Scaling of natural fracture patterns at Swift anticline, NW Montana: the influence of structural position, lithology, and observation scale
Abstract. Natural fracture patterns have long been associated with fold formation. Conceptual models of fold associated fractures are used to predict fracture networks and hence subsurface properties such as fracture connectivity, intensity and fluid flow. Subsurface datasets typically lack the resolution or coverage to adequately sample fracture networks in 3D, however, and geometric properties are typically extrapolated from available data (e.g., seismic data or wellbore image logs). Here we assess the applicability of extrapolating fracture properties (orientation, length and intensity) from one observation scale to another in a structurally complex setting and assess the interplay of fracture scaling with geological controls on fracture development. Fracture patterns are investigated at an outcrop exposure of layered carbonate rocks at Swift anticline, NW Montana. Data derived from high-resolution field images, medium resolution digital outcrop data, and relatively low resolution satellite imagery are leveraged to (i) assess interacting structural and stratigraphic controls on fracture development, and (ii) compare estimated fracture properties derived from multiple observation scales. We show that hinge-parallel and hinge-perpendicular fractures (i) make up the majority of fractures at the site, (ii) are consistently oriented with respect to the fold hinge and, (iii) exhibit systematic increases in intensity towards the anticline hinge. These fractures are interpreted as having formed during folding. Other fractures recorded at the site exhibit inconsistent orientations, show no systematic trends in fracture intensity, and are interpreted as unrelated to fold formation. Fracture orientation data exhibit greatest agreement across observation scales at hinge and forelimb positions where hinge-parallel and hinge-perpendicular fracture sets are well developed, and little agreement on the anticline backlimb, where fracture orientations are less predictable and more dispersed. This indicates that the scaling of fracture properties at Swift anticline is spatially variable and partly dependent on structural position. Our results suggest that accurate prediction and extrapolation of natural fracture properties in contractional settings requires assessment of structural position, lithologic variability, and spatially variable fracture scaling relationships, as well as consideration of deformation history before and after folding.
Adam J. Cawood et al.
Status: open (until 19 Jun 2023)
- RC1: 'Comment on egusphere-2023-812', Anonymous Referee #1, 09 May 2023 reply
- RC2: 'Comment on egusphere-2023-812', Amerigo Corradetti, 01 Jun 2023 reply
Adam J. Cawood et al.
Adam J. Cawood et al.
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The paper describes a study of fracture patterns across a range of observation scales in folded rocks. The topic is of general interest, despite decades of work on fold-related fractures, and is within the scope of the journal. The paper is clearly written and well-illustrated. This study builds on earlier work in the area (Watkins et al. 2019).
There are points that could use some clarification, more nuance, or additional referencing.
The use of the ‘scaling’ terminology needs some clarification. Does this refer to the different scales of image acquisition and image resolution and their effects on how pattern look? Or is it related to some intrinsic size-scaling of the fracture patterns? It seems like the authors mean the former (figure 1). But the tile (‘scaling of natural fracture patterns’) seems to imply the latter (as in Davy et al. 2010, A likely universal model of fracture scaling and its consequence for crustal hydromechanics). There don’t seem to be any classic looking ‘scaling’ plots or extrapolation from one scale to another for reservoir characteriztion (as in Hooker et al., 2009, Aperture-size scaling variations in a low-strain opening-mode fracture set, Cozzette Sandstone, Colorado, Journal of Structural Geology), unless this is what is meant by figure 14. Some clarification is in order.
The Arc map based fracture density mapping should be of interest to many readers.
The Introduction is clear and manages to cover the huge literature on the topic of fold-related fractures and fracture documentation pretty efficiently. But there are two related points that could be expanded. Under the impediments to fracture pattern analysis (45) the non-uniqueness problem of fracture analysis should be mentioned. In practical terms this is a central concern in using outcrop fractures for any purpose. Fracture attributes such as overall shape and even patterns that are independent of loading path or are not uniquely determined by the mechanical processes that formed them, obstruct clear interpretations of the structural history. Path-independent structures, as might be seen in individual fractures, either in outcrop or in core specimens, commonly cannot be ascribed to a unique mechanistic interpretation without additional information (like direct fracture dating). A path independent or nonuniqueness problem underlines some of the issues that limit fracture core analysis and is the main impediment to using outcrop fracture patterns reliably. Outcrop studies show that patterns can vary markedly with rock type and fracture formation mechanism, but the path independence that makes outcrop fracture patterns hard to interpret also makes these patterns impossible to infer from a sample of an individual fracture in either a core or in an outcrop. This is a case of fractures, individually, being too simple (are they fold related or due to uplift and unloading, for example). An issue related to this is, how to guard against well characterized but misleading outcrop fracture patterns? Both issues are addressed at length in a recent review (noted below) and so maybe these points can be mentioned mostly via reference to the literature. But the ambiguity of outcrop observations needs to be addressed in the Discussion as well.
