This last of four parts on the structured belt of the Western Canada Sedimentary basin describes in more detail the six structural domains and their exploration potential.
Liard/Mackenzie Fold Belt
The Liard/Mackenzie Gravity Slide Fold Belt is a regional Laramide gravity slide (Fig. 14).
It is characterized by a series of 100+ miles long and narrow anticlines and broad synclines, locally disrupted by strike-slip faulting. The slide butts against a NE-trending vertical basement strike-slip fault in NE BC and several vertical basement N-S strike-slip faults farther to the N in the Northwest Territories.
The present setting reflects the freezing of the main gravity slide, which while sliding contained several large anticlines and synclines that moved within the slide in the direction of its movement, much like ripples in a pond. The Beaver River, Kotaneelee, Pointed Mountain, Fort Liard, and P-66 anticlines contained 0.8 tcf of recoverable gas, some at 450°+ F. reservoir temperatures. Recently, liquids-rich gas was discovered in a fractured reservoir in an anticlinal core thrust structure in the N part of this fold belt.
In the northern portion of the region and extending N into the Yukon Territory and NWT, strike-slip in N-S direction over a width of some 100 miles produced E-W shortening and a regional N-trending structural depression in the filled-in Liard/Mackenzie basin.
A salt and shale layer, wet due to osmotic attraction of water, is present in the lower part of the basin’s sedimentary section. It rests on Cambrian and Precambrian sedimentary strata. It is the sliding surface for the overlying sedimentary section that slid E from the W flanks of the depression into its deeper N-trending central portion.
Surface geology demonstrates these anticlines are complex and change along their axes into faulted homoclines and both high-angle and low-angle thrust-cored anticlines with rollovers above their core thrusts, changing to rollovers beneath the thrusts. Anticlinal core thrusts in this domain are small, both in dip and strike directions, when compared with the massive thrusts in the Thrust Domain farther S.
Frequently, core-thrusts experienced substantial subhorizontal slip. The S portion of the slide was disturbed by a large deep-seated basement strike-slip fault, which even today brings up very hot fluids. It dragged the E-moving gravity slide and bent the N-S anticlinal axes in a SW-NE direction and created four-way closed culminations, many of which contain economic gas accumulations in fractured Devonian Nahanni dolomite reservoirs, with numerous crystal-lined vugs, resulting from fractures and dissolution.
Higher topographic elevations in the W, E dip of strata, and associated strong E regional fresh ground water flow considerably have diminished the exploration potential of the Liard/Mackenzie Gravity Slide Fold Belt. Drilling should only be considered for structures with 1,500+ ft of vertical reservoir closure. Lesser closure means exploitation will soon have to deal with increasing production of formation water.
There might be an opportunity in exploring deeper targets: closures on extension fault blocks, strike-slip fault closures, and flower folds below the decollement in presalt strata. Surface structural geology and well data suggest the possible presence of many (50+) large anticlinal structures in Cambrian and Precambrian clastics of the “basement.” It might be assumed the Precambrian section generates gas judging by the presence of giant gas fields in Precambrian strata in the NWT and Siberia.
Thrust Domain
The Thrust Domain contains thrusts formed by E-directed subhorizontal transpression/compression/gravity sliding forces exerted upon a gently W-dipping regionally extensive sediment package, overlying a solid igneous and metamorphic basement-the Canadian Shield (Figs. 6 and 7). This domain is divided into three lesser domains: Main Range, Front Range, and Foothills.
Multiple thrusts formed and moved rock bodies E into the undisturbed section of sedimentary rocks overlying the basement. It was pushed E and overridden by the thrusts.
This dynamic process includes several massive rock ploughs pushing E. The Main Range pushed the Front Range, which in turn pushed the Foothills. The latter in turn pushed E evidenced by structures in the Triangle Zone and Imbricate domains.
The E blade edge of the Triangle Zone was the leading edge of all the E-pushing rock ploughs. It slid into the undisturbed Plains Domain, moving on the basement and underthrusting (wedging) the younger sedimentary section of the Plains Domain. In addition, the impact of regional shear (strike-slip domain) on the formation of structures was immense (seismic transects ST 1-15, Part 3 of this article).
Main Range Domain
The W belt of the NW-trending Thrust Domain is the Main Range Domain (Figs. 6 and 7).
