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Home , Belly River Group, Exshaw Formation

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. 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/ 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 , 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 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 to , and 49.4° N a foreland basin between the Jurassic and Eocene. Far-field stress related to the purported Late Devonian–Early Carbonif- 3D Seismic Study Area erous Antler and the Jurassic-to-Early Eocene Cordilleran

CHW SSt. 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 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 affected by ex- dication of fault-associated fluid flow is inferential evidence of a tensional faults dipping steeply dipping down to the west (Fig. 2). Subvertical faults spaced 11–15 km apart cut through a potential permeable pathway through which pore-pressure pertur- prominent near-basement reflector. These faults offset Cam- bations can be transmitted quickly and over great distance. We brian, Devonian, , Jurassic, and strata speculate on the potential implications of this interpretation for and appear to be sealed by undeformed Belly River HF-related earthquakes, including the paucity of seismogenic Group strata (27). Lemieux et al. (28) inferred that the strike of wells at a basin scale (24, 25), and for fault-associated fluid- these faults observable on the SALT data in the WCSB cover is flow features as indicators of areas which may be prone to similar to the basement magnetic trends, and therefore their HF-induced seismicity. positioning would be basement-controlled.

-500 Rundle

-1000 lower Exshaw

TIME (ms) Precambrian

-1500

Reflectiv ity 1.5E+009 3D seismic 1E+009 -2000 5E+008 survey area 0 0 5 10 15 km -5E+008 (projected -1E+009 onto line) -1.5E+009

Fig. 2. LITHOPROBE seismic-reflection SALT line 30. The public reflection-seismic data from SALT line 30 have been marked with stratigraphic horizons (subhorizontal colored lines), previous fault interpretations (F1–F4 and red lines), and the projection of the southward 3D reflection-seismic survey (yellow box) onto the east–west–trending transect along the strike of the WSOF.

E10004 | www.pnas.org/cgi/doi/10.1073/pnas.1807549115 Galloway et al. Downloaded by guest on September 27, 2021 AA’AA’

A A’ Depth (masl)

+ SCIENCES ENVIRONMENTAL horizon curvature 0

Fig. 3. 3D reflection-seismic cross-section. Raw (Left) and interpreted (Middle) reflection-seismic cross-line data (gray area). Locations of earthquakes (red crosses), the 14-21-4-25W4 vertical well (black line), stratigraphic markers (subhorizontal colored lines), and lineament interpretations (red/pink lines/arrows) are shown. Errors in earthquake locations are on the order of 600 m vertically and 300 m laterally, which is reflected in the variability of hypocenter locations. (Right) Corresponding maximum positive curvature attributes (blue area) for each horizon with callouts to locations of the cross-line data (A–A′, red line), the lineament interpretations (red/pink arrows), and the vertical (black circles) and lateral extent (black line) of drilled wells. Subcircular / Wabamun Group anomalies can be seen east of the CHW in the lower Exshaw Formation curvature horizon. An enlarged view of the Precambrian curvature horizon (dashed box) more closely shows the lineament interpretations with respect to the earthquake epicenters.

Evidence from a 3D Reflection-Seismic Survey Synthetic seismograms were generated for 10 nearby wells To begin examining the geological controls of the Cardston with geophysical logs and were tied to the nearest observed earthquakes, proprietary 3D reflection-seismic data were utilized reflection-seismic arrival. This process incorporated data to characterize the subsurface around the CHW (Fig. 1). The 3D throughout the sedimentary sequence and resulted in a velocity reflection-seismic dataset, originally acquired in 1998, covers model of the sediment down to the Precambrian basement. 60 km2. This study used a version processed with surface- Below this, the crystalline basement lacks continuous interfaces of contrasting acoustic impedance; thus, the basement interval is consistent deconvolution, finite difference migration, and noise poorly suited for characterization by reflection-seismic data. removal by FXY deconvolution. The data were processed into × Furthermore, the reflections that appear below the Precambrian 50 50 m bins. The bandwidth of the interpretation volume horizon on the 2D and 3D data are interpreted as being seismic – decreases with depth: the bandwidth at 750 ms is 10 70 Hz (with multiples and should be ignored. Based on horizon correlations, a dominant frequency of 55 Hz), while the bandwidth at 1,500 ms we examined stratigraphic surface attributes including coher- is 11–65 Hz (with a dominant frequency of 40 Hz). Comparisons ence, curvature, and topography (see SI Appendix for details of with synthetic seismograms created from well logs suggest the analysis) (29, 30). Two significant structural features are de- phase of the reflection-seismic volume is reasonably stable and tected by the 3D reflection-seismic data near the CHW: a linear averages near 0°. feature trending approximately south–southeast (SSE) and a

