Review Reports - Ultra-Deep Oil and Gas Geological Characteristics and Exploration Potential in the Sichuan Basin

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The issue of harnessing new hydrocarbon resources is increasingly gaining significance both within China and globally. The proportion of conventional hydrocarbon reserves is declining annually, prompting a shift toward the exploration and development of more complex or alternative hydrocarbon sources. Consequently, a comprehensive review article on ultra-deep oil and gas basin is more pertinent than ever. However, the manuscript requires minor revisions to enhance its clarity and adherence to scientific standards. Primarily, these revisions pertain to the abstract. As is customary, the abstract should succinctly state the relevance of the research in a single sentence, followed by a detailed description of the methodologies employed. It should then present key conclusions and provide a concise summary of prospects for industry development, encompassing technological and scientific advancements. The current structure of the abstract does not fully meet these criteria; notably, only the final sentence indicates the aspects considered and discussed within the article.

To enhance its effectiveness, the authors should incorporate at least one sentence in the abstract addressing the following points: the geological processes involved in the generation, migration, and enrichment of ultra-deep hydrocarbons; the laws governing the distribution of ultra-deep oil and gas reservoirs; and an overview of the Sichuan Basin from a structural perspective, including source rocks, reservoirs, caprocks, closures, and preservation conditions.

In the introduction , the authors present a more logical and structured overview. They initially discuss regions worldwide characterized by the presence of particularly deep hydrocarbon deposits before narrowing their focus to China and its resource base. A brief description of these resources is provided. Subsequent sections address critical issues such as the geological conditions underpinning these deposits, their formation evolution, specific challenges faced by deposits in the Sichuan Basin, and recent advances in exploration techniques.

The manuscript references 39 scientific articles, over half published within the past five years. The discussion encompasses developments from 1992 to 2025, providing a broad temporal perspective on Sichuan Basin evolution. This extensive dataset allows for forecasting future development trends and assessing their industrial relevance. The review's strength lies in analyzing various factors previously considered separately by other authors into a cohesive overview. This approach facilitates a comprehensive understanding of both current conditions and historical changes within the basin.

The inclusion of illustrative material enhances comprehension by providing visual context for complex concepts.

Most importantly, the authors appropriately highlight challenges that the industry must confront when developing these deep hydrocarbon resources.

In conclusion, it would be beneficial to incorporate additional numerical data—beyond merely the gas-bearing area- to underscore both the relevance and feasibility of resource development. Such quantitative indicators would strengthen the overall assessment of potential industry prospects derived from this review. For instance, it would be prudent to append corresponding conclusions supported by numerical values to sections 2.1, 2.2, 3.1, 3.2, 3.3, and 4, thereby providing a more comprehensive and data-driven evaluation of each aspect discussed.

Author Response

Comments 1:The issue of harnessing new hydrocarbon resources is increasingly gaining significance both within China and globally. The proportion of conventional hydrocarbon reserves is declining annually, prompting a shift toward the exploration and development of more complex or alternative hydrocarbon sources. Consequently, a comprehensive review article on ultra-deep oil and gas basin is more pertinent than ever. However, the manuscript requires minor revisions to enhance its clarity and adherence to scientific standards. Primarily, these revisions pertain to the abstract. As is customary, the abstract should succinctly state the relevance of the research in a single sentence, followed by a detailed description of the methodologies employed. It should then present key conclusions and provide a concise summary of prospects for industry development, encompassing technological and scientific advancements. The current structure of the abstract does not fully meet these criteria; notably, only the final sentence indicates the aspects considered and discussed within the article.

Response 1：The abstract has been revised according to the review comments, and the detailed abstract can be found in the uploaded full text file.

Comments 2: To enhance its effectiveness, the authors should incorporate at least one sentence in the abstract addressing the following points: the geological processes involved in the generation, migration, and enrichment of ultra-deep hydrocarbons; the laws governing the distribution of ultra-deep oil and gas reservoirs; and an overview of the Sichuan Basin from a structural perspective, including source rocks, reservoirs, caprocks, closures, and preservation conditions.

Response 2：The abstract has been revised according to the review comments, and the detailed abstract can be found in the uploaded full text file.

Comments 3:In the introduction , the authors present a more logical and structured overview. They initially discuss regions worldwide characterized by the presence of particularly deep hydrocarbon deposits before narrowing their focus to China and its resource base. A brief description of these resources is provided. Subsequent sections address critical issues such as the geological conditions underpinning these deposits, their formation evolution, specific challenges faced by deposits in the Sichuan Basin, and recent advances in exploration techniques.

