Western Canadian Sedimentary Basin

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Introduction

The Western Canadian Basin is the second largest ​​intracratonic on the planet.It spans over 500,000 square miles from the eastern edge of the Precambrian shield, all the way to the western boundary of the Rocky Mountains. The basin can be as thick as 5 miles in some areas, containing an estimated reserves of 171 billion bbl of oil and 632 trillion cubic feet of gas. Today it produces 5 billion cubic feet of gas a day and 3.5 million bbl of oil a day.[1]

The basin holds major source rock, numerous reservoir horizons, as well as an abundance of data. Multiple hydrocarbon systems exist throughout the basin, a vast majority being weld in the western section, the hydrocarbons will migrate east. This lateral migration of hydrocarbons has created some of the richest hydrocarbon basins in the world, in places such as the Fort McMurray area. Exploration in this basin has spanned for over a century. However, the Western Canada Sedimentary Basin has appreciable untapped zones, and new plays continue to develop. With advancements in multistage fractured horizontal wells, the unconventional plays that make up the WCSB have revived the area.

File:600px-Canada geological mapWCSB.jpg
Outline of the Western Canadian Sedimentary Basin

Tectonic History

This basin's lateral variations reflect a long history of structural development involving a foreland basin that was superimposed on a cratonic platform and continental terrace wedge. Sometime during the Middle Ordovician to the Middle Jurassic phase, the cratonic platform split into an intersecting network of epeirogenic arches with intervening basins. These basins most likely formed due to uplift and erosion of the arches between the transgressive-regressive cycles.

The foreland basin was created in two different stages in the early Cretaceous to Palaeocene time. The first was the collisions between North America, the next was two sections of a tectonic collage of oceanic terranes that accreted to its western margin. This caused the continental terrace wedge to be compressed and displaced over the western margin of the craton. Part of this supracrustal cover was then pulled off of the craton, accreting to the overriding mass. This formed a wedge of imbricate thrust fault slices that would then propagate over the margin of the continental craton. These thrust slices allowed an outwash of clastic detritus from the evolving thrust belt to be trapped, thus forming the foreland basin.

Primary Geologic Risk and Uncertainties

There are always a large amount of risks associated with a drilling operation. When it comes to geology, there are risks of being incorrect of hydrocarbon migration, accumulation sites and volume of accumulation, and the paleo-thermal development of a basin, just to name a few. However, there are still many outside factors that contribute as well.

One of the major uncertainties in drilling Canada has been a lack of work force. After prices crashed during Covid-19, the companies in Canada have had a difficult time finding workers, even after prices have restored. Other environmental risks are associated with drilling in Canada as well. Mining operations have flattened forrest land to access the hydrocarbon beneath. With the leveling of trees and destruction of wetlands, endangered species such as migratory birds, caribou, bears, wolves and others are at risk.[2]

Petroleum Elements

Source Rock

The basin contains a variety of source rocks in addition to a sizable amount of coal. The source rocks were generally in the hydrocarbon-generating window in the western section of the basin, but to the east they were not mature enough because they were not buried deeply enough. In the WCSB, there are five significant source rock intervals.[3]

  1. Devonian Duvernay: The primary source rock for the Devonian petroleum system, including the Leduc reef reservoirs, is the Duvernay Formation, which is Frasnian in age. The Duvernay Formation's organic-rich mudstones, which contain up to 11 weight percent (wt.%) of total organic carbon (TOC), attracted industry attention as an unconventional shale resource potential in the early 2010s. The amount of recoverable oil in the deposit is thought to exceed 100 billion barrels. The Duvernay is in the gas/liquids window in the western portion of the basin and grades to immature in the eastern portion.[3]
  2. Devonian–Mississippian Exshaw and lower Banff: The TOC in the units may have been as high as 25% by weight, according to estimates. These units are important oil source rocks in the basin that most likely supplied a considerable amount of the oil to the oil sands and heavy oil in southeast Alberta and neighboring Saskatchewan. The Bakken Formation in the Williston Basin's core regions is where the majority of the oil found in southern Saskatchewan presumably originated before moving north into Saskatchewan.[3]
  3. Triassic Doig Formation: The Doig Formation sits immediately on top of the Montney or Sunset Prairie Formations and is a source rock rich in phosphate. Although there are other source rocks in the Montney Formation, this is probably one of the main sources for the Montney. The Doig is intermediate to high maturity in outcrop, and its TOC levels are 5% by weight. The Doig Formation is only present between latitudes 54° and 58° along the Alberta-British Columbia boundary.[3]
  4. Jurassic Fernie Formation: This is a significant WCSB source rock. The Fernie Formation in the Fernie Basin of southern Alberta has a thickness of up to 400 m. (a subbasin of the WCSB). Its TOC levels can reach 23.5% of its weight. Along with the Exshaw source, this is one of the main oil sands source rocks.[3]
  5. Upper Cretaceous Second Specks Formation (Colorado Group): This Turonian calcareous source rock infuses both the Albian Viking Formation beneath and the continuous reservoir sandstones of the Cardium Formation above.[3] 

Other source rock intervals exist in the basin as well as substantial coal units that generated methane and minor ethane. Also, abundant sulfur exists in the system, mostly in the Paleozoic section. The source rocks do not seem to generate sulfur-rich hydrocarbons. This suggests that the sulfur is introduced as part of the migration process and not the generating process.[3]

Schematic stratigraphy of the Western Canada Sedimentary Basin.