Galloway et al. PNAS | vol. 115 | no. 43 | E10005 Downloaded by guest on September 27, 2021 Depth Structure (masl) Isopach (m)

113°18'W 113°15'W 113°12'W 113°09'W

14-05-005-25-W4 49°22'N

49°22'N -1000 N -1040 -1080 -1120 -1160

-1200 49°20'N Rundle - lower Exshaw

Rundle -1240 49°20'N 09-29-004-25-W4 (CHW) -1280 420 14-21-004-25-W4 -1320 410 0 2000 4000m -1360 -1400 400 390 113°18'W 113°15'W 113°12'W 113°09'W 380 370 360 -1360 350 -1400 340 -1440 -1480 -1520 -1560 -1600 lower Exshaw - Precambrian -1640 -1680 845 lower Exshaw lower -1720 840 835 -1760 830 825 820 815 810 805 800 795 790 -2160 785 780 -2200 775 -2240 770 765 -2280 -2320 -2360 -2400 -2440 -2480 Precambrian -2520 -2560

Fig. 4. Processed seismic-reflection horizons. Maps of the topography of three reflection-seismic horizons (Left) are shown alongside the thickness between these intervals (Right). The locations of nearby wells (black circles/lines) and earthquake epicenters (red crosses) are displayed for geographic context.

small number of anomalous subcircular depressions of the lower imaged, a zone of strain manifested as flexure of strata along a Exshaw Formation reflector localized on the linear feature (Figs. linear trend is captured by the reflection-seismic horizons. The 3 and 4). anomalous trend of horizon flexure identified by the curvature The appearance of a linear, approximately SSE-trending fea- attributes is interpreted as the manifestation of the WSOF in the ture in the curvature attribute maps is persistent in numerous reflection-seismic data. stratigraphic horizons from the top of the (the The localized anomalous subcircular features visible on the shallowest interpreted horizon) down to the Precambrian base- lower Exshaw Formation curvature and depth structure maps as ment (Fig. 3). Due to its spatial overlap with the Cardston well as on the isopach maps for the Rundle-to-lower Exshaw earthquakes and CHW (Fig. 3), we interpret the linear feature as Formation and lower Exshaw Formation-to-Precambrian inter- the same West Stand-Off Fault (WSOF) that was previously vals vary in diameter and morphology (Figs. 3 and 4). The largest conjectured to host the Cardston earthquakes (11). In the 3D of these features is ellipsoid with major and minor axes of reflection-seismic dataset, we observe that the WSOF strikes at ∼1,600 m and 600 m, respectively; the depression appears to be ∼170° across the entire 3D reflection-seismic survey (∼4.8 km) and dips at ∼80° (to the west) through an interval of at least up to 40 m deeper than the adjacent area. We note that the 2,300 m. The reflection-seismic data have limited vertical reso- lateral positioning of these anomalies is spatially coincident with lution, so this feature does not create a detectable offset of an overall thinning of the interval between the lower Exshaw For- reflection-seismic horizons and therefore is not apparent in co- mation and Precambrian (Fig. 4). Given the limited vertical extent herence attribute volumes. Given the dominant frequencies of of the seismic anomalies, concomitant thickening and thinning is the interpretation volume and interval velocities measured in constrained to the Exshaw Formation/Wabamun Group. Finally, well 14-21-4-25W4, the vertical resolution of the reflection these anomalies are clustered and represent only a small portion seismic ranges from 15 m at 750 ms to 35 m at 1,500 ms, as- (∼2%) of the total area covered by the 3D reflection-seismic data. suming the vertical resolution is approximately one-quarter of In fact, the clustered distribution of the Exshaw Formation/ the dominant wavelength (31). Although distinct faults are not Wabamun Group anomalies and thinned strata underlying the

E10006 | www.pnas.org/cgi/doi/10.1073/pnas.1807549115 Galloway et al. Downloaded by guest on September 27, 2021 114º W R29 R25 113º W R20 R17W4 N T13

50º N

8-24-11-28W4 SALT 32 SALT

er n Riv T10 ma Old D

e f or m a F5 t io n r Lethbridge F e iv ro R n lly SALT 31 t e B Fig. 5. Map of drill core and well log data. Locations A' of wells with logs reaching as deep as the Wabamun Group are plotted as gray squares; wells which have

