Quantitative constraints to the complete state of stress from the combined borehole and focal mechanism inversions: Fox Creek, Alberta

Abstract

We develop a quantitative predictive model for the state of stress of a volume of crust encompassing the Duvernay Formation with a spatial extent of over 150 km  150 km centred near the municipality of FoxCreek. An average direction of the maximum horizontal compression S H is of N43°E, determined from analysis of 20 borehole image logs. A model of the vertical stress S V , corrected for topographic variations, is provided from the integration of a 3D density volume constructed from 1125 density logs. The minimum horizontal compressions S h and pore pressures P P are evaluated through analysis of 57 well tests carried out within the Duvernay Formation; 3D models developed by kriging of the observed values.Stress inversion of the focal mechanism solutions for the earthquakes nearby validated the assumed Andersonian stress regime and provided the shape-ratio of the stress which further allowed estimation of maximum horizontal compression S H . A program is provided allowing the user to calculate the full set of components necessary to describe the states of in-situ stress for a location of interest near the Duvernay formation within our study area.

Hydraulic Fracturing‐Induced Seismicity

“…These induced earthquakes have been interpreted as reactivation of basement‐rooted transtensional strike‐slip faults (R. Wang et al, 2017), which is consistent with fault inferences from 3‐D reflection seismic data (Chopra et al, 2017; Corlett et al, 2018; Eaton et al, 2018) and the present‐day tectonic stress field (Shen, Schmitt, & Haug, 2019; Shen, Schmitt, & Schultz, 2019; H. Zhang, Eaton, et al, 2019). Notably, foreshock microseismicity detected using a dense local network appears to show fault activation processes leading up to the mainshock (Eyre, Eaton, Zecevic, et al, 2019), including inferred aseismic slip (Eyre, Eaton, Garagash, et al, 2019).…”

“…Geomechanical factors such as target formation overpressure have been statistically linked to HF‐induced earthquake locations (Eaton & Schultz, 2018) and interpreted as a proxy for slip potential and fault‐valve processes (Sibson, 2020). Certainly, the state of in situ stress has a strong influence on which faults can host slip (Shen, Schmitt, & Haug, 2019; Shen, Schmitt, & Schultz, 2019; Zoback & Lund Snee, 2018). In many basins, the regional state of stress has been quantified (Lund Snee & Zoback, 2016, 2018), allowing for an indication of which faults may potentially host slip (Hennings et al, 2019).…”

“…Pore pressure propagates along this pathway, initiating slip on the weakest segment of the fault. In fact, cases that have accounted for the complete state of stress (Shen, Schmitt, & Haug, 2019) have found that slip initiation is guaranteed for most fault orientations under reasonable HF stimulation pressures (Shen, Schmitt, & Schultz, 2019; Lei et al, 2019), thus relaxing the requirement for a fault to be optimally oriented.…”

Sequential Fault Reactivation and Secondary Triggering in the March 2019 Red Deer Induced Earthquake Swarm

“…In other words, delayed fault slip at greater distances can result in post shut‐in seismicity. In our case, the delayed focal mechanisms show less favorable fault orientations relative to the direction of the present‐day maximum horizontal stress ( S Hmax ) in central Alberta (Reiter et al, 2014; Shen et al, 2019). This observation favors a second‐order triggering effect that was previously documented near Fox Creek (Schultz et al, 2017).…”

An open-access stress magnitude database for Germany and adjacent regions

“…However, we recommend not to use gradients or use them with caution and only for analysis at appropriate scales (see e.g. Shen et al 2019b). For the same reason, quantifying the goodness of linear fit is not necessarily appropriate.…”