Experimental Studies on Pore Structure and the Gas Content Evolution Mechanisms of Shale Gas Reservoirs at Different Burial Depths in the Longmaxi Formation, Southern Sichuan Basin

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The paper addresses a current issue of great relevance to unconventional hydrocarbon exploitation, namely the petrophysics of shale-type systems. The problem is excellently described in the introduction and is addressed using multiple analytical techniques that validate the results. I think it is a work of great interest for people working in the field.

My main criticism is associated with the presentation of the results, which makes the work difficult to understand. If the authors are willing to modify the presentation of the results, I will recommend the paper for publication.

There is an excess of figures (20 in total, and each of them with several subfigures) that are in some cases redundant graphs of the same results. This, coupled with the scarce descriptions at the bottom of the figures, makes the paper difficult to follow. The figure captions should clearly indicate from which tests and which calculations the results shown are derived.

I would also suggest deleting some sections that distract attention from the main results. For example:

- Section 3.4.1: bulk calibration of methane is something necessary to do, but not something necessary to show (the results are more than expected). 

- Section 3.4.2: the objective of the work is the analysis at reservoir temperatures and pressure, I don't understand what the ambient temperature test adds up to.

- Section 4.3.2 and discussion of figure 18: The linear correlations with only 3 points, small ranges in pore distributions and low R2 is not good enough to make conclusions.  They are definitively not "good correlations" and is not possible to affirm, for example, that figure 18c has a negative correlation. Same for figure 18d. Eliminate figure 18 and its discussion.

Other suggestions

Abstract

Line 26-29: There are some grammar mistakes and repetitions. Please rewrite these sentences

Introduction

Line 42: Put (China) in brackets.

Materials and methods:

Section 2.1. Some information regarding basically organic geochemistry of the formation (organic matter origin, kerogen type, etc) would be helpful to understand the context of the samples.

Line 159: Replace “therefore” for “that way” or something equivalent.

Line 175-176: It is not necessary to point out that you use excel for plotting results.

Discussion:

Is figure 11 a correction of figure 10 results? if that the case, just show figure 11. In addition, don’t use key brackets for indicate one area and arrows for the other areas. Use bracket for everything.

Figure 12b: What is the point of plotting the calibration data?

Lines 502-506: Reformulate the paragraph. It has some grammar errors.

Lines 533-537: OM pores accounts for 25-61% of total porosity according to figure 14. This is no “small”. Reformulate the phrase.

Figure 15: I recommend using the same scale on histograms to better show the differences. Use an inset for figure 14a if you want to zoom the 0-50 nm region.

Lines 549 and 551: I don’t understand the phrase “The OM pore size in Zhaotong area is basically less than 50 nm, and the pore size distribution shows two main peaks, namely 5-25 nm (Figure 15a).”. In the histogram, I see the two maximums at 10-15 nm and 30-35 nm.

Lines 582-584: “The thermal evolution degree of the Longmaxi Formation shale reservoirs in  Zhaotong, Weiyuan, and Luzhou areas is all in an over mature stage, indicating that the shale reservoirs have experienced the maximum ancient burial depth.” Is not correct do this affirmation only because the sample is in an over-mature stage. It might be true, but this is not the argument.

Figure 14 and 16: Are they the same results but in figure 14 you do an average per area and in figure figure 16 you graph them as function of the depth? If it the case, they are redundant.

Figure 17 and figure 20: are the same results but one plotted as a function of area and the other as a function of depth? Again, If that is the case, put a single graph and merge the discussions. Otherwise, clarify in the figure caption where each result comes from.

Comments on the Quality of English Language

English is generally apropiate. I have found only a couple of small grammar mistakes.

Author Response

Replies to the editor Reviewer #1:

I would also suggest deleting some sections that distract attention from the main results. For example:

- Section 3.4.1: bulk calibration of methane is something necessary to do, but not something necessary to show (the results are more than expected).

