Influence of Low-Temperature Hydrothermal Events and Basement Fault System on Low-Resistivity Shale Reservoirs: A Case Study from the Upper Ordovician to Lower Silurian in the Sichuan Basin, SW China

Round 1

Reviewer 1 Report

The MS entitled “Influence of low-temperature hydrothermal events and basement fault system on low-resistivity shale reservoirs: a case study from the Upper Ordovician to Lower Silurian in the Sichuan Basin, SW China”. This paper comprehensively discussed the connection of hydrothermal activity and low resistivity shale and firstly propose that the distribution of low-resistivity shale is directly controlled by the Paleozoic fault system and the late Permian hydrothermal fluids in the Southern Sichuan Basin.

The manuscript is detailed, and the topic is interesting for whoever is concerned with shale gas exploration. Moreover, this paper is well written, presents new information related to shale formation, and can be accepted with minor revisions.

General: The description of the composition characteristic and mineral morphology of hydrothermal mineral assemblage in the Section 4.3 and in the Section 5.11 is a little repetitive. Please recheck and delete the repetitive part.

Some minor errors of incomplete information or mistake in writing are needed further medicated:

(1)        Line 289: In Figure 4, braces of (f) in the Figure 4(f) are not shown in the picture and the feature description of hydrothermal veins of figure 4(i) is missing. Moreover, “i” should be capitalized in Figure 4(i).

(2)        Line 470-471: “whereas the surrounding host rocks are richer in Fe, Mn, and Al but poorer in Mn” should be corrected to “whereas the surrounding host rocks are richer in Fe, Mg, and Al but poorer in Mn”.

(3)        Line 480-481: “Marine carbonate rock”, “Mantle fluid flow”, these words have been repeated twice.

(4)        Line 862: It is suggested that the positions of Figure 17 and Figure 12 should be changed, there is a mistake with the figure number cited in the text (Line 489).

Author Response

Reviewer1:

The MS entitled “Influence of low-temperature hydrothermal events and basement fault system on low-resistivity shale reservoirs: a case study from the Upper Ordovician to Lower Silurian in the Sichuan Basin, SW China”. This paper comprehensively discussed the connection of hydrothermal activity and low resistivity shale and firstly propose that the distribution of low-resistivity shale is directly controlled by the Paleozoic fault system and the late Permian hydrothermal fluids in the Southern Sichuan Basin.

The manuscript is detailed, and the topic is interesting for whoever is concerned with shale gas exploration. Moreover, this paper is well written, presents new information related to shale formation, and can be accepted with minor revisions.

General: The description of the composition characteristic and mineral morphology of hydrothermal mineral assemblage in the Section 4.3 and in the Section 5.11 is a little repetitive. Please recheck and delete the repetitive part.

Answer: Thanks for your suggestions. We just adjust the repetitive descriptions about the mineral assemblages related to hydrothermal activity.

For Chlorite, we think the discussion about the temperature of the chlorite mentioned in the Section 4.3.1 is repetitive to the Section 5.1.1.(The high amount of Mn is the most recognizable geochemical indicator of this form of chlorite (Figure 7). As a result, it is designated as Al-Mn clinochlore for the purposes of the following discussion. According to certain studies, higher forming temperatures and high tetrahedral Al content in chlorite are strongly correlated. Therefore, the low concentration of tetrahedral Al in our samples suggested that the forming temperature of brunsvigite is not very high (Figure 7c). ). We think this paragraph is more suitable to be placed in the start of the Chapter 5.1.1 Section. Therefore, this description has been simplified.

For barite and hyalophane, some repetitive analysis of the distribution of REE patterns in the Section 5.1.1 has been shortened and polished. The descriptive and repetitive description of barium in the 5.1.1 has been deleted.

For calcite veins of low-resistivity shale samples, there are the same conclusions of calcite veins’ origin, driven by carbon and oxygen isotopes, both in the Section 5.1.1 and Section 4.3.1. Therefore, the repetitive conclusion has been deleted in the Section 4.3.1. (From the classical carbon and oxygen isotope fluid source chart, it can be seen that the calcite veins in Type II low-resistivity shale may be derived from marine sedimentary carbonate rocks, low-temperature hydrothermal solutions, hydrocarbon pyrolysis and other mixed fluids (Figure 11a, b, [51]). ).

Some minor errors of incomplete information or mistake in writing are needed further medicated:

(1)        Line 289: In Figure 4, braces of (f) in the Figure 4(f) are not shown in the picture and the feature description of hydrothermal veins of figure 4(i) is missing. Moreover, “i” should be capitalized in Figure 4(i).

