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{{Short description|Geological formation in Alberta, Canada}}
{{Infobox Rockunit
|
| image = Alberta tar sands.jpg
| caption = Outcrop along the [[Athabasca River]], ca. 1900
| type = [[Geological formation]]
| age = {{Fossil range|Barremian|Aptian|late [[Barremian]] to [[Aptian]]}}
| period = Aptian
| prilithology = [[Sand]], [[sandstone]]
| otherlithology = [[Silt]], [[mud]], [[coal]]
| namedfor = [[Fort McMurray]]
| namedby = F.H. McLearn,1917<ref name=McLearn />
| region = [[Alberta]]
| country = [[Canada]]
| coordinates = {{coord|56.995921|N|111.456612|W|region:CA-AB|name=McMurray Formation}}
| unitof = [[Mannville Group]]
| subunits = Upper, Middle and Lower members
| underlies = [[Clearwater Formation]] ([[Wabiskaw Member]])
| overlies = [[Waterways Formation]], [[Banff Formation]], [[Wabamun Formation]]
| thickness = up to {{convert|60|m|ft|-1}}<ref name=lexicon>{{Cite web|url=http://cgkn1.cgkn.net/weblex/weblex_litho_detail_e.pl?00053:009513|archive-url=https://archive.today/20120726105500/http://cgkn1.cgkn.net/weblex/weblex_litho_detail_e.pl?00053:009513|url-status=dead|archive-date=2012-07-26|title=McMurray Formation|author=Lexicon of Canadian Geological Units|access-date=2009-02-28}}</ref>
| extent =
| area = {{convert|140000|km2|sqmi|abbr=on}}<ref>{{Cite web|url=http://www.ags.gov.ab.ca/energy/oilsands/alberta_oil_sands.html|title=Alberta Oil Sands |author=Alberta Geological Survey|authorlink=Alberta Geological Survey|access-date=2009-03-02}}</ref>
| map =
| map_caption =
}}
The '''McMurray Formation''' is a [[Stratigraphy|stratigraphic]] unit of [[Early Cretaceous]] [[Geochronology|age]] (late [[Barremian]] to [[Aptian]] stage) of the [[Western Canadian Sedimentary Basin|Western Canada Sedimentary Basin]] in northeastern [[Alberta]].<ref name=Mossop19>{{Cite web|url=http://www.ags.gov.ab.ca/publications/wcsb_atlas/a_ch19/ch_19.html |title=The Geological Atlas of the Western Canada Sedimentary Basin, Chapter 19: Cretaceous Mannville Group of the Western Canada Sedimentary Basin |author=Mossop, G.D. and Shetsen, I., (compilers), Canadian Society of Petroleum Geologists and Alberta Geological Survey |year=1994 |access-date=2013-08-01 |url-status=dead |archive-url=https://web.archive.org/web/20130814133127/http://www.ags.gov.ab.ca/publications/wcsb_atlas/a_ch19/ch_19.html |archive-date=2013-08-14 }}</ref> It takes the name from [[Fort McMurray]] and was first described from [[outcrop]]s along the banks of the [[Athabasca River]] {{convert|5|km|mi}} north of Fort McMurray by F.H. McLearn in 1917.<ref name=McLearn>McLearn, F.H., 1917. Athabasca River section, Alberta. [[Geological Survey of Canada]] Summary Report 1916, p. 145-151.</ref> 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.

