Triassic Hydrocarbons in the Western Canada Sedimentary Basin: The Doig-Halfway Petroleum System

Luke Makowski

University of Calgary

and

Bradley Parkes

University of Calgary

 

Summary

The Triassic Doig and Halfway Formations of North East British Columbia and West Central Alberta are significant hydrocarbon producers in the Western Canada Sedimentary Basin (Campbell and Home 1986). Within the Doig Formation the source of hydrocarbon generation is identified as the basal Doig Phosphate Zone. The Phosphate Zone of the Doig Formation was deposited in a deep water distal shelf to middle shelf environment that transitions to a lower shoreface that is seen in the siltstones and sandstones of the Upper Doig Formation (Edwards et al 1992). The Halfway Formation was deposited during the second Transgressive-Regressive cycle of the Triassic and lies erosionally on the Doig (Dixon 2001). This resulted in a depositional environment of coarsening upward barrier island sands and created the reservoir sands of the Halfway Formation. The deposition of the coarsening upward fabric of the upper Doig resulted in the primary migration of the petroleum fluids upward in the carrier bed (Piggott and Lines 1991). This continued into secondary migration as the fluids entered the sandstones of the Halfway Formation. Due to the Cretaceous Laramide orogeny, the Doig-Halfway Formation dips toward the east allowing for extensive lateral up dip secondary bed migration of the petroleum. The Mannville Formation reservoir sands are the primary reservoir of the Triassic and are charged from numerous subcropping carrier bed systems (Piggott and Lines 1991). Multiple rich source rocks, including the Doig, create petroleum systems that are stratigraphically focused through secondary lateral bed migration into the reservoir. This accounts for massive tar sand accumulations of mixed sources at the terminus of the lateral carrier beds creating the Athabasca and Cold Lake tar sands (Piggott and Lines 1991).

 

Introduction

The Triassic Doig and Halfway Formations of North East British Columbia and West Central Alberta are significant hydrocarbon producers in the Western Canada Sedimentary Basin (Campbell and Home 1986). The Doig and Halfway Formations produce oil and gas from fields such as Sinclair, Valhalla, Stoddard, Cache Creek, Buick Creek, Wembley, Progress, Peejay, Milligan and Fireweed (Edwards et al 1992). The Triassic formations provide more oil and gas per volume of rock than any other source within the Western Canada Sedimentary Basin (Brack et al 1989). Estimates suggest that Triassic source rocks are responsible for the generation of 4% of the oil in the WCSB (approximately 750 mmbbl) and 8% of the gas in the basin (approximately 9.5 TCF) (Edwards et al 1992). The first successful well that identified Triassic hydrocarbons was drilled in 1950 in the Whitelaw area of Alberta (Edwards et al 1992). The long production profile and large resource in place makes the understanding of the hydrocarbon system of the Doig and Halfway Formations a priority for geologists looking to increase production and reserves. These two formations together create a hydrocarbon system that is sealed by the evaporites of the overlying Charlie Lake Formation. The lower Doig Phosphate Zone is considered to be the source rock with the overlying Upper Doig and Halfway sandstones as the reservoir units (Wright 2011).

 

Depositional Environment and Paleo-history of the Doig and Halfway Formations

Doig Phosphate Zone – Triassic Source Rocks

For a hydrocarbon system to be effective multiple requirements need to be met in the correct order and timing. The first of these requirements is a source rock buried to a depth that is of sufficient temperature to generate oil from the organic content. Within the Doig Formation the source of hydrocarbon generation is identified as the basal Doig Phosphate Zone, above the unconformity that terminates the transgressive Montney Formation of the lower Triassic. During the Triassic the western margin of North America had a paleo-latitude that was approximately 30°N of the paleo-equator and this resulted in a climate characterized by low precipitation, high evaporation and sparse vegetation, creating an arid environment with a mild temperature (Campbell and Home 1986). The Doig Phosphate Zone has a total organic carbon of 2-11% and a Hydrogen Index of up to 400 in the thermally mature areas, above the minimum levels required for the generation of kerogen (Walsh et al 2006). The kerogen in the Doig Phosphate Zone is of type II and is prospective for both oil and gas (Walsh et al 2006).

 

Upper Doig and Halfway Formation Reservoir Rocks

During the Triassic period, Northeast British Columbia and West Central Alberta consisted of a broad shallow marine cratonic shelf and epicontinental sea. The Triassic was a period of decreased tectonic activity in the Peace River Embayment and included three Transgressive-Regressive cycles of 3rd and 4th order (Edwards et al 1992). The Phosphate Zone of the Doig Formation was deposited in a deep water distal shelf to middle shelf environment that transitions to a lower shoreface that can be seen in the siltstones and sandstones of the Upper Doig Formation (Edwards et al 1992). The Phosphate Zone consists of granular phosphate and phosphatic pebble conglomerate that suggests the basin may have been subjected to an interval of submarine erosion or non-deposition prior to the resumption of the 2nd cycle of Transgression-Regression phases (Brack et al 1989). The Upper Doig was deposited in a regressive sea level period creating a coarsening upward sand facies, above the silts and shales of the lower Doig (Dixon 2001). The Top of the Doig is thin sandstone associated with a transgressive disconformity that is post the lowstand seen in the Doig (Campbell and Home 1986).