Answer: Thanks for your suggestion. Braces of (f) in the Figure 4 have been corrected, “i” in the Figure 4 has been modified. Moreover, the feature description of figure(i) has been added in the caption of Figure 4.

(2)        Line 470-471: “whereas the surrounding host rocks are richer in Fe, Mn, and Al but poorer in Mn” should be corrected to “whereas the surrounding host rocks are richer in Fe, Mg, and Al but poorer in Mn”.

Answer: Thanks for your suggestion. Following your suggestion, we have changed the sentence from “whereas the surrounding host rocks are richer in Fe, Mn and Al…” to “whereas the surrounding host rocks are richer in Fe, Mg, and Al but poorer in Mn”. This mistake has been corrected.

(3)        Line 480-481: “Marine carbonate rock”, “Mantle fluid flow”, these words have been repeated twice.

Answer: Thanks for your suggestion. We are very sorry for this mistake and redundant, repetitive words, such as “marine carbonate rock”, “mantle fluid flow” have been deleted.

(4)        Line 862: It is suggested that the positions of Figure 17 and Figure 12 should be changed, there is a mistake with the figure number cited in the text (Line 489).

Answer: Thanks for your suggestion. We really appreciated for this suggestion because we discovered that locations of Figure 12, Figure 13 and Figure 17 were wrong. Firstly, we changed figures of Figure 12 and Figure 17. Moreover, wrong names and captions of Figure 13 and Figure 12 have been corrected.

 

Author Response File: Author Response.pdf

Reviewer 2 Report

The work is based on a large amount of good analytical data. This is problem!!! As the results are overwhelming and the clear conclusion is not provided the way understandable for readers from a different field. The reader is lost in great detail descriptions and information. The use of different analytical techniques is beneficial in the work, however text needs to be rearranged the way that allows reader to use authors conclusions and findings. The main results are now lost in text.

The detailed description is often redundant and imprecise, eg lines 267-273, where the results are described with the words high and low. The variance of the values is not taken into account and the entire paragraph is unnecessary.

I recommend not to present the results in a table, figure and text in parallel, Discuss only the essential and most important results and clearly formulate conclusions.

Author Response

Reviewer2:

The work is based on a large amount of good analytical data. This is problem!!! As the results are overwhelming and the clear conclusion is not provided the way understandable for readers from a different field. The reader is lost in great detail descriptions and information. The use of different analytical techniques is beneficial in the work, however text needs to be rearranged the way that allows reader to use authors conclusions and findings. The main results are now lost in text.

The detailed description is often redundant and imprecise, eg lines 267-273, where the results are described with the words high and low. The variance of the values is not taken into account and the entire paragraph is unnecessary. I recommend not to present the results in a table, figure and text in parallel, Discuss only the essential and most important results and clearly formulate conclusions.

Answer: Thanks for your suggestions. Following you suggestion, Conclusion, Abstract, Section 4. Results and 5. Discussion have been significant adjusted. All data descriptions in the MS have been simplified, and data descriptions will not exist in both text and images. The interpretation of figures is mostly based on the geological significance derived from data analysis, while the text does not contain specific repeated descriptions of data (Figure 6, Figure 7, Figure 8, Figure 9, Figure 10, Figure 11, Figure 12, Figure 13, Figure 14, Figure 16). Meanwhile, the description of unimportant results has been significantly reduced in the Section “Conclusion”, “Abstract”, “Results”, “Discussion”. Moreover, “Conclusion” and “Abstract” have been re-rewritten for clearly formulate conclusion.

In the Section 4. Results, qualitative descriptions (e.g. high or low) have been significantly reduced, while many quantitative descriptions of plates and specific results of data have been added . Moreover, Section 4. Results has been re-organized, for comparing the mineral composition, geochemical element, thermal maturity, electrical resistivity, graphitization degree between samples with hydrothermal fine- veins (FV samples) and samples without hydrothermal fine- veins (NFV samples). The pattern of reorganization for Section 4. Results is to emphasize the impact of hydrothermal fluids on low resistivity shale reservoir.

In the Section 5. Discussion, all quantitative data descriptions have been transferred to Section 4. Results, while all discussion processes have been refined and simplified. Especially, we have added more discussion about the effects of hydrothermal activity on low resistivity shale.