Response 3：Thank you to the reviewers for their affirmation of the manuscript's content.

Comments 4:The manuscript references 39 scientific articles, over half published within the past five years. The discussion encompasses developments from 1992 to 2025, providing a broad temporal perspective on Sichuan Basin evolution. This extensive dataset allows for forecasting future development trends and assessing their industrial relevance. The review's strength lies in analyzing various factors previously considered separately by other authors into a cohesive overview. This approach facilitates a comprehensive understanding of both current conditions and historical changes within the basin.

Response 4：Thank you to the reviewers for their affirmation of the manuscript's content.

Comments 5:The inclusion of illustrative material enhances comprehension by providing visual context for complex concepts.

Response 5：Thank you to the reviewers for their affirmation of the manuscript's content.

Comments 6:Most importantly, the authors appropriately highlight challenges that the industry must confront when developing these deep hydrocarbon resources.

Response 6：Thank you to the reviewers for their affirmation of the manuscript's content.

Comments 7:In conclusion, it would be beneficial to incorporate additional numerical data—beyond merely the gas-bearing area- to underscore both the relevance and feasibility of resource development. Such quantitative indicators would strengthen the overall assessment of potential industry prospects derived from this review. For instance, it would be prudent to append corresponding conclusions supported by numerical values to sections 2.1, 2.2, 3.1, 3.2, 3.3, and 4, thereby providing a more comprehensive and data-driven evaluation of each aspect discussed.

Response 7：Thank you for your feedback. We have added statistics from China National Petroleum Corporation regarding advantageous exploration areas for deep and ultra-deep layers in China, to provide more comprehensive evidence.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Dear Authors,

I find the theme of Your review work interesting and I can see that You've invested time and effort to bring an overview of potential of deep and ultra-deep hydrocarbon reservoirs in Sichuan basin to geoscientific community. However, in its current state Your manuscript lacks some vital elements to be published. The structure of Section 3 is somewhat unclear, and the subsections would benefit from stronger connections to improve coherence. Additionally, the figures throughout the manuscript require substantial revision, as they are currently characterized by low resolution, inconsistent or confusing terminology, unsuitable captions, and in some cases, an unclear source. Furthermore, certain terms used in the text are unclear, and several references are missing across the manuscript. Also, overall number of references is too low for a review paper. It should be at least double.

All specific comments are in the attached file. Hope You'll find the motivation to improve this work as it would definitely be of interest to all experts interested in regional petroleum geological relations in Sichuan basin.

Kind regards

Comments for author File: Comments.pdf

Author Response

Comments 1:I find the theme of Your review work interesting and I can see that You've invested time and effort to bring an overview of potential of deep and ultra-deep hydrocarbon reservoirs in Sichuan basin to geoscientific community. However, in its current state Your manuscript lacks some vital elements to be published. The structure of Section 3 is somewhat unclear, and the subsections would benefit from stronger connections to improve coherence. Additionally, the figures throughout the manuscript require substantial revision, as they are currently characterized by low resolution, inconsistent or confusing terminology, unsuitable captions, and in some cases, an unclear source. Furthermore, certain terms used in the text are unclear, and several references are missing across the manuscript. Also, overall number of references is too low for a review paper. It should be at least double.

Response 1: Thank you to the reviewers for your valuable feedback. In response, we have added 13 new references to better support the arguments and enrich the content of the paper. We have also unified the expression of key terms in both the figures and the main text for consistency. Some of the figures have been redrawn, and the font sizes have been adjusted to improve clarity. Regarding the logical issues in Section 3, we have added a new “Database and Methodology” subsection to reorganize the placement of data and refine the logical flow. Transition sentences have been incorporated between paragraphs to enhance coherence. For a detailed view of all revisions, please refer to the full manuscript attached.

Comments 2:All specific comments are in the attached file. Hope You'll find the motivation to improve this work as it would definitely be of interest to all experts interested in regional petroleum geological relations in Sichuan basin.

Response 2: We sincerely thank the reviewers for their detailed and insightful comments on our manuscript. We have carefully addressed each suggestion and have revised the paper accordingly. For a complete overview of the changes made, please refer to the attached revised manuscript.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

There is no dedicated methodology section. How were the data in Tables 1 and 2 (e.g., TOC values, thicknesses, resource estimates) obtained or validated? Are they derived from original analysis, proprietary datasets, or previous publications?

Figures such as Figure 5 (Reservoir Model) and Figure 10 (Exploration Map) appear to compile existing knowledge. Are any of these figures based on new interpretations or original modeling? If so, please describe how they were created. If not, consider adding a predictive geological model or risk map.

