Geological Distribution of the Miocene Carbonate Platform in the Xisha Sea Area of the South China Sea, and Its Implications for Hydrocarbon Exploration

Round 1

Reviewer 1 Report

This paper sets out to examine the reservoir potential in an area in the South China Sea. The authors used well data and seismic data for their interpretation and relied on existing data in the region. Much of this data is not properly presented in the text. The bulk of the text deal with describing the evolution of the carbonate platforms in the study area. This information is not expended or contextualized before being discussed in the last chapter which attempts to discuss the reservoir aspects. Much of the aspected related to the reservoir potential are not properly presented in the results and there is a disconnect between chapters 4-5 and 6. Major revisions are needed including an expansion of the introduction, expansion of the results and rearrangement of the structure in order to make this manuscript publishable. 

Introduction

The introduction clearly presents the research motivation (carbonate reservoirs) and data gaps and why filling them is crucial. The Introduction is very brief and does not expend on the issues and fundamental problems with carbonate reservoirs (e.g. the effect of dolomitization, limitation of seismic interpreting while dealing with them, acquisition issues etc.) nor with the inherent issue of spatial/temporal variability in them which the rest of the test spends a fair amount of time addressing. To increase the general interest to readers, these should be plainly presented to highlight the problems the text is gaming to address rather than only stating there is a local data gap that the authors seek to fill.

Lines 49 to 53 should probably be in the geological settings.

Geological settings

while the authors established, there is already a wide database on the carbonate platforms in the South China Sea, this is glossed upon here which leaves the later parts of the paper without much-needed context. The description of the syn-rift and post-rift phases (lines 89 to 96) is worded oddly, treating these phases as seismic units rather than a temporal evolution. More information on the source units should be added.

Lines 80-81: what does "early in the south, late in the north, early in the east, and in the west" means? this sentence is not very clear nor informative on the pattern of evolution. 

Methods

The locations of seismic lines are not shown and their description is odd. If the data was collected for such a long period (26 years from 1989 to 2015) it is unlikely all of it will have the same parameters. Odds are some of this data is analogue, and some of it is digital. A supplement table listing the surveys and their parameters will help disambiguate this. There is also no information on 2017 reprocessed. 

Results

The results section is divided into two parts 4. Identification of carbonate platforms and 5. Results. In actuality, chapter 4 presents the actual observations (Results) whereas chapter 5 would be more accurately named Interpretation. 

In chapter 4.1 there is much information which is not clearly presented. Explain how the facies in Figure 4 were determined. What is in there? Describe the present facies, and explain their determination. At present, there is very little information in the text. The diagenetic facies (notably the dolomitization) is noted but with no explanation or description of the texture. All of these elements are critical for reservoir evaluation. 

Chapter 4.2 gives a brief description of the seismic facies; these could be expanded and shown in more figures. Notably, the channels and their transition across the platform (and to fans as implied in figure 10) could be shown in more detail. Clearer well ties would also be recommended as only one well is shown in the context of a seismic line while as of line 213 this is addressed in the text but not shown. I'd also note that from Line 213 to 280 there is a mixture of description, interpretation and discussion of the platform evolution that should be decoupled. Keeping the description and initial interpretation in this part and moving the temporal discussion to chapter 5. The volcanos shown in figure 10 are not addressed here although they have a critical role in controlling local uplift, topography and platform evolution. 

Lines 156 to 157: How was this identified? 

Line 175 (also 179): I think "significant" is not the right term here. Perhaps "more pronounced seismically"?

Line 273 to 274: this uplift and exposure is only reported with no supporting evidence.

Chapter 5 needs age information and not just lithostratigraphy. The authors already established in figure 2 that they have ages for the key horizons allowing them to infer the chronology in time. As the transition from phase 2 to phase 3 of the platform corresponds to a fundamental reorganization of the global platform growth pattern, having that context is imperative to the understanding of the changes and decline of the platform in subsequent phases. This chapter would also benefit from selected seismic lines illustrating the sequences and transitions. The presentation of this interpretation is missing greater context. Similarly, the diagenetic phases are not incorporated although they authors have information on them from the well data. 