For Abstract, and Conclusion, both parts have been re-written in the following 4 aspects: ① Firstly discovered that the low-resistivity shale in the Southern Sichuan Basin were modified by hydrothermal thermal and the characteristics of hydrothermal fluids, ② The effects of hydrothermal events on low-resistivity shale and the degree of organic graphitization, ③ The timing of hydrothermal fluid emplaced and modified the low- resistivity shale, and ④ The process of Paleozoic fault systems and the hydrothermal fluids controlled the distribution of low-resistivity shale in the Southern Sichuan Basin.

After sorting out the above, we firmly believe that the overall context of the article will be clearer. Please refer to the latest version of the manuscript for specific changes.

 

Author Response File: Author Response.pdf

Reviewer 3 Report

1. Fundamental and practical significance of the results must be added into the abstract and conclusions.

2. Abstract is too wordy, should by shortened.

3. Connection of low-resistivity and graphitization is not elaborated enough, please extend.

4. I suggest authors to add lithological and stratigraphic chart in the "Geological setting" section.

5. Have you separated organic carbon and calcite carbon for isotope analysis? Explanation is needed in the method section. Obviously these two carbons will have different isotope ratios. 

6. How TOC and Ro have been measured? It is not clear.

7. What is the source of Mn in your samples, what is the reason for its depletion? Brief explanation and discussion is needed.

 

8. More detailed discussion is needed regarding the influence of hydrothermal activity. 

Author Response

Reviewer3:

1. Fundamental and practical significance of the results must be added into the abstract and conclusions.

Answer: Thanks for your suggestions. Following your suggestion, we have added the fundamental and practical significance of the results, at the beginning of the abstract and conclusion.

For abstract, this description has been added for scientific significance illustration: Low resistivity and low contrast pay in shale reservoirs have persistently posed challenges in petrophysical evaluation, leading to missed opportunities in the oil and gas industry. It is crucial to understand the controlling factors and genetic mechanisms to select shale exploration targets accurately. In the Southern Sichuan Basin, the widespread development of low-resistivity shale with poor reservoir quality is mainly due to a high degree of organic graphitization. However, the process of organic graphitization in low-resistivity shale requires further discussion.

For conclusion, this description has been added for scientific significance illustration: Underestimating low resistivity and low contrast pay zones in shale has historically resulted in missed opportunities for shale oil and gas exploration and production. Four main reasons for low resistivity pay in shales are: organic graphitization, high water saturation, high content of special minerals, and finely laminated shale sequence. Highly graphitized, low-resistivity shale often has poor reservoir quality, affecting the screening of subsequent shale oil and gas targets. Understanding the causes of organic matter graphitization is crucial for effective exploration and exploitation of shale gas and oil in the Southern Sichuan Basin. Highly graphitized, low-resistivity shale often has poor reservoir quality, affecting the screening of subsequent shale oil and gas targets. Understanding the causes of organic matter graphitization is crucial for effective exploration and exploitation of shale gas and oil in the Southern Sichuan Basin.

2. Abstract is too wordy, should by shortened.

Answer: Thanks for your suggestion. Following your suggestion, we have shortened and refined the abstract of this manuscript. The word count of the abstract has been reduced from 477 words to 320 words, while added the content of research significance and scientific issues of low resistivity shale. The current abstract is as follows:

Low resistivity and low contrast pay in shale reservoirs have persistently posed challenges in petrophysical evaluation, leading to missed opportunities in the oil and gas industry. It is crucial to understand the controlling factors and genetic mechanisms to select shale exploration targets accurately. In the Southern Sichuan Basin, the widespread development of low-resistivity shale with poor reservoir quality is mainly due to a high degree of organic graphitization. However, the process of organic graphitization in low-resistivity shale requires further discussion. This paper clarifies the organic graphitization process in low-resistivity shale of the Changning Block, based on mineral composition, geochemistry, and petrophysical properties of 199 shale samples from 4 wells. The low-resistivity shale with an average resistivity of lower than 5 Ω. m often develops veins that are composed of hydrothermal minerals, such as brunsvigite -muscovite, barite-hyalophane-barium feldspar-potassium feldspar-anhydrite and calcite-ankerite. The existence of these hydrothermal mineral assemblages shows that the low-resistivity shale of the Wufeng- Longmaxi Formation in the Changning Block has generally undergone the hydrothermal reformation of small-scaled alkaline hot brine at the transition of P/T. Shale with organic matter Ro greater than 3.5% and graphitization degree greater than 25% develops hydrothermal veins widely, indicating small-scaled hydrothermal activities as a factor causing rapid increase in the degree of organic graphitization. Paleozoic fault systems and Late Permian hydrothermal activities, coeval with the Emeishan mantle plume, control the graphitization degree of low-resistivity shale. The formation of water and seawater infiltrated into the deep crust along the Paleozoic basement faults under gravity, formed alkaline hot brine through mantle plume heating and then occurred water-rock reaction with basement rocks. They migrated upward along deep and large Paleozoic faults through convective thermal circulation in the Tiangongtang area, Shuanglong-Luochang area and Xuyong area. This increased the formation temperature of the Wufeng-Longmaxi layer, as well as the thermal maturity and graphitization degree of organic matter in low-resistivity shale, resulting in the wide distribution of low-resistivity shale in the Changning Block.

