Research paperFracture abundance and strain in folded Cardium Formation, Red Deer River anticline, Alberta Foothills, Canada

•Folding and fracturing relationship is analyzed using field measurements and kinematic models.•Fracture strain measurements are consistent with horizontal strain calculated using Move.•Fracture strain varies little across the three structural domains of the fold.•Opening-mode fractures predate or formed during early onset of folding.•Cross-fold, opening-mode fractures were reactivated in shear and linked during folding.

The folded and thrusted Mesozoic clastic sequence of the Canadian Rocky Mountain foothills forms important hydrocarbon reservoirs; however, the generation of fractures and their timing relative to folding and thrusting have remained unclear. Understanding the distribution and evolution of natural fractures, especially their timing of formation relative to the evolution of the fold-and-thrust system, could potentially improve exploration and development outcomes in these otherwise tight, low-permeability reservoirs. We investigated the relation between folding and fracture formation in the Upper Cretaceous Cardium Formation by combining field structural observations and kinematic modeling of the fold-and-thrust belt evolution. We evaluated the effects of structural position and strain on the formation of predominantly opening-mode fractures by analyzing fracture intensity and aperture in the backlimb, the shallow forelimb close to the crest, and the steep forelimb, away from the crest of a reservoir-scale anticline, the Red Deer River anticline. Fracture strain determined from both outcrop and microscanlines varies little across the three structural domains of the fold, although somewhat lower fracture strain was measured in the forelimb close to the crest. These fracture strain measurements are qualitatively consistent with calculated horizontal strain in the tectonic-transport direction obtained from kinematic numerical models that simulate fold development associated with slip along the underlying Burnt Timber thrust. The kinematic models predict similar magnitudes of horizontal extension in both the back and forelimbs, and somewhat lower extension in the upper forelimb during early development of the Red Deer River anticline. Predicted early fracture formation during fold development is consistent with field structural observations of shear reactivation during later stages of folding. This combined kinematic modeling and field structural study demonstrates that deforming fold-and-thrust belts can undergo a complex evolution of bed-parallel extension in both space and time, resulting in spatially variable fracture formation in structurally complex subsurface reservoirs.