Shale Gas Exploration and Development Potential Analysis of Lower Cambrian Niutitang Formation and Lower Silurian Longmaxi Formation in Northwestern Hunan, South China, Based on Organic Matter Pore Evolution Characteristics
Round 1
Reviewer 1 Report
Numerous shortcomings are evident within the content of this paper, spanning across various sections. These issues can be categorized and addressed as follows:
1) Similarities with previous publications
Remarkable resemblances are observed between this paper's methodology, chosen sample sets, and ensuing experimental outcomes in comparison to a previously published work, duly cited within this document. The paper authored by Wang P, Yao S, Jin C, et al. titled "Key reservoir parameter for effective exploration and development of high over-matured marine shales: A case study from the Cambrian Niutitang formation and the Silurian Longmaxi formation, South China" (Marine and Petroleum Geology, 2020, 121: 104619)” showcases similarities.
2) References issues
Various issues concerning references have come to light, encompassing both missing references and inappropriately incorporated in-text citations. Instances of these issues can be found in the following text ranges: lines 55-57; 65-68; 69-71; 97-102; 105-110; and 147-153.
3) Methodological Implications
The analysis of the pore structure emerges as predominantly observational and qualitative in nature. This could be markedly enhanced by integrating pore size distribution metrics, particularly when addressing pores present within the organic matter. Critically, the rationale behind the choice of method for characterizing the organic matter remains inadequately substantiated. Aside from the assessment of maturity, a discernible lack of conceptual grounding is apparent, impeding the validation of the information gleaned in line with the stipulated hypothesis.
4) Hypothetical Framework and Experimental Evaluation
The mentioned hypothesis states; “The two sets of shales have distinct thermal maturities as a result of their various tectonic evolution process. Which could have an impact on the pore structure and gas storage capacity of the shales”. The test of the hypothesis was carried out according to the following paragraph. “The evolution of the pore structure between the two sets of shales is compared with the regional tectonic evolution characteristic, which can shed light on how the tectonic evolution has an impact on the pore structure and storage capacity.”
Throughout the manuscript, the explication of the hypothesis testing remains conspicuously vague, failing to establish a definitive conclusion regarding the correlation between tectonic activity and the typology and spatial dispersion of pores within both shale and organic matter. Notably absent are well-defined, original insights that could establish novelty in the context of this work, subsequently raising questions about its suitability for publication.
In light of these concerns, it is advised that further substantial revisions be undertaken to address the identified shortcomings and bolster the scholarly merit of this manuscript before considering its suitability for publication.
Author Response
Response to comments from Reviewer #1
Reply :
Thanks for your comments. Your proposed suggestions are of important significance to us.
Reviewer 1：
Comments and Suggestions for Authors
Numerous shortcomings are evident within the content of this paper, spanning across various sections. These issues can be categorized and addressed as follows:
1) Similarities with previous publications
Remarkable resemblances are observed between this paper's methodology, chosen sample sets, and ensuing experimental outcomes in comparison to a previously published work, duly cited within this document. The paper authored by Wang P, Yao S, Jin C, et al. titled "Key reservoir parameter for effective exploration and development of high over-matured marine shales: A case study from the Cambrian Niutitang formation and the Silurian Longmaxi formation, South China" (Marine and Petroleum Geology, 2020, 121: 104619)” showcases similarities.
Thank you very much for your valuable revision suggestions. The paper authored by Wang P, Yao S, Jin C, et al. titled "Key reservoir parameter for effective exploration and development of high over-matured marine shales: A case study from the Cambrian Niutitang formation and the Silurian Longmaxi formation, South China" (Marine and Petroleum Geology, 2020, 121: 104619) mainly discusses the difference of OM pore structure in the Lower Cambrian Niutitang Formation and the Lower Silurian Longmaxi Formation shale in Sichuan Basin and its surrounding areas, and proposes that the shale has evolved due to deep burial, resulting in the graphitization of organic matter and the disappearance of a large number of OM pores.