2. Connection of low-resistivity and graphitization is not elaborated enough, please extend.

Answer: Thanks for your suggestion. We have added the following paragraph to describe the connection of low-resistivity and graphitization in the Introduction: Typically, low-resistivity shale reservoirs are often featured a high content of graphite and paramagnetic minerals, with moderate to low porosities, showing extremely low resistivities that are often less than 5Ω.m. This is because that when shale reaches the mature-to-overmature stage (Ro > 3.5%), the aromatic conjugation of organic matter rapidly increases, and the orderliness and crystallinity of the carbon layer dramatically enhance, along with a rapid increase in the graphite content of organic matter. Graphite has a very high polarization rate, and the higher the graphite content in organic matter, the greater the polarization rate and the lower the electrical resistivity. Therefore, highly mature shale undergoes significant graphite maturation of organic matter, which increases conductivity of shale reservoir, resulting in a rapid decrease in shale resistivity.

4. I suggest authors to add lithological and stratigraphic chart in the "Geological setting" section.

Answer: Thanks for your suggestion. We have added lithological and stratigraphic figure in the “Figure 1”, as the following picture showing:

Figure 1. Structural position and stratigraphic columnar section of the Southern Sichuan Basin. (a) Map showing tectonic sub-units of the Southern Sichuan Basin, with the blue rectangular indications that the location of the Changning block. (b) lithological and stratigraphic figure of the Wufeng- Longmaxi Formation. ((a) and (b) modified after[7]).

5. Have you separated organic carbon and calcite carbon for isotope analysis? Explanation is needed in the method section. Obviously these two carbons will have different isotope ratios. 

Answer: Thanks for your suggestion. For analysing the effect of the low-temperature hydrothermal activity to shale reservoir, we measured the inorganic isotope data from the calcite veins developed in the low- resistivity shale. Meanwhile, we have described the experimental process of carbon and oxygen measuring in detail. The experiment is characterized by our first use of a diamond wire saw to cut along the edge of a calcite vein with a width of 1-2mm, exposing the calcite vein. Then, using a small dental drill, we carefully collect powder samples from the exposed calcite vein (This explanation has been added in the Section 3.1.2). The above method can completely separate the powder of calcite veins from bulk- rock samples, so that the measurement of carbon isotope of calcite veins can be unaffected by the surrounding shale rock.

In this study, the isotope analysis has not been conducted on the organic carbon. In this study, the analysis of TOC and Ro is enough for characterization of organic graphitization degree. Following your suggestion, maybe in the next time we would make some analysis about the connection of organic carbon isotope and organic graphitization degree. Therefore, the name of 3.1.2 has been changed from “Isotope geochemistry” to “Inorganic isotope geochemistry”.

6. How TOC and Ro have been measured? It is not clear.

Answer: Thanks for your question. The measuring process of TOC has been added in the Section 3.2:

All powdered samples with grain sizes between 200 and 400 mesh were measured for total organic carbon (TOC) using a LECO CS-200 carbon/sulfur analyzer. The powdered samples underwent treatment with a hydrochloric acid (HCl) solution with a ratio of 1:7 for volume of HCl to water. This step aimed to remove any carbonate minerals present in the samples, which can potentially interfere with the measurement of organic carbon content. After the removal of carbonate minerals, the remaining sample was neutralized using distilled water. Subsequently, the residue was dried and then subjected to combustion in order to determine the organic carbon content.

Details of measuring process of TOC has been added in the Section 3.2: To create pellets of 10 mm × 10 mm × 3 mm in size from shale samples, they were cut parallel to the bedding. Each of the resulting pellets was then meticulously polished with 4000 mesh sandpaper. A total of 48 pellets were selected for measuring the vitrinite reflectance of pyrobitumen using Raman spectrum. Raman Imaging and Scanning Electron Microscopy, developed by Zeiss and WITec, were used together for in situ Raman laser spectroscopy investigation of 928 OMs from 48 samples in order to assess the thermal maturity and the degree of organic graphitization of OMs found close to hydrothermal veins. In this investigation, a semiconductor laser with a wavelength of 532 nm and a power of 15 MW was employed, together with an energy-dispersive X-ray spectrometer (EDS, Ultim Max65) and a laser Raman spectrometer (Renishaw Company, UK,[34]). Since the Wufeng-Longmaxi marine shale is at a high to over-high maturity stage and lacks vitrinite, the equivalent vitrinite reflectance (Ro) was calculated from bitumen reflectance (BRr) using the formula…

7. What is the source of Mn in your samples, what is the reason for its depletion? Brief explanation and discussion is needed.

