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Quality of the jurassic-cretaceous Mist Mountain formation coals, southern Canadian cordillera: relationships to sedimentology and coalbed methane potential Vessey, Sarah Jane


A detailed investigation of the Jurassic-Cretaceous Mist Mountain Formation in the southern Canadian Cordillera was undertaken to determine what, if any relationships exist between coal quality parameters, stratigraphic stacking, depositional environment and coalbed methane potential. In the Line Creek area, a shift in depositional environment within the Mist Mountain Formation is recorded as a change in abundance and lateral continuity of lithofacies. The lower part of the formation was deposited in an interdeltaic coastal plain environment and is typified by alternation of laterally extensive coal seams and thick, widespread channel sandstones. In the upper Mist Mountain Formation, channel sandstones are replaced by an increasing abundance of thin coal seams, floodplain and crevasse splay facies sediments. The upper unit was deposited in a distal alluvial-fluvial floodplain environment. Coal seam geometry and quality data indicate that Mist Mountain Formation peat mires were not directly influenced by contemporaneous clastic sedimentation. The geometry of coal seams reflects the proximity of under- and overlying channel sandstones, but ash content and mineralogy of the coal do not parallel changes in thickness, proximity or composition of surrounding clastic sediments. Examination of lower unit characteristics suggests that peat mires developed only during breaks in clastic deposition; the upper unit coal appears to have formed in domed mires, raised above active clastic sedimentation. Mist Mountain Formation coals are of suitable rank and composition to host significant coalbed methane reserves, but exploration to date indicates only limited volumes of gas. The coal is commonly sheared and oxidised, features which generally enhance the permeability of coal, yet the effect of these factors on coalbed methane potential is not understood. Results from this investigation indicate greater volumes of methane are adsorbed with increasing vitrinite content in the Mist Mountain Formation coals. Additionally, oxidised coals have decreased methane adsorption capacity. The effects of maceral composition and oxidation on methane adsorption overshadow the effect of shearing. Groundwater is likely to travel preferentially through sheared and oxidised coal due to increased permeability, facilitating methane desorption and escape from the coal. It is probable that a combination of decreased adsorption due to oxidation and leakage of methane from sheared and oxidised zones account for the low volumes of methane in Mist Mountain Formation coals.

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