Giles R. Morrell
National Energy Board
Calgary
The Mackenzie River flows northward across the Northwest Territories of Canada to the Arctic Ocean. West of the river rise the Canadian Rocky Mountains; to the east, the northern interior plains stretch to the Canadian Shield 300 km distant.
The geographic region known as the Mackenzie plain lies in the relatively accessible mid-section of the Mackenzie Valley just south of the Arctic Circle (Fig. 1) (72846 bytes). The area has attracted exploration for oil and gas over several decades following the discovery of Norman Wells oil field in the first quarter of this century.
Norman Wells is now one of the largest fields in North America in terms of remaining reserves. Although exploration has been focused on the discovery of further pools of Norman Wells type, several other plays exist, notably in Cretaceous sandstones with interfingered oil prone source rock, and in Lower Paleozoic carbonates and clastics.
A high degree of structuring creates much variation in source rock maturity and juxtaposition of diverse potential reservoir units, both clastic and carbonate. Potential for further discoveries in this area ranges from moderate to high.
Pool sizes are expected to show an extreme range and contain a variety of hydrocarbon types ranging from heavy to light oil and possibly condensate or gas in the deepest parts of the basin. The area is close to an existing oil pipeline and service center at Norman Wells.
ln late 1994 and for the first time in many years, the Canadian government called for industry to nominate exploration parcels in this general area. The industry responded positively With several nominations, and the outlook for renewed exploration is promising.
EXPLORATION HISTORY
Oil seeps along the banks of the Mackenzie River have long been known and used by native people of the Dene Indian nation and were recorded by Alexander Mackenzie during his descent of the river in 1789.
The seepages at Norman Wells first attracted commercial interest in 1891 when they were pointed out to J.K. Cornwall of the Northern Trading Co. In 1919 Imperial Oil Co. acquired the Norman Wells prospect, and in the following year the first well in the region, Discovery No. 1, was drilled by the Northwest Co.
Subsequent delineation and development wells have indicated an accumulation of approximately 235 million bbl of recoverable oil trapped in the updip end of a Middle Devonian reef within 600 m of the surface (Fig. 1, 72846 bytes)(Fig. 2, 61121 bytes)(Fig. 4, 41752 bytes).
In the early 1940s, the Canol Project saw construction of a pipeline from Norman wells to a refinery at Whitehorse, Yukon Territory, in support of the war effort in the Pacific Theater. Flow through the pipeline peaked at 4,400 b/d but ceased after the war, and the pipeline was later dismantled.
Post-war production supported the refinery at Norman Wells, which processed an average of 2,675 b/d for northern consumption. In the early 1980s, a major expansion of Norman Wells was undertaken which, together with construction of 542 miles of 12 in. pipeline to Zama, Alta., completed in 1986, has enabled this field to become one of the top producing fields in Canada. Late 1994 production was about 31,500 b/d.
Exploration increased in the late 1960s and 1970s, and a total of 76 exploratory wells were drilled in the Mackenzie plain, most in a narrow corridor close to the river (Fig. 1) (72846 bytes)(Fig. 2) (61121 bytes). Away from the river in the rugged Mackenzie and Franklin mountains exploration has been sparser. In the early to mid-1980s exploration focused on an area of Middle Devonian reef development northwest of Norman Wells, but activity declined in the second half of the decade as land issuance was suspended pending native land claim negotiations.
The limited drilling in the late 1980s concentrated in the northern part of the region south of Fort Good Hope and on the Mackenzie Plain southwest of Fort Norman (Fig. 1) (72846 bytes).
Reconnaissance seismic has been shot over the extent of the Middle Devonian reef play in the vicinity of Norman Wells. Further northwest and south of Fort Norman there have been fewer seismic programs. Only one 3D seismic program has been shot in the area, at Norman Wells.