There are several differences between this manuscript and the above paper. First, this manuscript compares the shales of the Lower Cambrian Niutitang Formation and the Lower Silurian Longmaxi Formation in northwest Hunan. Although it also discusses the difference in burial depth between the two sets of areas caused by regional tectonic action in northwest Hunan, it is the main reason for the great difference in OM pores structure. But it is more the extension of the previous paper that proves that the above ideas are also applicable and universal outside China's Sichuan Basin;
Secondly, in the discussion part of this paper, by comparing the OM pores structure in the Lower Silurian Longmaxi Formation shale in Changning area, Sichuan Basin, it is proposed that the burial time effect may be the main reason affecting the difference of OM pores structure and hydrocarbon generation gas in the same set of shale, which provides a new research idea for shale gas accumulation in the future. It also provides the foundation for the follow-up continuous research.
2) References issues
Various issues concerning references have come to light, encompassing both missing references and inappropriately incorporated in-text citations. Instances of these issues can be found in the following text ranges: lines 55-57; 65-68; 69-71; 97-102; 105-110; and 147-153.
We appreciate your constructive suggestion. Issues concerning references have been revised in the manuscript.
3) Methodological Implications
The analysis of the pore structure emerges as predominantly observational and qualitative in nature. This could be markedly enhanced by integrating pore size distribution metrics, particularly when addressing pores present within the organic matter. Critically, the rationale behind the choice of method for characterizing the organic matter remains inadequately substantiated. Aside from the assessment of maturity, a discernible lack of conceptual grounding is apparent, impeding the validation of the information gleaned in line with the stipulated hypothesis.
Thank you for your valuable revision suggestions. In the revised draft, we used ImageJ software to digitize the SEM images, and calculated the size and quantity distribution characteristics of OM pores diameters. The results show that: The OM pores diameter in the Lower Cambrian Niutitang Formation shales in northwest Hunan is the smallest, and the OM pores diameter in the Lower Silurian Longmaxi Formation shales in northwest Hunan is larger than that of the Niutitang Formation shales in northwest Hunan, and the number of OM pores per unit view is larger than that of the Niutitang Formation shales. In contrast, the Longmaxi Formation shale in the Changning area of Sichuan Basin has the largest number of pores and the largest pore diameter, which indirectly indicates that for shale with similar burial depth, a longer period of evolution is more conducive to the development of OM pores. Please refer to the revised manuscript for details.
4) Hypothetical Framework and Experimental Evaluation
The mentioned hypothesis states; “The two sets of shales have distinct thermal maturities as a result of their various tectonic evolution process. Which could have an impact on the pore structure and gas storage capacity of the shales”. The test of the hypothesis was carried out according to the following paragraph. “The evolution of the pore structure between the two sets of shales is compared with the regional tectonic evolution characteristic, which can shed light on how the tectonic evolution has an impact on the pore structure and storage capacity.”
Throughout the manuscript, the explication of the hypothesis testing remains conspicuously vague, failing to establish a definitive conclusion regarding the correlation between tectonic activity and the typology and spatial dispersion of pores within both shale and organic matter. Notably absent are well-defined, original insights that could establish novelty in the context of this work, subsequently raising questions about its suitability for publication.
Thank you for your valuable revision suggestions. In fact, there have been many studies on the influence of tectonic evolution on the OM pores structure in shale. In combination with a large number of literatures and studies on OM pores in the Cambrian Niutitang shale in this region, and combined with the regional structural and geological situation, we have summarized the complete evolutionary process of the formation and eventual disappearance of OM pores in shale. It is also suggested that the evolution time effect plays an important role in controlling the formation of OM pores. The thermal evolution degree of the Longmaxi Formation shale in Changning area is similar to that of the Longmaxi Formation shale in northwest Hunan Province, but the evolution time of the Longmaxi Formation shale in Changning area is long, reaching 200Ma, while the evolution time of the Longmaxi Formation shale in northwest Hunan is only 100Ma. The OM pores structure of Longmaxi shale in Changning area is much better than that of Longmaxi shale in northwest Hunan. At the same time, we have obtained the OM pores evolution process of different well locations, different regions and different shale formations. Please refer to the revised manuscript for details.