- Section 3.4.2: the objective of the work is the analysis at reservoir temperatures and pressure, I don't understand what the ambient temperature test adds up to.

- Section 4.3.2 and discussion of figure 18: The linear correlations with only 3 points, small ranges in pore distributions and low R2 is not good enough to make conclusions.  They are definitively not "good correlations" and is not possible to affirm, for example, that figure 18c has a negative correlation. Same for figure 18d. Eliminate figure 18 and its discussion.

Reply: We have deleted above three Sections. But partial content of Section 3.4.1 and Section 3.4.2 move to Section 2.5.

Before each experiment, the chamber volume of the experimental equipment is calibrated using methane gas to avoid the impact of temperature changes on the experimental results. The methane in the sample chamber of the NMR system can be divided into three categories: free methane in shale pores, adsorbed methane in shale pores, and bulk methane free in the void space of sample chamber not related with sample.

The specific procedures for methane-saturated NMR T₂ spectrum tests are as follows:

(1) Sample pretreatment: dry the shale samples at 110 °C for 24 hours and record the mass of the dried core for gas content calculation.

(2) Test-parameter setting: the echo interval (TE) is set to 0.1 ms, the number of echoes (NECH) to 12000, the cumulative scanning number (NS) to 128 times, and the waiting time (TW) to 3000 ms.

(3) Calibration between methane mass and NMR signal in an empty sample chamber with different pressure and temperature: this work tested the NMR T₂ spectra at different temperatures (40°C, 60°C, 80°C and 100°C) and pressures (2 MPa, 4 MPa, 6 MPa, 8 MPa, 10 MPa and 12 MPa).

(4) Sample testing steps: place the dried sample in the sample chamber for vacuum extraction and loading the peripheral pressure of the core (approximate formation pressure); heat the entire testing system to the required temperature; inject a certain pressure of methane gas into the reference chamber, and record the pressure data (P₁) after equilibrium; open the valve between the reference chamber and the sample chamber to allow methane gas to expand into the sample chamber; after the system pressure is balanced, test the NMR T₂ spectrum and record the pressure data (P₂). The criterion for determining the saturated methane equilibrium of shale is to test the NMR T₂ spectrum every 1 hour after methane adsorption for a period of time, with three consecutive NMR T₂ spectra remaining unchanged to indicate full saturation of the sample.

(5) Result processing: calculate the saturated methane NMR signal intensity tested in the above steps. The conversion relationship between methane quality and NMR signal intensity can be expressed as[44] :

m = a×T₂                                    (1)

where m is the STP mass of methane (cm³), and T₂ is measured peak area of NMR T₂ spectrum, a is a parameter that varies with temperatures.

the mass of methane gas at standard temperature and pressure (STP) can be quantified based on a relationship with respect to the measured gas pressure and a given methane density at STP [44].

Line 26-29: There are some grammar mistakes and repetitions. Please rewrite these sentences

Reply: We have revised the writing style of this sentence as follows: The gas content of shale reservoir is influenced by both burial depths and pore structure. When the burial depth of shale gas reservoir is less than 2000 m, inorganic pores and microfractures develop, and the self-sealing ability of the reservoir to shale gas is weak, resulting in low gas content.. However, due to the small pore size of organic pores and low formation temperature, the content of adsorbed gas increases, accounting for up to 60%.

Line 42: Put (China) in brackets.

Reply: Currently, the burial depth of commercially developed shale gas reservoirs of Sichuan Basin (China) is between 500 m and 5000 m, which mainly concentrates in areas such as Zhaotong, Weiyuan, Fuling, Luzhou, and Yuxi [8].

Section 2.1. Some information regarding basically organic geochemistry of the formation (organic matter origin, kerogen type, etc) would be helpful to understand the context of the samples.

Reply: We have added some basic information. This set of Longmaxi Formation strata was deposited on the deep-water continental shelf, with black shale as the main lithology and a large amount of graptolites developed. The organic matter content is high, and the type is mainly I-type kerogen [8].

