Faults and associated karst collapse suggest conduits for fluid flow that influence hydraulic fracturing- induced seismicity

Elwyn Gallowaya, Tyler Haucka, Hilary Corlettb, Dinu Pana˘ a, and Ryan Schultza,1

aAlberta Geological Survey, Energy Regulator, Edmonton, AB T8N 3A3, ; and bDepartment of Physical Sciences, MacEwan University, Edmonton, AB T5J 4S2, Canada

Edited by Paul Segall, Stanford University, Stanford, CA, and approved August 21, 2018 (received for review May 1, 2018)

During December 2011, a swarm of moderate-magnitude earth- bore [up to hundreds of meters (18–21)]. Furthermore, geo- quakes was induced by hydraulic fracturing (HF) near Cardston, mechanical modeling reveals that poroelastic changes from HF Alberta. Despite seismological associations linking these two processes, are transmitted only locally (20). These results are in direct the hydrological and tectonic mechanisms involved remain unclear. In contradiction to HF-related earthquakes that are located kilo- this study, we interpret a 3D reflection-seismic survey to delve into the meters away from the closest well bore (e.g., refs. 2, 6, 11, 22, and geological factors related to these earthquakes. First, we document a 23). Specific to the Cardston case, earthquakes (located on re- basement-rooted fault on which the earthquake rupture occurred that gional arrays) were observed within the uppermost crystalline ∼ – extends above the targeted reservoir. Second, at the reservoir’s strati- basement, 1.5 km deeper than the target upper Stettler Big SI Appendix graphic level, anomalous subcircular features are recognized along the Valley Reservoir zone ( ,Fig.S1); this observation is fault and are interpreted as resulting from fault-associated karst pro- further complicated by the fact that the fault slip response oc- ∼ – cesses. These observations have implications for HF-induced seismicity, curred immediately (i.e., 1.5 3.0 h) after well stage stimulations as they suggest hydraulic communication over a large (vertical) dis- (11). Due to geological complexities/unknowns in the subsurface, tance, reconciling the discrepancy between the culprit well trajectory seismological and geomechanical approaches to explaining these and earthquake hypocenters. We speculate on how these newly iden- discrepancies remain speculative. SCIENCES

tified geological factors could drive the sporadic appearance of induced Based on these complications, we instead look for geological ENVIRONMENTAL indications of past fluid migration in a 3D reflection-seismic seismicity and thus be utilized to avoid earthquake hazards. survey that surrounds the Cardston earthquakes (Fig. 1). In this paper, we describe the methods by which we analyzed this dataset hydraulic fracturing | induced seismicity | hydraulically active faults | and then bolster our interpretation with additional earthquake, drill dissolution karst core, and well log data. Together, the intersection of these in- dependent datasets leads us to the conclusion that fault-associated nduced seismicity is a phenomenon by which stress accumu- fluid flow caused the dissolution of the Stettler Formation anhy- Ilating on a fault is suddenly released via an anthropogenically drites (Wabamun Group) (SI Appendix,Fig.S1) and resulted in triggered shear slip. This phenomenon is well documented, with karst features that affected accommodation patterns in the overlying numerous cases related to mining, waste-water disposal, reser- upper Stettler/Big Valley/ interval near the Cardston hori- voir impoundment, and geothermal development activities (1). zontal well (CHW) location. This interpretation has implications for More recently, induced earthquakes have also been recognized understanding the induced seismicity caused by the CHW: The in- as the direct result of hydraulic fracturing (HF) stimulation of unconventional reservoirs (2–10). One such case occurred