In light of these concerns, it is advised that further substantial revisions be undertaken to address the identified shortcomings and bolster the scholarly merit of this manuscript before considering its suitability for publication.
Author Response File: 
Author Response.doc
Reviewer 2 Report
1- Line 13-14: The authors should provide very brief introduction about the shale gas potentiality in these two formations. Also, highlight the ages of these rock units.
2- Line 23: Small number of what?
3- Lines 24-25: Very general statements and should not be included in the abstract.
4- The abstract should be rewritten with focus on the principle findings of the current study.
5- Line 44: Which data? Do you mean production data?
6- Line 50: “the enrichment and reservoir formation of hydrocarbon gas” confusing. Please rephrase.
7- Line 56: What is meant by good tectonic preservation tectonics? Please clarify.
8- Line 58-59: “In the investigation of the Longmaxi and Niutitang shales in western Hunan, the authors discovered that aside from significant differences in thermal maturity,…..” poor wording. Who are those authors? You should say previous studies if this is not from the results of the current work.
9- Line 61: “The Longmaxi shale has one that is between 2.2%” Has one of what? Again poor wording.
10- Line 70: “comparable tectonic preservation conditions” please explain.
11- Lines 71-73: You must add relevant references (e.g. https://doi.org/10.1016/j.marpetgeo.2022.105643; 10.1007/s11631-022-00576-5).
12- Lines 75-77: “The Longmaxi Formation in northwest Hunan still produces substantially less gas than the Longmaxi Formation in the Cahngning-Weiyuan region of the Sichuan Basin, south China (Fig. 1)” Figure 1 does not show that. This should be justified by production data.
13- Line 126: “Detailed stratigraphic log graph of the Paleozoic in Hunan, southern China (showing Longmaxi and Niutitang formations, revised from [33]).” What do you mean by revised from? Do you mean modified from? If yes please mention the modifications that were done.
14- Line 175: “In northwest Hunan, samples of the Longmaxi shale (SY1 well ) and Niutitang shale (CY1 well) are examined to better understand their reservoir, geochemical, and OM pore properties (Fig.1 and Table 2).” Please mention the examination techniques.
15- Lines 179-181: “Prior to testing, shale samples were ground into 0.15 mm-sized particles, removed the carbonate by dissolving in diluted hydrochloric acid, frequently rinsed with new water, and dried” poor English. Please rephrase. Also illustrate how the samples were dried.
16- Lines 183: “An important measure of the maturity of oil and gas reservoirs is the vitrinite reflectance” I think you mean oil and gas source rocks not reservoirs.
17- Line 207: “Solid kerogen and pyrobitumen are the two main types of organic matter (OM) found in shale.” Please refer to a figure to show that in your samples.
18- Lines 207-222: Results section should only contain the findings of the current study. Do not mix results with discussion. This part should be moved to the discussion section.
19- Line 236: R0 what?
20- Lines 244-245: “and its organic matter's physical and chemical character- istics are similar to those of graphite” what are these physical and chemical characteristics?
21- Line 249: Mineral what? Please use appropriate headings.
22- Lines 249-256: I wonder how the authors have determined the mineralogy of the samples. This has not been illustrated in the methodology section.
English is poor and needs moderate to major language polishing.
Author Response
Response to comments from Reviewer #2
Reply :
Thanks for your comments. Your proposed suggestions are of important significance to us.
Reviewer 2：
Comments and Suggestions for Authors
1-Line 13-14: The authors should provide very brief introduction about the shale gas potentiality in these two formations. Also, highlight the ages of these rock units.
2-Line 23: Small number of what?
3-Lines 24-25: Very general statements and should not be included in the abstract.
4-The abstract should be rewritten with focus on the principle findings of the current study.
We appreciate your constructive suggestion. The abstract has been rewritten according to the comments 1-4. Changes were marked with yellow background.
5-Line 44: Which data? Do you mean production data?
The “production data” do, however, clearly demonstrate that the Longmaxi Formation produces more shale gas than the Niutitang Formation. Changes were marked with yellow background.