To the W, it borders the Rocky Mountain Trench. To the E it borders the Front Range Domain at the Pipestone Thrust. Many shingled parallel major thrusts are recognized in this domain, and these are thought to cut deep into the Precambrian sedimentary section. Little is known about the exact nature of these thrusts and their exploration potential. With W-advancing exploration, this domain might become attractive as an exploration target.
Front Range Domain
The middle belt of the northwest-trending Thrust Domain is the Front Range Domain (Figs. 6 and 7).
To the W, it borders the Main Range Domain at the Pipestone Thrust. The McConnell Thrust separates this domain to the E from the Foothills Domain. Close to a dozen shingled parallel major thrusts are recognized in the Front Range. This domain is underexplored but supports numerous exploration targets: single and multistacked thrust sheet structures and such structures with stratigraphic anomalies.
Neither seismic acquisition nor drilling has advanced in this domain since accessibility is difficult and costly, distances are long to infrastructure, and risks are great. Despite all this, it is assumed that many undrilled structures await the drill bit.
Foothills Domain
The Foothills Domain is the E belt of the NW-trending Thrust Domain (Figs. 6 and 7-ST 1-14).
To the W, it borders the Front Range Domain at the McConnell Thrust. The Brazeau Thrust and sometimes subordinate thrusts separate this domain to the E from the Triangle Zone Domain. Up to five shingled parallel major thrusts are recognized in the Foothills Domain. It contains numerous local structures composed of multistacked thrust sheets with reservoirs that contain many gas fields.
Large parts of the Foothills Domain include up to six regionally extensive “horizontal detachment zones,” mainly in shale sequences, >1/2 to 1 mile vertically apart. Two such surfaces enclose a rock body-the detached body. In total there are five such rock bodies: The Cambro-Ordovician, Silurian/Devonian, Mississippian, Permian, and Triassic rock bodies (ST 12 and 13) each >½ to 1 mile thick. A complex structured post-Triassic section (>2 miles thick) is present. These detachment zones correspond somewhat with regional unconformities, marking periods of major structural upheaval.
In map view, the Foothills Domain is bordered to the E by a narrow Triangle Zone Domain. Single and multiple thrust sheets and associated rollovers were main exploration targets since 1913. Success has been rewarding, and failure was inspiring to drill the next prospect.
In single thrust sheets the target is often a single rollover (broken through thrust-end-fold). In the multiple thrust sheet structures, multiple rollovers are the prime targets. Even with current advanced seismic acquisitions and processing, the transformation from seismic record to three-dimensional structures is difficult. Drilling results seldom accurately agree with deep-well prognosis.
Both size and shape of Foothills thrusts were determined by the lithological character and affected stratigraphic level. Majestic thrusts developed in a section composed of siliciclastics overlain by a massive carbonate section in turn overlain by siliciclastics, in the southernmost region. Numerous thrusts/imbricates formed where the main section affected is a siliciclastic one and overlies the carbonates. ST 6 depicts an imbricated (>15 imbricates/thrusts) Cretaceous section shortened by some 20 miles. It is a gravity slide that moved on a clastic ball-bearing layer on top of a gently dipping carbonate section. The latter was shortened only by 1 mile.
There is an abundance of single sheet and multiple thrust sheet exploration targets throughout the Foothills Domain. There is no absence of targets, but to highgrade and select the successful plays is one of the most difficult tasks in exploration. Numerous four-way closed thrust rollover structures, disharmonic thrust-related folds, and rollovers are present in Jurassic/Cretaceous and younger reservoirs (resource play), Triassic, Permian, Mississippian, deep Devonian, and possibly HPHT older reservoirs (classic foothills play), separated by regional detachment surfaces.
A little known fact is that many anticlines and four-way closed thrusted anticlines contain vertical shear zones perpendicular to their hingelines. These shear zones are the conduits for gas generated in the deeper parts of the structured belt in Ordovician to Precambrian strata.
Triangle Zone Domain
The Triangle Zone Domain includes a rock body of E-thrusted WCSB sedimentary section (Figs. 6 and 7).
It moved on top of the basement and underthrusted the younger sedimentary section of the Imbricate Domain. The domain resembles a rock plough that overrode the deeper section and pushed the shallower section out of its way, upward and possibly over its top.
At the surface this domain is narrow, but it widens with increasing depth as shown on two-way time seismic transects from S to N (ST 1-3, 5-7, 9-14). There is a complex structural and spatial relationship between this domain and the Foothills and Imbricate domains.
