| 12 Sep 2025
A Fracture Never Comes Alone: Associations of Fractures and Stylolites in Analogue Outcrops Improve Borehole Image Interpretations of Fractured Carbonate Geothermal Reservoirs
Abstract. Natural discontinuity networks control convective fluid flow in carbonate geothermal reservoirs with low matrix porosity and permeability. The network can be separated into discontinuities that formed due to local drivers (e.g. faults/folds) and the background network formed by far-field stresses, each with different scaling behaviour. Borehole data are the only source to sample the subsurface network, as the majority of the discontinuities are of sub-seismic scale. Borehole images are the most cost-effective way of sampling the network, but the limited sample area and image resolution hamper the identification of the background network in this dataset. Analogue outcrops may complement the borehole data, but only after the analogy between outcrop and subsurface reservoir is established. In this study, we present a method that uses associations of discontinuity sets to establish a robust link between the outcrop and the subsurface. A discontinuity association comprises up to 4 discontinuity sets that can form coeval in a single stress field, a well-known concept that is rarely applied for subsurface characterization of discontinuities. We use the orientations and type of discontinuity associations as paleostress indicators in order to map out principal stress trajectories of regional discontinuity-forming events that created the background discontinuity network. We demonstrate this methodology in the Geneva Basin, Switzerland, where the naturally fractured Lower Cretaceous pre-foredeep carbonates are targeted for geothermal exploitation. Outcrops in the mountain ranges that surround the basin, consistently reveal two multiscale discontinuity-forming events that formed prior to Alpine fold-and-thrusting and thus constitute the regional scale background network. Therefore, based on the analogy principle, we predict that the target reservoir is also affected by these events. We use this prediction to isolate background-related discontinuities on image logs from two borehole that penetrate the target reservoir in the Geneva Basin. This analysis reveals that ∼45 % of the observed discontinuities can be understood in the framework of the regional-scale background. In this way, we demonstrate that DAs in outcrops are a powerful tool to predict the geometry of natural discontinuity networks in the subsurface and subsequently can be used to develop geothermal exploitation strategies in naturally fractured reservoirs.
General comments
Optimizing the use of outcrops and image log data to characterize subsurface fractures is a topic of widespread scientific and practical interest. This paper provides a useful example of fracture description that is relevant to geothermal applications. The approach is to obtain kinematically meaningful fracture and stylolite relations in various outcrops to identify regionally persistent patterns of fractures. Then the patterns are used to guide image log interpretation in two wells. Using fracture relationships for kinematic interpretation of fracture and stylolite patterns has long precedent (see the review by Hancock, 1985) but the specific approach used here of high grading a suite of key features from several outcrops regionally and the application to comparing outcrop fractures to the subsurface and as an aid to interpreting image logs for geothermal studies is sufficiently innovative and topical to be of interest. The work is within the scope of this journal. The illustrations are good, and the text is mostly well prepared. At 28 pages the MS is succinct.
The MS as it currently stands needs to be improved with moderate revision if the work is to have impact. The Introduction does not make the aims, assumptions, and claims sufficiently clear. Some of the material here can be reorganized. And the claims need to be made explicit. The Methods mix in arguments about interpretation that belong in the Discussion but leave out specifics of what methods were used and contains vague statements about the attributes of the outcrops. The use of well data is not evident from the Introduction or Methods even though two of the MS conclusions focus on claims about the subsurface. A short new but information-rich Methods section is needed. Stylolites are prominent in the MS title and the methods use fracture and stylolite relations, but because of the ‘discontinuity’ terminology the role of stylolites is not apparent until well into the MS. Although the approach of selecting key fracture relations seems to have worked, I don’t think the MS does a sufficient job of discussing the caveats—why did the method work here? What could go wrong if the approach is attempted elsewhere? And the MS needs to do a better job comparing or at least contextualizing the results with other recent attempts to use outcrops to guide fracture assessment for geothermal applications. And some parts of the latter part of the Discussion could be condensed as currently written they do not seem to be well linked to the results.