==Lithology==
The McMurray Formation consists of fine- to coarse-grained [[quartz]]itic [[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 [[Cementation (geology)|cemented]] with bitumen or, less commonly, with [[calcite]], [[iron oxide]]s, or [[quartz]].<ref name="CSPG_Lexicon">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}}.</ref>

==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 [[Conglomerate (geology)|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.<ref name=CSPG_Lexicon />

===Distribution===
[[File:McMurray Fm 2017.jpg|thumb|left|Outcrop of the McMurray Formation (black and dark grey) and underlying [[Waterways Formation]] (tan) on the Steepbank River.]]
[[File:McMurray Fm boulder.jpg|thumb|right|A boulder of McMurray Formation oil sand near the Athabasca River north of Fort McMurray.]]
The McMurray Formation outcrops along the Athabasca and [[Clearwater River (Saskatchewan)|Clearwater]] Rivers and their tributaries near Fort McMurray in the Athabasca Oil Sands of northeastern Alberta, where it averages about {{convert|60|m|ft|-1}} 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.<ref name=CSPG_Lexicon /><ref name="Map600">{{Cite web|url=http://www.ags.gov.ab.ca/publications/pubs.aspx?series=map|title=Bedrock Geology of Alberta. Alberta Geological Survey, Map 600|author=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.|year=2013|access-date=2013-08-13|url-status=dead|archive-url=https://web.archive.org/web/20130925225910/http://www.ags.gov.ab.ca/publications/pubs.aspx?series=map|archive-date=2013-09-25}}</ref>

===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 Formation|Banff]], [[Wabamun Formation|Wabamun]], and [[Winterburn Formation]]s farther west.

The McMurray Formation is equivalent to the lower [[Mannville Group]] of Alberta, the [[Dina Member|Dina Formation]] of eastern Saskatchewan, the [[Gething Formation]] of northwestern Alberta and northeastern British Columbia, and the [[Ellerslie Member|Ellerslie Formation]] and [[Ostracod Beds]] of central Alberta.<ref name=CSPG_Lexicon /><ref>{{Cite web|url=http://www.ags.gov.ab.ca/publications/wcsb_atlas/a_ch19/ch_19.html |title=The Geological Atlas of the Western Canada Sedimentary Basin, Chapter 19: Cretaceous Mannville Group of the Western Canada Sedimentary Basin |author=Mossop, G.D. and Shetsen, I., (compilers), Canadian Society of Petroleum Geologists |year=1994 |access-date=2013-08-22 |url-status=dead |archive-url=https://web.archive.org/web/20130814133127/http://www.ags.gov.ab.ca/publications/wcsb_atlas/a_ch19/ch_19.html |archive-date=2013-08-14 }}</ref>
[[File:Oil Sand Drill Cores.jpg|thumb|left|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 {{convert|30|m|ft|-1}}. 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.<ref name="Flach">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.</ref>

==Paleontology==
[[Ichnofossil]]s 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 scale|macroscopic]] fossils are rare,<ref name="Pemberton">Pemberton, S.G., Flach, P.D. and Mossop, G.D. 1982. Trace fossils from the Athabasca Oil Sands, Alberta, Canada. Science, 217: 825-827.</ref> although fragments of [[pinophyta|coniferous]] wood<ref name="Roy">{{Cite journal|title=Fossil wood of Taxaceae from the McMurray Formation (Lower Cretaceous) of Alberta, Canada. Canadian Journal of Botany, 50: 349-352.|author=Roy, S.K.|year=1972|doi=10.1139/b72-048}}</ref> and freshwater to brackish-water [[mollusca|molluscs]]<ref name="Mellon">{{Cite web|url= http://www.ags.gov.ab.ca/publications/REP/PDF/REP_72.pdf|title=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.|author1=Mellon, G.B. |author2=Wall, J.H. |name-list-style=amp |year=1956|access-date=2013-09-03}}</ref> have also been described. [[Microscopic scale|Microscopic]] fossils include [[foraminifera]], [[dinoflagellate]]s, [[pollen]] and [[spores]].<ref name="Mellon" /><ref>Vagvolgyi, A. and Hills, L.V. 1969. Microflora of the Lower Cretaceous McMurray Formation, northeast Alberta. Bulletin of Canadian Petroleum Geology, 17: 155-181.</ref>

