuscript entitled "Depositional structures and their reservoir characteristics in the Wufend-Longmaxi Shale in Southern Sichuan Basin, China", co-authored by Zhensheng Shi et al., is presenting a study of the mineralogical, sedimentological and transfer properties for shale gas exploration and production. The manuscript quality is overall quite good, but several points need to be improved to be ready for acceptance. The wording of several sentences is weird, and should be reworked.

In general, the introduction and section 4. should be merged.

In the results sections, the number of figures should be reduced to what is necessary.

In the discussion, it would be nice to extend it to sedimentary heterogeneity at the unit level, and its impact on migration pathways. Secondary, almost no mention of the deformation is integrated. The role of fault networks in such a process should be at least integrated into the discussion.

Thus, I recommend minor revisions.

The authors may find here attached the annotated pdf with specific comments.

Comments for author File: Comments.pdf

Author Response

[Answer-2-01]

Thanks. We have reworked the sentences carefully.

In general, the introduction and section 4 should be merged.

[Answer-2-02]

Thank you very much. We thank it is better not to merge the introduction and section 4.The reason is as follows. In the Introduction, we introduced the significance and existing problems of the Wufeng-Longmaxi shale research and the objectives of this paper. In the section 4, we introduced definitions of several shale terms. They are not very relevant and difficult to merge.

In the results sections, the number of figures should be reduced to what is necessary.

[Answer-2-03]

Thanks. We have reduced the number of figures to what is necessary.

In the discussion, it would be nice to extend it to sedimentary heterogeneity at the unit

level, and its impact on migration pathways.

[Answer-2-04]

Thank very much. We have revised the discussion and extent it to reservoir heterogeneity and impact on migration pathways.

Secondary, almost no mention of the deformation is integrated. The role of fault networks in such a process should be at least integrated into the discussion.

[Answer-2-05]

Thanks. We are very sorry to have no corresponding data. In the next research, we would do more work related to this topics.

Thus, I recommend minor revisions.

The authors may find here attached the annotated pdf with specific comments.

P.1 line 36-37: rephrase, key character for what? heterogeneity? pore network properties? architecture?

[Answer-2-06]

Thanks. We have reorganized the manuscript carefully.

P2 line 46: what about non phanerozoic black shales?

[Answer-2-07]

Thanks. We have revised the manuscript carefully.

P2 line 49: what do you mean here with biologic evolution?

[Answer-2-08]

Thanks. We have deleted the terms in the manuscript.

P2 line 51: please explain which link there is between mineral composition and depositional structure?

[Answer-2-09]

Thanks. We have reorganized the expression in the manuscript.

P2 line 54: rephrase, OM and clay are abundant in XX lamina

[Answer-2-10]

Thanks. We have replaced the sentences.

P2 line 56-59: this is not evident, please add ref. Bright laminae can be fully cemented, thus not permeable at all.

[Answer-2-11]

Thanks. We have added corresponding reference. In addition, we have reorganized the sentence.

P2 line 63-65: add ref, and rephrase, sentence is too complex.

[Answer-2-12]

Thanks. We have added corresponding reference and rewrite the sentence.

P2 line 66-70: please add reference.

[Answer-2-13]

Thanks. We have added corresponding reference.

P2 line 72-74: please be homogeneous with permeability units, either m² or mD, but be consistent.

[Answer-2-14]

Thanks. We have unified the units of permeability.

Figure 1: uplift?.

[Answer-2-15]

Thanks. We have revised the Figure 1 carefully.

P7 “4 Definition” should be rather presented in the introduction.

[Answer-2-16]

Thanks. We have presented the “4 Definition” into the “Introduction”.

Figure 4: poorly visible.

[Answer-2-17]

Thanks. We have changed the corresponding images.

Figure 5: quality of the images is poor, please check.

[Answer-2-18]

Thanks. We have changed the corresponding images.

Figure 9: should be presented before the thin sections images, and with a more developed description?

[Answer-2-19]

Thanks. In this manuscript, we describe the lamina, laminasets, and beds firstly. Then, we describe the characteristics of outcrops and cores. We think it is more suitable to place the Figure 9 after thin section images.

Figure 10: please add how many samples were measured, is this the overlap of the differences measures?

[Answer-2-20]

Thanks. We have added the number of samples measured and revised the corresponding sentences.

P10 line 564: clarify fractures and microfractures definition.

[Answer-2-21]

Thanks. We have clarified the definitions of fractures and microfractures.

Figure 17: what does this figure shows that other previous figures do not contain? is this figure necessary?

[Answer-2-22]

Thanks. We have deleted this figure.