Line 159: Replace “therefore” for “that way” or something equivalent.

Reply: We have replaced.

Line 175-176: It is not necessary to point out that you use excel for plotting results.

Reply: We have deleted.

Is figure 11 a correction of figure 10 results? if that the case, just show figure 11. In addition, don’t use key brackets for indicate one area and arrows for the other areas. Use bracket for everything.

Reply: We have deleted Figure 10 , and modified Figure 11. 

 

Figure 12b: What is the point of plotting the calibration data?

Reply: We have deleted it.

 

Lines 502-506: Reformulate the paragraph. It has some grammar errors.

Reply:The following two factors contribute to the low gas content tested by traditional methods. Firstly, There is still significant controversy over the accuracy of shale porosity testing results [45-46]. Inaccurate porosity measurement can lead to deviation in gas saturation, which affects the calculation results of free gas content. Secondly, in the confined space, the volume of methane under high temperature and pressure may no longer satisfy the Ideal Gas State Law. And as the temperature increases, the original adsorbed gas will transform into free gas [40, 52]. Therefore, we believe that the total gas content under high-temperature and high-pressure conditions using traditional method are smaller and have large errors.

Lines 533-537: OM pores accounts for 25-61% of total porosity according to figure 14. This is no “small”. Reformulate the phrase.

Reply: we have revised as “It can be seen that as the burial depth increases, the proportion of OM pores gradually increases, while the proportion of inorganic pores and MFs gradually decreases. This is because the burial depth is small in Zhaotong area, the overlying formation pressure of shale reservoirs is low, and the InterP pores are more developed, and the width and length of MFs are larger (Figure 5a-d), ultimately forming shale reservoirs with higher porosity (Table 1). Due to the similar TOC content in shale reservoirs with different burial depths (Table 1) and the small pore size of OM pores (Figure 4f-g), the contribution rate of organic pores to pore volume is not high. In the end, at higher porosity, the proportion of OM pores is 25% (Figure 14). As the burial depth increases, the pressure of the overlying strata increases, and inorganic pores and MFs are gradually compressed or even closed, especially the InterP pores. Due to the compression resistance of minerals and the overpressure of shale gas, the OM pores in the OM between minerals can be well preserved [62]. This results in shale reservoirs of Luzhou area with burial depths greater than 4000 m being dominated by OM pores, which proportion can be up to 60%.”

Figure 15: I recommend using the same scale on histograms to better show the differences. Use an inset for figure 14a if you want to zoom the 0-50 nm region.

Reply: We have modified as below:

 

Lines 549 and 551: I don’t understand the phrase “The OM pore size in Zhaotong area is basically less than 50 nm, and the pore size distribution shows two main peaks, namely 5-25 nm (Figure 15a).”. In the histogram, I see the two maximums at 10-15 nm and 30-35 nm.

Reply: We have modified.

Lines 582-584: “The thermal evolution degree of the Longmaxi Formation shale reservoirs in  Zhaotong, Weiyuan, and Luzhou areas is all in an over mature stage, indicating that the shale reservoirs have experienced the maximum ancient burial depth.” Is not correct do this affirmation only because the sample is in an over-mature stage. It might be true, but this is not the argument.

Reply: We have made the following adjustments. The thermal evolution degree of the Longmaxi Formation shale reservoirs in Zhaotong, Weiyuan, and Luzhou areas is all in an over mature stage, and they all experienced the maximum ancient burial depth and then uplifted to the current burial depth [55].

Figure 14 and 16: Are they the same results but in figure 14 you do an average per area and in figure figure 16 you graph them as function of the depth? If it the case, they are redundant.

Reply: We have deleted Figure 16.

Figure 17 and figure 20: are the same results but one plotted as a function of area and the other as a function of depth? Again, If that is the case, put a single graph and merge the discussions. Otherwise, clarify in the figure caption where each result comes from.

