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| 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.
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Status: open (until 30 Oct 2025)
- RC1: 'Comment on egusphere-2025-4175', Stephen Laubach, 22 Sep 2025 reply
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RC2: 'Comment on egusphere-2025-4175', Anonymous Referee #2, 22 Oct 2025
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The manuscript by Hupkes et al. proposes a new approach of using kinematically meaningful fracture and stylolite associations in various analog outcrops to identify regionally persistent patterns of fractures that can be used to guide image log interpretation in wells and are relevant to geothermal exploitation in the subsurface. This is in principle an interesting topic that deserves publication but the manuscript requires to me significant revision before being possibly further considered for publication.
My comments below :
*Title : the first sentence of the title should be removed. They are plenty of examples of occurrence of fracture populations without any stylolites owing to lithology for instance. I do not mean at all I disagree with the idea of working on stylolite-fracture associations, just that the first sentence clearly designed to appealing purpose is wrong in many instances.
*The Introduction mixes different aspects in a sometimes non logical order, each of which requiring more attention as well as support by references beyond very general considerations. Overall, some reorganization is needed to clearly state the scientific question at hand and to clarify the aims of the study.
*L24 : Addressing the role of discontinuities in forming preferred drains or barriers should also rely on papers from outside the academic circle from the authors (e.g., from host rocks different from carbonate rocks, and mineralizations of other types than calcite; see for instance Grare et al., 2018, Minerals, among others). Also, the respective importance of diffuse fracture networks on rock permeability and fluid flow compared to seismic-scale faults has already been addressed (e.g., Beaudoin et al., 2013; Smith et al., 2022).
* L29-30 : The authors use orientation and relative timing information derived from observations of geometric relationships to gather faults, opening-mode fractures and stylolites into kinematically or mechanically consistent patterns over a wide area in a fold-and-thrust belt and its foreland basin owing to their development in flat-lying strata before folding (e.g., during the layer-parallel shortening stage, see Tavani et al., 2015; Lacombe et al., 2021). In contrast, the authors seemingly consider that fractures related to fold development are only of local significance. This may be true for fractures formed in response to strata bending at fold hinges, but some mesostructures observed in folded strata may be unrelated to the contractional event that caused folding, instead having originated in response to strata burial (Lamarche et al., 2012; Lavenu et al., 2013), foreland flexure (Mercuri et al., 2022), strata exhumation (Bellahsen et al., 2006a), extensional collapse of fold-thrust systems (Tavani et al., 2012) or to other pre-or post-folding tectonic events (e.g. Bergbauer and Pollard, 2004), and some may be of regional significance. The appraisal of the local vs regional significance of fractures therefore may be scale-dependent and should be dealt with more caution eventhough I understand the wish of the authors to simplify the topic.
*Among the so-called local drivers of fracture development, faults deserve particular attention owing to the perturbations of the regional stress field they cause. The authors should have a close look to the works by e.g., Rispoli, 1981; Rawnsley et al., 1992; Homberg et al., 1997, 2004; Reiter et al., 2024. In particular, the works by Homberg et al. 1997 and 2004 are of prime interest to the study and deserve attention since they document stress perturbations and associated fracture patterns in the vicinity of major left-lateral strike-slip faults in the Jura (Pontarlier and Morez faults), keeping in mind that the area investigated is close to a very similar structure, the Vuache fault, which, on top of that is still seismically active. Again, this questions the scale at which one should consider the fracture patterns to be of local or regional significance in the kind of study carried out by the authors.
*L28 and 83-84 : The definition of a set of fractures by the authors is a bit simplistic and should be based instead on a more complete set of criteria, including a common orientation in either raw or unfolded attitude, common relative chronology with respect to other mesostructures and shared mode of deformation (e.g., Pollard and Aydin; Bellahsen et al., 2006a; Ahmadhadi et al., 2008; Lacombe et al., 2011; Sanderson et al., 2024). Due credit should be better given to such existing literature, including the statistics behind defining distinctive fracture sets.