Figure 23: should be placed in the results section, and not in the discussion?

[Answer-2-23]

Thanks. We have placed the figure in the results section.

Reviewer 3 Report

Dear Authors,

The manuscript has no potential to improve. I have added more comments bellow.

Some General Comments:

The text is lengthy and unnecessarily long for such a modest conclusion. Please reformat the text and suggest re-uploading.
It is necessary to shorten the text and rearrange the whole paper.
Please cite the paper in “[ ]” when citing authors in the text. Check the full text.
Please use SI units throughout the paper. Check the full text.
Add more concrete results (numbers) that confirm the hypotheses in the conclusions.
Correct the "References" section in accordance with the "Instructions for Authors".

Line-by-line comments:

L88 “180, 000” -> space

L143 The introductory text for " 3. Samples and methods" is missing.

L303 The introductory text for " 5. Result" is missing.

L513 The introductory text for " 5.3. Geochemical Parameters and Sedimentation Rate" is missing.

L564 The introductory text for " 5.4. Fractures and Nanopores" is missing.

L738 The introductory text for "5.5. Physical Properties" is missing.

L775 The introductory text for " 6. Discussion" is missing.

L868 The introductory text for " 6.3. Influence of Depositional Structures on Porosity and Permeability" is missing.

So unfortunately, my recommendation is to reject the article as it is outside the scope of the journal.

Regards,

Reviewer

Author Response

Dear Authors,

The manuscript has no potential to improve. I have added more comments bellow.

Some General Comments:

The text is lengthy and unnecessarily long for such a modest conclusion. Please reformat the text and suggest re-uploading.

[Answer-3-01]

Thanks. We have removed 3 unnecessary figures (Figure 8, Figure 10, and Figure 15) and Table 1 and corresponding texts.

It is necessary to shorten the text and rearrange the whole paper.

[Answer-3-02]

Thanks. We have shorten the text from 40 pages to 33 pages and rearranged the whole paper (See the Manuscript).

Please use SI units throughout the paper. Check the full text.

[Answer-3-03]

Thanks. We have used the SI units throughout the paper (for example, replaced the mD with ×10^-3μm²).

Add more concrete results (numbers) that confirm the hypotheses in the conclusions.

[Answer-3-04]

Thanks. We have used added more concrete numbers to confirm the hypotheses in the conclusions.

Correct the "References" section in accordance with the "Instructions for Authors".

[Answer-3-05]

Thanks. We have checked the “References” section in accordance with the “Instructions for Authors”.

Line-by-line comments:

L88 “180, 000” -> space

[Answer-3-06]

Thanks. We have revised the manuscript.

L143 The introductory text for " 3. Samples and methods" is missing.

[Answer-3-07]

Thanks. Introductory text is added for “3. Samples and methods”. (See the manuscript )

L303 The introductory text for " 5. Result" is missing.

[Answer-3-08]

Thanks. Introductory text is added for “4. Results”. (See the manuscript)

L513 The introductory text for " 5.3. Geochemical Parameters and Sedimentation Rate" is missing.

[Answer-3-09]

Thanks. Introductory text is added for “4.3. Geochemical Parameters and Sedimentation Rate”. (See the manuscript)

L564 The introductory text for " 5.4. Fractures and Nanopores" is missing.

[Answer-3-10]

Thanks. Introductory text is added for “4.4. Fractures and Nanopores”. (See the manuscript)

L738 The introductory text for "5.5. Physical Properties" is missing.

[Answer-3-11]

Thanks. Introductory text is added for “4.5. Physical Properties”. (See the manuscript)

L775 The introductory text for " 6. Discussion" is missing.

[Answer-3-12]

Thanks. Introductory text is added for “5. Discussion”. (See the manuscript)

L868 The introductory text for " 6.3. Influence of Depositional Structures on Porosity and Permeability" is missing.

[Answer-3-13]

Thanks. Introductory text is added for “ 5.3. Influence of Depositional Structures on Porosity and Permeability”. (See the manuscript)

Round 2

Reviewer 1 Report

The authors did not make most of suggested corrections and I could not find any clear copy of the revised MS. The MS is still too long and not easy to follow. The authors should check my previous comments and the attached MS in this review. I would like to reconsider the MS after the authros provide clear copy and corrections done. I recommend again major revision.

Comments for author File: Comments.pdf

Author Response

Review 1:

The authors did not make most of suggested corrections and I could not find any clear copy of the revised MS. The MS is still too long and not easy to follow.

