McMurray Formation

The McMurray Formation is a stratigraphic unit of Early Cretaceous age (late Barremian to Aptian stage) of the Western Canada Sedimentary Basin in northeastern Alberta.[4] It takes the name from Fort McMurray and was first described from outcrops along the banks of the Athabasca River 5 kilometres (3.1 mi) north of Fort McMurray by F.H. McLearn in 1917.[3] It is a well-studied example of fluvial to estuarine sedimentation, and it is economically important because it hosts most of the vast bitumen resources of the Athabasca Oil Sands region.

McMurray Formation
Stratigraphic range: late Barremian to Aptian
Outcrop along the Athabasca River, ca. 1900
TypeGeological formation
Unit ofMannville Group
Sub-unitsUpper, Middle and Lower members
UnderliesClearwater Formation (Wabiskaw Member)
OverliesWaterways Formation, Banff Formation, Wabamun Formation
Area140,000 km2 (54,000 sq mi)[1]
Thicknessup to 60 metres (200 ft)[2]
Lithology
PrimarySand, sandstone
OtherSilt, mud, coal
Location
Coordinates56.995921°N 111.456612°W / 56.995921; -111.456612 (McMurray Formation)
Region Alberta
Country Canada
Type section
Named forFort McMurray
Named byF.H. McLearn,1917[3]

Lithology

The McMurray Formation consists of fine- to coarse-grained quartzitic sand and sandstone, interbedded with lesser amounts of silt, mud, clay and, less commonly, thin coal beds. The sands are very loose and friable, unless they are partially or fully cemented with bitumen or, less commonly, with calcite, iron oxides, or quartz.[5]

Stratigraphy

Three members have been defined within the McMurray Formation. They can be differentiated in outcrops and, to a lesser degree, in the subsurface:

  • Lower Member: typically coarse-grained to conglomeratic sand, with minor beds of silt and mud; present only in depressions in the underlying Devonian formations; typically saturated with water, or poorly saturated with bitumen.
  • Middle Member: typically massive, well sorted, fine-grained sand at the base, with inclined beds of thick, rippled sand and thin shaley silt partings in the upper portion; usually well-saturated with bitumen.
  • Upper Member: typically horizontal beds of argillaceous, very fine-grained sand; usually saturated with bitumen.[5]

Distribution
Outcrop of the McMurray Formation (black and dark grey) and underlying Waterways Formation (tan) on the Steepbank River.
A boulder of McMurray Formation oil sand near the Athabasca River north of Fort McMurray.

The McMurray Formation outcrops along the Athabasca and Clearwater Rivers and their tributaries near Fort McMurray in the Athabasca Oil Sands of northeastern Alberta, where it averages about 60 metres (200 ft) thick. It thins eastward into Saskatchewan where, in most areas, it is devoid of bitumen. It has been removed by erosion north of the Athabasca Oil Sands area.[5][6]

Relationship to other units

The McMurray Formation is conformably overlain by the Wabiskaw Member of the Clearwater Formation. It is underlain by Devonian formations that were tilted and eroded prior to the deposition of the McMurray, resulting in a shallow angular unconformity between the McMurray and the underlying units. The McMurray therefore rests on different Devonian units in different parts of the basin. These range from the Elk Point Group in the east near the Saskatchewan border, to the Waterways Formation along the Athabasca River, and the Banff, Wabamun, and Winterburn Formations farther west.

The McMurray Formation is equivalent to the lower Mannville Group of Alberta, the Dina Formation of eastern Saskatchewan, the Gething Formation of northwestern Alberta and northeastern British Columbia, and the Ellerslie Formation and Ostracod Beds of central Alberta.[5][7]

McMurray Formation oil sand as seen in drill cores. Arrow indicates a fragment of fossil wood.