 

The Halfway Formation was deposited during the second Transgressive-Regressive cycle of the Triassic and lies erosionally on the Doig (Dixon 2001). To the north and east of the Peace River Embayment the Halfway Formation is a series of discontinuous sandstone bodies of multiple barrier island systems and to the west of the Peace River Embayment the Halfway is thick continuous sandstone representative of stacked sands of a W-SW prograding shoreface trend that was deposited along a S-SE depositional trend, analogous to the paleo-shoreline of the Triassic (Walsh et al 2006). This depositional environment of coarsening upward barrier island sands created the reservoir sands of the Halfway Formation. The subsequent Cretaceous Laramide orogeny resulted in loading, tectonism, folding and faulting with continuous sedimentation providing burial and further enhancing the required conditions for a successful hydrocarbon system (Brack et al 1989).

 

Migration of Triassic Hydrocarbons – Upper Doig, Halfway and Mannville Formations

Hydrocarbons analyzed from Triassic reservoirs in the Western Canada Sedimentary Basin are found to be sourced from the phosphatic facies of the Doig Formation (Creaney et al 1994). Doig sourced oils are reasonably distinctive; characteristically between 35 and 45 API gravity, moderately sulphur rich, pristine/phytane ratios of 1-1.5, with high tricyclic terpanes compared to pentacyclic terpanes. The Phosphate Zone ranges in maturity from mature to overmature, with the maturity line occurring with the onset of the erosionally-truncated eastern limit (Creaney and Allan 1990). Petroleum passage downward out of the Phosphate Zone into the thick Montney Shale occurs locally. These events however are limited, with the underlying Montney acting as a bottom seal for the Phosphate Zone. Since the source rock was deposited in a period of transgression, the following regression and subsequent deposition of the coarsening upward fabric of the upper Doig resulted in the primary migration and enhanced expulsion of the petroleum fluids upward in the carrier bed (Piggott and Lines 1991). The primary migration and expulsion of the petroleum upward continued into secondary migration as the fluids entered the sandstones of the Halfway Formation.

 

Due to the Cretaceous Laramide orogeny, the Doig-Halfway Formation and the Peace River Embayment dips toward the east and experiences extensive east to west thickening (Edwards et al 1992.) This allows for extensive lateral up dip secondary bed migration of the petroleum. The petroleum travels large distances and eventually charges the Mannville sandstone reservoir that subcrops the migration bed. This is one of many lateral carrier petroleum bed systems that eventually subcrop the Mannville sandstone (Piggott and Lines 1991). This allows the Mannville to act as a focus or gathering point for all the natural petroleum pipelines.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

(Creaney and Allan 1990)

Producing the Triassic Hydrocarbons - Stratigraphic Trap, Porosity and Seal – Upper Doig Formation, Halfway Formation and Mannville Formation

The Doig-Halfway petroleum system is sourced by the Doig Phosphate Zone with reservoir targets in the Upper Doig regressive fine grained shoreface sandstones and siltstones that coarsen upwards into the dolomitic sandstones and sandy dolostones of the Halfway Formation (Dixon 2001). The reservoir targets in the Upper Doig are NW and NE trending elongate fine grained sandstones encased in marine shales (Willis and Wittenberg 2000) and are up to 120m thick in the western portion of the basin. The Halfway productive zones are tidal inlet channel fills that scour the Upper Doig (Campbell and Home 1986). The effective porosity within the Doig sandstones is primarily intergranular porosity and enhanced by vuggy and moldic dissolution (Dixon 2001). The unproductive areas lacked early calcite diagenesis and where porosity was lost to compaction (Harris 2000). The main porosity within the Halfway Formation is the result of dolomitization, moldic dissolution, although there is significant intergranular porosity (Campbell and Home 1986). The Halfway reservoir quality is mainly affected by the degree of calcite cements (Dixon 2008). The Traps that seal the Doig and Halfway reservoir are mainly stratigraphic in nature. The Doig pinches out updip and to the east and is truncated by Coplin unconformity but is also sealed laterally by the tight barrier islands deposits and associated lateral facies (Campbell and Home 1986). The Halfway Formation is overlain by the evaporites of Charlie Lake Formation creating a stratigraphic updip seal (Campbell and Home 1986).

 

The Mannville Formation sands act as the primary reservoir of the Triassic hydrocarbons. The formation underlies the Joli Fou Shale, creating a regional shale seal. The reservoir sands are charged from numerous subcropping carrier bed systems (Piggott and Lines 1991). Multiple rich source rocks, including the Doig, create petroleum systems that are stratigraphically focused through secondary lateral bed migration into the reservoir. Extensive distances traveled results in biodegradation of the fluids. This accounts for massive tar sand accumulations of mixed sources at the terminus of the lateral carrier beds creating the Athabasca and Cold Lake tar sands (Piggott and Lines 1991).

