The Distribution of Gas Components within a Shale System and Its Implication for Migration

Round 1

Reviewer 1 Report

I really enjoy reading it. Please make these below minor revisions

1. Most of the units are in metric. since this paper is for broader audience. It will be great if you can include imperial unit next to the metric unit (in bracket , e.g. 100 meters (~328 ft.).
2. Label the stratigraphic shale consistently (e.g. some are labelled Chang7³ and some are Chang 7₃), make them consistent, use either only subscript or only superscript.
3. Have the grammar checked (e.g. line no 103; line 116 - for 3 hours heating).4
4. Line no. 114 and 123, Why 120deg C?, is it represent the reservoir temperature?
5. Table 3, change symbol of deltaDC2 into Delta13CO26.
6. Figure 2 is confusing. especially when you put accumulated gas, retained gas, and generated gas points into TOC, S1, S2, S3, and desorbed gas columns. Especially when you put the Figure 2 before your explanation about gas types on Line 257. So please add explanation in the caption. Caption should self-explain a figure without having a reader going to the main body text.
7. I suggest you re-write the discussion part. Currently it is vaguely written. E.g. discuss why samples with  TOC less than 2% has low volume of desorbed gas.
8. Line no. 299; Fig 5a and 5b.
9. A lot of redundancy in the discussion part that were already mentioned in the result section.  

Good Luck

 

Author Response

Most of the units are in metric. since this paper is for broader audience. It will be great if you can include imperial unit next to the metric unit (in bracket , e.g. 100 meters (~328 ft.).

We have change all the units

Have the grammar checked (e.g. line no 103; line 116 - for 3 hours heating).

We have revised all of these errors 4 placing the sealed tank into desorption system and heating for 3 hours .

Line no. 114 and 123, Why 120deg C?, is it represent the reservoir temperature?

The reservoir temperature is between 50-60 ℃ and We have explained this in the text. We changed this as “120 ℃, for nearly all the adsorbed gases can be desorped near 120 ℃)[4]”

Table 3, change symbol of deltaDC2 into Delta13CO26.

We have changed all the deltaDC2 to Delta13CO2

Figure 2 is confusing. especially when you put accumulated gas, retained gas, and generated gas points into TOC, S1, S2, S3, and desorbed gas columns. Especially when you put the Figure 2 before your explanation about gas types on

Line 257. So please add explanation in the caption. Caption should self-explain a figure without having a reader going to the main body text.

We have revised the information of Figure 2. and we also revised the Caption as “Figure 2. Histogram showing the cores, geological parameters and gas type classifications

I suggest you re-write the discussion part. Currently it is vaguely written. E.g. discuss why samples with  TOC less than 2% has low volume of desorbed gas.

We have revised some parts of discussion. For samples with  TOC less than 2% has low volume of desorbed gas, we revised as follows.

As shale with low TOC usually have low hydrocarbon generation ability, the shale samples with TOC less than 2% may generated a small amount of shale gas, resulting the low volume of desorbed gas. However, the low volume of desorped gas does not indicate that the shale gas is in-situ generated gas, for a part of gas generated by shale with higher TOC may migrated and stored within the samples with TOC less than 2%. In order to judging the in-situ generated gas or migrated gas, the gas dryness can be used as a reference.

Line no. 299; Fig 5a and 5b.

We have checked and revised all these information.

A lot of redundancy in the discussion part that were already mentioned in the result section.  

We have revised both some parts of discussion and the conclusion.

Author Response File: Author Response.pdf

Reviewer 2 Report

Title: The distribution of gas components within shale system and its
implication for migration

 

The manuscript presented the overall procedure and results obtained from the desorption and adsorption experiments for YC24 Well in the Chang-7 shale gas field in China. Then, the study attempted to get a full understanding of 59 distribution characteristics of different gas components, to explore the migration ability 60 of different gas components, and to explain the accumulation  characteristics of shale gas.

 

Overall, the study presented various experimental methods and procedures required to characterize shale gas formation, adopting several recently presented theories. Thus, I would believe the contents could be interesting to a readership in Minerals and the experimental results regarding the Chang-7 shale gas field in China would be also useful for some readers.

 

Otherwise, I couldn't find any new theoretical findings or technical advances from the manuscript; and it rather looked like a geological report specialized to geochemical experiments. Moreover, most methods and procedures adopted into the study were based on the well known general approaches.

Hence, I am afraid whether we scarcely find any scientific values and improvements from the manuscript. Then, I would strongly recommend the authors should supplement the unique, important and specialized matters  achieved in this study, such as theories, methods or procedures. This is the main points to be useful for the novel readership of Materials journal.

Particularly, you presented several conclusions including the different migration abilities of different components. However, I would think it is natural and widely known phnomena in terms of migration. Also you presented the quantitative values of some properties and types. Here, I could not easily agree to your results because you have not shown any proofs or reasonabe explanations about them; if you would think they have been shown, I would recommend you to supplements them.

 

Minor revisions:

(1) The experimental procedures would rather be explained using flow-charts or others to be effectively presented to the readers.

(2) Most discussions about the experiment results could be improved using graphs or charts instead of tables; the tables could be moved to appendix section.

(3) 19 Chang 7 shale samples collected from YC24 well should be expained more concisely also using another map or chart.

