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Structural-Diagenetic Evolution of Fractures in Folds: Nikanassin and Cardium Fms. Alberta Foothills, Canada


In the Alberta Foothills, the Nikanassin and Cardium Fms. are generally characterized as tight gas sandstones with submilidarcy values of permeability and matrix porosities typically less than 6%. They produce gas and/or oil at commercial rates where they contain a network of open fractures, but exploration and development outcomes are variable, underlining the necessity for a better understanding and characterization of the more fractured and potentially more productive regions. These formations provide an opportunity to study the relationship between folding and fracture formation through time. An outcrop study of fractures associated with two reservoir-scale anticlines delineated by Nikanassin Fm. reveals the presence of two main fracture sets in both anticlines: fracture set 1 is perpendicular to the fold axis, whereas fracture set 2 is parallel. Both sets have an associated oblique fracture set. Scanline data indicate higher fracture intensity in the steeply-dipping limbs of the folds than in the shallower-dipping limbs. Based on homogenization temperatures of two-phase aqueous inclusions in crack-seal quartz cement, synkinematic fracture opening and cement precipitation occurred during prograde burial for set 1 fractures (90°-175°), which may have continued during exhumation; whereas, set 2 fracture opening (120°-175°) occurred concurrent with thrusting and exhumation toward the end of the Laramide orogeny (∼50 Ma). Structural models constructed using MOVE predict a higher strain accumulation in the steep limb of the Sternie Creek anticline than in the fold hinge and shallow limb. Models also predict an early opening of fracture set 1, which is in accordance with the general observation that set 2 cross-cuts set 1 in outcrop. Fractures in Cardium strata exposed in three anticlinal structures occur in two main fractures sets parallel and perpendicular to the fold axes and in two oblique sets. The two main fracture sets appear to be of regional extent. Scaling analyses indicate no systematic variation of size-frequency distributions with respect to structural position for most fracture sets. Fractures may predate folding. For both formations we speculate that fracture formation may be related to processes such as loading or gas generation and charge during deep prograde burial. Fracturing in the Nikanassin Fm. may have temporally overlapped with folding and thrusting resulting in the spatial correlation of fracture intensity with folds.