I appreciate the attempt in the MS title to grab the readers’ attention. But the implicit claim of the title: ‘A Fracture Never Comes Alone: Associations of Fractures and Stylolites…’ is not sustainable. In the literature there are many fractures described that are not associated with stylolites. I don’t think that the 1988 fracture review by Pollard and Aydin even mentions stylolites. Plenty of fractures ‘come alone’. In many cases where stylolites are present, they are bed-parallel structures that are not necessarily associated with fracture formation and that do not have usable kinematic significance for fracture analysis. The association of fractures are stylolites is not unique to this example, but the pattern is also not universal. The kind of stylolite appealed to here (MS fig. 1), effectively a kind of widely spaced disjunctive cleavage, is a feature of certain carbonate rocks (and rarely, sandstones) and settings, but the MS (and MS title) never makes this much narrower scope clear. The title of the MS ought to be revised to reflect this. For stylolites, in addition to the Hancock review, the authors should consider the classic paper on the topic by Marshak and Engelder (Marshak, S., & Engelder, T. 1985. Development of cleavage in limestones of a fold-thrust belt in eastern New York. Journal of Structural Geology, 7(3-4), 345-359.) Posing the issue as one of kinematic analysis rather than in terms of stresses (or paleostresses) may have some advantages (e.g., Friedman, 1964; Groshong, 1988; Marrett and Peacock, 1999). Marrett, R., & Peacock, D. C. (1999). Strain and stress. Journal of Structural Geology, 21(8-9), 1057-1063.
The MS’s generalization of fracture and stylolite assemblages from figure 1 (from Hancock 1985) also needs to be presented with more nuance. Although all the structures shown in the figure can form together, the literature suggests that in many cases only one or some of the structures are present (see regional studies by Engelder; and the recent geothermal-related outcrop study by Elliott et al.). And even if all the elements are present, they may not be contemporaneous or even related at all. Such an assemblage relationship needs to be demonstrated, not assumed. In other words, opening-mode fractures do not necessarily bisect arrays of small faults or en echelon fracture arrays (or transect stylolites). I think the authors appreciate this, but the point isn’t clear from the text.
Specific comments
The Introduction could also do a better job of leading up to the claims. If I have the claims right, these steps should be:
With each bullet point equal to a paragraph with background citations.
The focus on needing to find evidence for ‘geomechanical drivers’ seems like it could be hard to support and is probably not needed. Showing why fractures formed is notoriously challenging and the evidence for doing so is probably lacking in this instance. Where is the precise fracture timing and depth information or the rock mechanical properties history? For the approach used in this MS to work, that kind of geomechanical argument is probably not needed. The authors use orientation and relative timing information from geometric relations to classify faults, opening-mode fractures, and stylolites into kinematically compatible groupings and show that over a wide area these groupings have consistent patterns. The regionally consistent patterns can then be used to improve interpretation of fractures visible on image logs in some wells.
The writing could use work to make the text clearer. I’ve marked some of these concerns below keyed to lines in the text. The use of terms, particularly in the Introduction, is confusing and the text there and elsewhere ought to be rationalized for clarity. I don’t think there is a meaningful difference between ‘fractures’ and ‘fracture sets’ and ‘discontinuities’ and ‘discontinuity sets’ except that the latter two are less widely used. And, on line 86, the text finally mentions that stylolites are also being measured. The paper title uses ‘fracture’ and nothing would be lost by sticking with this and related terms throughout, or ‘fractures and stylolites’.
Comments keyed to lines in the text
1-8 (Abstract) The text here is all correct but it seems out of place in an Abstract, which I think ought to get to the findings more directly. This text seems more suitable to the Introduction or the start of the Discussion. Consider condensing or moving this text.