Inside the Triangle Zone its leading edge thrust and large Foothills domain thrusts are cut off by the bounding underthrust of the Imbricate Domain. This demonstrates the dynamic nature of this “rock plough.” In the S, the domain includes large thrusts that incorporate Mississippian (ST 3-billion-barrel Turner Valley field) and Devonian carbonate sections. Farther N, it includes folded age-equivalent carbonates, albeit much thinner, and folded Permian and Triassic carbonates separated by substantial shale sections, and several regional detachment zones (ST 12-13).
High-density thrusting gradually gives way to undulating folds with decreasing amplitude in the E direction. Eventually and farther E, the low-amplitude folds merge with the gently westerly dipping sedimentary section of the Plains Domain. When the rock plough stopped moving and froze in place, its leading edge E upward-cutting thrust also froze in place. This thrust cuts at a low angle forward and upward into the younger overlying section.
In the Turner Valley field area, the Triangle Zone Domain includes a large thrust with a rollover that contains giant Turner Valley field (ST 3), where gas was flared for many decades, decreasing the oil recovery factor. It might be a thought to inject carbon dioxide from the Kevin-Sunburst Dome in Montana to energize the oil.
Exploration in this domain will concentrate on major thrust rollovers, strike-slip fault closures, flower structures, microshear faulted carbonate reservoirs that are thrusted, rolled over, and flower folded, and much deformed and sheared sections butting against the bounding underthrust fault plane.
Strike-Slip Domain
The Strike-Slip Domain is a wide belt characterized by numerous strike-slip faults and associated structures (Figs. 6 and 7).
Historically, strike-slip faults were not recognized and this belt was included in the Plains Domain. It contains many folds that gave rise to the term “Outer Foothills.” Eventually, strike-slip faults and associated flower structures were recognized and the name “Strike-Slip Domain” was born (this article). This domain is divided into a north-south domain and a NW-SE domain. The latter also includes many NE-SW and some E-W strike-slip faults.
Strike-slip faults (Fig. 10) are abundant in this belt, which extends to the W and dives beneath the Rocky Mountain thrust belt. With increasing depth of the basement, however, high pressures and high temperatures are present, and the basement rock does not fault under shear stress but instead plastically deforms (Fig. 5).
The domain (ST 4-15) includes a) numerous parallel trending vertical to subvertical deep-seated dextral strike-slip faults (Type I) and associated negative and positive flower folds, b) many dextral strike-slip faults (Type Ia) and associated positive and negative flower folds, striking parallel to a) and confined to the thickened sedimentary section and not penetrating the basement, and c) a body of structurally thickened sedimentary intervals (ST 10), containing vertical faults, between which strata were squeezed (thinned), and positive and negative squeeze folds formed at the shallow end and deep ends of these faults, respectively. The domain includes the vertical Bovie, Liard, Ferrier (ST 8), Okotoks, Pine River, and many other strike-slip faults ranging in length from several miles to over 100 miles.
The strike-slip domain experienced little movement in its easternmost region (ST 1, 6-9, and 14), where it is anchored to the massive Canadian Shield. S to N movements progressively increased going from E to W through the domain. Farther W, SE-to-NW movements increased.
The combined subhorizontal S-to-N and SE-to-NW slips along these faults are estimated to be in the tenths of miles in the basement. W-to-E shortening due to shear of the domain might be calculated in miles. It is important to note that the effect of strike-slip movement in the basement on the overlying sedimentary cover is determined by the thickness of that cover and the type of sediments included in that cover.
The Strike-Slip Domain comes in from beneath the Liard/Mackenzie Gravity Slide Fold Belt in the N and extends some 1,000 miles to the US border in the S. It cuts SE across the W portion of the Plains Domain and runs parallel to the Imbricate Domain. Farther S, it cuts through the Imbricate Domain and at a low angle intersects the Triangle Zone Domain.
From there, it cuts into the Foothills Domain just W of Calgary and continues S underneath and past Turner Valley and Waterton fields. Then it turns SE and extends into the US. Strike-slip faults had a very important function in the initiation and formation of thrust ramps throughout the region (ST 2-4, 6, and 12-14).
A combination of complex strike-slip and Triangle Zone structures (ST 5) is evident 30 km NW of Calgary. From there, the strike-slip strike changes to S and just W of the city was instrumental in determining ramping and degrees of rollover in shingled thrusts that contain Sarcee gas field (100 bcf). Continuing S, it “dives” beneath giant Turner Valley field and onward to the US border.