Reply: The prediction model established by this research mainly relies on experimental test results under different temperature and pressure conditions. Therefore, the exploration and development guidance for deep shale gas reservoirs is not good. At present, deep shale is the main battlefield for increasing shale gas storage and production in southern Sichuan. So, deep shale samples with porosity close to the average value in Luzhou area were selected for in-situ gas content prediction under geological conditions, which is more applicable for guiding the exploration and development of deep shale gas with burial depths of 3500m~4500m. Therefore, separate discussions were conducted.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The research, entitled "Experimental Studies on Pore Structure and Gas Content Evolution of Shale Gas Reservoirs with Burial Depth in the Longmaxi Formation, Southern Sichuan Basin," offers a valuable contribution to our understanding of unconventional hydrocarbon resources. However, several issues demand the author's attention and correction before this work can be considered for publication.

Graph Clarifications: It is important to provide the temperature for the pressure sweep graphs and pressure for the temperature sweep graphs, within the figures.

Line 147-148: When referencing the methodology for porosity determination, it would be beneficial to cite the method by its specific name as listed in the reference, which is "Water immersion porosity."

Line 113: The term "clarify" could be rephrased for enhanced clarity to avoid confusion.

Line 250: The presence of pyrite and paramagnetic particles can interfere with NMR signals and influence the distribution of relaxation times, the authors may consider referencing and analyzing the following reference: Livo, K., Saidian, M., & Prasad, M. (2020). "Effect of paramagnetic mineral content and distribution on nuclear magnetic resonance surface relaxivity in organic-rich Niobrara and Haynesville shales," Fuel, 269, 117417.

Lines 289-292: The text mentions microfractures and refers to Figure 4, but Figure 4 does not illustrate microfractures. This inconsistency should be addressed.

Lines 230-236: The usage of a consistent variable for excess gas adsorption capacity, such as a unique symbol, can facilitate reader comprehension. Vex and Mex are used.

Lines 331-333: The relationship established in Equation 6 should be accompanied by a reference for validation purposes.

Lines 348-350: It is unclear whether the values of the chamber volumes in the NMR system were measured or standardized by the instrument, and this should be clarified.

Lines 364-366: The authors should consider providing a reference or stronger evidence to support the statement, "This also confirms that NMR testing has certain limitations on adsorbed gas, but it can test free gas in shale by subtracting the void space between the sample chamber and the sample."

Line 358: The meaning of "saturated methane" is not clear, especially considering that methane is a saturated hydrocarbon. This terminology should be clarified.

Lines 357-366: The paragraph lacks clarity and coherence. A revision is needed to enhance it.

Line 389: A revision of the sentence is advisable as pores themselves do not relax; rather, they affect relaxation times.

Figures 10 and 11: It is suggested to specify "area" in place of "aera," and the arrows in the figures should be clearly associated with the relevant curves.

Lines 457-459: When mentioning the requirement for extrapolation for the sample at 4000 m, it would be beneficial to specify the extrapolation method used.

Equation 7: It appears to be identical to Equation 8; clarification or correction is needed.

Lines 468-483: The authors should provide a more comprehensive explanation regarding the acquisition of the A, B, and C parameters.

Statistical Consideration: For statistical purposes, authors should contemplate expanding Figure 12 and incorporating the analysis for additional samples.

Figure 15: In Figure 15, it may be advantageous to depict both inter and intraorganic matter pores for a more comprehensive representation.

Lines 640-647: The information presented in this section might be better organized and presented in a table, including the values of constants and parameters used for calculating free gas.

Lines 634-648: The paragraph needs a restructured and clearer presentation, as the current form is confusing.

 

Figure 18: Arrows should be either added to all graphs or removed from Figure 18(a) for consistency and clarity.

