Structural Architecture and Permeability Patterns of Crystalline Reservoir Rocks in the Northern Upper Rhine Graben: Insights from Surface Analogues of the Odenwald
Round 1
Reviewer 1 Report
Review of “Structural architecture and flow properties of crystalline reservoir rocks in the Northern Upper Rhine Graben: insights from surface analogues of the Odenwald” by Claire Bossennec et al.
Dear Authors and Ediotr,
In the followings, you will find my comments to the manuscript presented by Bossennec et al. In order to be suitable for publishing, the manuscript need to undergo to major revisions.
The submitted manuscript present a dataset of structural data about the geometry and network properties of some selected outcrops if the NURG, claimed representative of the structural diversity of the region. The Authors then present the results of stochastic DFN models computed to constrain the subsurface properties of some selected outcrops.
The main strength of the presented manuscript consists in its integrated research approach involving combined structural (multi-scale) quantifications and DFN modelling. However, the manuscript suffers from several weaknesses in the explanation of research methods, data presentation and integration between field/virtual structural data and DFN models. And it lack several fundamental references related to the adopted methods. The comments and suggestion I provide below aim at improving the data and methods presentations and to aid the Authors in specifying what are the data adopted for DFN models computation.
General comments:
The Authors need to clearly describe and specify all the methods involved in the acquisition and processing of data presented in this paper. If the methods are explained in another reference, please provide in any case a brief description of the methodology and the necessary amount of technical specifics to understand what the data retrieved could mean. See Line comments for the details.
Methods and data need to be clearly separated into two sections. At present, some method description is included in the data section.
The Authors need to specify in details the data adopted as input to compute the DFN models (fracture sets, orientation, length distribution) and how they have been retrieved.
I don’t understand the adoption of Kxx, Kyy, … I would describe the permeability tensor in terms of its principal components magnitude (K1, K2, K3) and orientation of each component. These are the parameters that defines the permeability tensor. Kxx, Kyy, … give only a partial representation of the permeability tensor, the principal components are likely inclined with respect to the geographical coordinates (x,y,z); thus Kxx, Kyy, Kxy,… will not provide any information to the general shape and orientation of the permeability tensor. (The principal components can be easily calculated in Fracman ticking the “Compute Eigenvectors” option in the ODA permeability tab).
In addition, I don’t understand the choice of the cell size of the DFN models. What is the main aim of the DFN modelling? To retrieve the permeability of a equivalent porous medium? Then the cell size is too small. Please specify in detail what are the aims of the DFN modelling in terms of analysed parameters.
In the discussion you need to consider the limitations of the manual extraction of lineaments onto the final results of network topology.
Detailed comments.
Line 35. Petrophysical rock properties.
Line 43. Please consider also Ceccato et al., 2020 https://doi.org/10.1016/j.jsg.2021.104444
Line 47: replace e.g. with “i.e.”
Line 54 (and elsewhere): what does “semi-artificial” mean? Are you referring to stochastic models?
Line 57: “a-priori”.
Line 62: delete “typical” or explain why they are typical.
General comment on the Geological Setting: Line 66-136. Could it be summarized better? Is al this detail necessary to the comprehension of the paper?
Line 145: what do you mean with profiles? Outcrop exposure? Please clarify.
Line 158: LiDAR. Please add the specs of the resolution dimensions and typology of LiDAR Is that airborne or ground-based?
Line 159: Bas syntax, please rephrase.
Lines 157-168: This section needs clarifications. First of all: explain Ransac and its output. Then explain why you need to hillshade the rastered version of the LiDAR dataset. Why do you need then to work on GIS? How do you extract and analyze the fracture network properties? (Need detailed description of the methods).
Line 169 – General comment on this section. The Authors need to explain better from where the data adopted in the DFN modelling are retrieved and what are the input parameters for the stochastic computing.
Line 175: 4000m
Lines 185-186: This sentence does not make any sense to me. Terzaghi correction is usually applied on data retrieved from line sampling methods. Are you adopting line sampling methods here? This is not clear.
Line 189: dependEnt.
Line 196-198: move “… with n, the Poisson ratio, […] the maximum stress” after the equation.
Line 204: explain the parameters in the equation.
Line 204-207: The reason behind DFN modelling is not clear at all. This cell size is rather small if you want to capture the equivalent porous medium properties of the crystalline basement, and the meaning of the sentence “to quantify permeability influence between fractures” is not clear to me.
Line 208: deterministic models are usually based on some outcrop maps, please specify and show them.
General comment to section 4.1 Lines 222-233: From where do these data come from? Report it in the main text, either in the method section or here in the data referring to a previous publication. Not in the figure caption.
Line 254: need to explain better the location of the selected outcrops. Then, are fracture network properties related to lithology or to the vicinity of faults and fracture zones?
Lines 274-276: why do you need this description here?
Line 292: Figure 6a represent the DFN model….
Line 297: where does the P10 data comes from? How have they been retrieved?
