McMurray Formation

McMurray Formation Stratigraphic range: late Barremian to Aptian PreЄ Є O S D C P T J K Pg N
Outcrop along the Athabasca River, c. 1900
Type	Geological formation
Unit of	Mannville Group
Sub-units	Upper, Middle and Lower Member
Underlies	Clearwater Formation (Wabiskaw Member)
Overlies	Waterways Formation, Banff Formation, Wabamun Formation
Area	140,000 km2 (54,000 sq mi)[1]
Thickness	up to 60 metres (200 ft)[2]
Lithology
Primary	Sandstone
Other	Siltstone
Location
Coordinates	56°46′24″N 111°24′13″W / 56.77328°N 111.40374°W / 56.77328; -111.40374 (McMurray Formation)
Region	north-eastern  Alberta
Country	Canada
Type section
Named for	Fort McMurray
Named by	F.H. McLearn,1917

The McMurray Formation is a stratigraphic unit of Early Cretaceous age (late Barremian to Aptian stage) in the Western Canada Sedimentary Basin in northeastern Alberta. It takes the name from Fort McMurray, and was first described in the outcrops exposed 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 the vast bitumen resources of the Athabasca Oil Sands.

Lithology

The McMurray Formation consists of fine- to coarse-grained quartzitic sand and sandstone, interbedded with lesser amounts of silt, mud and clay. The sands are very loose and friable, unless they are partially or fully cemented with bitumen, calcite or, rarely, quartz.^[4]

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.^[4]

Distribution

The McMurray Formation outcrops along the Athabasca and Clearwater Rivers 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.^[4][5]

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.^[4][6]

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. The drainage system was encroached upon from the north by a rising sea, 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 lies 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.^[7]

Paleontology

Ichnofossils such as Skolothios 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,^[8] although fragments of coniferous wood^[9] and freshwater to brackish molluscs^[10] have also been described. Microscopic fossils include foraminifera, dinoflagellates, pollen and spores.^[10][11]

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, 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.^[12]

Hydrogeology

The Lower Member of the McMurray Formation consists predominantly of poorly consolidated sands. These sands are commonly saturated with water, rather than with bitumen, and are referred to as the Basal Water Sand (BWS) aquifers.^[13] Where these aquifers are recharged from the underlying Devonian formations they may be saline (Griffiths & 2003 9), but the aquifers are non-saline where recharge occurs from surface water.(FMFN & 2012 31,105)^[14]

References