*L29-31 : again, references should be provided from out of the academic circle of the authors : e.g., Bergbauer and Pollard, 2004; Casini et al., 2011; among others
*L55 : That multiple discontinuity sets can form coeval in a single stress field is now nicely supported by absolute dating of syn-tectonic calcite mineralizations, which allows for a better appraisal of the local vs regional tectonic significance of fracture networks (e.g, Beaudoin et al., 2018; Parrish et al., 2018; Zeboudj et al., 2025). However, in some instances, the use of geochronology may either simplify or sometimes increase the apparent complexity of the fracture network analyzed on the sole basis of orientation and kinematic criteria. Geochronology may reveal very similar ages for veins of significantly contrasting orientations, which questions the perfect stability of the direction of the maximum principal stress during a fracturing event and the range of orientations allowed to define a fracture set considering possible stress variation at the scale of the strata or close to faults, in a time span lower than the few My of uncertainties of the geochronology technique. Conversely, absolute dating can, in some cases, complicate the interpretation of deformation history particularly when multiple distinct ages are obtained from fractures belonging to the same set defined on the qualitative criteria above. I would suggest much more caution with the hypothesis of constant principal stresses at the regional scale (L119).
*L107-110 : the authors should be more careful when using the Andersonian stress hypothesis according to which one principal stress is generally vertical, especially in fold-and-thrust belts. As stated in the discussion paper by Lacombe, 2012 (section 4), mesostructures that yield one stress axis perpendicular to bedding while the other two lie within the bedding plane (e.g., bed-perpendicular joints/veins or stylolites, or Layer-Parallel Shortening (LPS) – related microfaults) yield a sub-vertical paleostress axis only after backtilting to their prefolding attitude (unfolding), which in turn implicitely leads to consider them as pre- or early-folding. However, it has been argued that such structures could have developed within tilted layers, hence possibly under a non vertical principal stress, if bedding anisotropy was able to significantly reorient stresses or if flexural slip occurred at very low friction so that the principal stresses rotated but remained either parallel or perpendicular to bedding (e.g., Tavani et al., 2015). A kind of circular reasoning may thus be involved when chronology is based on an Andersonian assumption only. As a result, the classical fold test must be preferred.
*L292 : I suggest to change into : … predates the onset of localized deformations occurring during fold growth and late stage fold tightening.
*Stylolites are expected to be important players according to the title of the manuscript and the approach is supposed to use fracture and stylolite relations, but the ‘discontinuity’ terminology used throughout the manuscript leads to overlook the role of stylolites. Poor use is made of stylolites and the most recent literature on stylolites is properly ignored (eg. Toussaint et al., 2018). Especially, it has been shown that the roughness of a stylolite brings some signal that can be treated and used to derive the state of stress prevailing at the time the stylolite ended its development. This applies to compaction-related sedimentary stylolites as well as to tectonic stylolites. In contrast to sedimentary bedding-parallel stylolites used to derive paleodepth constraints (eg., Beaudoin et al., 2019; 2020; see also the work by a senior author of the manuscript), tectonic stylolites the manuscript seemingly focuses on can bring very useful kinematic information for fracture analysis, beyond the classical papers (e.g., Marshak and Engelder). Tectonic stylolites can be either compressional or strike-slip in type (e.g., Beaudoin et al., 2016, 2020). In this case, the accurate interpretation of the stylolites in terms of stress is highly relevant and potentially predictive in fracture network studies. Adding one or two sentences on this topic may strengthen the ‘stylolite’ aspects of the manuscript which otherwise remains weak and nearly useless as is.
*The role of fractures in fluid flow in unclear, especially the role of diffuse fracture networks compared to major drains (eg, seismic scale faults), see for instance Beaudoin et al., 2013. Also, the authors discuss in which situations fractures may enhance fluid flow, but all the field examples document sealed mode I fractures. In the vast majority of studies as well as in modeling works, the features conducting fluid are commonly assumed to be open fractures (e.g., Wennberg et al., 2016). Would the authors expect that in the subsurface, the fractures of interest (i.e., conductive) be open or only partly sealed while those in the outcrops are filled with calcite ? Or do they consider that already sealed fractures may be re-opened and made conductive under the current stress field ? It is unclear since authors speak about either mode I fractures or open fractures. It should be kept in mind that mineralization of mode I fractures and faults requires some specific chemo-physical conditions (see the synthesis by Laubach et al, 2019) unlikely to be met very close to the surface. Open fractures are more likely to be found close to the surface than under a certain overburden. In other words, the structural, burial and fluid flow history of exposed rocks and rocks in the subsurface may differ. This point requires an even brief discussion.