The Abstract could start with the text in line 7 (with edits): “We present a method that uses associations of fractures and stylolites, which we call discontinuity sets, to link outcrop and subsurface structures. Discontinuity sets are associations of kinematically compatible structures—faults, opening-mode fractures, and stylolites—that can form broadly contemporaneously. Relative timing can be obtained from crossing and abutting relations. Although such associations are commonly described in outcrop fracture studies they are rarely used to link outcrop observations to structures in geothermal targets or to help guide classification of sparse structural observations made using image logs. We use the orientations and type of discontinuity associations as indicators to map out principal paleostress trajectories of regional discontinuity-forming events that created a background discontinuity network…’ (See the comments on the Introduction structure above).
8 ‘robust’? It seems like a plausible link, but what do you mean by robust? Maybe you could claim robustness if you had independent timing information. Seems overstated.
21-22 This line struck me as sounding a bit circular: ‘[fractures] control…in fractured reservoirs…’ It’s also potentially confusing and convoluted since most of the ‘discontinuities’ are fractures but this isn’t stated. Can this line be revised to be more straightforward? Also, it might be worthwhile to mention that not all geothermal reservoirs are fractured.
22 Berre et al. and Medici et al. both talk about ‘fracture networks’. So why call these ‘discontinuity networks’? These are also review papers about modeling, where the features conducting fluid are assumed to be open fractures. That seems to differ from what you are dealing with: partly or fully sealed fractures (veins) and stylolites.
24 These La Bruna et al. papers are great (only one of them is in the reference list). But they are about outcrop fractures or fracture attributes that might influence flow rather than, as the sentence implies, being about studies that demonstrate with evidence such as production data that fractures actually influence flow. Many papers describe features that might influence flow, so it’s not clear why these two papers would be singled out (an e.g., at least is needed). But what you want to support the statement is one of the papers that uses well data to make this point. The number of papers that demonstrate that some aspect of subsurface fractures influence flow is pretty small, owing to the problems of characterizing subsurface fracture arrays. A paper that uses production data evidence wrt fractures is Solano et al. 2011 SPE Res Eval Eng. I suggest that the line and referencing be modified to reflect this. One example in the literature that links a specific fracture attribute to flow response is open versus sealed fractures (e.g. Weisenberger et al. 2019 Petroleum Geoscience). So if your fractures are fully or partly sealed this ought to be addressed in the Discussion.
28 For definitions of fracture sets see the review by Hancock 1985, J. Struct. Geol. Another aspect of sets is ‘relative timing’. Why omit it here?
29 I suggest that you tone down the geomechanical aspect here as unneeded. All you need to know, or assume, is that the structures are kinematically compatible and broadly contemporaneous. That’s what figure 1 shows. You use the relative timing between structures, from crossing and abutting relations, and their orientation with respect to tilted beds to group structures. The claim here is that fractures ought to be separated by ‘geomechanical driver’ and although this approach has precedent going back at least to Nelson’s 1985 book using a mechanism or ‘driver’ is a problematic way to classify fractures since the cause of fractures is notoriously hard to specify. Fold- and fault-related fractures and regional fractures have been recognized in the literature since at least the 1950s (as call outs to the literature ought to reflect) but unless you already know what the distribution and timing of fractures is how does an appeal to a ‘geomechanical driver’ help? If you are looking at a fracture in core (or a trace on an image log) you probably will not be able to accurately classify the fracture as ‘fold related’ or ‘regional’. See the discussion of equifinality in Revs. Geophys. 2019. Maybe the driver material belongs in the Discussion.
The entire paragraph from 27 to 34 seems out of place.
Background or regional fractures are not necessarily more evenly or uniformly distributed than other types of fractures. The literature has excellent examples of clustered fractures within regional sets. See the 2018 J. Struct. Geol. theme issue on spatial arrangement for examples.
30 ‘regional’ fractures have been recognized in the literature at least as far back as Balk, 1936. Balk, R. (1936). Structure elements of domes. AAPG Bulletin, 20(1), 51-67. And there are studies that identify regional fracture patterns in outcrop and compare them to sparse core observations.
