Fracture abundance and strain in folded cardium formation, Alberta fold-and-thrust belt, Canada
The folded and thrusted Mesozoic clastic sequence of the Canadian Rocky Mountain foothills forms important hydrocarbon reservoirs. Understanding the distribution of natural fractures, their evolution, and timing of formation relative to the evolution of the fold-and-thrust system could potentially improve exploration and development outcomes in these otherwise tight unconventional reservoirs. However, the formation of fractures and their timing relative to folding and thrusting have remained unclear. I investigated the relation between folding and fracture formation in the Upper Cretaceous Cardium Sandstone by combining field structural observations and kinematic modeling of the fold-and-thrust belt evolution. I explored the relationship between fracture intensity and fracture strain with structural position by analyzing fracture spacing or frequency and aperture data collected along outcrop and micro-scanlines in the backlimb, in the forelimb close to the crest, and in the steeper dipping forelimb away from the crest of the Red Deer River anticline. Fracture frequency and aperture data collected both at the outcrop and micro scales indicate that variation in fracture strain is small across these three structural domains of the fold, with somewhat lower fracture intensity in the forelimb close to the crest. These fracture strain measurements are qualitatively consistent with calculated horizontal strain in the tectonic transport direction obtained through kinematic numerical models that simulate fold development associated with slip along the underlying Burnt Timber thrust. The models predict roughly similar amount 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. Fracture formation early during fold development is consistent with the 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 such structurally complex subsurface reservoirs.