April 8, 2013
The Energy Resources Conservation Board 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. In total, it is estimated there are 14 trillion cubic metres (500 tcf) of gas in place in all the coal in Alberta.
Alberta contains vast amounts of coal distributed throughout the southern Plains, Foothills and Mountains. Originally deposited in relatively flat-lying peat swamps, organic matter (peat) was buried by sediments derived from uplift (mountain building), in the west, and gradually changed into coal with increasing heat and pressure of burial. Over time, the coals were uplifted and partially eroded away, resulting in the present distribution of coal across the Plains.
Coal-bearing strata gently dip westward toward the Mountains, where the coals are folded and abruptly turn toward the surface to be exposed in the Foothills (cross-section image).
The coal-bearing succession in Alberta has older coals overlain by progressively younger coals, except in some locations within the Foothills/Mountains where older coals may be thrust over younger coals by mountain-building tectonic processes. Uplift and erosion of coals dipping toward the west has resulted in younger coals coming to surface at locations farther west than older coals. Commonly, mines are located where the coal comes to surface; for example the Whitewood mine and Cardinal River mine.
Coal typically occurs within a 'coal zone,' as discrete coal seams and/or packages with several thin and thick seams interbedded with non-coaly rock layers or beds . A coal zone may be traceable over a large geographic area. 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 potential as CBM exploration targets.
The oldest and deepest coals of the Alberta Plains belong to the Lower Cretaceous Mannville Group coals. The Mannville coals are widely distributed across the Alberta Plains, are thick, continuous and contain some of the highest gas contents 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 ranging from about 800 metres up to 2800 metres.
Upper Cretaceous through to Tertiary-aged coal also occur across the Plains, with older coals being overlain by progressively younger rocks and coals. 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. Compared with Mannville, Horseshoe Canyon and Ardley coals, the overall thin coals and restricted lateral continuity of Belly River Group coal seams have resulted in limited exploration efforts in these coals.The Bearpaw Formation represents a marine encroachment over the Belly River Group. When the Bearpaw Sea retreated, sediments of the Horseshoe Canyon Formation were laid down. Extensive peat swamps developed, which resulted in the Drumheller Coal Zone. The Drumheller Coal Zone, located in the lower part of the Horseshoe Canyon Formation, has been a primary CBM target for industry. 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 multi-completion CBM target for drilling companies.
Thin, discontinuous coals overly the Drumheller Coal Zone, with the Carbon-Thompson Coal Zone present near the top of the Horseshoe Canyon Formation. Silty sands and shales of the Whitemud and Battle formations were deposited over the Horseshoe Canyon Formation. These formations were, in turn, covered by fluvial sediments and associated peat swamps of the Scollard Formation. The Ardley Coal Zone occurs near the top of the Scollard Formation and contains thick, widespread coals. 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.
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. These coals are generally higher in rank and have greater CBM content than Plains coals. These coals commonly show much deformation, which disrupts their continuity and thickness. Locating thick coals with limited deformation is a challenge in exploring for CBM in the Mountains and Foothills, and currently there is some limited exploration in some areas.