GEOLOGIC SETTING
Characteristics of Mackenzie Plain, N.W.T. (48740 bytes)
The Mackenzie plain overlies the southern part of the geological feature known as the Peel trough between the arc of the Cordillera (Mackenzie mountains) to the west and the flank of the Keele arch (Franklin mountains) to the east (Fig. 1) (72846 bytes).
The Peel trough comprises a westward thickening wedge of Cretaceous-Tertiary strata overlying a broad Lower Paleozoic syncline with a gently dipping eastern limb and a more steeply dipping western limb rising to outcrop in the front ranges of the Mackenzie mountains. Lower Paleozoic strata outcropping in the Franklin mountains flank the Peel trough to the east.
The trough widens to the northwest, where the Mackenzie mountains swing westward. The Mackenzie mountain fold belt in this northern area extends beneath the Mackenzie plain. To the south the trough becomes increasingly constricted as the Keele arch reaches a terminus close to the Mackenzie mountain front at about 64 N. Lat.
The entire region has been affected by compressional tectonics, expressed as long wavelength folds (especially evident in the north), bedding-parallel detachments (beneath the Mackenzie plain), and thrust faults outcropping in the Franklin mountains.
STRATIGRAPHY
The generalized stratigraphy of the Mackenzie plain is shown in Fig. 3 (157980 bytes).
A basal Cambrian clastic section overlies Proterozoic rocks throughout the region-at depth in the Peel trough and rising to outcrop east of the region.
Sandstones of the Mount Clark formation are gas bearing in the Colville Hills to the northeast and are likely to exist at depth beneath the Mackenzie plain. Continental deposition culminated in the deposition of Cambrian evaporites-the Saline River formation-which were superseded by widespread carbonate deposition for the remainder of the Early Paleozoic.
The Lower Paleozoic carbonate platform in the Northwest Territories comprises the Ordovician Franklin Mountain and Silurian Mount Kindle formations overlain by Lower Devonian platform carbonates and reefs-the Bear Rock, Arnica, Landry, and Hume formations. The Bear Rock also contains an extensive evaporitic facies.
Reef-forming carbonates of Keg River, Sulphur Point, and Slave Point formations, present in the Western Canada sedimentary basin and in the southern Northwest Territories, are represented by the shaly Hare Indian formation in the Norman Wells area.
Conditions favorable to reef development returned to the Norman Wells area in the late Middle Devonian which saw the growth of Ramparts formation ("Kee Scarp") reefs. Reef development was terminated by deposition of the Canol shales in the late Devonian, followed by the thick clastic wedge of the Imperial formation. The Jungle Ridge formation is a thin limestone marking a mid-Imperial hiatus in clastic input to the basin.
Albian and Late Cretaceous strata are widely preserved and overlie the Imperial formation above a major unconformity.' Potential reservoir sandstones are equivalent in age to the Mannville group of Alberta and include the Slater River, Little Bear, and East Fork formations. Local deltaic influx is evident from clinoforms visible on seismic in certain of these units.
The early Tertiary Summit Creek formation is locally preserved in the vicinity of Fort Norman. Cretaceous depositional patterns may have been influenced by syn-depositional structuring related to limited mobilization of Saline River salt. Permo-Triassic, Jurassic, and pre-Albian strata are absent from the area.
RESERVOIRS
The Middle Devonian "Kee Scarp" reef at Norman Wells is the sole producing reservoir in the region. The field is in foreslope, reef margin, and reef interior lagoonal facies of an atoll-type reef which built up to 150 m above a regional limestone platform.
Porosity development in the Kee Scarp reservoir at Norman Wells is unusual - micro-leaching has developed a chalky porosity ranging from 12-20% with fine but consistent pore throat size. The reservoir has good horizontal but poor vertical permeability, and production is closely tuned to geological zonation of the reservoir.
Thin bioclastic shoals are associated with the leeward side of the Norman Wells reef. These may have a more widespread albeit discontinuous distribution across the regional platform and may be comparable to patchily distributed bioclastic sandstone encountered immediately above the reef and below the Canol shale - the Charrue sandstone. Where highly fractured the Canol shale has potential as a low volume producer in its own right.
