Outcrop to Core Comparison of Natural Fractures in a Tight Gas Sandstone Reservoir, Alberta Foothills, Canada
Ukar, Estibalitz; Eichhubl, Peter; Fall, András; Hooker, John
In tight gas reservoirs, understanding the attributes, orientation and distribution of natural open fractures, and how they relate to the structural and stratigraphic setting are important for exploration and production. Outcrops provide the opportunity to sample fracture characteristics that would otherwise be incomplete due to the limitations of sampling by cores and well logs thus filling the information gap between seismic and core data and establishing the relationship between fracture attributes and subseismic structures. However, fractures in exhumed outcrops are frequently not representative of fractures in the reservoir because of differences in burial and exhumation history. Late Jurassic-Early Cretaceous Nikanassin Formation exposed near Grande Cache, Alberta, share many structural and diagenetic attributes with cores of the same formation from producing reservoirs ~80 km away, thus offering an 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, and 4) predict reservoir properties based on structural position. 1-D scanlines in outcrop and core samples indicate fractures are most abundant within small parasitic folds within larger, mesoscopic folds. Fracture intensity is lower away from parasitic folds; intensity progressively decreases from the faulted cores of mesoscopic folds to their steep limbs, with lowest intensities within relatively undeformed shallow-limb strata. Fracture apertures locally increase adjacent to reverse faults without an overall increase in fracture frequency. Fracture opening strain (1D extension) is higher in more deformed areas such as steep limbs and anticline cores. 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. Fluid inclusion analyses of synkinematic crack-seal quartz cement indicate both aqueous and methane-rich inclusions are present. Homogenization temperatures of two-phase inclusions indicate synkinematic fracture cement precipitation and fracture opening under conditions of 120-160°C in outcrop samples, and near maximum burial of 190-210°C in core samples.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013