Models for hydraulic fracturing–induced earthquakes in shales typically ascribe fault activation to elevated pore pressure or increased shear stress; however, these mechanisms are incompatible with experiments and rate-state frictional models, which predict stable sliding (aseismic slip) on faults that penetrate rocks with high clay or total organic carbon. Recent studies further indicate that the earthquakes tend to nucleate over relatively short injection time scales and sufficiently far from the injection zone that triggering by either poroelastic stress changes or pore pressure diffusion is unlikely. Here, we invoke an alternative model based on recent laboratory and in situ experiments, wherein distal, unstable regions of a fault are progressively loaded by aseismic slip on proximal, stable regions stimulated by hydraulic fracturing. This model predicts that dynamic rupture initiates when the creep front impinges on a fault region where rock composition favors dynamic and slip rate weakening behavior.

No abstract

Focal-time analysis is a straightforward data-driven method to obtain robust stratigraphic depth control for microseismicity or induced seismic events. The method eliminates the necessity to build an explicit, calibrated velocity model for hypocenter depth estimation, although it requires multicomponent 3D seismic data that are colocated with surface or near-surface microseismic observations. Event focal depths are initially expressed in terms of zero-offset focal time (two-way P-P reflection time) to facilitate registration and visualization with 3D seismic data. Application of the focal-time method requires (1) high-quality P- and S-wave time picks, which are extrapolated to zero offset and (2) registration of correlative P-P and P-S reflections to provide [Formula: see text] and [Formula: see text] time-depth control. We determine the utility of this method by applying it to a microseismic and induced-seismicity data set recorded with a shallow-borehole monitoring array in Alberta, Canada, combined with high-quality multicomponent surface seismic data. The calculated depth distribution of events is in good agreement with hypocenter locations obtained independently using a nonlinear global-search method. Our results reveal that individual event clusters have distinct depth distributions that can provide important clues about the mechanisms of fault activation.

We have developed an interpretive seismic workflow that incorporates multicomponent seismic inversion, guided by structural mapping, for characterizing low-permeability unconventional reservoirs. The workflow includes the determination of a calibrated time-depth relationship, generation of seismic-derived structural maps, poststack inversion, amplitude-variation-with-offset analysis, and PP-PS joint inversion. The subsequent interpretation procedure combines structural and inversion results with seismic-derived lithologic parameters, such as the Young’s modulus, Poisson’s ratio, and brittleness index. We applied this workflow to a 3D multicomponent seismic data set from the Duvernay play in the Kaybob area in Alberta, Canada. Subtle faults are discernible using isochron maps, horizontal time slices, and seismic stratal slices. Fault-detection software is also used to aid in the delineation of structural discontinuities. We found that seismic-derived attributes, coupled with structural mapping, can be used to map reservoir facies and thus to highlight zones that are most favorable for hydraulic-fracture stimulation. By imaging structural discontinuities and preexisting zones of weakness, seismic mapping also contributes to an improved framework for understanding the induced-seismicity risk.

Injection-induced seismicity in Western Canada and elsewhere in North America has drawn considerable recent interest. Current models indicate that induced earthquakes occur on reactivated basement faults, which can be challenging to detect using seismic-reflection data. Here we use regional gravity and magnetic datasets, together with LITHOPROBE crustal seismic profiles, to investigate basement tectonics and crustal structure in an area of Western Canada that is prone to induced seismicity. Previously mapped basement faults that were active during the Paleozoic can be recognized on the basis of pronounced curvature, truncations and/or offsets of stratigraphic marker horizons. Within the Precambrian crystalline basement, however, brittle faults are poorly imaged by seismic data due to various factors such as the obscuring effect of multiples. Regional potential-field fabrics are critical to establish the tectonic setting of basement domains, with complementary information provided by magnetic, Bouguer and isostatic residual gravity anomalies based on 2D modelling constrained by seismic profiles. However, individual faults appear to lack diagnostic expression in regional potential-field anomaly data, since the anomalies are dominated by the effects of larger-scale crustal structures. We show evidence that large-scale basement faults can potentially be recognized on the basis of truncation and offset of distinct horizons within the Winagami Reflection Sequence (WRS), which is interpreted as a regionally-extensive mid-crustal sill complex emplaced during a Proterozoic magmatic pulse. An abundance of caution is necessary to interpret these features, due to complications arising from out-of-plane reflections at long reflection times.