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, Alberta Energy Regulator, Edmonton, AB T8N 3A3, Canada; 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. tified geological factors could drive the sporadic appearance of induced Based on these complications, we instead look for geological 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- SCIENCES SI Appendix Ilating on a fault is suddenly released via an anthropogenically drites (Wabamun Group) ( ,Fig.S1) and resulted in ENVIRONMENTAL 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/Exshaw 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 in Significance southern Alberta, Canada, near the town of Cardston (Fig. 1) (11). Starting in December, 2011, anomalous earthquakes oc- Induced earthquakes can be caused by hydraulic fracturing curred contemporaneously with the completion of a multistage (HF). However, the exact means by which stress changes are horizontal well in the uppermost Wabamun Group (also referred transferred to seismogenic faults are unknown. This paper “ ” to as the Exshaw play ). These induced events were of mod- provides evidence that a case of induced earthquakes in erate magnitude (up to moment magnitude 3), with hypocenters southern Alberta responded to increased pore pressure on a located in the shallow crystalline basement (11). fault in hydraulic communication with the HF operation. Induced earthquakes are thought to require three geo- Reflection-seismic and drill core data provide evidence that mechanical conditions: (i) the presence of a fault, (ii) nearly iii fluid flow along this fault caused strata underlying the target critical slip-orientation of the fault, and ( ) a means to perturb reservoir to dissolve, causing a karst collapse in the geo- stress on the fault past the critical condition (12). In general, the logical past. We suggest that seismogenic and hydraulically first two conditions are well-established requirements for fault active faults are geologically rare and that the injection rupture nucleation. However, the third point is contentious, since there are multiple anthropogenic means to communicate of fluid directly into them is even rarer, potentially explain- stress changes. For example, effective stress changes can be ing the small percentage of HF wells that cause induced communicated to the fault either through pore-pressure per- earthquakes. turbations transmitted along the damage zone of a mechanically active fault (13) or poroelastically through a (impermeable) rock Author contributions: R.S. designed research; E.G., T.H., H.C., D.P., and R.S. performed research; E.G., T.H., H.C., and R.S. analyzed data; and E.G., T.H., H.C., D.P., and R.S. wrote matrix (14). For disposal cases, pore-pressure diffusion is often the paper. the favorite speculative explanation, since continued injection The authors declare no conflict of interest. into permeable formations allows plausible communication over great distances until the intersection with a critically unstable This article is a PNAS Direct Submission. fault (e.g., refs. 15 and 16). However, for HF, this reasoning is Published under the PNAS license. complicated by additional conceptual hurdles. For example, HF 1To whom correspondence should be addressed. Email: [email protected]. wells inject into impermeable formations to stimulate reservoir This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. productivity (17); thus, reasonable pressure perturbations are 1073/pnas.1807549115/-/DCSupplemental. restricted to the stimulated reservoir zone proximal to the well Published online October 8, 2018. www.pnas.org/cgi/doi/10.1073/pnas.1807549115 PNAS | vol. 115 | no. 43 | E10003–E10012 Downloaded by guest on September 27, 2021 113.4° W 113.2° W 113.0° W 112.8° W Tectonic Setting The area under investigation is in the southern Alberta Plains, 49.6° N ∼10 km east of the triangle zone of the foothills, which repre- sents the approximate eastern limit of the Cordilleran defor- mation front. The sedimentary succession comprises Cambrian to Paleocene strata of the Western Canada Sedimentary Basin (WCSB), which overlie the Precambrian crystalline basement of St. Mary River the North American craton. The strata of the WCSB were de- SALTSALT 30 posited in three different general tectonic settings: a passive continental margin from the Neoproterozoic to Middle Devo- WSOF nian, a back-arc setting from the Late Devonian to Jurassic, and 49.4° N a foreland basin between the Jurassic and Eocene. Far-field stress related to the purported Late Devonian–Early Carbonif- Belly River 3D Seismic Study Area erous Antler and the Jurassic-to-Early Eocene Cordilleran CHW St.S Mary Reservoir contraction to the west may be responsible for faulting in the investigated area. Postorogenic isostatic reequilibration of the Cordilleran foreland system was accompanied by the removal of two kilometers of strata in the Alberta Plains (26), which may have triggered minor Late Cenozoic faulting or fault reactivation. 49.2° N Quaternary deposits cover the bedrock. Cardston First, we considered public 2D reflection-seismic transects from Canada’s LITHOPROBE study, thereby placing the study 0 5 10 15 km area into a regional context (Figs. 1 and 2). Specific to southern Alberta, the Southern Alberta Lithospheric Transect (SALT) Fig. 1. Map of the study area. The location of the HF well responsible for includes 290 km of reflection-seismic lines acquired in 1995 be- the induced earthquakes (yellow star), 3D reflection-seismic survey (black tween the Cordilleran deformation front and the western side of box), SALT 2D reflection-seismic line (black line), and a previously inferred the Sweetgrass Arch in southern Alberta and northern Montana fault (the WSOF) (red dashed line) are displayed alongside local municipal- (27). This portion of the SALT consists of two east–west lines: ities (gray areas), roads (gray lines), and waterbodies (blue). (Inset) Map line 30 (110 km) and line 31 (140 km), tied by the roughly north– shows the location of the study (white star) in North America. south line 29 (40 km). The SALT data revealed a subhorizontal layer-cake structure of the WCSB stratigraphy affected by ex- dication of fault-associated