Turner Valley field resulted from a combination of N-S and SE-NW dextral strike-slip movements and simultaneous E-directed compression. The latter created a thrust-end fold, up against a dextral strike-slip fault block, that subsequently broke through and ramped up to create the Turner Valley thrust and four-way closed rollover (ST 3).
In the S and W parts of the disturbed belt, the E-thrusted thick overlying sedimentary cover prevented strike-slip faults from penetrating upward from the basement into and through that sedimentary section. Instead, many vertical shear zones are present, and these are observable on seismic sections and in the field. These shear zones do not contain an actual strike-slip fault but instead are zones in which shear was intense.
Such shear zones are present, although most often not recognized, in many NW-SE trending thrusts and enhance reservoir characteristics through intense shear microfaulting (fracturing). Not recognizing the existence of these shear zones, including strike-slip faults, puts in doubt the validity of many structural contour maps of thrust sheets.
In some instances, reservoirs have 1% original porosity and no permeability. Strike-slip shearing zones created substantial fracture numbers and permeability and some wells have produced over 290 bcf from such reservoirs (Waterton field).
The N-S Okotoks field is a closure against a large strike-slip fault. Its main reservoir is in dolomites of the Devonian Wabamun formation, which, undeformed, contains many irregular globules of anhydrite and little porosity and permeability. The high E side of the fault contains a narrow, elongated fault closure in which dolomite is heavily shear-fractured, contains crystal-lined dissolution vugs (leached fractured anhydrite globules), numerous fractures, and a permeability of up to 7 darcies. Farther E, the undeformed formation has little porosity and no fractures.
The most interesting aspect of the western strip of the Strike-Slip Domain is that dextral movement along many of the faults was simultaneous with E thrusting of the sedimentary section, starting in the Thrust Domain and continuing into the Triangle Zone Domain. The combined E-W and N-S movements resulted in many rather complex structures in the sedimentary section. All these structures, even negative/synclinal flower structures, are exploration targets.
Numerous strike-slip faults were cut off from their basement roots, during continued eastward thrusting, but when thrusting slowed, continued movement along the strike-slip faults in the basement penetrated upward and formed new strike-slip faults, flower structures, and strike-slip shear envelopes in the overlying sedimentary section.
In the Deep Basin, a common structure was formed when an E-moving thrust (sometimes a subhorizontal shear plane) with rollover encountered an active dextral N-S or NW-SE strike-slip fault. The collision sheared the E limb of the rollover against the vertical strike-slip fault and a vertical E limb (box fold) resulted.
Most expressions of strike-slip faulting and folding are in the subsurface; however, in some areas, imbricates associated with strike-slip faulting are present but are mapped as parallel thrusts. Their strike-slip provenance was not recognized. Few structural geologists have field-mapped strike-slip structures, including “fractures,” in the subject region.
The most common fractures and fracture patterns investigated in the field are associated with thrusting in the Thrust, Triangle Zone, and Imbricate domains. Therefore, analyses and interpretations of shear “fractures” in cores or deduced from seismic interpretations should be handled with great care pending the strike-slip experience of the interpreter. Structures associated with strike-slip faulting were recognized some 35 years ago by the author in the WCSB.
A decade ago, the author field-mapped the classical N-trending Toumarolin strike-slip fault where it shears through the W flank of the Murzuk basin in southwestern Libya. This fault is well exposed for many miles in a desert terrain nearly void of vegetation. It cuts through a flat-lying sedimentary package resting on a gently E-dipping crystalline basement.
E-W seismic transects show the fault to evolve into a classical flower structure, where the sedimentary package reaches a thickness of 3-4 km. The two strike-slip faults that bound that flower fold do not reach the surface. Fault plane, fault zone, bed dips, micro to macro shear faults, and associated en-echelon complementary fault and associated fold structures, including “imbricates” with structural closures at low angles to the fault plane, were mapped in the field and are reference material for the current structural assessment.
In the Plains Domain, several similar large classical strike-slip structures are seismically documented well beyond the eastern boundary of the Strike-Slip Domain proper. In some areas, vertical shear movements resulted not in the formation in faults but gentle folds and imbricates (ST 1-7).
Exploration in this domain will focus on recognition of the strike-slip structures, amplitude anomalies associated with geothermal dolomitization in limestone, and both carbonate and siliciclastic reservoirs in flower structures.