6-Line 50: “the enrichment and reservoir formation of hydrocarbon gas” confusing. Please rephrase.
The sentence has been rewritten and was marked with yellow background in the manuscript.
7-Line 56: What is meant by good tectonic preservation tectonics? Please clarify.
“Good tectonic preservation conditions” means shallow burial at early stage and rapid settlement at late stage. Changes were marked with yellow background.
8- Line 58-59: “In the investigation of the Longmaxi and Niutitang shales in western Hunan, the authors discovered that aside from significant differences in thermal maturity,…..” poor wording. Who are those authors? You should say previous studies if this is not from the results of the current work.
The sentence has been rewritten and was marked with yellow background in the manuscript.
9-Line 61: “The Longmaxi shale has one that is between 2.2%” Has one of what? Again poor wording.
The sentence has been rewritten and was marked with yellow background in the manuscript.
10-Line 70: “comparable tectonic preservation conditions” please explain.
The statement has been replaced with a more precise expression.
“Previous studies stated that, Longmaxi and Niutitang shales in western Hunan present similar depositional settings and mineral components.”
11-Lines 71-73: You must add relevant references (e.g. https://doi.org/10.1016/j.marpetgeo.2022.105643; 1007/s11631-022-00576-5)
According to the suggestion, the relevant literature has been cited.
12-Lines 75-77: “The Longmaxi Formation in northwest Hunan still produces substantially less gas than the Longmaxi Formation in the Cahngning-Weiyuan region of the Sichuan Basin, south China (Fig. 1)” Figure 1 does not show that. This should be justified by production data.
We do believe that production data is the best evidence for the differences between the two shales. However, the publication of the paper must be subject to internal confidential review, and the exact location and output of the region cannot be publicly published. Thank you for understanding.
13-Line 126: “Detailed stratigraphic log graph of the Paleozoic in Hunan, southern China (showing Longmaxi and Niutitang formations, revised from [33]).” What do you mean by revised from? Do you mean modified from? If yes please mention the modifications that were done.
The illustration has been modifies according to the comment.
14-Line 175: “In northwest Hunan, samples of the Longmaxi shale (SY1 well ) and Niutitang shale (CY1 well) are examined to better understand their reservoir, geochemical, and OM pore properties (Fig.1 and Table 2).” Please mention the examination techniques.
The examination techniques have been added and marked with yellow background in the manuscript.
15-Lines 179-181: “Prior to testing, shale samples were ground into 0.15 mm-sized particles, removed the carbonate by dissolving in diluted hydrochloric acid, frequently rinsed with new water, and dried” poor English. Please rephrase. Also illustrate how the samples were dried.
The method of “Total organic carbon content (TOC) test” has been reorganized and marked with yellow background in the manuscript.
16-Lines 183: “An important measure of the maturity of oil and gas reservoirs is the vitrinite reflectance” I think you mean oil and gas source rocks not reservoirs.
We do agree with the comment and the expression has been revised.
17- Line 207: “Solid kerogen and pyrobitumen are the two main types of organic matter (OM) found in shale.” Please refer to a figure to show that in your samples.
Solid kerogen belongs to the product of low maturity stage of organic matter. All the samples in this paper are at high maturity stage, and Solid kerogen cannot be observed. pyrobitumen is a highly mature product of organic matter and is indicated in Figure 7-8.
17-Lines 207-222: Results section should only contain the findings of the current study. Do not mix results with discussion. This part should be moved to the discussion section.
This part has been moved to the discussion section. We appreciate your constructive suggestion. It is really helpful to emphasize the points of this paper.
18-Line 236: R0 what?
An appropriate heading has been used in the manuscript.
19-Lines 244-245: “and its organic matter's physical and chemical character- istics are similar to those of graphite” what are these physical and chemical characteristics?
The statement has been replaced with a more precise expression.
20-Line 249: Mineral what? Please use appropriate headings.
An appropriate heading has been used in the manuscript.
21-Lines 249-256: I wonder how the authors have determined the mineralogy of the samples. This has not been illustrated in the methodology section.