Author Response

Replies to the editor Reviewer #2:

Graph Clarifications: It is important to provide the temperature for the pressure sweep graphs and pressure for the temperature sweep graphs, within the figures.

Reply: We reviewed and modified the temperature and pressure related drawings in the pictures.

Line 147-148: When referencing the methodology for porosity determination, it would be beneficial to cite the method by its specific name as listed in the reference, which is "Water immersion porosity."

Reply:The porosity test was obtained using the water immersion porosity (WIP) method [46].

Line 113: The term "clarify" could be rephrased for enhanced clarity to avoid confusion.

Reply: We modified it to determine.

Line 250: The presence of pyrite and paramagnetic particles can interfere with NMR signals and influence the distribution of relaxation times, the authors may consider referencing and analyzing the following reference: Livo, K., Saidian, M., & Prasad, M. (2020). "Effect of paramagnetic mineral content and distribution on nuclear magnetic resonance surface relaxivity in organic-rich Niobrara and Haynesville shales," Fuel, 269, 117417.

Reply: Although shale contains paramagnetic mineral pyrite, the content is less than 5%, which has little impact on nuclear magnetic response [59].

Lines 289-292: The text mentions microfractures and refers to Figure 4, but Figure 4 does not illustrate microfractures. This inconsistency should be addressed.

Reply: We have made modifications to the citation of the Figures.

Lines 230-236: The usage of a consistent variable for excess gas adsorption capacity, such as a unique symbol, can facilitate reader comprehension. Vex and Mex are used.

Reply: We have uniformly modified it to m_ex.

Lines 331-333: The relationship established in Equation 6 should be accompanied by a reference for validation purposes.

Reply: Due to suggestions from other reviewers, we have deleted the Section 3.4.1 and 3.4.2. So Equation (6) is written using literature citation.

Lines 348-350: It is unclear whether the values of the chamber volumes in the NMR system were measured or standardized by the instrument, and this should be clarified.

Reply: Before each experiment, the chamber volume of the experimental equipment is calibrated using methane gas to avoid the impact of temperature changes on the experimental results.

Lines 364-366: The authors should consider providing a reference or stronger evidence to support the statement, "This also confirms that NMR testing has certain limitations on adsorbed gas, but it can test free gas in shale by subtracting the void space between the sample chamber and the sample."

Reply: Reply: Due to suggestions from other reviewers, we have deleted the Section 3.4.1 and 3.4.2.

Line 358: The meaning of "saturated methane" is not clear, especially considering that methane is a saturated hydrocarbon. This terminology should be clarified.

Reply:Due to the large amount of methane adsorption in shale, the "saturated methane" mentioned in this experiment includes adsorbed methane and free methane injected into the core pores under certain pressure conditions

Lines 357-366: The paragraph lacks clarity and coherence. A revision is needed to enhance it.

Reply: Due to suggestions from other reviewers, we have deleted the Section 3.4.1 and 3.4.2.

Line 389: A revision of the sentence is advisable as pores themselves do not relax; rather, they affect relaxation times.

Reply: we have revised.

Figures 10 and 11: It is suggested to specify "area" in place of "aera," and the arrows in the figures should be clearly associated with the relevant curves.

Reply: We have deleted Figure 10. And Figure 11 was revised as below:

 

Lines 457-459: When mentioning the requirement for extrapolation for the sample at 4000 m, it would be beneficial to specify the extrapolation method used.

Reply: This indicates that the equipment cannot meet the requirements and needs to be extrapolated. The method of extrapolation is explained in detail in the following paragraphs.

Equation 7: It appears to be identical to Equation 8; clarification or correction is needed.

Reply:               

Where a, b are constants. T is experimental temperature, °C. 

Lines 468-483: The authors should provide a more comprehensive explanation regarding the acquisition of the A, B, and C parameters.