Most recent wells targeting the Middle Devonian have penetated back reef or lagoonal facies; one well flowed salt water at good rates (Morrow Creek J-71), and cores from PCI Hoosier Ridge N-22 and Carcajou 025 bled oil. Gas with heavy saltwater spray flowed from a fractured zone in Carcajou D-05 (Fig. 1) (72846 bytes).
The Lower Devonian Bear Rock carbonates and evaporates occur extensively in the subsurface of Mackenzie plain. Cavernous porosity is developed in the subsurface. Minor oil staining has been reported in the Bear Rock near the western facies transition from anhydrite to carbonate. All porous zones tested to date have flowed water, but this unit is potentially an excellent reservoir if isolated from the regional aquifer.
Other Lower Devonian formations also have potential - either in intergranular and vuggy porosity developed in platform carbonates or locally as pinnacle reefs building from the Hume platform. A shallow and breached example of the latter was drilled by the Manitou Lake L-61 wells near Fort Good Hope, a community 125 km downstream from Norman Wells and just north of the area of interest shown in Fig. 1 (72846 bytes).
East Mackay B-45 recovered 20 gravity oil in pipe from fractured cherts of the Late Cambrian-Ordovician Franklin Mountain formation. There is potential for enhancement of the reservoir properties of brittle Lower Paleozoic units by fracturing during Laramide deformation. There is also a possibility of deeper clastic reservoirs in the Cambrian beneath the Saline River but, if similar to the Colville Hills gas reservoirs, they are unlikely to have porosities exceeding 12%.
STRUCTURE, TRAPS, SEAL
Laramide deformation of the Paleozoic margin has created a variety of fold, thrust, and wrench structures, each of which is quite localized and separated by areas where deformation has been minimal.
The area and style of deformation is linked to the distribution of the Saline River salt, which forms a major decollement surface. Bedding-parallel detachment and eastward translation of broad panels of post-Cambrian strata are demonstrated by mapped over-thrusts in the Franklin Mountains.
A schematic diagram (Fig. 4) (41752 bytes) is largely geo-fantasy but is included to suggest the very important role that the salt must have played in regional deformation.
Large amplitude folds related to a deep seated detachment are apparent in the Mackenzie mountains which border on the west of the region. These structures extend beneath the Mackenzie plain in the north but are usually deeply truncated by the basal Cretaceous unconformity or are breached at surface.
In the central part of the valley, west and southwest of Norman Wells, the regional dip is to the west. As the Mackenzie mountain front is neared, a dip reversal occurs above a deep triangle zone that appears to comprise imbricate thrust panels.
South of Fort Norman a major discontinuity is apparent in the alignment of mountain ranges and the course of the river. This discontinuity marks a zone of wrench faulting that runs at an oblique angle across the fold axes of the Mackenzie and Franklin mountains towards the Smith arm of Great Bear Lake (the "Fort Norman structure"). Major thrust folds are associated with this wrench system, which aligns with a trend of pre-Cretaceous extensional faulting, in itself a reflection of an older crustal lineament. Structural deformation in the area is influenced by the Cambrian salt with swells and withdrawals causing shallow-and possibly counter-regional-dips in overlying strata.
Lower Paleozoic strata are upturned and truncated along the western flank of the Keele arch: sub-unconformity traps may be created in this area by overlying Cretaceous shales.
SOURCE ROCKS
Norman Wells oil is sourced from the Canol shale draping the reef."
The Canol is responsible for most of the oil seeps along the Mackenzie River. The Canol was deposited extensively, and geochemical studies demonstrate that its potential as a source rock for oil is sustained through most of the region.
A lithologically similar but older unit-the Bluefish member of the Hare Indian formation-is also a rich oil prone source rock.