Answer: Thanks for your suggestion. We have added the content about the source of Mn. And explained the reasons for the difference in Mn enrichment in calcite veins and surrounding rocks. Following explanation has been added in the Section 5.1.1:

The calcite veins found in the low resistivity shales were formed by low-temperature hydrothermal fluids that underwent water-rock interactions. The formation environment of the veins was a closed hydrocarbon-water-rock system. An Al-Fe-Mn ternary diagram suggests that the calcite vein forming system was relatively closed and had little influence from hydrothermal fluids. There was no influx of distal fluids, such as meteoric waters.

The mass fraction of Mn in acidic, neutral, and basic igneous rocks is between (600-800) × 10^-6, (1200-1500) × 10^-6, and (1500-2500) × 10^-6, respectively. The mass fraction of Mn in atmospheric and sea water is very low (generally less than 0.1 × 10^-6). Therefore, the Mn element in the calcite veins may have originated from magma-related hydrothermal fluids. In the late Permian, during the process of mantle plume rising to the continental lithosphere, the strata were subjected to large-scale baking, causing a large amount of metal elements related to intermediate-basic igneous rocks to enter the formation water. Hence, the calcite veins were enriched with iron and manganese elements.

Before reaching the oil window stage, a significant amount of organic acid was produced, which reacted with the surrounding host rocks. This reaction promoted the migration of Mn, Ca ions, and other metallic elements, reduced the content of carbonate minerals in the surrounding host rocks, enriched the metallic elements in the veins, buffered the pH of the carbonate system, and caused dissolution of Ca²⁺ and HCO^3-.

8. More detailed discussion is needed regarding the influence of hydrothermal activity. 

Answer: Thanks for your suggestion. Following your suggestion, we have added more discussion about the effects of hydrothermal activity on low resistivity shale. The degree of hydrothermal activity has an important impact on the thermal maturity and graphitization degree of organic matter in low-resistivity shale, especially reflected in 3 aspects: increasing the thermal maturity of shale to matured- over matured, increasing graphitization degree, and decreasing organic pore volume in samples with hydrothermal modification. The study found that low-resistivity shale samples with hydrothermal fine veins contain organic matter with higher thermal maturity compared to those without hydrothermal fine veins (Figure 12). The statistics of 508 organic matters from low-resistivity shale samples with hydrothermal fine veins showed an average Ro value of 3.82%, while those of 420 organic matters from samples without hydrothermal fine veins showed an average Ro value of 3.38%. The organic graphitization widely occurs in low-resistivity shale samples with hydrothermal fine veins in the Changning Block, especially in the Type II samples. The Dh/Gh values of 23 organic matters in Type II samples range from 0.72 to 0.84, with an average of 0.74 (Figure 13a-d). In contrast, the organic matters in low-resistivity shale samples without hydrothermal fine veins have relatively low degrees of organic graphitization. For example, Dh/Gh values of OMs in Sample 3946.01 and Sample 3946.53 from NX2 are 0.56 and 0.59, respectively, with no obvious graphite peak (G 'peak), indicating that the samples have not undergone graphitization. Additionally, the modification process of low-temperature hydrothermal activities affects the development of organic pores. The organic matter wrapped by hydrothermal minerals has a high maturity (average Ro value is 3.82%). The organic matter is generally graphitized to a certain extent, resulting in almost no organic matter pores with radii ranging from 2nm to 53nm (Figure 14). However, the organic matters in samples at a similar depth but without hydrothermal fine veins have an average Ro value of 3.38% and relatively developed organic matter pores, with an average pore diameter of 823nm (Figure 14).

The above discussion has been added in the Section 5.2.

Moreover, Section 4. Results has been re-organized, for comparing the mineral composition, geochemical element, thermal maturity, electrical resistivity, graphitization degree between samples with hydrothermal fine- veins and samples without hydrothermal fine- veins. The pattern of reorganization for Section 4. Results is to emphasize the impact of hydrothermal fluids on low resistivity shale reservoir.

 

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Good work.