Mineral compositions were measured using a Bruker D8 DISCOVER diffractometer equipped with a Cu x-ray tube and a Sol-X solid-state detector. The detailed method has been added in the Section 3.
Author Response File: Author Response.doc
Reviewer 3 Report
Dear authors of the manuscript "Shale gas exploration and development potential analysis of Lower Cambrian Niutitang Formation and Lower Silurian Longmaxi Formation in Northwestern Hunan, South China".
I read your manuscript with interest. I liked it and my comments can be found in the appendix to this letter.
Regards,
reviewer
Comments for author File: 
Comments.pdf
Attention should be paid to technical words related to professional terms
Author Response
Response to comments from Reviewer #3
Reply :
Thanks for your comments. Your proposed suggestions are of important significance to us.
On the other hand, some comments (point 2-8) are confused to us. Like “The viscosity of the fracturing fluid/fracking simulator/hydraulic fracturing techniques” are not the relative points in this paper.
For the recommended reference, we will read it carefully and add it where it fits in the manuscript.
Reviewer 3：
Comments and Suggestions for Authors
Point 1: The abstract needs to be rewritten. It will not be clear to the reader what kind of research the authors conducted in the paper. The purpose of the paper and the result
should be presented.
We appreciate your constructive suggestion. The abstract has been rewritten according to the comment. Changes were marked with yellow background.
Round 2
Reviewer 1 Report
Some aspects of the paper have been addressed. However, some key issues remain.
1) The authors say:
"First, this manuscript compares the shales of the Lower Cambrian Niutitang Formation and the Lower Silurian Longmaxi Formation in northwest Hunan. Although it also discusses the difference in burial depth between the two sets of areas caused by regional tectonic action in northwest Hunan, it is the main reason for the great difference in OM pores structure. But it is more the extension of the previous paper that proves that the above ideas are also applicable and universal outside China's Sichuan Basin;
Secondly, in the discussion part of this paper, by comparing the OM pores structure in the Lower Silurian Longmaxi Formation shale in Changning area, Sichuan Basin, it is proposed that the burial time effect may be the main reason affecting the difference of OM pores structure and hydrocarbon generation gas in the same set of shale, which provides a new research idea for shale gas accumulation in the future. It also provides the foundation for the follow-up continuous research."
The author mentions that this work is a continuation of the previously mentioned work. However, a significant part of the data shown here shows similarities with the previous work. Raising the question about the Novelty.
The author notes that the current work represents an extension of their prior paper. Nevertheless, a substantial proportion of the data showcased herein reveals parallels with their preceding research, thereby engendering a pivotal inquiry into the facet of novelty.
2) The most significant findings in this study relate to the analysis of organic matter pore structures. However, this analysis is mainly based on observation and qualitative assessment. The absence of a quantitative analysis, particularly in terms of characterizing pore structures (i.e using pore sizes and distribution), makes it challenging to provide strong statistical support for testing the proposed hypotheses. One example of the importance of using quantitative/semiquantitative analysis for analyzing porosity in shale can be found in.
Medina-Rodriguez, B. X., Frouté, L., Alvarado, V., & Kovscek, A. R. (2023). Multimodal study of the impact of stimulation pH on shale pore structure, with an emphasis on organics behavior in alkaline environments. Fuel, 331, 125649.
Incorporating quantitative or semi-quantitative analyses would notably enhance the overall quality of the paper.
Author Response
Response to comments from Reviewer #1
Reply :
Thanks for your comments. Your proposed suggestions are of important significance to us.
Some aspects of the paper have been addressed. However, some key issues remain.
1) The authors say:
"First, this manuscript compares the shales of the Lower Cambrian Niutitang Formation and the Lower Silurian Longmaxi Formation in northwest Hunan. Although it also discusses the difference in burial depth between the two sets of areas caused by regional tectonic action in northwest Hunan, it is the main reason for the great difference in OM pores structure. But it is more the extension of the previous paper that proves that the above ideas are also applicable and universal outside China's Sichuan Basin;
Secondly, in the discussion part of this paper, by comparing the OM pores structure in the Lower Silurian Longmaxi Formation shale in Changning area, Sichuan Basin, it is proposed that the burial time effect may be the main reason affecting the difference of OM pores structure and hydrocarbon generation gas in the same set of shale, which provides a new research idea for shale gas accumulation in the future. It also provides the foundation for the follow-up continuous research."