Discussion 

At present, the discussion only discusses the potential reservoir properties while the bulk of the data presented deal with the stratigraphic evolutions of the system. As such no discussion actually deals with the bulk of the results presented while chapter 5 which integrated the stratigraphic aspects is rather descriptive and does not deal with the mechanistic aspects controlling this evolution. The description in chapter 5 does not relate these major changes in the platform structure to significant changes occurring at that time including changes in monsoon intensity, nutrient availability or ocean currents. 

I would strongly either add this element to chapter 5 or devote a separate sub-chapter in chapter 6 for the larger context. 

The present discussion of reservoir parameters is not supported at all by the information presented in period parts of the paper and presents new information not presented before. As such this information is not contextualized by the stratigraphic framework presented before. Essentially, this part of the paper is disconnected from what came before it. If the Auther wish to have a detailed discussion of the reservoir potential, the key information related to that including fluid migration pathways, relation to the kitchen area, HC charging and sill needs to be properly presented in the results. 

Chapter 6.2 is missing information on the thermal gradient and potential for gas vs. oil generation and when the source rocks would have entered the key windows. 

Line 355: how do you know that?

Line 360 to 362: not established until this point. also note that diagenetic process in carbonate can destroy porosity if occurring before HC have entered the pores. 

I would refer the author to some references relevant to the evolution of carbonate platforms and reefs around south and east Asia (Including from the south china Sea) during the study period which show similar patterns to the ones described and provide mechanisms that would be relevant to the observations here:

Betzler, C. and Eberli, G.P., 2019. Miocene start of modern carbonate platforms. Geology, 47(8), pp.771-775.

Mathew, M., Makhankova, A., Menier, D., Sautter, B., Betzler, C. and Pierson, B., 2020. The emergence of Miocene reefs in South China Sea and its resilient adaptability under varying eustatic, climatic and oceanographic conditions. Scientific Reports, 10(1), pp.1-16.

Wang, R., Yu, K., Jones, B., Wang, Y., Zhao, J., Feng, Y., Bian, L., Xu, S., Fan, T., Jiang, W. and Zhang, Y., 2018. Evolution and development of Miocene “island dolostones” on Xisha Islands, South China Sea. Marine Geology, 406, pp.142-158.

Wu, S., Chen, W., Huang, X., Liu, G., Li, X. and Betzler, C., 2020. Facies model on the modern isolated carbonate platform in the Xisha Archipelago, South China Sea. Marine Geology, 425, p.106203.

Wilson, M.E., 2008. Global and regional influences on equatorial shallow-marine carbonates during the Cenozoic. Palaeogeography, Palaeoclimatology, Palaeoecology, 265(3-4), pp.262-274.

Fournier, F., Montaggioni, L. and Borgomano, J., 2004. Paleoenvironments and high-frequency cyclicity from Cenozoic South-East Asian shallow-water carbonates: a case study from the Oligo-Miocene buildups of Malampaya (Offshore Palawan, Philippines). Marine and Petroleum Geology, 21(1), pp.1-21.

Liu, L., Eronen, J.T. and Fortelius, M., 2009. Significant mid-latitude aridity in the middle Miocene of East Asia. Palaeogeography, Palaeoclimatology, Palaeoecology, 279(3-4), pp.201-206.

Holbourn, A., Kuhnt, W., Clemens, S.C. and Heslop, D., 2021. A∼ 12 Myr Miocene record of East Asian Monsoon variability from the South China Sea. Paleoceanography and Paleoclimatology, 36(7), p.e2021PA004267.

Shahzad, K., Betzler, C., Ahmed, N., Qayyum, F., Spezzaferri, S. and Qadir, A., 2018. Growth and demise of a Paleogene isolated carbonate platform of the Offshore Indus Basin, Pakistan: effects of regional and local controlling factors. International Journal of Earth Sciences, 107(2), pp.481-504.

Bialik, O.M., Reolid, J., Betzler, C., Eberli, G.P. and Waldmann, N.D., 2020. Source shifts to periplatform deposits during the early to middle Miocene in response to climatic and oceanographic forcing, Maldives, western Indian Ocean. Palaeogeography, palaeoclimatology, palaeoecology, 559, p.109969.

Shao, L., Li, Q., Zhu, W., Zhang, D., Qiao, P., Liu, X., You, L., Cui, Y. and Dong, X., 2017. Neogene carbonate platform development in the NW South China Sea: Litho-, bio-and chemo-stratigraphic evidence. Marine Geology, 385, pp.233-243.