4-24-7-22W4 29 SALT 6-16-6-22W4 an Exshaw Formation logged thicker than 10 m are highlighted in red around a gray square. Reviewed F1 F2 SALT 30 cores are labeled with their unique well identifier (e.g., 6-16-6-22W4), except for well 14-21-4-25W4, T5 Glenwood Magrath F3 F4 which had no core but exhibits Exshaw Formation A overthickening nearby the CHW. The CHW that St. Mary Reservoir caused the HF-induced earthquakes (star) is dis- Canada CHW W 4-21-4-25W4 (no core) played alongside the extent of the Lower Banff– S 13-5-4-25W4 O Exshaw–Big Valley administrative pool boundaries F (green area) and all other HF wells completed in this Cardston play (black dots). For geographic context, the 3D reflection-seismic area (hatched area), SALT lines

r

e –

v (black lines), and previously identified faults (F1 F5, i

R y r ar 1-20-1-24W4 ive T1 red stars; and WSOF, dashed red line) have been in- M lk R . i t M – ′ S 49º N cluded. The log section from A A (blue line) is dis- cussed further in Fig. 6.

Exshaw Formation (coincident with the WSOF) is suggestive of 18 analog wells where the Exshaw Formation is thicker than 10 m some localized process being the causal mechanism. were identified in the study area (Fig. 5) (a thickness of >10 m was used as a metric to identify potential wells with analogous core) Evidence from Well Logs and Drill Cores (Fig. 6). From these analog wells, four drill cores were available SCIENCES

The stratigraphic interval of interest for this part of the study within the study area (Fig. 5). In these overthickened Exshaw ENVIRONMENTAL includes the Devonian Wabamun Group and the Exshaw and Formation wells, there is commonly an associated decrease in the Banff Formations (SI Appendix, Fig. S1). The Wabamun Group thickness of the underlying Stettler Formation anhydrite, although is divided into the Stettler and Big Valley Formations, which the lack of deeper well control prohibits the thorough quantification comprise predominantly evaporites with lesser carbonates and of the thickness of these evaporites within the study area [only 91 of carbonates, respectively (SI Appendix, Fig. S1). The Stettler the 187 total wells fully penetrate the Stettler Formation (SI Appendix)]. Formation can be further subdivided into a lower Stettler an- First, to observe examples of an intact succession, areas out- hydrite and a relatively thin upper Stettler carbonate. The latter side (well 13-5-4-25W4) and on the edge (well 4-24-7-22W4) of is overlain by of the Big Valley Formation, which may Exshaw Formation overthickening were reviewed within two drill be erosionally absent in the study area. The Wabamun Group cores. The lower Stettler Formation in these areas comprises strata are unconformably overlain by the black of the sabkha- and/or salina-type anhydrite with interbedded dolola- Exshaw Formation, which can be subdivided into an informal minites. The upper Stettler, typically 5–8 m thick, comprises lower Exshaw Formation (black shales), and an upper Exshaw dololaminites and microbial laminites, with common organic- Formation comprising a silty carbonate (SI Appendix, Fig. S1). rich laminations and peloid grainstones. The Stettler Forma- This is in turn overlain by assigned to the Banff Formation tion is overlain by the mid- to distal-ramp deposits of the Big (Banff A in SI Appendix, Fig. S1). Valley Formation, typically less than 2 m thick or absent beneath The 3D reflection-seismic dataset revealed the presence of a the lower Exshaw Formation and comprising dark-colored shales series of subcircular anomalies at the Exshaw Formation/Wabamun with bioclastic nodular lime mudstone-wackestone and common Group interval (Fig. 3). Because the anomalies are spatially limited crinoid columnals. The Big Valley Formation is abruptly overlain in extent (Fig. 4), it was conjectured that they may be features as- by the laminated organic-rich black shales of the lower Exshaw sociated with karst-related dissolution of the underlying Stettler Formation, which in turn are overlain by the silty dolostones of Formation anhydrite (herein we refer to karst as being a process the upper Exshaw Formation. In areas outside the overthickened operating only in the anhydrite of the Stettler Formation). We Exshaw Formation, the thickness of the combined lower and inferred concomitant subsidence within the overlying carbonates of upper members ranges from 3.2 to 6.7 m, although this is also the upper Stettler and Big Valley formations through collapse of the variable due to absence of the upper Exshaw Formation member karst-related dissolution features in the Stettler anhydrite, resulting in parts of the study area (32). The interpretations in this study of in increased accommodation and overthickening of the overlying the sedimentology and stratigraphy from these drill cores are Exshaw Formation, which is 22.4 m thick in well 14-24-4-25W4. To consistent with prior studies (32, 33). substantiate this, we have considered geological drill core and well Four drill cores were examined in areas where the Exshaw log data. Unfortunately, no drill cores from this interval exist from Formation thickness is greater than 10 m (e.g., well 6-16-6- the CHW area; in their absence, nearby analogs have been in- 22W4) (Fig. 6). In three of the drill cores, the upper Stettler and vestigated instead. To find these analogs, we have considered Big Valley Formation are dominated by intervals of intraclast stratigraphic correlations and isopach mapping of the Exshaw, Big breccia (Fig. 6 B and C) or dipping and offset laminations where Valley, and Stettler Formations within 184 wells (Fig. 5). At least unbrecciated. Many of the unbrecciated intervals are highly