31 ‘the’ background set. This seems to imply that there might just be one regional set. But regional studies (like papers be Engelder from outcrops in NY) document multiple regional sets.
40 The fracture sampling issue needs to be mentioned. Part of this concerns gaps in fracture observations that are inevitable when using wellbores to sample dispersed features like fractures and another is the problem of putting the sparse fracture samples into broader context: in other words, how easy is it, for example, to specify that a trace on an image log corresponds to certain features seen in outcrop? Part of this latter issue is how similar fractures look that formed by different processes (a situation called ‘equifinality’ where these issues are extensively discussed in a recent review: Laubach et al., 2019, Reviews of Geophysics). Since this MS proposes a solution to this issue by isolating specific kinds of kinematically meaningful relationships from the outcrop and using those geometric and relative timing inferences to guide image log interpretation, it would strengthen the argument to describe this sampling issue explicitly.
46-53 This paragraph struck me as vague and having a mixed message. The previous paragraph established that wellbore data has limitations. If you have fracture/stylolite relations in core or visible on image logs, that tells you something about the structures in the subsurface that would not obviously be improved by seeing that relationship in a distant outcrop. A useful thing about outcrops is being able to see features that can never be directly observed in the subsurface, like length or connectivity, which by their nature cannot be captured by wellbore probes.
And the referencing could be more extensive. There have been several studies that specifically address the issue of how to compare outcrop fractures to the subsurface, including specifically for geothermal applications. Note them. Or cover the topic, with references, in the Discussion.
What do you mean by ‘analogy’ and there is more involved in a useful comparison that just similar rock types, age, and structural setting (including diagenesis/rock property history).
For one thing, outcrops by definition have different loading histories than rocks that are still in the subsurface. It’s well established that uplift and unloading commonly do produce fractures (e.g., Engelder, 1985; English, 2012) as do a wide range of near subsurface and geomorphic processes (e.g., Eppes et al., 2024, Earth Surface Dynamics 12, 35-66. https://doi.org/10.5194/esurf-12-35-2024) so these differences may not be trivial. The first step in outcrop fracture studies aimed at guidance for the subsurface is usually trying to identify these.
I suggest you provide a broader assessment of how exposed rocks are judged to be appropriate analogs for the subsurface target (see papers by Agosta et al., 2010; Sanderson, 2016; Ukar et al., 2019). Possibly in the Discussion. A range of factors go into selecting a good analog for a subsurface geothermal target, including matching rock types and—broadly—structural history (Bauer et al., 2017; Busch et al., 2022, Peacock et al., 2022; Elliott et al., 2024). Some studies have questioned the viability of using outcrops for making specific predictions about key subsurface parameters l (Peacock et al., 2022) whereas others claim that such assessments are possible in some instances (Elliott et al., 2024). Since what you are doing is a contribution to solving this problem, the Discussion is a good place to contextualize your work. Many of the other approaches such as using chemical aspects of the fracture system (e.g. Elliott et al. 2025) seem like they would be a good compliment to your approach.
55 Genetic relations between fractures and stylolites have long been appreciated. See references in Groshong (1975). And that multiple fracture orientations can form in a single deformation goes back at least to Stearns. See also: Olson, J. E., 2007, Fracture aperture, length and pattern geometry development under biaxial loading: a numerical study with applications to natural, cross-jointed systems. In Couples, G & Lewis, H., eds., Fracture-Like Damage and Localization, Geological Society of London, Special Publication. 289, 123-142.
Groshong Jr, R. H. (1975). Strain, fractures, and pressure solution in natural single-layer folds. Geological Society of America Bulletin, 86(10), 1363-1376.
68 ‘carbonate rocks’; just saying carbonates sounds slangy.
80 (Methods section) This section is confusing. The second part of it (2.2) seems like it belongs in the Discussion. In 2.2 you are making the case that your outcrop data can be linked to the subsurface. This is an interpretation, not a method. The point is best addressed in the Discussion.