Lüdmann, T., Kalvelage, C., Betzler, C., Fürstenau, J. and Hübscher, C., 2013. The Maldives, a giant isolated carbonate platform dominated by bottom currents. Marine and Petroleum Geology, 43, pp.326-340.

Shiming, W., Anchun, L., Kehui, X. and Xueming, Y., 2008. Characteristics of clay minerals in the northern South China Sea and its implications for evolution of East Asian Monsoon since Miocene. Journal of China University of Geosciences, 19(1), pp.23-37.

Figures

Figure 1: in subfigure a, show all claimed EEZ for the region as this is still a disputed matter. UNCLOS note both Chinese and Vietnamese claim for the Paracel Islands. 

Figure 3: Please make this figure larger, perhaps by tilting in on its side. At present, it is very hard to see the features described. Also, note that the well in the figure is named Y35-1-1 while in the text it is called YC35-1-1 which may lead to confusion. This should be uniformed. 

Figures 4: Please add a mini map showing borehole locations to that the reader won't have to go back and forth. 

Figure 5: Please add a mini map showing borehole locations to that the reader won't have to go back and forth. Why is the lithological marking in this figure different then the ones in Figure 4? 

Figure 6 to 9: Please add maps showing the local bathymetry and configuration of these lines; they are very small in figure 1b. 

Figure 10: This interpretation far exceeds the presented data. The author should include the full coverage of seismic lines available to them (as reported in the Methods chapter) in figure 1b to illustrate that they are able to go into the level of detail presented in this figure. Additionally, the boundary interfaces described in chapter 4.2 should probably be included. 

Figure 12: Please note ages and not just formation names.

Author Response

Response to Reviewer 1 Comments

We are very glad to receive the comments from the review #1. Such kind suggestions and comments are of great significance to the manuscript revision and research level improvement, that we tried our best to revise our paper carefully. We appreciate it very much that most of the comments are very helpful to reproduce and highlight the core value of our paper.

The AIM of our paper is using high-resolution seismic profiles and drilling data to identify the edges of carbonate platforms, and to clarify their spatiotemporal distribution. Based on the spatial and temporal distribution, establish the mode of the evolutional stage of carbonate platform, and then discuss their potential hydrocarbon reservoir. The significance of its hydrocarbon reservoir is only a preliminary discussion.

Such research TECHNICAL ROUTE is feasible: 1) Seismic data have been validated to be used for the identification of carbonate platform edges, related researches have been accepted by the peer experts; 2) Drilling data in Xisha and other relevant sea areas prove that these boundaries do exist.

The preliminary discussion about SIGNIFICANCE of carbonate reservoir is to point out the direction for the next step of hydrocarbon exploration: 1) Drilling data evidence shows that the carbonate reservoir processes good physical conditions for oil and gas (see Shi 2016). 2) The dolomite / recrystallization parts develop only in partially exposed layers, which are distributed in islands and reefs areas (e.g. Xisha Uplift). Deep water submerged carbonate platform are relatively rare. 3) The significance of hydrocarbon reservoir is preliminarily discussed from the hydrocarbon source rock reservoir, cover layer, dredging system and the speculated oil and gas transportation channel. The idea that deep water carbonate has the potential of oil and gas accumulation is a qualitative discussion, not a precise quantitative analysis.

Therefore, according to the comments, our ms was revised as follows.

 

Point 1: Introduction

The introduction clearly presents the research motivation (carbonate reservoirs) and data gaps and why filling them is crucial. The Introduction is very brief and does not expend on the issues and fundamental problems with carbonate reservoirs (e.g. the effect of dolomitization, limitation of seismic interpreting while dealing with them, acquisition issues etc.) nor with the inherent issue of spatial/temporal variability in them which the rest of the test spends a fair amount of time addressing. To increase the general interest to readers, these should be plainly presented to highlight the problems the text is gaming to address rather than only stating there is a local data gap that the authors seek to fill.

Lines 49 to 53 should probably be in the geological settings.

 

Response 1: We have added related content on research status, significance of hydrocarbon exploration of the carbonate platform at the very beginning to avoid misleading to the emphases on the reservoir property. The purpose of Line 49-53 is to describe the current research status and then introduce fundamental problems needed to soloved.