The author mentions that this work is a continuation of the previously mentioned work. However, a significant part of the data shown here shows similarities with the previous work. Raising the question about the Novelty.
The author notes that the current work represents an extension of their prior paper. Nevertheless, a substantial proportion of the data showcased herein reveals parallels with their preceding research, thereby engendering a pivotal inquiry into the facet of novelty.
First, the MPG paper (Marine and Petroleum Geology, 2020, 121: 104619) discusses the differences between the two shale reservoirs in Chongqing and Hubei (the Lower Cambrian Niutitang Formation shale in Yichang) Area, whereas this manuscript uses the Longmaxi Formation shale in Changning, Sichuan, as a comparative unit, and focuses on the development of organic matter pore in Longmaxi and Niutitang Formations shale in Hunan area. There is a huge difference in the study area and argument, compared to the previous paper.
Meanwhile, in this manuscript, we found that the Niutitang Formation shale lacks paleotectonic uplift zones to maintain a moderate degree of thermal evolution, while the Longmaxi Formation shale uplifted at an earlier period, losing the sealing conditions to preserve the porosity. Therefore, the storage capacities of both shales in Hunan is poor compared to the Longmaxi Formation shale in Changning, Sichuan. Due to the complexity of tectonic evolution in China, different regions/areas need their matching study. This paper can provide a reasonable explanation for low production of shale gas in Hunan Area, in aspect of paleotectonic influence on shale reservoir.
2) The most significant findings in this study relate to the analysis of organic matter pore structures. However, this analysis is mainly based on observation and qualitative assessment. The absence of a quantitative analysis, particularly in terms of characterizing pore structures (i.e using pore sizes and distribution), makes it challenging to provide strong statistical support for testing the proposed hypotheses. One example of the importance of using quantitative/semiquantitative analysis for analyzing porosity in shale can be found in.
Medina-Rodriguez, B. X., Frouté, L., Alvarado, V., & Kovscek, A. R. (2023). Multimodal study of the impact of stimulation pH on shale pore structure, with an emphasis on organics behavior in alkaline environments. Fuel, 331, 125649.
Incorporating quantitative or semi-quantitative analyses would notably enhance the overall quality of the paper.
The quantitative comparison can be realized by the pore volume (PV) test of isolated OM after extraction, and the interference of pores associated with mineral components can be avoided. It can be seen, in Fig. 19, that the PVs of OM pores are similar at the closed thermal degrees and burial conditions, while are various at the different thermal degrees and burial conditions. In Hunan Area, the PVs of OM pore in the Niutitang Formation shale is the smallest, with averaging value of 0.09 ml/g. The PVs of OM pores in the Longmaxi shale is 1.5 times of that in the Niutitang Formation shale, with averaging value of 0.15 ml/g. In contrast, OM pores of Longmaxi Formation shale in Changning area of Sichuan is the most developed, with averaging PV reaching to 0.34 ml/g, which is equivalent to 4 times that of the Formation Niutitang Formation shale in Hunan Area. Meanwhile, it can also be seen from Fig. 19 that OM pores with small apertures (<300nm) share high percentages in both shales in Hunan Area, while OM pores with small apertures (300nm~1000nm) share high percentages in Longmaxi Formation shale in Changning area of Sichuan. This indicates that, with the decrease of thermal evolution and the delay of uplift time, the small OM pores are enlarged and gradually merged into large pores. The quantitative comparison of the pore volume of OM pores after extraction is in high agreement with the results of semi-quantitative statistics under the SEM observation.
Reviewer 2 Report
The authors have addressed all the comments raised during the previous review. I think the article is currently ready for publication.
Author Response
Thanks for your comments. Your proposed suggestions are of important significance to us.