Galloway et al. PNAS | vol. 115 | no. 43 | E10007 Downloaded by guest on September 27, 2021 100/14-21-004-25W4 100/07-08-006-22W4 100/06-16-006-22W4 100/04-24-007-22W4 A 31.9 km 2 km 11.3 km A' Gamma Density porosity Gamma Density porosity Gamma Density porosity Gamma Density porosity ray (limestone) ray (limestone) ray (sandstone) ray (limestone) 30 15 0 30 15 0 30 15 0 30 15 0 Depth m Depth m Depth m Depth m 1750 1750 1775 2000 1775 1775 1800 2025 Banff 1800 Upper 7.8 m 11.3 m 22.4 m

1800 Exshaw 26.2 m

2050 Lower 1825

Exshaw 1825 Big Valley 1825 A Stettler ? B C 2075

Stettler 1850 anhydrite

A B C

2 cm 2 cm 2 cm

Fig. 6. Cross-section A–A′ and images of drill cores. (Upper) The cross-section shown in Fig. 5 is displayed on the upper half of this figure, datumned on the upper Exshaw Formation. Well log 100/06-16-006- 22W4 is labeled with the stratigraphic nomenclature used in this study and displays overthickened Exshaw Formation (>10 m) relative to nearby well 100/07-08- 006-22W4. (Lower) Core photos show microfaulted brecciation (arrows, A) with possible flow banding recognized by faint matrix laminations and subtle alignment of clasts (B) and vertical brecciated clasts (C) from the upper Stettler and Big Valley Forma- tions in well 100/06-16-006-22W4, which are inter- preted as resulting from dissolution-related karst in the underlying Stettler Formation anhydrite.

microfractured (Fig. 6A). Clasts within brecciated units of the compare (or reject) depositional case scenarios against the ob- upper Stettler Formation comprise faintly bedded to microbial served reflection-seismic waveform amplitudes. Using wireline laminated dolostones and common dolomudstones. Clasts are logs for interval transit time (sonic logs) and bulk density, an dominantly angular, with some subrounded occurrences, and acoustic impedance log is calculated for a well and convolved with range widely in size. Some breccias are poorly sorted (Fig. 6C), a zero-phase wavelet to produce synthetic seismograms (see SI whereas others display more common clast size grading and Appendix for details). The acoustic impedance log of well 14-21-4- possible flow banding (Fig. 6B). Within drill cores where the 25W4 was used as the basis for describing geological scenarios in lower Exshaw Formation was recovered, inclined laminations the forward modeling because of the proximity of the well to the were observed in the lower parts of the unit. Furthermore, one Exshaw Formation/Wabamun Group anomalies and the CHW. drill core from an overthickened Exshaw Formation occurrence Well log interpretation indicates considerable thickening (22.4 m) of the Exshaw Formation at 14-21-4-25W4, which is supported by (well 8-24-11-28W4) (Fig. 5) displayed pervasive secondary do- ’ lomite overprinting in the Big Valley Formation and possibly the well s apparent penetration of the edge of a seismic isopach part of the upper Stettler Formation. The dolomitization is anomaly (Fig. 4). The sonic and density logs of 14-21-4-25W4 were recognized by a medium-to-dark gray coloration relative to the therefore modified to better represent the background geology by compressing the Exshaw Formation interval to a locally typical beige dolostones of other observed breccias. The dolomitization value (5 m). results in partially fabric-destructive textures that obscure the Three scenarios and their seismic responses were considered appearance of brecciation except where intraclast ghosts and SI Appendix (Fig. 7). In each scenario, 35 m of extra accommodation is created partially corroded rims of clasts are common ( , Fig. by removing the uppermost portion of the Wabamun Group. The S2). Overall, these observations are suggestive of fluid flows i aspect that distinguishes the three scenarios is the interval which is along faults and are responsible for ( ) dissolution-related karst of thickened to fill the extra accommodation created by the disso- evaporites and associated accommodation patterns in the over- lution of underlying anhydrite: (i) syndepositional thickening of lying Exshaw Formation interval, which manifest as the subcircular only the upper Exshaw Formation member, (ii)syndepositional reflection-seismic anomalies and (ii) localized secondary dolomi- thickening of both the lower and upper Exshaw Formation, and tization of carbonates in the uppermost Wabamun Group. (iii) syndepositional thickening of only the lower Exshaw Forma- tion member. The cases are differentiated by the forward seismic Reflection-Seismic Modeling of the Exshaw Formation/ models. Although the Exshaw Formation/Wabamun Group transi- Wabamun Group Anomalies tion is marked by a strong amplitude peak in all cases, a significant To further explore our conjecture regarding the Exshaw Formation/ amplitude increase is only associated with the Exshaw Formation/ Wabamun Group anomalies, we employed forward modeling to Wabamun Group anomaly in cases two and three. In addition, only