Reply: Organize the NMR signal intensity of the same sample tested under different temperature and pressure conditions in Excel, and import it into Origin function analysis software. Use equation (9) for tens of thousands of iterations to ensure the convergence and strong correlation of the function. After fitting the actual experiment data, the values of A, B and C are 3.57, 3.20 and 4552, respectively. The square of the correlation coefficient is 0.86. Then NMR signal intensity of L214 sample at 130°C and 60 MPa was calculated to be 5787 a.u. So the free gas content at STP is 16.38 m³/t and total gas content is 18.88 m³/t.

Statistical Consideration: For statistical purposes, authors should contemplate expanding Figure 12 and incorporating the analysis for additional samples.

Reply: The main introduction here is the method. We have tested a large number of nuclear magnetic resonance response characteristics under different temperature and pressure conditions, all of which have consistency. The intensity of nuclear magnetic signal is linearly related to pressure and exponentially related to temperature. If the results of these samples are presented, it will take up a lot of space and appear cumbersome. Other reviewers have suggested reducing the number of images and sections in the article, and deleting some sections. In response to this issue, explanations have also been provided in the text.

Figure 15: In Figure 15, it may be advantageous to depict both inter and intraorganic matter pores for a more comprehensive representation.

Reply: A large number of scholars classify the pores of organic matter and minerals into inorganic pores, which are mainly influenced by compaction. Asphalt, which develops pores within organic matter, often fills the existing intergranular pores and is also the main pore in the Longmaxi Formation shale reservoir. Therefore, it is believed that studying organic pores in asphalt is representative.

Lines 640-647: The information presented in this section might be better organized and presented in a table, including the values of constants and parameters used for calculating free gas.

Reply: Due to the fact that these research wells come from different blocks, there are significant differences in burial depth and reservoir properties, and the gas-bearing parameters have been presented in Table 1. If presented in a table, there are many parameters that appear to be all data, which is not intuitive enough to reflect the differences in gas bearing properties of shale reservoirs with different burial depths.

Lines 634-648: The paragraph needs a restructured and clearer presentation, as the current form is confusing.

Reply: we have modified. In this study, the adsorption gas contents were measured by methane isothermal adsorption experiments, and free gas contents were measured by saturated methane NMR experiments. Due to the limited temperature and pressure of NMR equipment, the free gas content in Luzhou area cannot be directly tested, which was calculated by Equations 8-9. Thus, the total gas content of shale reservoirs in Zhaotong, Weiyuan, and Luzhou area were obtained (Figure 17). The total gas content of shale reservoir in the Zhaotong area ranges from 6.91 m³/t to 7.93 m³/t, with high proportion of adsorbed gas, up to 60%. The total gas content of shale reservoir in the Weiyuan area ranges from 9.59 m³/t to 10.28 m³/t, with an average value of 9.86 m³/t. The proportion of free gas is approximately 65.05%. The total gas content of shale reservoir in the Luzhou area ranges from 11.76 m³/t to 18.88 m³/t, with an average value of 14.31 m³/t, with free gas accounting for 84.47 %. It can be seen that as the burial depth increases, both the total gas content and the proportion of free gas increase.

Figure 18: Arrows should be either added to all graphs or removed from Figure 18(a) for consistency and clarity.

Reply: Due to the small number of data points in different areas in Figure 18 and low credibility, section 4.3.2 has been deleted.

 

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

My comments are in the attached file.

Comments for author File: Comments.pdf

Comments on the Quality of English Language

The article needs proofreading. 

Author Response

Replies to the editor Reviewer #3:

1.The introduction section needs to be enriched by the discussion of recently published studies for shale gas characterization. I would ask the authors to read the below studies and provide a comparison or at able of these studies and a few other groups. Although the below studies deal with CO2, however, they are beneficial to understanding the pore structure analysis of shales.