Line 305: what is Cv? Is it the coefficient of variation of spacing? How do you have computed it? How is it defined? Please add references.
Line 306: what is CL? How it is calculated?
Author Response
Dear rewiever,
Thank you very much for your valuable comments. Please find attached the detailed list of implemented revisions.
Our best regards,
Claire Bossennec and co-authors.
Author Response File: 
Author Response.docx
Reviewer 2 Report
This manuscript characterizes the fracture network on the surface of crystalline rocks in the NURG and uses the obtained fracture statistics to examine the flow properties of the rocks at depth. This would be of practical value and be interesting to industry. However, it is better to add to the “2. Geological context” section a large-scale tectonic map with recent maximum horizontal stress trajectories. This figure helps understand the extensional or transtensional state of a fault in the region or the study area for readers.
The manuscript is well written and well organized. There are still a small number of unclearings and strange expressions as follows:
Line 28: “DFN”？
Lines 54-55: needs citation about the DFN models.
Lines 78-80: What is the central segment? What is the relationship between compressional shear and extensional shear? In this paragraph the authors seem confused with the basic terms, stress and strain.
Line 97: “by by”?
Line 99: delete “compressional phases”.
Lines 95-106: replace Units (I), (II) and (III) with Units I, II and III.
Lines 104-105 and 106: “tectonic sinistral shearing” >> “sinistrally strike-slip”?
Line 108: “the divergent strike-slip regime” has not been described before.
Line 113: “NE-SW sinistral strike-slip shearing”? “shearing” is a kinematic term. It is better replaced by faults or ductile shear zones. Use the dash rather than the hyphen.
Lines 128-129: “transtensive sinistral shear stress regime”? “strike” >> “in direction”. (Structural geologists use strike to describe the orientation of planar structures.)
Line 132: delete “strike” or replace it with “directed”.
Line 158: “lidar”?
Lines 189 and 189: Different symbols about fracture aperture were used.
Lines 196-197: Symbols should be italic.
Line 204: Equation (2) is missing. Both kf and e are not defined.
Line 205: “*”?
Figure 2: How do the authors separate natural fractures from artificial fractures formed during exploration? This needs be discussed in the text.
Line 258: “discordance” >> “contact”?
Figure 4: Unclear about whether the projection is on the lower or upper hemisphere. At many places in pink, there is no point available. Why?
Figure 6: There is a gap between the fracture statistics and the DFN models. For example, the distribution in space and time of fracture seeds was not mentioned. Needs a brief description about the procedure of fracture generation.
Author Response
Dear reviewer,
Thank you very much for your very helpful review. Please find attached the detailed list of implemented revisions.
Our best regards,
Claire Bossennec and co-authors.
Author Response File: Author Response.docx
Reviewer 3 Report
This manuscript presents an integrated field- and digital-based fracture analysis aimed at reducing uncertainties related to petrophysical properties heterogeneities in crystalline rocks. The fracture network properties of two case studies, at different depths, have been documented by applying different depth-dependent methodological approaches. The resultant DFN modeling hence the understanding of transfer properties distribution within the studied crystalline reservoirs.
This study presents new important data and models that are worthy of publication. The approach is novel and overall, the manuscript was a pleasant read. However, some parts of the text lack on fluentness, affecting the readability of the paper (please cf. the annotated manuscript where you can find notes requesting "rephrasing"). Fracture data are well presented and coupled with DFN modelling, although the results need to be further discussed in the meaning of the main achievements of this work.2. Comments and Suggestions for Authors
The annotated manuscript contains 50 notes, most of which are minor comments or suggestions.
The presented study would certainly match the interest of the "Energies" journal audience. Therefore, considering the overall quality of this work, I recommend the publication of this manuscript prior to minor revision.
|
1) Abstract The abstract is well written and contains all the relevant information of the presented study in order to focus the attention of the journal audience. 2) Introduction The introduction section needs minor editing in the sense of readability. Furthermore, I believe the introduction would benefit from emphasizing the novelty of this work and making clear statements regarding the structural related findings of this study. 3) Main part The applied methodology is well described and the fracture analysis consists of robust datasets. However, some parts of the text seem to be out of place, i.e., section 4.4 and parts of the discussion section (please cf. the annotated manuscript for further explanation). The discussion section contains too many general statements lacking reference to the presented results. 4) Conclusion The conclusion section requires minor editing. It would benefit by better constraining the main findings of this work, perhaps using a bullet point format. 5) Reference The authors made good use of the referenced literature. However, there are very few references related to DFN modeling and reservoir petrophysical properties analysis, despite the great number of works present in literature. |
Comments for author File: 
Comments.pdf
Author Response
Dear reviewer,
Thank you very much for your very detailed and helpful review. Please find attached the detailed list of implemented revisions.
Our best regards,
Claire Bossennec and co-authors.
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
I appreciate the effort of the Authors in answering and implementing the Reviewer's comments in the new manuscript. I have no further suggestions.