E10008 | www.pnas.org/cgi/doi/10.1073/pnas.1807549115 Galloway et al. Downloaded by guest on September 27, 2021 overthickened upper Exshaw

u. Exshaw overthickened l. Exshaw Exshaw Fig. 7. Forward modeling of the Exshaw Formation/ Wabamun Wabamun Group anomalies. Forward reflection- seismic modeling results are shown for three sce- narios proposed for infill of the subcircular Exshaw Formation/Wabamun Group anomalies. (Left) The sonic log from well 14-21-4-25W4 has been modified to approximate regional Exshaw Formation thick- overthickend ness and is shown with elevations of the Wabamun lower Exshaw Group and the upper and lower units of the Exshaw Formation. (Center Left) Three geological models for depositional infill of the anomalies are shown in time. (Center Right) Respective zero-offset seismic reflectivity sections. (Right) An actual reflection- seismic reflectivity section is shown for comparison.

the forward reflectivity models for cases two and three exhibit a Exshaw Formation/Wabamun Group anomalies would appear continuity of the amplitude trough immediately above the Exshaw smaller than expected. Therefore, caution should be taken when Formation/Wabamun Group peak. Last, cases one and two exhibit an comparing horizon amplitudes in the anomalies with those out- extra peak–trough of moderate amplitude above the Exshaw For- side the anomalies. mation/Wabamun Group interface within the area of anomalously thick Exshaw Formation. Discussion An equivalent portion of real reflection-seismic data exhibits Evidence of Faulting in Southwestern Alberta and the WSOF. Pre- some of the characteristics identified in the forward seismic vious studies in the southern Alberta Plains have identified (or models (Fig. 7): The amplitude of the reflection from the Exshaw inferred) extensional faults, which suggest subsequent accom- SCIENCES Formation/Wabamun Group interface is consistent across the modation of compression. For example, clear section; the amplitude trough immediately above the Exshaw evidence of extensional faulting in the study area was provided ENVIRONMENTAL Formation/Wabamun Group peak is continuous; and an extra by the SALT (27, 28). Normal faults (Fig. 2) show obvious breaks peak–trough of moderate amplitude above the Exshaw Forma- (up to 450 m of vertical throw) in reflections in the Paleozoic tion/Wabamun Group interface within the area of anomalously strata. The trends of these faults were inferred to be subparallel to thick Exshaw Formation is evident. From these qualitative ob- the local northwest–southeast grain of the crystalline basement servations, case three can be dismissed as a viable model for the (27), suggesting that at least some of these faults may be rooted in Exshaw Formation/Wabamun Group anomalies. Although cases the basement and thus may be reactivated basement structures. one and two both resemble the real reflectivity, they each dem- These previous studies anticipate the presence of extensional onstrate some aspect that differentiates it from the actual re- tectonism in southern Alberta in general. Specific to the study sponse. Case one lacks continuity of the amplitude trough above area, inferential evidence points to the possibility of local ex- the Exshaw Formation/Wabamun Group peak. The amplitude tensional faulting, interpreted as the WSOF, in the study area increase along the Exshaw Formation/Wabamun Group horizon (Fig. 1): (i) basement lineaments derived from magnetic and in the anomaly in case two is not evident in the real reflection- Bouguer gravity maps which may represent Precambrian base- seismic field data. Qualitatively, cases one and two both show ment discontinuities prone to reactivation (34–36), (ii) gradients reasonable similarity to the reflection-seismic field data. The in maps of formation tops (11), and (iii) evidence of earthquake forward modeling of the reflection-seismic data demonstrates hypocenters induced by the CHW (11). that dissolution of Stettler Formation anhydrite and continuous Specific to the 3D reflection-seismic data analyzed here, our infill of sediments during Exshaw Formation time can produce confidence that the identified linear feature represents a genuine anomalies similar to those seen in the field data. Although the geological structure is bolstered by several factors. First, the orien- modeling does not discern the details of the timing, it suggests tation of the anomaly is not aligned to the acquisition geometry or to that overthickening occurs throughout Exshaw Formation time. any geographical factors. Second, the plane of flexure dips to the These results are consistent with the dissolution timing suggested west, translating the curvature anomalies westward for deeper hori- by the brecciation of uppermost Wabamun Group carbonates in zons, suggesting that the anomaly has geological causes and is not a the overthickened Exshaw Formation drill cores. processing error. Finally, the lineament stretches across the entire 3D However, we recognize that this simplistic approach to for- survey, indicating that this feature is significant in at least the in- ward seismic modeling has ignored any seismic acquisition illu- vestigated region. Considering these supporting pieces, the in- mination effects. Contrary to the zero-offset simplification used terpretation of the newly imaged ∼SSE lineament (Figs. 3 and 4) as a for this modeling, reflection-seismic data are acquired with some basement-rooted extensional fault is likely. offset between source and receiver. This can become an issue Evidence for the commonly invoked Late Devonian–Early when the geometry of the source–receiver anomaly precludes Antler orogeny (e.g., ref. 37) is limited and con- source energy from reaching or reflected energy from returning troversial at this latitude in the Cordillera (38, 39). However, from the anomalous structural feature. Should the reflection- Paleozoic tectonism at the western margin of ancient North seismic data be affected by these illumination complications, America is recorded by multiple and the magnitude of the reflections in the structural lows of the (40). Therefore, the possibility that far-field