[Answer-1-1]

Thanks very much. We re-organized the manuscript carefully. In this version, we deleted Figure 8, 10, 15-20, 23 and integrated their contents into the “Results” section. In addition, we re-organized sections “5.4.2. Nanopores types”, “5.4.3. Pore−Size Distribution”, “5.4.4. Porosity Distribution”, and integrated their contents into section “4.5. Nanopores and porosity distribution”.

The authors should check my previous comments and the attached MS in this review.

[Answer-1-2]

Thanks very much. We have checked the previous comments (See following answers) and the attached MS in this review.

I would like to reconsider the MS after the authors provide clear copy and corrections done. I recommend again major revision.

Previous comments is as follows:

The theoretical and experimental background of MS are quite well and reported data have high scientific soundness; however, I cannot say it for the results section.

The results section should be re-organized by the authors, because this section is quite long.

[Answer-1-01]

Thanks very much. We re-organized the manuscript carefully. In this version, we deleted Figure 8, 10, 15-20, 23 and integrated their contents into the “Results” section. In addition, we re-organized sections “5.4.2. Nanopores types”, “5.4.3. Pore−Size Distribution”, “5.4.4. Porosity Distribution”, and integrated their contents into section “4.5. Nanopores and porosity distribution”.

and section 5.1 report data should be reported firstly. Therefore, the authors should firstly report mineralogical compositions, TOC values and sedimentary petrography data of studied samples then report layering and related fundamental data of each layers.

[Answer-1-02]

Thanks very much. In this revision, we first reported mineralogical compositions, TOC contents and sedimentary petrography data of studies samples, then we reported their layering and related fundamental data of each layers. We hope this revision is well organized and easy to understand (See section 4.1. and section 4.2.).

Secondly, in the section 5.3, the authors should report only the results of ICP-MS, and the reported data in the sections 5.3.1 and 5.3.2 could be combined with sections 6.1 and 6.2.

[Answer-1-03]

Thanks very much. In the section 4.3, we have deleted the discussion parts in the section and only report the results of ICP-MS. We have combined the related discussion in sections 5.1 and 5.2. We hope this revision can help to improve the concision and clarity of this manuscript.

[Answer-1-04]

Thanks very much. We all agree that porosity is not only controlled by the depositional setting, especially meso- and micro-pores are more related with mineralogical and maceral compositions. In this study, we found that clayey lamina and silty lamina has similar porosity. But the mineral compositions and nanopore types differ greatly. In the Wufeng-Longmaxi shale, clayey laminae is mainly composed of quartz (70%~90%), OM (10%~20%), clay minerals (3%~5%), dolomite (2%~3%), calcite (1%~2%), and other minerals (1%~2%). The minerals are dominantly clay-sized with grain size less than 3.9μm. In contrast, silty laminae is dominantly composed of dolomite (25%~35%), calcite (25%~35%), quartz (10%~20%), pyrite (3%~5%), and OM (5%~10%) and their minerals are dominantly silt-sized with grain size in the range 4 φ to 8 φ. Both in clayey lamina and in silty lamina, clay minerals contents are very limited. In addition, clayey lamina abound in organic pores whereas silty lamina abound in inorganic pores. Organic pores are mainly located in OM which is dominantly dispersed among quartz, carbonate minerals, and pyrite particles, with minor OM combined with clay minerals forming organomineralic aggregates. Inorganic pores are mainly inter-particle pores and inter-particle dissolved pores which are located among mineral particles such as calcite, quartz, and dolomite. In addition, clayey lamina have 2-3 times more organic pores than silty lamina, whereas silty lamina have 3–4 times more inter-particle pores and 1−2 times more inter-particle dissolved pores than clayey lamina. Furthermore, clayey lamina have relatively less microfractures than silty lamina. In general, for the Wufeng-Longmaxi shale, porosity is less related with clayey matrix in the Wufeng-Longmaxi shale.

Therefore, reporting brief and well-organized mineralogical compositions, XRD and SEM-EDS data is important. Because the clay matrix of studied shale samples are illitic- and smectite composition, which have higher surface areas than other clay minerals. Therefore, the SEM-BSE images should be re-organized as like Fig. 23.

[Answer-1-05]

Thanks very much. We described mineral types and their contents in detail based on XRD and SEM-EDS data in this revision (See “4.1. Layering”, Figure 5, Figure 6, and Table A1). In addition, we re-organized Figure 5 as like Figure 23.

[Answer-1-03]

Thanks for your suggestion. We have re-organized the corresponding sections. (See “4 Results”). But the influence of shale maturity on the type of OM pore is not obvious. By counting the types of organic pores, results showed that there were a large number of organic matter pores in pyrobitumen of the Wufeng- Longmaxi shale, with large diameter and strong connectivity,and solid kerogen had a little organic matter pores (Figure 13). Pores developed in organic matters contributed more to the total organic porosity. This paragraph has added to the Section 4.5.