Depositional History

The McMurray Formation was deposited by an extensive drainage system that flowed northward following a regional north-south depression. The depression was created by the dissolution of thick salt deposits in the Devonian Elk Point Group deep within the subsurface. A rising sea encroached upon the drainage system from the north, and as this marine transgression progressed, depositional environments changed from fluvial at the base (Lower Member), to estuarine (Middle Member), to shore-face at the top (Upper Member), and finally to open marine in the overlying Clearwater Formation.

The channel sand deposits host the majority of the bitumen which is contained in the pore spaces between the sand grains. The largest sand channels occur in the Middle Member where, in places, they reach thicknesses of more than 30 metres (100 ft). They are flanked by off-channel deposits that consist primarily of mud and silt that accumulated in the floodplain, tidal flat, swamp, and brackish-bay environments that existed contemporaneously with the channels.[8]

Paleontology

Ichnofossils such as Skolithos and Teichichnus that were created by burrowing organisms are common in the middle to upper portions of the McMurray Formation. Other macroscopic fossils are rare,[9] although fragments of coniferous wood[10] and freshwater to brackish-water molluscs[11] have also been described. Microscopic fossils include foraminifera, dinoflagellates, pollen and spores.[11][12]

Origin of the Hydrocarbons

The original petroleum was probably generated from the organic shales of the Exshaw Formation, according to organic biomarker studies and isotopic ratios that act as a fingerprint for specific rock units. It then migrated up dip toward the northeast.[13] Radiometric dating by the rhenium-osmium method indicates that it was emplaced in the McMurray and other formations at 112 ± 5.3 Ma (million years ago), not long after the deposition of the McMurray sediments.[14] Biodegradation by bacteria then converted the oil to immobile bitumen. This may have contributed to the trapping mechanism for the hydrocarbons, as well as hampering the lithification of the host sediments.[13][15]

Hydrocarbon Production
McMurray Formation mined in Syncrude's Mildred Lake mine site

Bitumen has been produced from the McMurray Formation in the Athabasca Oil Sands since 1967, at first by open-pit mining, and later from the subsurface as well, using in-situ techniques such as Steam-Assisted Gravity Drainage (SAGD). As of 2010, the output of oil sands production had reached more than 1.6 million barrels per day (250,000 m3/d); 53% of this was produced by surface mining and 47% by in-situ methods. The Alberta government estimates that production could reach 3.5 Mbbl/d (560,000 m3/d) by 2020 and possibly 5 Mbbl/d (790,000 m3/d) by 2030.[16]

Hydrogeology

The Lower Member of the McMurray Formation consists predominantly of poorly consolidated, discontinuous sand bodies. These sands are commonly saturated with water rather than with bitumen, and they lie beneath the bitumen-saturated sands which are aquitards.[15] They are commonly referred to as the Basal Water Sand (BWS) aquifers.[17] Where these aquifers are deep-lying, they are recharged with saline water from the underlying Devonian formations, but where they lie at relatively shallow depths, recharge occurs from meteoric (surface) water and they are non-saline.[15][18]

The BWS aquifers are commonly depressurized prior to open-pit mining because high pore water pressures can reduce pit-wall stability, and seepage onto the pit floor can reduce trafficability. They also have a negative impact on SAGD operations if they are in hydraulic communication with a steam chamber, resulting in heat loss.[15]

There are also aquifers within the Upper McMurray Member and the overlying Wabiskaw Member of the Clearwater Formation. These can also interfere with SAGD operations if cool water from these aquifers invades the steam chambers.[15]