Factors affecting shale microscopic pore structure variation during interaction with supercritical CO2

CO2-water-shale interaction induced shale microstructural alteration

Supercritical CO2-water-shale interactions and their effects on element mobilization and shale porestructure during stimulation Geochemical and physical alteration of clay-rich shales under supercritical CO2 conditions

Reply: We have added references to the above literature, enriching the significance of pore structure research. After hydraulic fracturing development of shale gas, a large amount of fracturing fluids will remain in the shale reservoir, causing changes in the pore structure of the shale reservoir and affecting shale gas seepage and development [9-11]. Therefore, it is of great significance to clarify the pore structure and gas-bearing characteristics of shale reservoirs at different burial depths for understanding the occurrence mechanism, flow behavior, and exploitatation after fracturing of shale gas.

2.In the introduction,and last paragraph, you need to connect the state of the art to your paper goals, and clearly state the novelty of this work. Many previous studies worked on shale gas. Please reason both the novelty and the relevance of your paper goals and what are the Research Gaps/Contributions.

Reply: We have made corresponding modifications as below:

The purpose of this study is to clarify gas content characteristics in situ of shale gas reservoirs at different burial depths more efficient and accurate and to identify controlling factors. The pore structure of shale reservoirs with different burial depths was characterized by using FE-SEM followed with image splicing and processing technique. The pressure-holding coring for on-site desorption results were used as an accurate way in determining total gas content of shale reservoirs. In response to the lack of experimental testing for the total gas content of shale reservoirs in situ, especially for free gas testing, a combined characterization of free gas and adsorbed gas in shale reservoirs was established using saturated NMR for plug-sized samples and isothermal methane adsorption for crushed samples under high-temperature and high-pressure conditions. On this basis, a geological extrapolation model was established to predict the gas-bearing characteristics and controlling factors of shale reservoirs with different burial depths. The results will provide useful guidance for evaluating the mechanism and mode of gas enrichment for deep shale gas reservoirs in China.

3.During saturation for NMR, how do the authors ensure full saturation of the sample?for how long are the samples kept in saturation cells?

Reply: Under different pressure conditions, the amount of saturated methane in shale cores varies. The time required for shale cores with different pore structures to reach saturation also varies. So The criterion for determining the saturated methane equilibrium of shale is to test the NMR T₂ spectrum every 1 hour after methane adsorption for a period of time, with three consecutive NMR T2 spectra remaining unchanged to indicate full saturation of the sample.

4.Does the authors have which specific clay minerals are present in the samples?Are they swelling ornon-swelling clay minerals?

Reply: The content of clay minerals in shale reservoirs of shale gas development targets is about 15%, with illite as the main clay mineral, accounting for 80%. The swelling capacity is relatively small, less than 5%.

5.The conclusion and abstract can be improved.Please make sure your conclusions section underscores the scientific value-added of your paper,and/or the applicability of your findings/results.Highlight the novelty of your study.In addition to summarizing the actions taken and results,please strengthen the explanation of their significance. It is recommended to use quantitative reasoning compared with appropriate benchmarks,especially those stemming from previous work.

Reply: We have made some modifications to the abstract and conclusion, highlighting the significance of this study. For example “This work establishes a NMR method of saturated methane on plug-sized samples to test the free gas content and a prediction model of shale reservoirs at different burial depths. The gas content of shale reservoir is influenced by both burial depths and pore structure. When the burial depth of shale gas reservoir is less than 2000 m, inorganic pores and microfractures develop, and the self-sealing ability of the reservoir to shale gas is weak, resulting in low gas content.. However, due to the small pore size of organic pores and low formation temperature, the content of adsorbed gas increases, accounting for up to 60%. As the burial depth increases, the free gas and total gas content increase; at 4500 m, the total gas content of shale reservoirs is 18.9 m³/t, and the proportion of free gas can be as high as 80%. The total gas content predicted by our method is consistent with the results of pressure-holding coring technique, which is about twice the original understanding of gas content, greatly enhancing the confidence in accelerating the exploration and development of deep shale gas.”

Author Response File: Author Response.pdf