 

 

 

 

 

 

 

 

 

 

 

 

 

(Edwards et al 1992)

Conclusion

The Triassic Doig and Halfway Formations of North East British Columbia and West Central Alberta are significant hydrocarbon producers in the Western Canada Sedimentary Basin (Campbell and Home 1986). Within the Doig Formation the source of hydrocarbon generation is identified at the basal Doig Phosphate Zone, above the unconformity that terminates the transgressive Montney Formation of the lower Triassic. The Doig Phosphate Zone has a total organic carbon of 2-11% and a Hydrogen Index of up to 400 in the thermally mature areas (Walsh et al 2006). The Phosphate Zone of the Doig Formation was deposited in a deep water distal shelf to middle shelf environment that transitions to a lower shoreface (Edwards et al 1992). The Halfway Formation was deposited during the second Transgressive-Regressive cycle of the Triassic and lies erosionally on the Doig (Dixon 2001). Petroleum passage downward is limited, with the underlying Montney acting as a bottom seal for the Phosphate Zone. The deposition of the coarsening upward fabric of the upper Doig resulted in the primary migration of the petroleum fluids upward in the carrier bed (Piggott and Lines 1991). This continued into secondary migration as the fluids entered the sandstones of the Halfway Formation. Due to the Cretaceous Laramide orogeny, the Doig-Halfway Formation dips toward the east allowing for extensive lateral up dip secondary bed migration of the petroleum. The Mannville Formation sands act as the primary reservoir of the Triassic hydrocarbons. The reservoir sands are charged from numerous subcropping carrier bed systems (Piggott and Lines 1991). Multiple rich source rocks, including the Doig, create petroleum systems that are stratigraphically focused through secondary lateral bed migration into the reservoir. This accounts for massive tar sand accumulations of mixed sources at the terminus of the lateral carrier beds creating the Athabasca and Cold Lake tar sands (Piggott and Lines 1991).

 

References

Brack, L., Abbott, G.M, Noble, I. Tang, R.C.W. (1989): "Triassic/Jurassic Fields. Geophysical Atlas of Western Canadian Hydrocarbon Pools". Canadian Society of Exploration Geophysicists. URL < http://www.cseg.ca/publications/atlas.cfm >, [February 13, 2013]

 

Campbell, C., Horne. J.C.: "Depositional Facies of the Middle Triassic Halfway Formation, Western Canada Basin". The Society of Economic Paleontologists and Mineralogists (SEPM). Modern and Ancient Shelf Clastics. Vol 9. (1986): 413-460.

 

Creaney, S., and J. Allan. "Hydrocarbon generation and migration in the Western Canada sedimentary basin." Geological Society, London, Special Publications 50.1 (1990): 189-202

 

Creaney, S., et al. "Petroleum generation and migration in the Western Canada sedimentary basin." Mossop, GD, and Shetsen, I., comps., Geological atlas of the Western Canada sedimentary basin: Calgary, Canadian Society of Petroleum Geologists and Alberta Research Council (1994): 455-468.

 

Dixon, J. (2001): "Stratigraphic Setting of Middle Triassic Strata in the Spirit River-Rycroft Area of Northwest Alberta". Geologic Survey of Canada, Open File 3524. Calgary, Alberta.

 

Dixon, J. "Stratigraphic Relationships of the Triassic Halfway Formation in the Western Canada Sedimentary Basin". Bulletin of Canadian Petroleum Geology. Vol. 56. No. 1. (2008) P. 62-68.

 

Edwards, D.E, Barclay, J.E., Gibson, D.W., Kvill, G.E, Halton, E. (1992): "Triassic Strata of the Western Canada Sedimentary Basin; in Geological Atlas of the Western Canada Sedimentary Basin". G.D. Mossop and I. Shetsen (comp.), Canadian Society of Petroleum Geologists and Alberta Research Council, URL <http://www.ags.gov.ab.ca/publications/wcsb_atlas/a_ch16/ch_16.html>, [February 13, 2013].

 

Harris, R.G (2000): "Triassic Doig Formation Sandbodies in the Peace River Area of Western Canada: Depositional and Structural Models, and the Impact of Diagenesis on Reservoir Properties". University of British Columbia, Faculty of Graduate Studies. Department of Earth and Ocean Studies.

 

Piggott, N., and M. D. Lines. "A case study of migration from the West Canada Basin." Geological Society, London, Special Publications 59.1 (1991): 207-225

 

Walsh, W., Adams, C., Kerr, B., Korol, J. (2006): "Regional “Shale Gas” Potential of the Triassic Doig and Montney Formations, Northeastern British Columbia". British Columbia, Ministry of Energy, Mines and Petroleum Resources, Oil and Gas Division, Petroleum Geology Open File 2006-02. Victoria, British Columbia.

 

Wright. C. (2011): "Hydrodynamics of the Doig Formation, Potential Extensive and Prolific Deep Basin Gas". Canadian Discovery. Calgary, Alberta

 

Willis, A., Wittenberg, J. "Exploration Significance of Healing-Phase Deposits in the Triassic Doig Formation, Hythe, Alberta”. Bulletin of Canadian Petroleum Geology. Vol. 48.(2000): No 3. P 179-192.