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by
  • Meihua Yang 1,
  • Yinhui Zuo 1,* and
  • Jiazhen Zhang 1
  • et al.

Reviewer 1: Anonymous Reviewer 2: Anonymous

Round 1

Reviewer 1 Report

Dear Authors,

I have read your valuable contribution and find your manuscript interesting. While I think your data is good and your research design is appropriate, you have not fully "mined" the potential of your own research. I think the introduction needs some improvement to "sell" your research to a broad audience including various fields of geology and put your research questions upfront. In the discussion I think you made only discussions points that are internally relevant (e.g., within your own problem). This is not so elegant thing to do as restricts the potential impact your work may cause. I would suggest to rethink the discussion and frame your research results into a much more globally relevant perspective. For instance, provide some sort of comparison, evaluation and scaling of your research to similar sedimentary basins and highlight the differences and similarities. I also think it would be important to do the same within the Chinese sedimentary basins/oil basins as it would show some sort of trends for the readers such work. I think the conclusion section then should reflects some points around the above mentioned discussion points as well.

I have provided an annotated PDF where you can see my other comments.

Kind regards

Comments for author File: Comments.pdf

Author Response

  1. We have already added the clarification. i.e. “The quality of hydrocarbon source rocks is affected by the sedimentary environment.”

  2. We added the beneficial paleoenvironment of the development of source rocks. i.e. “Paleoenvironment with anoxia, high paleoproductivity is beneficial to the development of source rocks.”

  3. We have already modified the wording. i.e.” These parameters are affected by the paleoclimate and sedimentary environment, such as paleosalinity, paleoredox, and paleoproductivity.”
  4. We have already added the location of the Sichuan Basin and its adjacent areas (Figure1).

  5. We have already cited the map figure (Figure1a).

  6. We have already added the location of the Guizhou-Chongqing-Xiang-Hubei fold belt and the Qianzhong paleo uplift in Figure 1b.

  7. We have added the stratigraphic column map of the Ordovician of the Sichuan Basin and its adjacent areas to clarify the formations.

  8. We have already added the coordinates in Figure 1.

  9. Figure 1a is the petroliferous basins of China and the position of the Sichuan Basin and its adjacent areas and Figure 1b is the structural division and TOC of the Sichuan Basin and its adjacent areas. We have already noted the research area in the China overview map.

  10.  We have already explained the results of polysolution of paleoredox. i.e. “However, researchers have realized the limitations of element ratios to evaluate the paleoredox [26-28]. The threshold values corresponding to the same index are not uniform. For example, the threshold of U/Th to evaluate the paleoredox proposed by Jones and Manning [11] and Wignall and Hallam [16] are quite different. This is because the geological conditions of different research areas, such as geological ages, provenance, paleoactivities, are quite different, and these thresholds do not have global applicability. Except for the redox environment, the organic matter types, depositional rate, and late diagenesis may affect the enrichment of trace elements. Thus, these indexes have multiple conclusions for the determination of redox. Therefore, trace element discrimination can only be used as a reference in the recovery of paleo-redox, but also combined with the actual lithology and color of samples. In 50 mud-stone samples, 90% of the samples are dark grey and black mudstones, indicating the oxygen-depleted and anoxic environments. Combined with the sample color and trace element ratios, the mudstones of the Meitan Formation were deposited under an oxygen-depleted and anoxic environment.”

  11. We have already added the citation to the kerogen type classification.

  12. We supplemented the geological significance and guiding suggestions of the research results for oil and gas exploration. i.e. “(3) In general, the source rocks of the Meitan Formation have hydrocarbon gener-ation potential, and oil and gas can migrate and accumulate to upper reservoirs. The Meitan Formation and its adjacent reservoirs can be de concurrent layers during the petroleum exploration in the Ordovician. In the next Ordovician petroleum exploration, gas and source rocks correlation in the upper Baota Formation can be considered to reveal whether there is a contribution of the Meitan Formation. In addition, for the source rocks deposited in similar paleoenvironment, the possibility of small-scaled oil and gas discovery can be considered.”

  13. We have already added the DOI numbers in the references list.

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript titled: “Paleoenvironment of the Lower Ordovician Meitan Formation in the Sichuan Basin and adjacent areas, China” presents an interesting study about the restoration of paleoproductivity and different paleoenvironmental parameters that occurred during the development of the hydrocarbon bearing Meitan Formation. For this scopes, major and trace elements of the collected samples were determined using XRF and ICP-MS, ICP-OES analysis, respectively. In both cases, the analyzed samples were taken from wells and geological sections. Total organic carbon percent (TOC %) was also determined, as well as carbon isotopic composition of kerogen. For the paleoclimate restoration, the chemical index alteration (CIA) equation and the Sr/Cu ratio were used. The Rb/K ratio was implemented to study paleosalinity conditions, while the Th/U ratio and V/(V+Ni) ratio were used for paleoredox evaluation. Finally, the paleoproductivity was qualitatively evaluated by a marine paleoproductivity equation.