Providing such data could also increase scientific soundness of the MS. I added several notes and some suggested studies in the revised MS. Overall, I could suggest a minor correction.

Author Response File: Author Response.doc

Reviewer 3 Report

Dear authors,

The corrections I requested for the paper " Depositional structures and their reservoir characteristics in the Wufeng–Longmaxi shale in southern Sichuan Basin, China" were successfully made. The quality and clarity of the text and results has been significantly improved. The scientific contribution is visible and applicable and, following the proposed corrections, future research on this topic can be compared and developed. I wish successful further research.

Best regards,

Reviewer

Author Response

Thanks for your suggestion. In the future, we decided to do some research in the following 3 aspects:

(1) Classification of shale laminae. Currently, lamina is subdivided dominantly based on grain size or mineral components of shale. Both classifications have their own advantages. Grain size classification can effectively reflect the hydrodynamic conditions of laminae formation whereas mineral components classification can effectively reflect provenance characteristics. In the future, grain size and mineral components classifications should be integrated to solve problems according to the particularity of the study area.

(2) Assemblage styles of lamina and their influences on porosity and permeability of shale. Lamina assemblage styles involves factors including inter-layering types of lamina, thickness of single lamina, lamina continuity, lamina inclination and so on. Subtle changes of their factors will have a great impact on shale reservoir quality. Therefore, it is necessary to carry out systematic numerical simulations to explore the influence of their factors on the porosity and permeability of shale.

(3) Depositional structure types and their genetic mechanism. Shale is widely distributed on earth and depositional structures are of great significance to discussing the formation environment of shale. Therefore, depositional structure types are very important for future research. Based on flume and numerical simulations, the process of erosion, transport and deposition of fine-grained sediments is reproduced to interpret the formation conditions and environments of various depositional structures.

These paragraghs are added to the 5.5. in the manuscript.

Round 3

Reviewer 1 Report

The revised MS is far better than the previous revised MS. The scientific discussion is quite good, and the authors should combine element proxies (e.g., Sr/Ba or U/Th) and mineralogical compositions with discussions about climate and redox conditions in Section 5.1. Furthermore, section 5.5 should be combined with the conclusion section. The authors could report it in a very brief way in the conclusion. Although the MS reported an enormous amount of data, it could be written in a shorter way than the current format. because MS's current format is still too long and difficult to follow. Furthermore, some paragraphs are repeated three times. The authors should re-check their revised MS. I think this problem is due to the review mode of MS Office. Therefore, it is always better to report clean copy together with copy with tracked changes. Overall, I could advise moderate minor revision.

Comments for author File: Comments.pdf

Author Response

Dear MS. Kristina Simunovic,

We would like to thank you very much for the kind effort on reviewing our manuscript submitted to Energies, and we are grateful for the constructive comments raised by the reviewers and editor. We have revised the manuscript carefully. The following is a summary of our replies with respect to the reviewers' comments. We list the reviewers' comments followed by our answers. We prefix the answers with the term “[Answer-x-xx]”.

Best Regards,

The Authors

Review 1:

[Answer-1-1]

Thanks very much. We have combined element proxies such as CIA, U/Th, Ni/Co, and V/Cr with discussions about climate and redox conditions in Section 5.1.

Furthermore, section 5.5 should be combined with the conclusion section. The authors could report it in a very brief way in the conclusion.

[Answer-1-2]

Thanks very much. We have combined section 5.5 with the conclusion section briefly.

Although the MS reported an enormous amount of data, it could be written in a shorter way than the current format. because MS's current format is still too long and difficult to follow.

[Answer-1-3]

Thanks very much. We have deleted the unnecessary and repeated words and figures in the manuscript. The total pages of current version is 29 pages. We hope this version is refined and concise.

Furthermore, some paragraphs are repeated three times. The authors should re-check their revised MS. I think this problem is due to the review mode of MS Office. Therefore, it is always better to report clean copy together with copy with tracked changes. Overall, I could advise moderate minor revision.

[Answer-1-4]

Thanks very much. We have re-checked the MS. In addition, we have uploaded clean copy together with tracked changes.

As for the section 4.5, we should answer this question: Ok it could be! but it is not clear that this data belongs to authors or from a previous study! Please clarify it!

[Answer-1-5]

Thanks very much. We draw this conclusion ourselves based upon systematical analysis of SEM photos.

Author Response File: Author Response.doc