The first part of the Methods (2.1) also needs to be clarified. Mixed in here are incomplete descriptions of the outcrop sizes and what can be measured in them, data collection methods like circular scanlines that may have only been collected at one outcrop (line 100), a distribution of 10×10 m outcrop stations that is supposed to be “…as evenly as possible over the studied area”, and a method for selectively extracting kinematically significant fracture/stylolite relations. These elements need to be separated out and described clearly and quantitatively. Some aspects, like outcrop sizes, maybe ought to be in the Geological Setting. A useful approach would be to build a table and use that as a guide to revising the section. The Methods section also should mention that you had access to and described two wells (line 220).
My suggestions above for the Introduction are based in part on the impression I had from this section that the outcrops you had to work with are small, and not amenable to the type of analysis of large clean outcrops as for example in Elliott et al. 2025.
81 How do you use this approach if you don’t have independent measures of the (paleo)stress directions? This is a problem with emphasizing the stress or paleostress aspects. You just needed to observe kinematically significant structural relations, like orientation and crosscutting or abutting relations and map the patterns regionally. In the 1990s a similar approach was used to map coal fracture patterns regionally in outcrop and guide interpretation of core, image log, and production data from coalbed methane wells (see this review paper: 1998, Characteristics and origins of coal cleat: a review: International Journal of Coal Geology, 35, 175-207, their figures 1, 15, 16, and 19).
81-90 Much of this material seems like it belongs in the Discussion.
81 ‘…mode I and mode II fractures, vein arrays…’ I suggest that you rethink your terminology here. The mode terminology seems to add unnecessary jargon. A simpler descriptive terminology appropriate to natural examples is to just call these features ‘opening-mode fractures and faults’. Besides, the mode I, II etc terminology refers to where you know the crack mouth opening displacement, which is why these terms are typically found in experimental or theoretical treatments where this aspect can be observed or specified. According to Pollard and Aydin: “Broadly speaking, joints are associated with the opening whereas faults are associated with the shearing modes. Because the mode may vary along the fracture front and may involve mixtures of modes I, II, and III, however, one should not be too categorical about these associations.” The terms joint and vein have connotations about mineral deposits that are unhelpful (see Rev. of Geophys. 2019). Veins can form in several ways (they can be filled opening mode fractures or dilatant parts of faults, in addition to some being replacement deposits), but mixing this term that relates to mineral deposits with the mode terminology is confusing. Why not just say ‘mineral deposits in the fractures’ if that is what you mean? Both opening-mode fractures and faults commonly contain mineral deposits.
83 “Discontinuity sets are defined on the basis of both orientation and discontinuity type.” I believe that you said this already (line 28). In any case, relative timing is also typically a component of defining sets.
86 Here is the first indication that your ‘discontinuities’ include stylolites. That you are using stylolites ought to be mentioned earlier. And why not just say throughout ‘fractures and stylolites’ instead of the awkward ‘discontinuities’?
90-100 The size and degree of exposure of the outcrops is hard to parse from this description. You mention stations that are 10x10 m but in line 101 you seem to use circular scanlines with radius 1 m and say that only one outcrop had ‘quality pavements’ to allow circular scanlines. Describe what the outcrops are like, probably in the last part of the Geological Setting.
99 ‘seven’ (small number convention). Check the MS throughout.
106 (section) The argument that the data you collected can be used to link the outcrop and the subsurface belongs in the Discussion (and may occur in the claims at the end of the Introduction).
There are other studies in the literature that have the goal of identifying outcrop analog fractures that can be used as guides to geothermal reservoir extrapolation. Some of these provide different perspectives on the issue and ought to be mentioned in the Discussion to give balance to your conclusions: Elliott, S.J., Forstner, S.R., Wang, Q., Corrêa, R., Shakiba, M., Fulcher, S.A., Hebel, N.J., Lee, B.T., Tirmizi, S.T., Hooker, J.N., Fall, A., Olson, J.E., Laubach, S.E., 2025. Diagenesis is key to unlocking outcrop fracture data suitable for quantitative extrapolation to geothermal targets. Frontiers in Earth Science 13, 1545052.