 

Point 2: Geological settings

while the authors established, there is already a wide database on the carbonate platforms in the South China Sea, this is glossed upon here which leaves the later parts of the paper without much-needed context. The description of the syn-rift and post-rift phases (lines 89 to 96) is worded oddly, treating these phases as seismic units rather than a temporal evolution. More information on the source units should be added.

 

Response 2: The syn-rift and post-rift phases: the preliminary research results show that the post-rift stage presents a tectonic stable period, which is suitable for the development of the carbonate platform.

Lines 80-81 have been revised.

 

Point 3: Methods

The locations of seismic lines are not shown and their description is odd. If the data was collected for such a long period (26 years from 1989 to 2015) it is unlikely all of it will have the same parameters. Odds are some of this data is analogue, and some of it is digital. A supplement table listing the surveys and their parameters will help disambiguate this. There is also no information on 2017 reprocessed.

 

Response 3: The data are mainly collected in 2010-2015, and reprocessed in 2017, combined with the newly collected data from 2020-2021, only very few data is collected before year 2010. We believe that our data quality is relatively high.

 

Point 4: Results

The results section is divided into two parts 4. Identification of carbonate platforms and 5. Results. In actuality, chapter 4 presents the actual observations (Results) whereas chapter 5 would be more accurately named Interpretation.

In chapter 4.1 there is much information which is not clearly presented. Explain how the facies in Figure 4 were determined. What is in there? Describe the present facies, and explain their determination. At present, there is very little information in the text. The diagenetic facies (notably the dolomitization) is noted but with no explanation or description of the texture. All of these elements are critical for reservoir evaluation.

Chapter 4.2 gives a brief description of the seismic facies; these could be expanded and shown in more figures. Notably, the channels and their transition across the platform (and to fans as implied in figure 10) could be shown in more detail. Clearer well ties would also be recommended as only one well is shown in the context of a seismic line while as of line 213 this is addressed in the text but not shown. I'd also note that from Line 213 to 280 there is a mixture of description, interpretation and discussion of the platform evolution that should be decoupled. Keeping the description and initial interpretation in this part and moving the temporal discussion to chapter 5. The volcanos shown in figure 10 are not addressed here although they have a critical role in controlling local uplift, topography and platform evolution.

Lines 156 to 157: How was this identified? This description is based on former researches, e.g. Fyhn et al. 2013

Line 175 (also 179): I think "significant" is not the right term here. Perhaps "more pronounced seismically"?

Line 273 to 274: this uplift and exposure is only reported with no supporting evidence.

 

Response 4: We have renamed the Chapter 4 as “Results” section and combined the Chapter 5 and 6 into the “Discussion” section with some necessary revision of the subtitles and related content.

The description of lines 156 -157 and line 273-274, is based on previous researches, e.g. Fyhn et al. 2013.

Line 175 has been revised.

 

Point 5: Chapter 5 needs age information and not just lithostratigraphy. The authors already established in figure 2 that they have ages for the key horizons allowing them to infer the chronology in time. As the transition from phase 2 to phase 3 of the platform corresponds to a fundamental reorganization of the global platform growth pattern, having that context is imperative to the understanding of the changes and decline of the platform in subsequent phases. This chapter would also benefit from selected seismic lines illustrating the sequences and transitions. The presentation of this interpretation is missing greater context. Similarly, the diagenetic phases are not incorporated although they authors have information on them from the well data.

 

Response 5: The ages of seismic surfaces are based on previous research results, Wu et al., 2009.

 

Point 6: Discussion

At present, the discussion only discusses the potential reservoir properties while the bulk of the data presented deal with the stratigraphic evolutions of the system. As such no discussion actually deals with the bulk of the results presented while chapter 5 which integrated the stratigraphic aspects is rather descriptive and does not deal with the mechanistic aspects controlling this evolution. The description in chapter 5 does not relate these major changes in the platform structure to significant changes occurring at that time including changes in monsoon intensity, nutrient availability or ocean currents.

I would strongly either add this element to chapter 5 or devote a separate sub-chapter in chapter 6 for the larger context.

The present discussion of reservoir parameters is not supported at all by the information presented in period parts of the paper and presents new information not presented before. As such this information is not contextualized by the stratigraphic framework presented before. Essentially, this part of the paper is disconnected from what came before it. If the Auther wish to have a detailed discussion of the reservoir potential, the key information related to that including fluid migration pathways, relation to the kitchen area, HC charging and sill needs to be properly presented in the results.