The presented research corresponds to the aims and scopes of the Journal Minerals. The manuscript is quite well written; however, some sections are recommended to be revised by a professional English language user. Τhe research design and presentation is quite structured. The authors have used the appropriate citation throughout the manuscript. The presented methods are adequately described, however clarifications are recommended. The presented Tables and Figures are sufficiently described and well displayed; however some additions and changes are required prior to publication in the Journal Minerals. Based on the aforementioned comments, Minor Revision is suggested.

Specific comments:

  1. It has been observed that the manuscript has some English language issues. Specifically, the “Abstract” is recommended to be revised as the meaning is not clear for the reader.
  2. In section “Geological Setting”, paragraphs in lines 57 – 67, are recommended to be rephrased in a more understandable way for the reader.
  3. In section “Sample and Experiment”, is suggested to be renamed as follows “Materials and Methods”. It is advised to add the acronyms of the implemented analyses, for example X-ray fluorescence spectrometry (XRF). In addition, would it be possible to include the range of the detection limits for the analyzed trace elements with the ICP-MS and ICP-OES methods? The same is suggested for the XRF method.
  4. In the same section, in line 85, “… The main and trace elements…”, please replace the main with major.
  5. As far as the presented Figures are concerned, Figure 1 is missing. Since in the manuscript, you refer, for example in line 176, to Figure 5c, you do not provide such numbering in Figure 5. Please, make the appropriate changes for Figures 2, 4 and 5, which are consisted of multiple graphics.
  6. As far as the presented Tables are concerned, you use two different numbering formats throughout the manuscript. It is recommended to have a consistent format along the manuscript. Please make the appropriate changes.
  7. The equation that was used to evaluate the paleoproductivity, is not sufficiently explained. The reader has to read the next sentence in order to understand to which elements the numerator of the fraction X/Ti corresponds to. Please, proceed to the appropriate changes.
  8. In line 50-52 you state “In this paper, the paleoclimate, paleosalinity, paleoredox, and paleoproductivity of the Meitan Formation were analyzed combined with geological, geophysical, and geochemical parameters”. Throughout the research none geophysical data are presented. Could you clarify why you mention geophysical parameters? Is possible that you are referring to the physical properties of the samples?

Author Response

  1. We have already modified the Abstract. i.e. “The quality of hydrocarbon source rocks is affected by the sedimentary environment, and paleoenvironment with anoxia, high paleoproductivity is beneficial to the development of source rocks. The paleoenvironment of the Lower Ordovician Meitan Formation of the Sichuan Basin and its adjacent areas are lack, restricting the oil and gas exploration of the Ordovician in the Sichuan Basin and its adjacent areas. To clarify the paleoenvironment and quality of the development of source rocks, the major and trace elements of 50 samples were tested to analyze the paleoclimate, paleosalinity, paleoredox, and paleoproductivity during the deposition of the Meitan Formation. The control of the paleoenvironment on the source rocks development was discussed, and which is the favorable paleoenvironment for source rock development of the Meitan Formation was pointed out. The results show that the paleoenvironment of the Meitan Formation is characterized by humid, brackish water, oxygen-depleted, anoxia environment and high paleoproductivity, which is conducive to the development of poor and moderate source rocks. The source rocks of the Meitan Formation were developed in the north, west, and south of the Sichuan Basin and its adjacent areas. The organic matter of the source rocks is mainly typed II1 kerogen and the quality is evaluated as poor-medium, which has the potential to generate oil and gas. This study can provide fundamental parameters for the further exploration of Ordovician petroleum.”

  2. We rephrase the geological setting. i.e. “The Sichuan Basin and its adjacent areas are located in southwestern China (Figure 1a). The Songpan-Ganzi fold belt and the Longmenshan fault zone are in the west of the research area, the Chengkou-Fangxian fault is in the north, the Guizhou-Chongqing-Xiang-Hubei fold belt is in the east, and the Qianzhong paleo uplift is in the south. The Sichuan Basin is distributed in the study area in a NE direction, showing the characteristics of a multi-tectonic system under multi-stage structures (Figure 1b) [7].

    From Late Cambrian to Early Silurian, several strata developed in the Sichuan Basin and its adjacent areas. The Lower Ordovician Tongzi Formation, Honghuayuan Formation, Meitan Formation, Middle Ordovician Shizipu Formation, Upper Ordovician Baota Formation, Linxiang Formation, Wufeng Formation were deposited from bottom to top (Figure 2). However, the Middle-Upper Ordovician were missing in the north, west, and southwest of the Sichuan Basin under the influence of the Caledonian movement [5,8].” Moreover, to make the words more understandable for reader, we added the Figure 2 to reveal the strata which was deposited in the Sichuan Basin and its adjacent areas.

  3. We have already modified the section “Sample and Experiment” into “Materials and Methods” and added the acronyms of the implemented analyses.

  4. We have already replace the main with major。

  5. We have already modified the Figures 2, 4 and 5。

  6. We have already modified the numbering of the tables in the manuscript.

  7. We refined the meaning of (X/Ti)PAAS. i.e. “(X/Ti)PAAS is a constant, representing the ratio of the average content of X and Ti in the Neoarchean Australian shale. The (Ba/Ti)PAAS, (Zn/Ti)PAAS are 0.1083 and 0.0083, respectively [24]”.

  8. It is the physical properties of the samples. We have already modified the mistakes.

Author Response File: Author Response.pdf