Seal

The dense unconventional reservoirs do not have much of a problem sealing themselves off, especially when compared to other conventional wells. Due to this good seals are found throughout the basin, normally being source rock. Since hydrocarbon cannot migrate up towards the surface, it tends to move horizontally through porous and permeable layers, or through fractures and faults. [3]

Trap

The majority of the hydrocarbon accumulations take place in stratigraphic traps (pinch-outs) in the plains or structural traps in the thrust belt.[3]

Migration

The sedimentary fill of this asymmetric basin is thicker on its western side and thinner on its eastern side. Due to this asymmetry, a unique migration system is created, in which the source rocks are heated to maturity on the western side of the basin and the produced hydrocarbons migrate to the eastern side of the basin to form large oil and gas accumulations and vast bitumen deposits.[3] In the WCSB, hydrocarbon migration primarily occurs laterally from the southwest to the northeast. Although buoyancy would encourage vertical migration, a number of variables prevent buoyancy from playing a large role.[3]

Heavy Oil Migration System

Porosity and Permeability

While the vast majority reservoirs in the WCSB are unconventional, this means that they have both a low porosity and permeability. Porosity refers to the space in-between rock, how much "storage" a reservoir has, if you will. While permeability refers to the ability in which hydrocarbon can flow through the rock. Until recent years, unconventional wells were never even considered. However, with advancements in hydraulic fracturing, individuals are now able to commercialize these tight formations.

However, it is thought that there are some areas in the WCSB that are higher permeability. These formations are most likely carbonates, with the most productive units seemingly in the hydrothermal dolomite reservoirs of Devonian and the Mississippian age.In the Northeast part of WCSB, carbonate formations such as Leduc and Swan hills are known for potentially being highly permeable. However, permeability in these locations are extremely variable. [4]

Reservoirs

The majority of hydrocarbon within the Paleozoic section is said to be in the middle of the upper Devonian strata and even overlapping the Mississippian strata. Due to the large size of the basin, no single depositional environment is dominate. While in the middle and upper Devonian, reef building organisms dominate with stromatoporoid reefs transitioning into coral reefs in the Upper Devonian. At the Frasnian/Famennian boundary, the coral reef builders switch to a grainstone-wackestone-mudstone depositional system.[3]

Structural Styles

Lithological character of the deformed stratigraphic sequences largely influenced the structural style of the fold and thrust belt. Thick sandstone/carbonate succession favored the emerging thick thrust sheets. However, other rock successions such as shale and sandstone favored the formation of folds between detachments.

Changes in structural style change as you move from north to south. While the south is dominated by thrusts, the north is fold dominated. However there is a very broad transition zone, where folds are more common at the surface and thrust faults lie in the subsurface. This occurs between the Athabasca River and Williston Lake-Peace River. The eastern foothills part of the fold and thrust belt contain most of the hydrocarbon exploration, and will continue to host for over the next decade.

Production

The giant Leduc discovery in 1947 was the beginning of oil in western Canada. Higher outputs began to grow, and by 1973 it peaked to 1.5 million bbl/d.This was short lived, from 1973 to 2007, conventional oil production in the WCSB has declined around 3% annually.[5]

However, since the early 2000’s, higher crude prices have driven companies to exploit tight oil-bearing formations, which had seemed impossible historically. Horizontal drilling and multi-stage hydraulic fracturing tight oil formations are the key to produce oil and gas from these unconventional formations. The use of this technology has saved low-producing or unproductive oil reserves in the WCSB.

Drilling rig in the Greater Sierra field

References

Hickson, Catherine, et al. A Relook at Canada’s Western Canada Sedimentary Basin for Power , https://publications.mygeoenergynow.org/grc/1034257.pdf.

Laureshen, Catherine & Plessis, Duke & Chung, Keng & Xu, Chunming. (2013). Asian Pacific Markets - A New Strategy for Alberta Oil. Journal of Canadian Petroleum Technology. 45. 10.2118/06-01-GE.

Greenfield, Nicole. “10 Threats from the Canadian Tar Sands Industry.” NRDC, 1 Oct. 2019, https://www.nrdc.org/stories/10-threats-canadian-tar-sands-industry.

Paul MacKay, Per Kent Pedersen; The Western Canada Sedimentary Basin: A confluence of science, technology, and ideas. AAPG Bulletin 2022;; 106 (3): 655–676. doi: https://doi.org/10.1306/12032121141

Wright, G N, M E McMechen, and D G Potter. “Canadian Society of Petroleum Geologists.” Structure and Architecture of the WCSB. Accessed November 2, 2022. https://www.cspg.org/common/Uploaded%20files/pdfs/documents/publications/atlas/geological/atlas_03_structure_and_architecture_of_the_WCSB.pdf.

W., P. J., J. W. Porter Google Scholar Find this author on PubMed Search for more papers by this author, Porter, J. W., Google Scholar Find this author on PubMed Search for more papers by this author, Price, R. A., McCrossan, R. G., & This text was harvested from a scanned image of the original document using optical character recognition (OCR) software. As such. (1982, May 5). The Western Canada Sedimentary Basin. Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences. Retrieved November 2, 2022, from https://royalsocietypublishing.org/doi/10.1098/rsta.1982.0032

External Links

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