The Deep Basin portion of this domain is a prolific exploration region. Additional targets, both stratigraphic and structural might be found in deeper Cambrian and Precambrian clastics truncated by an angular unconformity and underlying the younger WCSB section.
Imbricate Domain
The Imbricate Domain includes most of the shallower sedimentary section that overlies the Triangle Zone Domain (Figs. 6 and 7 and many STs).
While moving east, the Triangle Zone “rock plough” underthrusted the Imbricate Domain, lifted it, and in places turned beds to vertical at the bounding thrust. This domain is bounded by a large thrust (ST 1). It is cut from below by the leading edge thrust of the Triangle Zone. It is the last one in a series of many that developed in parallel fashion, stepping forward, and that were sequentially cut off by the underthrust when the rock plough moved E. Remnants of earlier leading edge thrusts are shown on ST 1.
The rock body of the Imbricate Domain includes many smaller thrusts/imbricates that trend parallel with the first bounding thrust and dip easterly. These imbricates have W-directed rollovers, a direction opposite that of the major thrusts in the Thrust Domain. The combination of thrusts, imbricates, the series of cutoff leading edge thrusts demonstrate the dynamic underthrusting nature of the rock plough (Triangle Zone Domain) and the Imbricate Domain.
Exploration in this domain will concentrate on more precisely defining the structural framework of this domain and the role of structure in combination with the more than 30 regional Plains exploration plays advancing into the zone from the E. Furthermore, the Late Jurassic/Cretaceous and younger resource plays require immediate attention. Large bodies of rock butt against the underthrust probably are severely fractured and might contain exploitable natural gas.
Plains Domain
This domain includes the “undisturbed” gently-dipping sedimentary Precambrian to Tertiary section of the WCSB that rests on igneous and metamorphic rocks of the Canadian Shield (Figs. 6 and 7 and ST 1-14).
This section extends from outcrops of the shield in the east and “dives” beneath the first main thrusts of the Foothills in the W. Historically it supported intense and successful exploration. Current drilling activity demonstrates exploration of the Plains Domain continues unabated.
Numerous play types are augmented by many unconformities, providing a wide array of traps for hydrocarbons. The unconformities correspond with a series of worldwide mountain-building episodes and continental drifting events, of which the most striking is the one that occurred at the end of the Cretaceous epoch some 60 million years ago.
More than 30 regional oil and gas exploration plays extend W from the undisturbed Plains Domain. The first addition of structure to a portion of these regional exploration plays is in the Strike-Slip Domain. This resulted in discovery of 8+ tcf of gas in place to date.
Numerous plays await the drill bit and will sharply increase the overall undiscovered gas volumes. Faults in this domain had a great impact on the formation of gentle structures (traps) and acted as conduits for high-pressured hot fluids to facilitate reservoir enhancement through dolomitization.
Future exploration will have to increasingly concentrate on the relationship between structure and stratigraphy. Successful exploration of the 30+ stratigraphic plays in the structured belt will be taxing for the oil and gas industry as it requires an understanding of the complex combinations of structure and such plays.
Acknowledgments
The author thanks M. West, C. Winter Williams, and Pres. G. Smith of Olympic Seismic for permission to use some of their numerous WCSB 2D seismic transects in this article. Thanks to geophysicists D. Slater, H. Klingensmith, M. Marshall, T. Sartorelli, T. Bell, S. Beatty, C. Curtis, N. MacKeith, T. Podivinsky, H. Westbroek, T. Galeski, D. Poley, W. Reed, M. Lack, G. MacLean, J. Sluggat, K. Mitchell, and C. Xhufi, and to geologists P. Goetz, A. El Sogher, T. Taleb, K. Nachtigall, J. Phillips, J. Dobson, D. Green, A. Wolff, B. Zhu, D. Sparks, C. Hughson, K. Wallace, D. Campbell, C. Sproule, G. Jones, K. Waunch, M. Zander, P. Haynes, J. Peachy, J. Beck, and K. Rath. Contributions by engineers M. Abougoush and R. Cech (CBM), completion engineer D. Gunn, and drilling engineer J. Hyatt are gratefully acknowledged, and special thanks go to petrophysicists S. Bleue and C. Macfarlane, designer F. Wennekers, and draftsmen J. Daunhauer and B. Wyatt.”
References
A list of references is available from the author.