Also with respect to the Introduction, in my opinion the last paragraph (96-102) would be more compelling if it were to be restated as specific claims along the lines of ‘here we show that…’ Get the reader’s attention by teasing the actual findings here. With so many papers on fold-related fractures, drawing the reader in seems like it should be a priority.
Intensity variations and 1d and 2D clustering can occur in the absence of folding (see the 2018 J Struct. Geol. issue on fracture spatial arrangement and Correa et al., 2022, J. Struct. Geol.). So I’m concerned with the impression that the variability observed in this example is necessarily fold related.
The Discussion and Conclusions could use reorganization and both sections can be more compact.
43 There is another major impediment that is worth mentioning because it is in some ways more fundamental than variability and complexity or sampling. This is the inherent lack of inherent complexity in individual fractures and fracture patterns that makes it very challenging to say when and why fractures formed even with good sampling. This is issue of equifinality and the research directions needed to overcome it is described in a recent review: Laubach et al. 2019, Reviews of Geophysics. This issue is fundamental to what this MS is trying to accomplish. Worth addressing explicitly.
Laubach, S.E., Lander, R.H., Criscenti, L.J., et al., 2019. The role of chemistry in fracture pattern development and opportunities to advance interpretations of geological materials. Reviews of Geophysics, 57 (3), 1065-1111. doi:10.1029/2019RG000671
56-65 Some waning comments about the pitfalls and ways that outcrop fractures can be misleading could perhaps be added here. For using outcrop data to assess specific locations in the subsurface requires more than good exposures and imaging.
71 ‘kinematically consistent’?
96-102 Where are the specific claims?
164 Although it probably is a good place to study fold-related fracturing, it seems like demonstrating that these (or some of these) fractures are related to folding is a key point you need to demonstrate. So this statement sounds a bit like the start of a circular argument. Consider revising to clarify your argument.
177 Consider writing ‘Twenty-two’ etc as first word in sentence.
193 (end of methods section) So, no petrology or microstructure of fractures? Seems like a gap in the analysis protocol.
In many Paleozoic beds in the Rockies, the fractures are calcite filled, and these deposits can be informative. See for example: Amrouch, K., Lacombe, O., Bellahsen, N., Daniel, J. M., & Callot, J. P. (2010). Stress and strain patterns, kinematics and deformation mechanisms in a basement‐cored anticline: Sheep Mountain Anticline, Wyoming. Tectonics, 29(1). And Beaudoin, N., Leprêtre, R., Bellahsen, N., Lacombe, O., Amrouch, K., Callot, J. P., ... & Daniel, J. M. (2012). Structural and microstructural evolution of the Rattlesnake Mountain Anticline (Wyoming, USA): new insights into the Sevier and Laramide orogenic stress build-up in the Bighorn Basin. Tectonophysics, 576, 20-45.
Fractures formed in the subsurface are in chemically reactive environments where cement deposits are to be expected. So inspecting outcrops for mineral deposits seems like it ought to be step in verification that the sampled outcrop fractures are indeed related to subsurface folding. After all, these rocks are in surface settings where a range of loading conditions could produce fractures. Also, with respect to scaling of open fractures, the diagenesis can set a scale that is worth noting. See Forstner, S.R, Laubach, S.E., 2022. Scale-dependent fracture networks. Journal of Structural Geology, 165, 104748. https://doi.org/10.1016/j.jsg.2022.104748, which is another Rockies example.
213 ‘thrusting’ is a process. Consider rephrasing.
219 The ‘outer arc’ extension fractures seems like a premature interpretation. What’s the evidence for this?
265-275 The patchy and possibly clustered arrangement of some of these fractures is interesting. Clustered backlimb fractures in another Rockies anticline is described by Wang et al. 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. The analysis methods used there may be of interest.
290 Will you come back in the discussion to assess the biases in measured lengths? Lots of time the higher resolution the image, the shorter the lengths owing to the segmentation of opening-mode fractures that are increasingly hard to see with standoff.
340 (section) Do you take fractured layer thickness into account in this? Seems like this could be an element in map-view intensity patterns if you have something approaching a fracture / bed thickness relationship. See also 470. How important is it to have omitted this consideration?
393 (section) ‘Scaling of fracture orientations’ seems like a strange way to say ‘impact of observation scale on apparent fracture orientation’.
400 These are interesting graphical representations and maybe should be noted as a feature of the MS in the Introduction.
411 But fractures in different structural positions may show differences in actual orientation dispersion in different structural positions. For example, Wang et al. 2023 Mar. & Pet. Geol.) in horizontal well data over a Rockies fold, show more orientation dispersion in the steep limb, ascribing in to shear of pre-existing fractures (wing crack formation). I’m not sure I follow your point here. May need clarification.
449 So would you expect intensity to scale? Why? Is this taken up in the discussion. What to these (seemingly) very different exponents signify?
420 (figure) so have these orientations/rose diagrams been restored to a pre-folding configuration (has a fold test been done?). How would you do this with just satellite data?
