Geochemical Characteristics of Elements and Main Controlling Factors of Organic Matter Enrichment in Shale from Wufeng Formation to Submember Long-11 in Southern Sichuan
Round 1
Reviewer 1 Report
The authors present a systematic analysis of the major and trace elements of the shales from the Wufeng Formation to Submember Long-11 in Changning area. Based on the geochemical indices, the redox conditions, depositional environment and organic matter enrichment of the shales were discussed. The results are certainly valuable and worthy of publication after major revisions. Major problems that I consider are:
1. English writing which needs to be polished by native speakers, and many idioms of Chinese in the manuscript;
2. DOPT index, which the content of Fe element in pyrite is calculated through the content of S element, however, in the paper, the measurement of morphological sulfur has not been found;
3. Only use elemental geochemical indexes to discuss the redox conditions, depositional environment and organic matter enrichment of the shales were unconvincing, please supplement the previous research on geological background and petrological documents.
4. Explain the validity and margin of error of TOC content in the paper.
5. More detailed revision points are marked in the annotated PDF uploaded with this review.
Comments for author File: 
Comments.pdf
English writing which needs to be polished by native speakers, and many idioms of Chinese in the manuscript
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Reviewer 2 Report
The work by Xiong et al. describes geochemical characteristics (TOC, major and trace elements) of the Paleozoic shales that are a significant source of shale gas in China. As Section 2 informs, the authors studied 70 samples from 7 drill cores of the Changning area. These data may be of interest for the Applied Science readers. However, they are presented in the form of figures only that significantly decreases the paper value.
The MS organization is OK. 3 figures and 1 table are of good quality, although Fig. 1 may be in higher resolution, more informative and better explained. The language needs at least polishing.
The obtained data are used to decipher a depositional environment of the studied shales including its paleoproductivity, REDOX conditions, input of terrigenous material, and sea level change. Tectonic setting, possible volcanic and hydrothermal influences, and sediment sources have not been discussed. In addition, some significant works discussing the topics of this study (see, for instance, References below) have not been even mentioned. These are major flaws of this study.
For interpretation of REDOX conditions, the authors use the traditional geochemical indexes such as U/Th (Hatch and Leventhal, 1992), Ni/Co (Jones and Manning, 1994), V/Sc (Kimura, 2001), δU (Wignall, 1994), and DOPT (Raiswell, 1988), all of which reflect anaerobic environment that is common for black and gray shales like the studied ones. This conclusion seems reasonable, but routine. Other conclusions are that organic matter enrichment in the studied shales was mainly controlled by low sea level and a weak terrigenous input, while bioproductivity did not play a significant role. The authors’ argumentation for these seems not solid.
Total REE concentration and distribution pattern in terrigenous sediments reflects these characteristics of the source rocks rather than the total amount of terrigenous input (as the authors suggested).
δCe value in the shales reflects the REDOX conditions during and after their deposition rather than rise and fall of sea level (as the authors suggested).
If high TOC value in the studied shales was not related to high bioproductivity in the basin of deposition, what was a source of their organic matter? Terrigenous input, which was low as the authors suggest? This question is not discussed.
The authors use BaXS as a proxy of paleoproductivity. However, reducing conditions can lead to barite undersaturation (McManus et al., 1998). The relatively good correlation between DOPt and TOC (Fig. 3e) probably indicates a significant role of productivity for organic matter enrichment of the studied shales as Wang et al. (2017) suggested using P2O5/TiO2 ratio and SiO2 content.
More specific comments may be found in the attached pdf file.
Overall, I recommend a major revision including presentation of all the obtained data as supplementary tables, and significant reconsideration of the study discussion and conclusions.
References
Wenbo Su, Yongbiao Wang, D Cramer Bradley, Munnecke Axel, Zhiming Li, Lipu Fu. Preliminary estimation of paleoproductivity via TOC and Habitat Types: Which method Is more reliable?-A Case Study on the Ordovician-Silurian Transitional Black Shales of the Upper Yangtze Platform, South China[J]. Journal of Earth Science, 2008, 19(5): 534-548.
Fan, C., Zhong, C., Zhang, Y., Qin, Q., and He, S. (2019). Geological factors controlling the accumulation and high yield of marine–facies shale gas: case study of the Wufeng– Longmaxi formation in the Dingshan area of southeast Sichuan China. Acta Geol. Sin. (Eng. Ed.) 3, 536–560.
Tang, X., Jiang, S., Jiang, Z., Li, Z., He, Z., and Zhu, D. (2019). Heterogeneity of Paleozoic Wufeng–Longmaxi formation shale and its effects on the shale gas accumulation in the Upper Yangtze Region China. Fuel 239, 387–402.
Zhao, D., Guo, Y., Wang, G., Bai, W., Zeng, C., Jiao, W., et al. (2020). Sedimentary characteristics of shales graptolite and Its implications for high–quality shale gas reservoirs in the Wufeng-Longmaxi formation shales in southeast Chongqing. J. Henan Polyt. Univ. (Nat. Sci.) 39, 26–36. (in Chinese).
McManus J, Berelson WM, Klinkhammer GP, et al. (1998) Geochemistry of barium in marine sediments: Implications for its use as a paleoproxy. Geochimica et Cosmochimica Acta 62: 3453–3473.
Wang, Y., Li, X., Dong, D., Zhang, C., Wang, S. (2017) Major controlling factors for the high-quality shale of Wufeng–Longmaxi Formation, Sichuan Basin. Energy Exploration & Exploitation, 35/ 4, 444-462.
Comments for author File: Comments.pdf
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The author has not provided a reasonable explanation of the issues previously raised, rather than just selectively explaining and avoiding the issues raised, and it is essential that the previous issues are explained reasonably and revised in the text before publication. In the current version, I don't see that the author has provided a reasonable explanation for the issues raised earlier, including the data requested, which could be considered as an attachment if there is too much data.
Minor editing of English language required
Author Response
Dear reviewer, thank you very much for your feedback. I have done my best to study the research content you mentioned, and some of the content that needs to be studied will be further studied in the future.
Reviewer 2 Report
I have not found any satisfactory reply to my comments in the authors’ responses. Generally, the used geochemical indexes depend on much more factors than discussed in the study. This makes the actual study conclusions unsolid. The authors seem going to resolve this problem in future. Let's wait for results of this work.
Then, I recommend rejection with a possibility to resubmit a really revised manuscript.
Author Response
Dear reviewer, thank you very much for your feedback. I have done my best to study the research content you mentioned, and some of the content that needs to be studied will be further studied in the future.Thank you very much.