Core photographs (e.g., Figure 6) and stratigraphic diagrams lack essential elements such as fault labels, or interpretive overlays. Can you revise these figures to improve interpretability and scientific value?

Some sections, particularly the reservoir and source rock discussions, are dense and repetitive. Would you consider simplifying these paragraphs and improving transitions to enhance clarity?

Resource estimates in Table 2 are presented as single values without uncertainty bounds or classification (e.g., PRMS, deterministic vs. probabilistic). Can you explain why uncertainty analysis was omitted and whether any volumetric modeling was conducted?

It is unclear which data are from your own field work or analysis and which are sourced from literature or reports. Can you clearly attribute data sources in the tables, text, and figure captions?

The paper briefly mentions other ultra-deep basins such as Tarim, Gulf of Mexico, and Adriatic Sea. A structured comparison (e.g., table or paragraph) benchmarking Sichuan Basin against those would strengthen the context. Would you consider adding one?

Although the English is generally acceptable, many sentences are long and complex. Reducing sentence length and avoiding redundancy could improve clarity. Would you consider a language and structure revision for readability?

The conclusion section largely repeats content from earlier sections. Could you use this section to more critically reflect on limitations, uncertainties, and future directions for ultra-deep exploration in the Sichuan Basin?

Author Response

Comments 1:There is no dedicated methodology section. How were the data in Tables 1 and 2 (e.g., TOC values, thicknesses, resource estimates) obtained or validated? Are they derived from original analysis, proprietary datasets, or previous publications?

Response 1：We appreciate your suggestions on the paper. We have added a *Database and Methodology* section to supplement the sources of data used in this study. The results concerning source rocks, resource estimates, and the interpretive profiles and maps are mainly derived from a new round of research and statistical work on favorable deep to ultra-deep exploration intervals in the Sichuan Basin, conducted by the Exploration and Research Institute of PetroChina Southwest Oil & Gasfield Company. Additional information was supplemented by reviewing the literature and consulting the IHS reserves bulletin.

Comments 2:Figures such as Figure 5 (Reservoir Model) and Figure 10 (Exploration Map) appear to compile existing knowledge. Are any of these figures based on new interpretations or original modeling? If so, please describe how they were created. If not, consider adding a predictive geological model or risk map.

Response 2：Thank you for raising this question. In Figure 5 (Reservoir Model) and Figure 10 (Exploration Map), we have incorporated the latest interpretation by PetroChina regarding deep to ultra-deep reservoirs in the Sichuan Basin. During the drafting process, we referred to the most recent seismic data and drilling interpretations to ensure the figures align more accurately with current exploration understanding.

Comments 3:Core photographs (e.g., Figure 6) and stratigraphic diagrams lack essential elements such as fault labels, or interpretive overlays. Can you revise these figures to improve interpretability and scientific value?

Response 3：Thank you for pointing out this issue. We have added and improved the missing elements, such as fault labels, in the figures.

Comments 4:Some sections, particularly the reservoir and source rock discussions, are dense and repetitive. Would you consider simplifying these paragraphs and improving transitions to enhance clarity?

Response 4：We fully agree with your suggestion. Therefore, we have added a *Database and Methodology* section to present and analyze the reservoir and source rock data in a unified way. At the same time, we adjusted the logical flow of the content to enhance the coherence between paragraphs. Please see the attached revisions for details.

Comments 5:Resource estimates in Table 2 are presented as single values without uncertainty bounds or classification (e.g., PRMS, deterministic vs. probabilistic). Can you explain why uncertainty analysis was omitted and whether any volumetric modeling was conducted?

Response 5：Thank you for raising this issue. Our resource estimates are based on the new round of research and statistical results from the Exploration and Research Institute of PetroChina Southwest Oil & Gasfield Company regarding favorable deep to ultra-deep exploration intervals in the Sichuan Basin, as well as the IHS reserves bulletin. In oil and gas resource studies in China, data are usually presented as single values, and uncertainty analyses are typically not conducted.

Comments 6:It is unclear which data are from your own field work or analysis and which are sourced from literature or reports. Can you clearly attribute data sources in the tables, text, and figure captions?

Response 6：Thank you for raising this issue. We have now provided a detailed explanation of the data and figure sources in the *Database and Methodology* section. For some referenced figures, the corresponding source documents are cited in the paper.

Comments 7:The paper briefly mentions other ultra-deep basins such as Tarim, Gulf of Mexico, and Adriatic Sea. A structured comparison (e.g., table or paragraph) benchmarking Sichuan Basin against those would strengthen the context. Would you consider adding one?