*Like the physical properties (e.g., permeability) of fault rocks from exhumed ancient faults may hardly be extrapolated to the in situ hydraulic behavior of deep active faults because during uplift these rocks underwent unloading, decrease in temperature and changing fluid-rock interactions (including weathering), one can wonder whether fracture networks identified in exposed rocks are reliable analogs for the mechanical and hydraulic properties of subsurface fracture networks despite similar regional tectonic history and host rocks.
*Since veins contain nearly pure calcite material compared to the host rock, one could expect some kind of strength change in the rock due the structural diagenesis, with the vein network causing possibly some strength anisotropy at the scale of the reservoir. Would it be an alternate parameter to usefully consider during exploitation ?
*What about stylolites in fluid flow studies ? The authors report that stylolites may behave as drains or barriers. First, again, citing only Bruna et al. is not a fair acknowledgement of previous work on the topic (see for instance, Koehn et al., 2016; Heap et al., 2014; Braithwaite, 1989). In addition, when speaking about compartmentalization of the reservoir in term of fluid flow, I guess the authors also consider sedimentary, i.e., non-tectonic bedding-parallel stylolites, the development of which has nothing to do with the topic of the manuscript. It may be misleading to mix features as different as fractures and stylolites, or tectonic and non-tectonic features.
*If I understand well, the authors show that in the area investigated, ~40-50% of the fractures identified by borehole imaging correspond to natural fracture patterns, and discard the uncorrelated fractures as being more recent structural objects (L 321). The authors do not explain which process the formation of these uncorrelated fractures can be ascribed to, or if (why?) they necessarily postdate the development of the regionally consistent fracture sets. In addition, should they predate or postdate the formation of the natural fracture sets, I would expect some of the uncorrelated fractures to be of possible interest by enhancing connectivity of the regional natural fracture sets despite being of local significance, hence permeability of the reservoir . This would deserve a bit more attention for the interested reader.
* I think that accepting a maximum deviation of ~30° for azimuth and dip of fractures recognized on the BHI is reasonable, but there is no justification for this acceptability ‘threshold’. Is it purely arbitrary and does it rely on observations or established statistics ? Please justify.
* What about the interpretation of the clockwise rotation with depth of the dip direction of bedding planes in GEO-01 ? The authors relates it to the probable presence of a fold, itself potentially related to a fault, at ~480m depth. First, the authors should substantiate this interpretation and possibly discuss the significance of this fold (geometry, consistency with regional features). Second, following the authors’ logic, this fold and associated fault have expectedly caused localized brittle deformation, for instance in the form of extensional fractures at fold hinge. Did the authors try to incorporate this aspect in their interpretation / scenario ? How do they explain that the % of uncorrelated fractures is similar with Geo-02 ? Third, several studies have shown that stress may be perturbed close to the tip of a reactivated / propagating fault even in still flat-lying strata, causing local directional perturbation of both stress orientation and magnitudes during LPS which may strongly influence the distribution of fractures in advance of fold development (e.g., Bellahsen et al., 2006b; Amrouch et al., 2010). I would expect the authors not to simply brush aside this point. This comment is also relevant to fractures formed in the vicinity of the Vuache fault which formed prior to the Alpine LPS (Oligocene extension, Homberg et al., 2002) and hence was prone to reactivation during the subsequent Alpine compression.
* The authors make a big deal about the use of directional and kinematic characteristics of fracture –stylolite associations in outcrops, but I am also wondering whether additional information such as fracture dimensions (length, vertical persistence across strata) or fracture connectivity owing to the type of fracture intersections, cross-cutting relationships with stylolites could not be also useful to built a reliable fracture and permeability model of the reservoir since these attributes can hardly be assessed using well bores.
*The authors conclude about the success of their approach in the particular case of the Geneva basin, and encourage people to apply it to other case studies, but what could be the limitations of this approach if applied blindly to a different setting ? I guess the change in lithology with depth, the stress discrepancy between above and below a mechanically weak decoupling layer or stress perturbations caused by large-scale structures could be part of the answer, but the authors should elaborate a little bit on that point.
I hope that these comments will help the authors improve their manuscript.
Citation: https://doi.org/10.5194/egusphere-2025-4175-RC2
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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