Galloway et al. PNAS | vol. 115 | no. 43 | E10009 Downloaded by guest on September 27, 2021 Interpretation of the Exshaw Formation/Wabamun Group Anomalies. AB Localized overthickening of the Exshaw Formation suggests syndepositional accommodation (49–51). The creation of this accommodation space is associated with brecciation of the Big Valley and upper Stettler Formations in the examined analog drill cores (Fig. 6). There is no evidence of subaerial karst within the Big Valley and upper Stettler Formation carbonates in the cores examined. Rather, the brecciation is associated with col- located overthickening of the Exshaw Formation and decreased C D thicknesses in the underlying anhydrite of the lower Stettler Formation. This suggests that dissolution of anhydrite by fault- associated fluids within this interval led to the collapse or sub- sidence of the overlying carbonates of the Big Valley and upper Stettler Formations, resulting in the brecciated textures observed. This interpretation is consistent with the subcircular sinkhole-type morphology of Exshaw Formation/Wabamun Group anomalies in the 3D reflection-seismic dataset (Fig. 3). The interpretation of a solution–evaporite collapse origin for E the breccias of the Big Valley and upper Stettler Formations is LEGEND Lethbridge supported by (i) the wide ranges in clast sizes, (ii) poor sorting, Limestone Possible fault iii iv v Shale Normal fault ( ) large rotated clasts, ( ) thickness of brecciated intervals, ( )

Exshaw Fm. Ascending fluids high frequency of microfracturing and offset sedimentary struc-