131 ‘excellent exposures’ is vague. How big, how complete is the exposure? Are fractures that formed in the subsurface readily separated from surface-related fractures here? How?
140-146 (In the Geological Setting) It would be useful to mention, even if qualitatively, how the structural and burial history or outcrops and rocks in the subsurface differ. Also mention the current state of stress/ stress regime (could cite world stress map papers). In some areas surface fractures relate to current stresses (see the pop ups described by Engelder in the 1980s; references in Elliott et al. 2025).
149 (In the Results) It might be helpful to start by describing the structural elements that are present in the entire area.
156 Note and consider the strong condemnation of the term ‘shear fracture’ in the Pollard and Aydin 1988 GSA Bulletin review. Maybe ‘small displacement faults’?
168 Consider adding a star or other mark to the stratigraphic column to show which unit is being analyzed.
170 Dissolution along the fractures. Does this play a part in the interpretation? This may be of interest to readers concerned with some of the deep carbonate fractured reservoirs in China, where this kind of dissolution is a key element. Is there any evidence of this process in outcrop? This seems like it could be part of your Discussion.
198 ‘is composed of’ but ‘comprises’. You use this weird English convention correctly in line 190.
224 (figure 6) Nice way to do the scales on these images.
225 These wells need to be anticipated in the Introduction and Methods.
232 Help the reader understand the Doesberg 2023 reference (an unpublished MS thesis). Did you do image log interpretation or just use some kind of compilation from this reference? Line 236 makes it seem like you interpreted the images. You might be interested in how Wang et al. 2023 handled references to reinterpreted archival image log data: Wang, Q., Narr, W., Laubach, S.E., 2023. Quantitative characterization of fracture spatial arrangement and intensity in a reservoir anticline using horizontal wellbore image logs and an outcrop analog. Marine & Petroleum Geology 152, 106238. https://doi.org/10.1016/j.marpetgeo.2023.106238
239 How can you know that veins are ‘invisible’ if you don’t have core? Filled fractures do commonly show up on image logs.
239 ‘feat’ > feature?
250-252 Hmm. What if these picks are wrong? Is this discussed further?
281-285 This is confusing.
285 ‘On the contrary’ > ‘in contrast’
303 I assume that by ‘the only way’ you mean given the type of data that has been collected to date? Maybe instead ‘a practical, widely used, and relatively inexpensive way’? But one with several important drawbacks.
304 Maybe start the line with ‘In the subsurface of the Geneva basin…’ to make it clear that this is a location specific issue.
307-321 In 307 you say that image log bias has rarely been investigated, but this isn’t really the case, although I guess it depends on what you mean by ‘bias’. There have been many studies of the capabilities and limitations of image logs. Bias is a systematic distortion of a result due to some factor. Unless you mean the bias of a specific analyst, the problem is one of inherent ambiguity rather than bias. The kind of reproducible rules, such as in the Andrews et al. reference, are good. But excellent discrimination rules were worked out in the 1990s based on wells with both image logs and core; these are the basis for commercial log picks. There have been many core-to-log comparisons published since 1988 and they mostly come to the same sad conclusion that there is a lot of inherent ambiguity in this aspect of image log interpretation. The reason for this is that many features on image logs look alike. Drilling induced fractures may not have the characteristic shapes and distributions that would allow rules to reliably differentiate them, mineral deposits in natural fractures can be microns thin and undetectable on image logs, and in some case in core inspection. Or fill in sealed fractures can be eroded out. Open natural fractures are not necessarily aligned with current day SHmax. The problem of correctly differentiating drilling and natural fractures or open and sealed fractures has been the focus of several studies since the late 1980s. This section of text can probably be reduced to a short paragraph.