References
  1. Alberta Geological Survey. "Alberta Oil Sands". Retrieved 2009-03-02.
  2. Lexicon of Canadian Geological Units. "McMurray Formation". Archived from the original on 2012-07-26. Retrieved 2009-02-28.
  3. McLearn, F.H., 1917. Athabasca River section, Alberta. Geological Survey of Canada Summary Report 1916, p. 145-151.
  4. Mossop, G.D. and Shetsen, I., (compilers), Canadian Society of Petroleum Geologists and Alberta Geological Survey (1994). "The Geological Atlas of the Western Canada Sedimentary Basin, Chapter 19: Cretaceous Mannville Group of the Western Canada Sedimentary Basin". Archived from the original on 2013-08-14. Retrieved 2013-08-01.CS1 maint: multiple names: authors list (link)
  5. Glass, D.J., editor, 1997. Lexicon of Canadian Stratigraphy, vol. 4, Western Canada. Canadian Society of Petroleum Geologists, Calgary, Alberta, 1423 p. on CD-ROM, ISBN 0-920230-23-7.
  6. Prior, G. J., Hathaway, B., Glombick, P.M., Pana, D.I., Banks, C.J., Hay, D.C., Schneider, C.L., Grobe, M., Elgr, R., and Weiss, J.A. (2013). "Bedrock Geology of Alberta. Alberta Geological Survey, Map 600". Archived from the original on 2013-09-25. Retrieved 2013-08-13.CS1 maint: multiple names: authors list (link)
  7. Mossop, G.D. and Shetsen, I., (compilers), Canadian Society of Petroleum Geologists (1994). "The Geological Atlas of the Western Canada Sedimentary Basin, Chapter 19: Cretaceous Mannville Group of the Western Canada Sedimentary Basin". Archived from the original on 2013-08-14. Retrieved 2013-08-22.CS1 maint: multiple names: authors list (link)
  8. Flach, P.D. and Mossop, G.D., 1985. Depositional Environments of Lower Cretaceous McMurray Formation, Athabasca Oil Sands, Alberta. American Association of Petroleum Geologists Bulletin, 69: 1195-1207.
  9. Pemberton, S.G., Flach, P.D. and Mossop, G.D. 1982. Trace fossils from the Athabasca Oil Sands, Alberta, Canada. Science, 217: 825-827.
  10. Roy, S.K. (1972). "Fossil wood of Taxaceae from the McMurray Formation (Lower Cretaceous) of Alberta, Canada. Canadian Journal of Botany, 50: 349-352". doi:10.1139/b72-048. Cite journal requires |journal= (help)
  11. Mellon, G.B. & Wall, J.H. (1956). "Foraminifera of the upper McMurray and basal Clearwater Formations. In: Geology of the McMurray Formation, Part I. Alberta Geological Survey, Report 72, p. 5-28" (PDF). Retrieved 2013-09-03.
  12. Vagvolgyi, A. and Hills, L.V. 1969. Microflora of the Lower Cretaceous McMurray Formation, northeast Alberta. Bulletin of Canadian Petroleum Geology, 17: 155-181.
  13. Schaefer, B.F. 2005. When do rocks become oil? Science, vol. 308, p. 1267-1268.
  14. Selby, D. and Creaser, R.A. 2005. Direct radiometric dating of hydrocarbon deposits using rhenium-osmium isotopes. Science, vol. 308, p. 1293-1295.
  15. Barson, D., Bachu, S. and Esslinger, P. 2001. Flow systems in the Mannville Group in the east-central Athabasca area and implications for steam-assisted gravity drainage (SAGD) operations for in situ bitumen production. Bulletin of Canadian Petroleum Geology, vo. 49, no. 3, p. 376-392.
  16. "Facts and Statistics". Alberta Energy. Alberta Government. 2008. Archived from the original on 2017-05-28. Retrieved 2013-01-31.
  17. Joslyn North Mine Project: Environmental Impact Assessment Hydrologeology (PDF) (Report). Edmonton, Alberta: Deer Creek Energy. December 2005. Archived from the original (PDF) on 2013-12-02.page=4
  18. Griffiths, Mary; Woynillowicz, Dan (April 2003). Oil and Troubled Waters: Reducing the impact of the oil and gas industry on Alberta's water resources (PDF) (Report). Edmonton, Alberta: Pembina Institute.CS1 maint: ref=harv (link)
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