Here, I am a little suspicious the rapid and strong changes between the samples are reasonable, as you also indicated the shale system has very strong heterogeneity.

As I could not find any special deffects in your experimental procedures, I would believe the experimental results might be accurate; however, in my experiences, I have not seen such strong and rapid changes in the neighboring sections.

(4) In page 13, there is no Figure 8, so you should check the figure numbers . Also, the notation for figures should be coinsident. (Fig. 1. & Figure 1)

 

I appreciate your hard works for the experiments and manuscript. I also expect you would efficiently improve the quality to be published in the novel journal, Minerals so that widely refered to various readership.

 

Best regards and thanks,

Reviewer.

Author Response

I admit that most methods and procedures adopted in the study were based on the well known general approaches. I think the most important thing is that there still exist many strange characteristics ( the strange distribution of gas content and gas components in this text) to be explored, but no former researches hardly noticed the strange distribution of gas content and gas components. For this manu, its just like clue to remind the reader that many things to be explored. For this content of this paper, we have revised it.

Minor revisions:Minor revisions:

(1) The experimental procedures would rather be explained using flow-charts or others to be effectively presented to the readers.

We have tried our best to using graphs or charts. However, each experiment is too simple, Thus we drew as new Figure as “Figure 2. Chart showing the flows and purposes of experiments”

(2) Most discussions about the experiment results could be improved using graphs or charts instead of tables; the tables could be moved to appendix section.

We have revised this. We added two Figures. Figure 3. The volume percentage of different gas component.     Figure 4. The carbon isotopes of different gas components.

(3) 19 Chang 7 shale samples collected from YC24 well should be expained more concisely also using another map or chart.

Here, I am a little suspicious the rapid and strong changes between the samples are reasonable, as you also indicated the shale system has very strong heterogeneity.

As I could not find any special deffects in your experimental procedures, I would believe the experimental results might be accurate; however, in my experiences, I have not seen such strong and rapid changes in the neighboring sections.

This is the same opinion as that of Reviewer 2. I totally agree with the Reviewer. Indeed most of the wells have little changes. During my past five years work in petroleum company, we just found a few wells have this rapid changes. Once we tested the H2S and N2 (most of the gas components are them)in the sheer shale section. Thus i think the shale gas geology is very complex, many works need to be explored. For this manu, its just like clue to remind the reader that there still exist many strange characteristics to be explored, as no former researches hardly noticed this.

(4) In page 13, there is no Figure 8, so you should check the figure numbers . Also, the notation for figures should be coinsident. (Fig. 1. & Figure 1)

We have revised them in the text.

 

 

Author Response File: Author Response.pdf

Reviewer 3 Report

The paper discusses topical research in the field of gas components formation and their migration in shale formations. It is noted in the paper that shale rocks are capable of gas adsorption, the development of such rocks can provide gas production with different composition. The authors argue with the need for targeted research on gas adsorption and desorption of specific shale formations. Such studies will make it possible to describe the distributions of various gas components in rocks as a result of the different migration ability of gas components and to explain the characteristics of shale gas accumulation.

It should be noted a large amount of research carried out by the authors, which allows us to fully characterize the origin of Chang 7 shale gas and to explain the diversity of the produced gas composition. The experimental techniques described in the paper represent a scientific interest in the field of shale gas development and will be useful for staff in the oil and gas industry.

Comments:

1. All the conclusions have based on experimental data from one well in a particular depth interval and therefore in my opinion they cannot claim to be “global”. It would be good to include the comparison between the obtained results and the data from other wells/fields/reservoirs.
2. Based on data shown in Fig. 4 (b, d) authors conclude that “In YC24 well, TOC shows a good positive correlation with S2 and volume of desorbed gas”. To prove this statement (and similar ones in the paper) it’s highly recommended to calculate the correlation coefficients.
3. It's desirable to revise the overall text of the paper to avoid numerous repetitions. I hope it will make the paper more understandable and readable.

Author Response

Reviewer 2

All the conclusions have based on experimental data from one well in a particular depth interval and therefore in my opinion they cannot claim to be “global”. It would be good to include the comparison between the obtained results and the data from other wells/fields/reservoirs.

This is the same opinion as that of Reviewer 3. I totally agree with the Reviewer, and We tried this even before our writing paper. However, two conditions are need for comparison: one is to carry out desorption experiments on a profile with the unique rock combination, and the other is that the gas composition of desorption gas is consistent with that of the study well. But we can't find this research that meets the conditions.

During my past five years work in petroleum company, we just found a few wells have this rapid changes. Once we tested the H2S and N2 (most of the gas components are them)in the sheer shale section. Thus i think the shale gas geology is very complex, For this manu, its just like clue to remind the reader that there still exist many strange characteristics to be explored, as no former researches hardly noticed this.

Based on data shown in Fig. 4 (b, d) authors conclude that “In YC24 well, TOC shows a good positive correlation with S2 and volume of desorbed gas”. To prove this statement (and similar ones in the paper) it’s highly recommended to calculate the correlation coefficients.

We have revised the Figure and calculated the correlation coefficients.

It's desirable to revise the overall text of the paper to avoid numerous repetitions. I hope it will make the paper more understandable and readable.

We have revised the text.

Author Response File: Author Response.pdf