The point I guess is that the image logs are widely used but have mostly intractable limitations, so the kind of outcrop inferences and guidance for log interpretation you provide can be helpful in trying to get reliable data from the logs. Your discussion ought to talk about how general your guidance might be or is it specific to this unit or rock type in this basin.
316-320 This section of text describes an important contribution of this MS. But the message seems buried. A clearer description is needed.
327 ‘barren’ and ‘mode I’ are not equivalent things. And image logs cannot tell if a fracture is barren or not. The mineral deposit veneers on some natural fractures are microns thin and require an SEM to detect, so they (and even thicker deposits) are invisible to current image log technology.
A point that I don’t see considered is that the outcrop images you show seem to be mostly sealed fractures. Are these fractures filled with calcite deposits (the Results ought to describe this). If the fractures (or at least some of them) in outcrop are calcite filled, that at least is some evidence they are not near surface features but are representative of subsurface deformation. Do you mention this? And if they are sealed, how do they contribute to fluid flow? Or show up as open on image logs? If sealed, is their main role as weaknesses for reactivation during stimulation (Cao et al. point to this as a major uncertainty)? Earlier in the text you mention dissolution along fractures in this basin. Is this an issue worth discussing?
323-341 I agree with the points here, but this section of text could use some work for clarity.
340 (section of Discussion). I think this section ought to be condensed such that it focuses of issues you cover in your Results.
360 If you include the effects of fracture abundance in your Results or geological background you should describe what porosity and permeability the host rock has. If host-rock permeability is appreciable then closely spaced fractures (if open) could affect overall permeability due to flow through the host rock between fractures (Philip et al. 2005, SPE Res. Eval. Eng.) If the host rock is impermeable, but the open fractures are not interconnected then the closeness of the fractures to each other should matter. There is a large literature on connectivity and flow (e.g. Long and Witherspoon 1985). Connectivity is not necessarily a function of fracture abundance. But you don’t describe connectivity in your outcrop description. Maybe the best move is to make this entire section much shorter and just say that once you have established that the outcrops are representative of the subsurface with your outcrop to image log comparison, you could go back to the outcrops to get this other information that would be useful for modeling.
363 You mention ‘saturation’ without putting this concept into context. Maybe best to just leave it out. Where in your Results is there evidence one way or the other to argue for some degree of saturation?
396 The conclusion “Outcrop study is a time and cost-efficient method to obtain a first-order evaluation of the contribution of the background network in the subsurface”. I’m sure that this is a true statement. But you have not done a time or cost analysis or a value of information assessment, so I question whether this is a valid conclusion. Maybe the remark belongs at the end of the Discussion along with some ballpark estimates of costs and time of field data acquisition and the potential value of improved image log interpretation. For an example of this and a spreadsheet that can be used to make your calculation, see: Almansour et al. 2020. Value of Information analysis of a fracture prediction method. SPE Reservoir Evaluation & Engineering, 23 (3), 811-823. doi: 10.2118/198906-PA.
Check the figure captions for the word ‘legenda’; should be ‘legend’.
The titles in the reference list are formatted inconsistently.
Some reference mentioned in the review
Elliott, S.J., Forstner, S.R., Wang, Q., Corrêa, R., Shakiba, M., Fulcher, S.A., Hebel, N.J., Lee, B.T., Tirmizi, S.T., Hooker, J.N., Fall, A., Olson, J.E., Laubach, S.E., 2025. Diagenesis is key to unlocking outcrop fracture data suitable for quantitative extrapolation to geothermal targets. Frontiers in Earth Science 13, 1545052.
Rysak, B.R., Gale, J.F.W., Laubach, S.E., Ferrill, D.A., Olson, J.E., 2022. Mechanisms for the generation of complex fracture networks: observations from slant core, analog models, and outcrop. Frontiers in Earth Science, v. 10, Section Geohazards and Georisks. In Li, Y, Rutter, E.H., Shang, J. and Ji, Y., Eds., Special Issue, Recent Advances in Mechanics and Physics of Rock Fractures across Scales. doi.org/10.3389/feart.2022.848012