Chapter 6.2 is missing information on the thermal gradient and potential for gas vs. oil generation and when the source rocks would have entered the key windows.

Line 355: how do you know that?

Line 360 to 362: not established until this point. also note that diagenetic process in carbonate can destroy porosity if occurring before HC have entered the pores.

 

Response 6: On the study of reservoirs, Shi Zhiqiang (2016) has done a lot of work. Shi Zhiqiang has done a lot of work on the reservoir research. This paper is only a preliminary discussion, the layers of good physical property is calculated in the whole region based on the evolution stage. In previous studies, there are no structural influence or large scale of dolomitization, which theoretically concluded as good physical properties, so the most critical influencing factors are the hydrocarbon source rock and its migration system. Therefore, in the course of the reservoir discussion, we cite precious studies, and only discuss the distribution of the carbonate platform, and then discuss its reservoir according to it in this study.

On the possibility of missing information, I think we can continue to discuss it. However, the research presented in Chapter 5, clearly shows that the distribution of reservoirs can determine the basic direction of oil and gas exploration. We use the distribution characteristics of carbonate platform to clarify the matching relationship with regional source and migration system.

The number and description of line 355 are calculated according to the seismic profiles.

 

Pont 7: Comment of Figures

 

Response: 7

Figure 3 has been revised by redrawing the lithologic interfaces and retiled the well name as YC35-1-1.

Figures 4 and Figure 5 has been add a mini map. The lithological markings have been revised.

Figure 1B has been revised by adding locations of seismic profiles.

In figure 12, the ages of seismic surfaces are based on Wu et al, 2009.

Author Response File: Author Response.pdf

Reviewer 2 Report

Generally the paper presents good characterization of analysed oil system from data used in interpretation to final conclusions. There are only sone small details to correct:

1. To frequent use of the word "distibution"

2. Litological interface -better to use geological terminilogy

3. 6.1 title: Reservoir rocks: physical properties....

Author Response

Response to Reviewer 2 Comments

We are very glad to receive the comments from the review #2. Such kind suggestions and comments are of great significance to the manuscript revision and research level improvement, that we tried our best to revise our paper carefully.

 

Point 1. To frequent use of the word "distibution".

 

Response 1: have been revised.

 

Point 2. Litological interface -better to use geological terminilogy.

 

Response 2: has been revised as “Litologic interface”.

 

Point 3. 6.1 title: Reservoir rocks: physical properties....

 

Response 3: has been revised as “Physical Properties of the Reservoir Rock”.

Author Response File: Author Response.pdf

Reviewer 3 Report

My specific comments are included in the annotated file. I have marked the places that are difficult to understand, probably because the people responsible for the English (which is very good considering the grammar) was not a specialist in geology. Hence the paper requires correction and resubmission. 

Comments for author File: Comments.pdf

Author Response

Response to Reviewer 3 Comments

We are very glad to receive the comments from the review #3. Such kind suggestions and comments are of great significance to the manuscript revision and research level improvement, that we tried our best to revise our paper carefully.

 

Point: My specific comments are included in the annotated file. I have marked the places that are difficult to understand, probably because the people responsible for the English (which is very good considering the grammar) was not a specialist in geology. Hence the paper requires correction and resubmission.

 

Response:

We have revised the “abstract” section to make it more clear.

We have also comprehensively revised all the highlighted content shown in the review.pdf by review #2. ( Line 22, 23, 27, 29, 61, 81, 82, 92, 122, 123, 132, 140, 143, 145, 146, 150, 153, 191, 213, 216, 225, 230-234, 246, 256, 270, 274, 284, 294, 300, 303, 309, 311, 336-340, 365, 384-387, 412, 432, 436, 451, and etc. the number of line is referred to the original ms).

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

I strongly insist to change the references 1-3 (these are not relevant to the content of sentences).

You apply "prosperous phase" - perhaps "bloom phase"?

Comments for author File: Comments.pdf

Author Response

Response to Reviewer 3 Comments

Thanks again for the comments from reviewer 3.

 

Point: I strongly insist to change the references 1-3 (these are not relevant to the content of sentences).

You apply "prosperous phase" - perhaps "bloom phase"?

 

Response:

We have revised the references 1-3 and change all the “prosperous phase" to "bloom phase”.

Author Response File: Author Response.docx