Hydrogeology and Stress Regime of the Upper Cretaceous-Tertiary Coal-Bearing Strata in Alberta

The coalbed methane potential and producibility of any coal-bearing strata are strongly affected by the hydrogeological regime of formation waters and by coal permeability, which in turn depends on the effective stress regime of the coal seams. Peat accumulated in the Alberta basin during the late Cretaceous and early Tertiary led to the formation of coal deposits in the Upper Cretaceous Belly River Group and Horseshoe Canyon Formation, and the Upper Cretaceous-Paleocene Scollard and Paskapoo formations. The flow of formation waters in these strata is driven by gravity (topography) and erosional rebound, and is controlled by rock absolute permeability, gas generation and capillary pressure to gas (relative permeability).

The permeability of coal seams decreases west-southwestward with increasing burial depth, from the order of several darcies (D) and higher in the shallow (<50 m) zones, to millidarcies and less in the deep zones (>1500 m). The minimum effective stress, which affects coal permeability by closing fractures, increases west-southwestward from zero at the erosional edge of the strata to approximately 20 MPa near the Rocky Mountain deformation front. Fractures, including those in coal seams, will generally be vertical, and will propagate on a southwest-northeast axis along the direction of the maximum horizontal stress.

The flow of formation waters indicates that the coalbed methane in deep coal seams in west-central Alberta (Edmonton and Belly River groups) is most likely of thermogenic origin. The gas content of the coal may be quite low, as the underpressuring caused by erosional rebound could have drawn the gas out of coal into the adjacent sandstone units where it has accumulated in stratigraphic traps created by a changing depositional environment. The coalbed methane in shallower coal in and near the subcrop regions of the Upper Cretaceous-Tertiary strata is probably of thermogenic and biogenic origin. These coals seam, although of low rank, may contain significant amounts of late-stage biogenic methane.

From the point of view of produced water, the salinity of formation water in shallow coal seams, where the flow is driven by topography, is low, generally less than 1500 mg/l, although in places it may react 3000-5000 mg/l. The salinity of the formation water in the deeper strata in west-central Alberta, where the flow is driven by erosional rebound, is significantly higher, reaching up to 18 000 mg/l. This affects treatment and/or disposal strategies with regard to the water produced concurrent to coalbed methane.

From a strictly hydrogeological and permeability/stress regime point of view, the region that probably has good CBM potential and producibility from coal seams in the Upper Cretaceous-Tertiary strata of the Alberta basin extends from the west-northwest, at the top of Scollard-Paskapoo succession, to central and southern Alberta, along and near the subcrop area of the stratigraphically deeper Edmonton and Belly River groups. The deep Edmonton and Belly River strata in western and central Alberta most likely have a reduced CBM potential due to possibly lower gas content and low permeability. These considerations need to be applied against studies of coal thickness, rank and gas content top identify the best targets for CBM exploration and production in Alberta.