==Origin of the Hydrocarbons==
The original [[petroleum]] was probably generated from the organic [[shale]]s of the [[Exshaw Formation]], according to organic [[biomarker (petroleum)|biomarker]] studies and [[isotope|isotopic]] ratios that act as a fingerprint for specific rock units. It then migrated up [[dip (geology)|dip]] toward the northeast.<ref name=Schaefer>Schaefer, B.F. 2005. When do rocks become oil? Science, vol. 308, p. 1267-1268.</ref> [[Radiometric dating]] by the [[rhenium-osmium dating|rhenium-osmium method]] indicates that it was emplaced in the McMurray and other formations at 112 ± 5.3 [[megaannum|Ma]] (million years ago), not long after the deposition of the McMurray sediments.<ref name=Selby>Selby, D. and Creaser, R.A. 2005. Direct radiometric dating of hydrocarbon deposits using rhenium-osmium isotopes. Science, vol. 308, p. 1293-1295.</ref> [[Biodegradation]] by [[bacteria]] then converted the oil to immobile bitumen. This may have contributed to the [[petroleum reservoir|trapping mechanism]] for the hydrocarbons, as well as hampering the [[lithification]] of the host sediments.<ref name=Schaefer /><ref name=Barson />

==Hydrocarbon Production==
[[Image:Syncrude mildred lake plant.jpg|thumb|right|200px|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|Steam-Assisted Gravity Drainage]] (SAGD). As of 2010, the output of oil sands production had reached more than {{convert|1.6|Moilbbl/d|m3/d}}; 53% of this was produced by surface mining and 47% by in-situ methods. The Alberta government estimates that production could reach {{convert|3.5|Moilbbl/d|m3/d|abbr=on}} by 2020 and possibly {{convert|5|Moilbbl/d|m3/d|abbr=on}} by 2030.<ref>{{cite web
| title = Facts and Statistics
| work = Alberta Energy
| publisher = Alberta Government
| year = 2008
| url = http://www.energy.alberta.ca/OilSands/791.asp
| doi =
| access-date = 2013-01-31
| archive-url = https://web.archive.org/web/20170528162132/http://www.energy.alberta.ca/oilsands/791.asp
| archive-date = 2017-05-28
| url-status = dead
}}</ref>

==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 [[aquifer|aquitards]].<ref name=Barson>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.</ref> They are commonly referred to as the Basal Water Sand (BWS) [[aquifer]]s.<ref name="Total2005">{{cite report|url=http://www.total-ep-canada.com/upstream/documents/application/CR_4_Groundwater_Report.pdf|title=Joslyn North Mine Project: Environmental Impact Assessment Hydrologeology|location=[[Edmonton]], Alberta|date=December 2005|publisher=Deer Creek Energy|url-status=dead|archive-url=https://web.archive.org/web/20131202222021/http://www.total-ep-canada.com/upstream/documents/application/CR_4_Groundwater_Report.pdf|archive-date=2013-12-02}}page=4</ref> 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 water|meteoric (surface) water]] and they are non-saline.<ref name=Barson /><ref name="Pembina">{{cite report|url=https://www.ualberta.ca/~ersc/water/links/pembina.pdf|title=Oil and Troubled Waters: Reducing the impact of the oil and gas industry on Alberta's water resources|first1=Mary |last1=Griffiths|first2=Dan |last2=Woynillowicz|date=April 2003|publisher=Pembina Institute|location=Edmonton, Alberta }}</ref>

The BWS aquifers are commonly depressurized prior to open-pit mining because high [[pore water pressure]]s 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.<ref name=Barson />

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.<ref name=Barson />

==References==
{{Reflist}}

{{Western Canadian Sedimentary Basin|Northeast_Plains=yes}}

{{Authority control}}

[[Category:Geologic formations of Alberta]]
[[Category:Athabasca oil sands]]
[[Category:Fort McMurray]]
[[Category:Western Canadian Sedimentary Basin]]
[[Category:Cretaceous Alberta]]
[[Category:Sandstone formations of Canada]]
[[Category:Fluvial deposits]]
[[Category:Tidal deposits]]
[[Category:Reservoir rock formations]]

McMurray Formation - Revision history

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