V V

V vi V

V Anhydrite Descending fluids tures, and ( ) proximity to evaporate strata interpreted as being Dolomite partially removed (Fig. 6) (52, 53). Although previous interpre- Breccia Earthquake tations of some of these breccias includes formation in tidal Sandstone channels (51), the above observations coupled with a lack of N brecciation outside overthickened Exshaw Formation areas is more consistent with brecciation by subsidence through dissolution of underlying lower Stettler Formation anhydrite. Furthermore, intraclast brecciation within the Big Valley Formation—interpreted Fig. 8. Proposed depositional and dissolution sequence. (A) Salina/sabkha as being deposited in a mid-to-distal ramp setting—precludes a tidal depositional setting of the Stettler Formation overlying Paleozoic carbonates channel origin for the brecciation. The lack of brecciation in the (blue bricks) and Cambrian sandstones (deepest yellow layer). (B) Following Exshaw Formation and the presence of inclined bedding within deposition of the Big Valley Formation , late Devonian extensional the lower Exshaw Formation black shales above areas of thinned faulting provide a conduit for (possibly) ascending hydrothermal fluid (red anhydrite suggest that minor subsidence continued during Exshaw arrows along faults) and fault-associated dissolution of the lower Stettler Formation deposition (Devonian–Mississippian). These drill core Formation anhydrite (cavity in pink layer). (C) Collapse of karst cavity in the findings are consistent with the results of our waveform modeling Stettler Formation anhydrites affecting the upper Stettler and Big Valley (Fig. 7), which suggest collapse was initiated before and continued Formations (purple and blue bricks, respectively) and causing brecciation and accommodation. (D) Syndepositional overthickening of the Exshaw Forma- through Exshaw Formation deposition. Based on the lack of de- tion (black layer). (E) Present-day horizontal HF well (drill rig and black line) formational features (brecciation, convolute bedding, and micro- drilled in the Big Valley Formation and causing an earthquake (red circle). fracturing) in the Exshaw Formation, it is unlikely that periods of significant collapse in the underlying units continued during the deposition of the overlying Banff Formation. stresses associated with the Antler tectonism triggered faulting of The localized nature of the ellipsoid anomalies recognized on the Paleozoic stratigraphy in southern Alberta is not precluded. the 3D reflection-seismic data (Figs. 3 and 4) and drill core On the other hand, the west-dipping normal faults depicted by observations (Fig. 6) suggest that undersaturated fluids may have been migrating along the WSOF. This fault-associated fluid flow SALT that extend at least into the Upper Cretaceous strata likely dissolved the Stettler Formation anhydrites local to the constitute undisputed evidence for downflexing of the North CHW, creating the Exshaw Formation /Wabamun Group sub- American craton during the well-documented Cordilleran tec- circular anomalies (Fig. 8). It is apparent from Figs. 3 and 4 that tonic loading (41). The trend of the WSOF identified here is the ellipsoid anomalies can occur at some distance from the similar to that of the extensional faults in the Cordilleran fore- interpreted faults associated with the CHW. This distance dis- land. Furthermore, the WSOF is subparallel to and just west of a crepancy may be explained by the nature of speleogenesis within basement magnetic lineament (11), a spatial relationship that evaporites, whereby relatively narrow horizontal or subhorizontal may indicate a genetic relationship. We speculate that the karst passages through more permeable beds (such as the dolo- WSOF may be the shallow expression of a Precambrian base- laminites in the case of the lower Stettler) connect upward to large ment discontinuity, most likely reactivated during the foreland perched caves at some distance from the potential source of fluids stage of the basin’s evolution. We infer this relationship because (Fig. 8B) (54, 55). Intervals of secondary dolomitization in the – the fault can be traced upwards through the Campanian-aged Stettler Big Valley Formation carbonates indicate that, in at least Belly River Group. some parts of the basin (e.g., well 8-24-11-28W4 in Fig. 5), fluids migrated along faults. Both of these observations—dissolution and Given the presence of a fault, its orientation with respect to in — situ stress conditions dictates its degree of mechanical activity. dolomitization are consistent with hydraulic communication along faults. This type of dissolution and dolomitization may be similar to The stress regime in the WCSB (42) likely supports a reac- ∼ fracture-associated hydrothermal karst and fault-associated hydro- tivatable stress state on the SSE-orientated WSOF (Fig. 3), as thermal karst and dolomite, respectively, as noted in the vicinity of evidenced by the induced earthquakes (11). Often, mechanically the Peace River Arch in west- (56). Devonian– – active faults are also hydraulically active (43 45), since fluid flow Mississippian (Antler) tectonism has been documented in different is established along the damage zone of the shear slip (13, 46, parts of the WCSB by fault-associated dissolution, dolomitization, 47). For example, fault-associated fluid flow has significantly and collapse of the uppermost Devonian strata (e.g., refs. 56 and influenced the spatial distribution of localized “pockmark” fea- 57) and in the study area are inferred from unconformities in the tures in strata overlying faults in paleo-basins (48). uppermost Wabamun Group and Exshaw Formation (40). In

