08 May 2023
 | 08 May 2023
Status: this preprint is open for discussion.

Scaling of natural fracture patterns at Swift anticline, NW Montana: the influence of structural position, lithology, and observation scale

Adam J. Cawood, Hannah Watkins, Clare E. Bond, Marian J. Warren, and Mark A. Cooper

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)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • 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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Short summary
Conceptual models are often used to predict subsurface fracture properties. Here we test conceptual models by investigating fractures across multiple scales. We find that most fractures increase in abundance towards the fold hinge and we interpret these as fold related. Other fractures at the site show inconsistent orientations and are unrelated to fold formation. Our results show that predicting fracture patterns requires consideration of multiple geologic variables.