Response 7：We fully agree with your suggestion. Therefore, we have added a table of statistical data from PetroChina on favorable exploration blocks in deep and ultra-deep settings in China. This table more clearly illustrates the advantageous exploration position of marine carbonate rocks in the Sichuan Basin in ultra-deep domains.

Comments 8:Although the English is generally acceptable, many sentences are long and complex. Reducing sentence length and avoiding redundancy could improve clarity. Would you consider a language and structure revision for readability?

Response 8：We fully agree with your suggestion. Therefore, we have revised the sentence structures throughout the paper to make the expression clearer and the arguments more explicit. Please see the revised manuscript for details.

Comments 9:The conclusion section largely repeats content from earlier sections. Could you use this section to more critically reflect on limitations, uncertainties, and future directions for ultra-deep exploration in the Sichuan Basin?

Response 9：We fully agree with your suggestion. Accordingly, we have divided the conclusion into three parts, separately discussing the current strengths and limitations of ultra-deep exploration in the Sichuan Basin, the uncertainties, and the directions for future research.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The article could be recommended for publication

Author Response

Comments1: The article could be recommended for publication

Response1: Thank you for your contribution to revising the article and for your recognition of it. Wishing you all the best in your work.

Reviewer 2 Report

Comments and Suggestions for Authors

Dear authors,

I was glad to see improvements in Your manuscript. However, I must admit I still find the text difficult to follow. Language editing would be beneficial. Figures still need focused effort for better alignment with text (Figure 2), better alignment between Figures (Figures 2, 3 and 5), as well as better definition of mapped units (Figure 10). Also, there appears to be a significant error in the reference list, with entries from number 25 onward seemingly disordered.

Specific comments are in the pdf file.

I was surprised to find that only the revised manuscript without tracked changes was provided. Including a version with tracked changes would have made it significantly easier to follow the modifications made in response to my comments. Additionally, the file labeled "Author Response" contained only the revised manuscript and did not address the specific questions I raised. In places where no changes were made following my remarks, no explanation was given, leaving me to assume that my comments may have been disregarded.

Kind regards

Comments for author File: Comments.pdf

Comments on the Quality of English Language

Dear Editor,

Improvements of the manuscript are visible. However, I must admit I still find the text difficult to follow. Language editing would be beneficial. Figures still need focused effort for better alignment with text (Figure 2), better alignment between Figures (Figures 2, 3 and 5), as well as better definition of mapped units (Figure 10). Also, there appears to be a significant error in the reference list, with entries from number 25 onward seemingly disordered.

Kind regards,

Iva

Author Response

Comments1：Improvements of the manuscript are visible. However, I must admit I still find the text difficult to follow. Language editing would be beneficial. Figures still need focused effort for better alignment with text (Figure 2), better alignment between Figures (Figures 2, 3 and 5), as well as better definition of mapped units (Figure 10). Also, there appears to be a significant error in the reference list, with entries from number 25 onward seemingly disordered.

Response：Thank you for your feedback on the manuscript. We have addressed the issues you raised in your attachment and provided our revisions and responses in our revised manuscript. Please take a moment to review it. Regarding the references, we have reorganized them accordingly. Once again, we appreciate your valuable suggestions for improving the paper. Wishing you all the best in your work.

Comments2：I was surprised to find that only the revised manuscript without tracked changes was provided. Including a version with tracked changes would have made it significantly easier to follow the modifications made in response to my comments. Additionally, the file labeled "Author Response" contained only the revised manuscript and did not address the specific questions I raised. In places where no changes were made following my remarks, no explanation was given, leaving me to assume that my comments may have been disregarded.

Response：We sincerely apologize that in our previous response to the initial review comments, we did not provide point-by-point replies in the comments and instead submitted the final version directly. In the newly uploaded file, we have now included detailed responses to both rounds of review comments, clearly indicating the revisions made and supplementing the previously overlooked parts. Please kindly review the updated document.

Author Response File: Author Response.pdf

Round 3

Reviewer 2 Report

Comments and Suggestions for Authors

Dear Authors,

Thank You for Your answers. I find Your manuscript much improved, especially regarding the Figure 2, now everything can be easily followed in the text. However, I still find maps on Figure 4 difficult to read due to poor resolution. Also, there are some sentences that still need to be revised. The suggestions are in the comments in the file.

Kind regards

Comments for author File: Comments.pdf

Author Response

Thank you again for your suggestions.The manuscript has been meticulously revised in accordance with your recommendations to enhance its quality. Please find the updated document attached for your review.

Author Response File: Author Response.pdf