460 Ortega et al. (2006, AAPG Bulletin) make the case that you need to specify a scale to have a meaningful, comparable metric of intensity. Is this what you are getting at here?
Is the ‘intensity’ here for all sets in aggregate? Even genetically unrelated sets?
470 Bed thickness effects. This is such a well attested effect on average spacing that it seems strange to write it off like this. With your field data you should be able to at least say what the range of fractured bed thicknesses are and what the height distributions looks like. You cite Hooker et al. 2013, which has a useful fracture height classification. (Although in my experience with fractured Paleozoic carbonates of the western US, they commonly are not well described by a fractured layer/bed thickness relation, being tip bounded in the classification of Hooker et al. within many meters thick fractured units with narrow spacing). It would be good to have some assessment in the results for your outcrop.
494 (section) So is the intensity variation with image resolution a consequence of scaling of fracture sizes or is it an artifact of image resolutions? Do you mean the average orientation depending on the volume of rock over which you aggregate measurements? This seems confusing. For fracture sets having power law size distribution (as in the Hooker et al. 2009 reference noted above) fractures in the same set may share common (identical) orientations over orders of magnitude in fracture size. But this wouldn’t mean that the orientations ‘scale’ would it?
495-500 But the prominent title of the paper is ‘scaling’. Does this mean the scaling is not so important? Clarify.
500-505 Hmm. This seems a bit inconclusive.
510 Although really, the Stearns and Price models have long been questioned including the hkl fractures in Hancock (1985) and more specifically for the Rockies by Hennings et al. 2000. Does anyone really still rely on the Stearns model? (the hkl scheme also captures ‘variable and dispersed’ although that ends up a criticism of such an ‘expanded’ Stearns model, since every orientation can be fit in.)
A related point is that for angular-hinged folds with rotated but otherwise undeformed limbs, the fold-related fracture strain can be quite localized in a narrow (and frequently poorly exposed) hinge region.
512-513 and to 518. These arguments do not seems overly convincing. (i) Is the ‘in general’ increase in intensity toward the fold hinge quantitatively different from intensity variations in backlimb setting distant from hinges? Can you even test this with the degree of exposure? (ii) The ‘parallel’ and ‘perpendicular’ to the fold hinge configuration are also orientations that are in kinematic compatibility with other regional and local features, including former basin margins (as postulated for some other Rockies fractures by Lorenz in the ‘90s), the Cordilleran extensional province boundary and many young normal faults in the norther Rockies, and local topography. In some Rockies folds these seemingly kinematically compatible orientations are fully or in part out of sync with the timing of folding (Laubach, S.E., Fall, A., Copley, L.K., Marrett, R., Wilkins, S., 2016. Fracture porosity creation and persistence in a basement-involved Laramide fold, Upper Cretaceous Frontier Formation, Green River Basin, U.S.A. Geological Magazine 153 (5/6), 887-910. doi:10.1017/S0016756816000157).
Is there any crossing and abutting relations evidence to see if the two sets formed in the expected sequence?
519 The passive construction here obscures who it is doing the interpreting (‘are generally interpreted to…) If this is your interpretation, construct the sentence to make this clear then say why you think so. Why could these not be (at least in part) pre- or post-folding regional fractures? (Regional fractures having about this orientation are present elsewhere in the Rockies distant from fold hinges, and some of these arrays are clustered).
527 I think this process was explicitly postulated for fractures in the Sevier fold belt by Lorenz. But may be published in an obscure source.
530 Hmm. Maybe you should say this first and then provide the speculation.
538 But regional opening-mode fractures arrays are very low strain features and do not require a tectonic event to account for them. A constant plate-scale stress field and a perturbation like gas generation can account for such fractures (For example, Fall et al. 2015, GSA Bulletin for regional fractures elsewhere in the Rockies).
540-545 But these caveats also apply to the fracture sets that happen to coincide with your fold geometry. Maybe it would be better to provide this broader context first then make the case for linking some fractures to the fold.
Overall, I think the Discussion can be more compact.
575 (figure) is there a reason that the fractures shown don’t have any particular crossing and abutting relations? The text makes it seem like the fold contains temporally and genetically distinct groups of fractures and wouldn’t you expect such patterns from a fold-related progression of fracture? Especially if (as is likely) preexisting fractures exist in the folded layers?
591 Is this really the appropriate topic sentence for this conclusion? Some hinge-parallel and hinge-perpendicular fractures systematically increase in abundance towards the fold hinge and are thus presumably fold related. But many fractures/sets are not clearly related to folding and you say they are probably unrelated to folding. The composite pattern from the outcrop is some fold-related fractures amidst others of unknown origin.
580-90 The ‘are attributed’ construction is not helpful. Instead say ‘we attribute’ and then say why (what about the ‘stratigraphic level’?). Your claim here is not clear. The highest intensity would be in the fold hinge, but some units have higher overall intensity because of rock type or bed thickness and where these are exposed away from the fold hinge it results in a discrepancy between intensity and structural position?
There are some formatting corrections needed in the reference list.