The Cardston Earthquake Swarm and Hydraulic Fracturing of the Exshaw Formation (Alberta Bakken Play)

Abstract

More than 60 small earthquakes ( M L 0.7–3.0) were detected from December 2011 to March 2012 north of Cardston, Alberta, an area with little evidence for previous seismic activity. The timing of these events closely correlates (>99.7% confidence) with hydraulic fracturing completions of the Devonian–Mississippian-age Exshaw Formation at a nearby horizontal well. Unanimous waveform multiplicity within the swarm suggests that the events share a similar origin and source mechanism. This observation is corroborated by the point-like collocation of hypocenters within the crystalline basement during robust, double-difference relocations. Furthermore, the presence of a pre-existing fault is confirmed via formation-top offset mapping and interpreted to be a Late Cretaceous extensional fault. The confirmation of this fault at depth provides a plausible pathway for rapid hydraulic communication from the fracturing interval into the crystalline basement. Consistent with structural interpretations and available stress information, moment tensor inversion of the largest magnitude event ( M w 3.0) indicates reactivation of a basement fault with normal slip. We conclude that the genesis of this earthquake swarm was likely caused by increased pore pressure, within the basement fault, as a result of fracturing stimulation. Online Material: Figure showing frequency–magnitude distribution of earthquakes, and table of the velocity models used in the study.