A Regional Evaluation of Coal Quality in the Southern and Central Foothills/Mountains Region of Alberta

The raw coal quality variations within the Lower Cretaceous Luscar and Jurassic - Cretaceous Kootenay Groups are documented by using several approaches. Techniques include classical descriptive and exploratory statistics, weighted means of coal quality parameters plotted on regional geological maps, and detailed in-seam vertical chemical profiles of coals. A geological understanding of these coal quality variations is gained by relating some parameters (e.g. ash, sulfur) to the original depositional environment, and others to depth of burial and/or structural deformation (volatile matter, fixed carbon, carbon). Some coal quality parameters can be related to both depositional environment and later burial/deformation history (ash, sulfur and calorific value).

Data were obtained from the ERCB coal hole database and consist of some 333 coal quality analyses from approximately 77 coal exploration holes. Coal quality parameters available include; ash, fixed carbon (FC), volatile matter (VM), moisture, sulfur (S) and calorific value (CV). Very few ultimate analyses, carbonization parameters, or detailed coal combustion properties are available for study. Many of the variables examined are normally distributed. Mean values for foothills/mountains coals in the central and southern Rockies are as follows (most on a dry basis): equilibrium moisture - 16.25%, ash - 25.90%, FC - 51.49%, VM-22.61, CV - 25582.05 Kj/kg. And S - 0.56%. Volatile matter, calorific value and sulfur are the only variables that show a significant statistical difference between the Luscar and Kootenay Groups.

Because the data is clustered within three or four small geographic areas within the region and because the exact stratigraphic horizon of the data is not known, interpretation of coal quality trends from the regional maps should only be done with extreme care. For example, fixed carbon and volatile matter show a systematic system increase in rank to the north.

Measured stratigraphic sections of the Kootenay group together with coal seams sampled in the Crowsnest Pass area, suggest that mean in-seam values of carbon (C) , FC and VM can be used to assist in coal seam correlation in local areas. In-seam coal quality profiles show varying patterns. Higher sulfur values at the top and base of the seam are thought to be related to the early diagenetic environment and generally not to the influences of possible overlying marine strata. Ash is inversely related to sulfur content at some locations. Ash increases with a higher degree of tectonic shearing at the center of some coal seams, while carbon values are reduced by shearing. Three distinct depositional environment and related coal quality facies can be recognized in the Crowsnest Pass area: 1) mid-coastal plain-mire facies that tend to have overall poor coal quality parameters, 2) distal fluvial/lacustrine-mire facies that has produced very thick, and high quality economic coals, and 3) a proximal fluvial-mire facies that often produces thick seams with very high ash contents.

Coal rank variations, depth of burial and structural influences on rank were explored by examining data from the ERCB database, existing published vitrinite reflectance measurements and by collecting additional samples (for vitrinite reflectance) in regions where data were missing. Volatile matter values, plotted for the base of the Kootenay Group, show a graduation from high-volatile bituminous "A" coals in the area south of Crowsnest Pass to semi-anthracite in the Canmore area. The increase in coalification from south to north must be explained in terms of depths of burial , geothermal gradients, and possible additional loading by thick thrust sheets. The inferred depth of burial for coals at the base of the Kootenay Group varies from 5 km in eastern regions to about 7 km in the west. Local tectonism causes hearing of the coal which in turn makes it more susceptible to oxidation, and at times, causes increased ash contents. Rank does not seem to be enhanced by proximity to major thrust faults. Structural thickening of coals occurs in anticline and syncline axes and through imbricate thrust sheets.