E10010 | www.pnas.org/cgi/doi/10.1073/pnas.1807549115 Galloway et al. Downloaded by guest on September 27, 2021 contrast to the localized karst-related subcircular anomalies near drill core analysis that are indicative of these processes could the CHW, dissolution features related to basin-scale fluid flow provide cost-effective insight to the geological susceptibility for are often associated with more regionally pervasive karsting, for these types of earthquakes (69). example, as recognized in the Prairie Evaporite Formation in We note that documenting these geological factors helps de- northeastern Alberta (58). Finally, the last supporting evidence for velop a conceptual understanding of induced seismicity, with the potential hydraulic conductivity of a deep-seated fault in the ramifications for location models in forecasting induced seismic study area comes from induced earthquakes linked to HF of the hazard (65) and for identifying future sites (during exploration) overthickened Exshaw Formation (11). that may be prone to these events. For example, pre-HF reflection- seismic site assessments may be unable to resolve the offsets/geom- Implications for HF-Induced Earthquakes. Together, our findings etry of deep-seated faults to confidently establish their propensity for suggest the presence of fault-associated fluid flow in the geological reactivation. The results of this paper suggest, instead, that geo- past. However, remineralization and cementation of fault-damage logical indicators of hydraulic connectivity may be utilized during zones can eventually seal faults, drastically reducing their perme- site assessment to characterize fault influences and that proxies for ability (46). The stress regime in southern Alberta (42) has likely fault-associated fluid flow may bolster confidence during pre-HF kept the WSOF mechanically/hydraulically active since the Lar- assessment or provide some level of input when reflection-seismic amide orogeny, as evidenced by natural earthquakes presently is either unresolvable or unfeasible. Scaled up to the basin level, occurring in southern Alberta (59). Additionally, repeated over- the acquisition of 3D reflection-seismic surveys for the entire re- pressurization from HF stimulations has the potential to propa- gion becomes unfeasible. Instead, based on our findings, it is gate shear/tensile failures along the fault pathway, reopening any – possible to utilize geological proxies at this scale to glean any (semi)sealed segments encountered (23, 60 62). In fact, shallow in potential influence on the spatial distribution of induced earth- situ measurements of fault hydraulic diffusivities computed during quakes (e.g., refs. 64, 65, and 69). reactivation experiments estimate values of 3,500 m2/s (63). Be- cause of these considerations, it is possible that the WSOF is Conclusions permeable enough to support the large hydraulic diffusivity esti- mated seismologically (4.5–64 m2/s) (11). We note, however, that In conclusion, we find that geological factors indicating fault- denser arrays would have defined the seismologically estimated associated fluid flow have consequences for HF-induced earth- hydraulic diffusivity more faithfully. Based on these findings, fu- quakes. The coincidence of the CHW, evaporite karst-related ture geomechanical studies wishing to understand HF-induced overthickening in the targeted Exshaw Formation/Wabamun earthquakes should seriously consider the influence of fluid flow Group interval, and deeper induced earthquakes share a common along faults and fractures on modeled outputs. hydraulic connection: a basement-rooted fault. The CHW oper- Given these prior arguments, it is natural to presume that ation targeted the thickened reservoir, allowing an express route these indicators of fault-associated fluid flow have had a causal of hydraulic communication from the stimulated fractures along influence on the expression of induced earthquakes at the CHW the damage zone of the fault. This resulted in an increase in pore (Fig. 8) and possibly for all HF wells in the upper Stettler–Big pressure in the shallow basement fault segment, which caused Valley Reservoir play. By this reasoning, the relative paucity of fault slip nucleation. The spatial coincidence of the numerous geological conditions conducive to induced seismicity could re- geological conditions required for HF-induced seismicity is rare, flect the small fraction of seismogenic operations (24, resulting in their sporadic documentation in the Exshaw Forma- SCIENCES

25). To test this conjecture with available data for the Exshaw tion play (and possibly also in other plays, globally). Recognition ENVIRONMENTAL Formation, we consider Fig. 5. In this plot, wells with a relatively of geological indicators of fault-associated fluid flow may help us thickened Exshaw Formation (>10 m) are potentially diagnostic understand induced seismicity potential in a given area. of the conditions required for induced seismicity. On the other hand, we note that very few regions indicating thick Exshaw Data Availability Formation overlap with HF development of the play; the CHW Additional earthquake-related data are available online through is one of the few nearby cases (Fig. 5). While we recognize this the Alberta Geological Survey Earthquake Catalogue at the In- assertion is hardly definitive for this case, it supports an estab- corporated Research Institutions for Seismology (https://www.iris. lished idea that geological factors influence the likelihood of edu/hq/) and in prior published material (11, 59). All well log and encountering induced earthquakes (62, 64–70). For example, HF drill core data used in this study are publicly available through the operations susceptible to induced seismicity in the Duvernay play Alberta Energy Regulator (www1.aer.ca/ProductCatalogue/230.html; display a statistically significant spatial correspondence to the and https://aer.ca/providing-information/about-the-aer/contact- margins of a fossil reef (65). Some of these carbonate margins us/information-services-and-facilities/core-research-centre). Input have undergone hydrothermal dolomitization, a process related 3D reflection-seismic survey data from which our results were to structurally controlled fluid flow (68, 71, 72). These consid- derived are proprietary. All data needed to evaluate the con- – erations are anecdotally supported by north south strike-slip clusions of the paper are present in the paper. earthquakes (8, 9) and evidence of a transtensional flower structure (68, 73), a structure known to be conducive to vertical ACKNOWLEDGMENTS. We thank ExxonMobil for the donation of the 3D fluid flow (72). In this light, considering any features recognized reflection-seismic dataset license that this study describes and Matt Grobe during the reflection-seismic survey, wire-line log mapping, and and two anonymous reviewers for critiques that helped improve this paper.

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