STRUCTURAL-DIAGENETIC CONTROLS ON FRACTURE OPENING IN THE NIKANASSIN FORMATION, ALBERTA FOOTHILLS

The Jurassic to Lower Cretaceous Nikanassin Formation produces gas at commercial rates where it contains a network of open fractures. Fractures from outcrops have the same diagenetic attributes as those observed in cores <100 km away, thus offering an ideal opportunity to 1) evaluate the distribution and characteristics of opening mode fractures relative to fold cores, hinges and limbs, 2) compare the distribution and attributes of fractures in outcrop vs. core samples, 3) estimate the timing of fracture formation relative to the evolution of the fold-and-thrust belt. Cathodoluminescence images of cemented fractures in both outcrop and core samples reveal several generations of quartz and ankerite cement that is synkinematic and postkinematic relative to fracture opening. Crack-seal textures in synkinematic quartz are ubiquitous, and well-developed cement bridges abundant. Fracture porosity may be preserved in fractures wider than ~100 microns. 1-D scanlines in outcrop and core samples indicate fractures are most abundant within small parasitic folds in larger, tight, mesoscopic folds. Fracture intensity progressively decreases from the faulted cores of mesoscopic folds to their forelimbs, with lowest intensities within relatively undeformed backlimb strata. Fracture apertures locally increase adjacent to reverse faults without an overall increase in fracture frequency. Fluid inclusion analyses in crack-seal quartz cement indicate the presence of both aqueous and methane-rich inclusions. Based on homogenization temperatures of two-phase aqueous inclusions synkinematic fracture cement precipitation and fracture opening occurred under conditions at or near maximum burial of 190-210°C in core samples, and 120-160°C in outcrop samples, during exhumation. These results suggest a strong control of regional tectonic processes on fracture generation. One fracture set, perpendicular to fold axes and the main structural trend formed as a regional set, whereas a second orthogonal fracture set, parallel to fold axes, formed related to individual structures.