Alberta contains vast amounts of coal distributed throughout the southern Plains, Foothills, and Mountains. The AER estimates there are 91 billion tonnes of coal resources at depth suitable for mining. There is an additional 2 trillion tonnes of coal at depth in the Alberta Plains that may be suited for coalbed methane (CBM) exploration. Coal in the Foothills and Mountains would also significantly add to this tonnage.
Coal typically occurs within a 'coal zone,' as discrete coal seams or packages of several thin and thick seams interbedded with non-coaly rock layers or beds. A coal zone may be traceable over a large geographic area.
In Alberta, the coal zones are found in strata ranging in age from Late Jurassic (approximately 145 million years old) to Tertiary (approximately 65 million years old; see the stratigraphic chart). Most of the coal zones in Alberta have CBM potential.
The coal-bearing succession in Alberta has older coals overlain by progressively younger coals, except some locations in the Foothills and Mountains where older coals may be thrust over younger coals by mountain-building tectonic processes. Uplift and erosion of coals dipping towards the west has resulted in younger coals coming to surface at locations farther east. Commonly, mines are located where the coal comes to surface (e.g., the Whitewood mine, Paintearth mine and Cardinal River mine).
Coal-bearing strata in the Mountains and Foothills are folded and abruptly turn towards the surface to be exposed in the Foothills.
The coal rank (organic matter maturity) in Alberta ranges from very low (lignite) to high (anthracite). Coal near the surface in the Plains is generally of sub-bituminous rank, with lignite occurring in the north and northeast part of the Plains, and high volatile bituminous C in the northwest and southwest areas of the Plains. In the Plains, coal rank increases towards the west, as coal zones dip and become progressively deeper towards the mountains.
Coals near the surface in the Foothills generally are of high volatile bituminous C, with minor amounts of high volatile bituminous B and A. In the Mountains area west of the Foothills, coal rank occurs in the range of medium and high volatile bituminous with local occurrences of anthracite near the Canmore area. Low rank, shallow coals in the Plains are used mainly for heating and power generation (so called ‘thermal coal’). Higher-rank coals of the Mountains and Foothills are suitable for the steel making industry (so called ‘metallurgical’ or ‘coking coal’). Weathered coal occurring near the surface, known as humalite and leonardite, is mined for agricultural use (e.g. fertilizer).
In the Plains, coal cleat systems in outcrop can be traced along two main directions SW-NE and NW-SE within 60 degrees. Coal cleats in the shallow Plains are generally wider than cleats deeper in the subsurface of the Plains due to high pressure in deeper coals that serve to close cleats.
The oldest and deepest coals of the Alberta Plains are the Lower Cretaceous Mannville Group coals. The Mannville coals are widely distributed across the region. Some of the Mannville Group coal seams are thick, continuous (i.e., Medicine River Coal), and contain some of the highest gas content of any coals in the Alberta Plains. Typically six or more seams with cumulative coal thickness ranging from 2 to 14 metres occur over a stratigraphic interval of 40 to 100 metres. The thickest coals extend from southeast of Grande Prairie in a widening wedge between Edmonton and Calgary to the Coronation area, with coals occurring at depths between 800 metres and 2800 metres.
Upper Cretaceous through to Tertiary-aged coal zones also occur across the Plains, with older coals being overlain by progressively younger rocks and coal beds. Three coal zones are recognized within the Upper Cretaceous Belly River Group: the McKay coal zone, near the base of the Belly River Group; the Taber coal zone, located in the middle; and the Lethbridge coal zone, at the top of the Belly River Group. The Belly River Group coal seams exhibit minor thickness, a smaller number of coal seams, higher ash content, and inconsistent lateral continuity, which has resulted in limited CBM exploration efforts.
Compared with the Mannville coals, the younger Horseshoe Canyon and Ardley coal zones consist of thinner and less regionally distributed coal seams. Both Horseshoe Canyon and Ardley coal seams cover smaller, but well-delimited, areas in the Alberta Plains.
A primary CBM target for industry has been the cyclic occurrence of numerous coal seams of the Drumheller coal zone of the Horseshoe Canyon Formation, developed in backshore peat swamps environments. For instance, in the area between Bashaw and Rockyford, the Drumheller coal zone is relatively shallow (about 300 metres), with 10 to 20 metres cumulative coal within a 70 to 120 metre coal zone thickness. The coal zone may contain 20 or more individual thin seams and interbedded sandstone and shale, which combine to make an attractive multicompletion CBM target for drilling companies.
Near the top of the Horseshoe Canyon Formation, one very thin and discontinuous coal seam of fluvial origin, known as the Carbon-Thompson coal zone, marks the end of coal development in the succession.
The Ardley coal zone occurs near the top of the Scollard Formation and contains thick and continuous coals of fluvial origin distributed in a relatively narrow region in front of the Foothills.
Two areas of the Ardley coal zone have been the main focus of CBM exploration: the northeastern Pine Creek (north of Edson) and central Pembina (Buck Lake-Drayton Valley) areas. These areas include some of the thickest and most continuous Ardley coals (15–25 metres net coal) and occur at depths between 300 and 700 metres.
Sand, silt and shale of the Paskapoo Formation were deposited over the Scollard Formation. Thin coals occur throughout the Paskapoo Formation.
Shallow coals of the Plains (i.e., Ardley, Horseshoe Canyon) have a low rank and have not generated large quantities of methane. Deeper coals (i.e., Mannville) have experienced a greater degree of coalification (high rank) and have produced and retained greater quantities of methane than younger coal zones. Conversely, shallow coal zones experience more intense bacterial methanogenetic processes.
During the mountain-forming process, compressional tectonic stress contributed to the formation of coal cleats even further east in the Plains.
Maps of Coal Zones distribution and characteristics
The below maps are from AER/AGS reports and the AER/AGS coal database. The maps illustrate the general coal distribution, net coal thickness, and CBM potential for each major coal zone in the Alberta Plains.
|Ardley Depth to Top||Ardley Net Coal Thickness|
|Carbon-Thompson Depth to Top||Carbon-Thompson Net Coal Thickness|
|Drumheller Depth to Top||Drumheller Net Coal Thickness|
|Lethbridge Depth to Top||Lethbridge Net Coal Thickness|
|Taber Depth to Top||Taber Net Coal Thickness|
|McKay Depth to Top||McKay Net Coal Thickness|
|Mannville Depth to Top||Mannville Net Coal Thickness|
Coals of the Luscar and Kootenay groups, as well as correlative strata of the Scollard Formation, Horseshoe Canyon Formation, and Belly River Group, occur throughout the Foothills and Mountains. Mountain-building processes in the western parts of Alberta and into British Columbia (Rocky Mountains) have produced great compressional stress in the coal-bearing rocks. Consequently, these coals are generally higher in rank and have greater CBM content than Plains coals. The intense structural deformation disrupted coal seam continuity and thickness, and locating thick coals with limited deformation is a challenge in exploring for CBM in the Mountains and Foothills.