Both of these source rocks are just within the oil window at the current subsurface depth of Norman Wells field, although the reservoired oil is more mature, indicating greater depth of burial in the past. Higher maturation levels, possibly to beyond the lower limit of the oil window, are likely in deeper parts of the basin nearer the Mackenzie mountains.
Oil recovered from the Franklin Mountain formation at East Mackay correlates to a source rock in the Early Cretaceous Slater River formation .6 This unit is regionally extensive, and its variable depth through the basin suggests a spectrum of maturity.
Oil staining has also been observed in Cretaceous sandstones in several wells; for instance, 25-30 gravity oil in mud was recovered on drillstem test from Hanna River J-05.
POTENTIAL
Large reefal developments north of Norman Wells have been partially delineated by existing seismic and wells without success.' However, exploration opportunities remain for oil pools along the updip edges of the reef masses and within the complex architecture of the reef where source seal, and porosity development coincide. Proximity to outcrop in the Franklin mountains heightens the risk of reservoir flushing. This and uneven porosity development are the principle risks in this play.
In the vicinity of the Norman range, the prospective Middle Devonian section may be repeated beneath the thrust sheet carrying Norman Wells field. The possibility exists for Kee Scarp reefs in such a structural position. Low relief shoals developed above the regional carbonate platform may be additional targets for small oil accumulations.
Imbricate thrusting close to the Mackenzie mountain front, Laramide age thrust folds, and pre-Laramide folds and fault blocks are less explored structural targets and could involve many different ages and types of rock. Reservoir development must be highly risked for these plays.
The Cretaceous has potential for small oil pools that may be of interest with the close proximity of production facilities at Norman Wells. Cretaceous pools are likely to have a stratigraphic component to the trap, but the potential for good quality reservoir is high.
Final well history reports for wells drilled in the Northwest Territories and Yukon Territory, as well as geophysical and geological program reports, are available for inspection in the Frontier Information Office, National Energy Board, 311-6th Ave. S.W., Calgary, Alta., Canada T2P 3H2.
REFERENCES
- Gabrielse, H., and Yorath, C.J., Geology of the cordilleran orogen in Canada, Geology of Canada, No. 4, GSA Decade of North American Geology Project, Vol. G-2, 1992, 844 p.
- Aitken, J.D., Cook, D.G., and Yorath, C.J, Upper Ramparts River and San Sault map areas, District of Mackenzie, GSC Memoir 38, 1982, 48 p.
- Cook, D.G., and Aitken, J.D., Ontaratue Lake, Travaillant Lake, and Canot Lake map areas, District of Mackenzie, Northwest Territories, GSC Paper 74-17, 1975.
- Yorath, C.J., and Cook, D.G., Cretaceous and Tertiary stratigraphy and paleogeography, Northern Interior Plains, District of Mackenzie, GSC Memoir 398, 1981.
- Aitken, J.D., and Pugh, D.C., The Fort Norman and Leith Ridge structures: major, buried Precam-brian features underlying Franklin mountains and Great Bear and Mackenzie plains, Bull. Canadian Petroleum Geology, Vol. 32, No. 2,1984, pp. 139-146.
- Feinstein, S., Brooks, P.W., Fowler, M.G., Snowdon, L.R., and Williams, G.K., Families of oils and source rocks in the central Mackenzie corridor: a geochemical oil-oil and oil-source rock correlation, in James, D.P., and Leckie, D.A. (eds.), Sequences, stratigraphy, sedimentology: surface and subsurface, CSPG Memoir 15,1988, pp. 543-552.
- Snowdon, L.R., Rock-Eval/TOC data for 55 Northwest and Yukon Territories wells (60-69 N.), GSC Open File 2327, 1990 (available on diskette).
- Williams, G.K., Kee Scarp play, Norman Wells area, N.W.T., GSC Open File 1228, 1986, 5 sheets, text 5 p.
THE AUTHOR
Copyright 1995 Oil & Gas Journal. All Rights Reserved.