Coal quality and rank variation within Upper Cretaceous and Tertiary sediments, Alberta plains region

Near-surface coals in the Alberta plains occur in Upper Cretaceous and Tertiary sediments of the Belly River Group, Horseshoe Canyon, Wapiti and Paskapoo Formations. These coal bearing units contain significant coal resources of varying quality and rank. Statistical techniques were used to document both the distribution of, and the interrelationships between the components of proximate and ultimate analysis and calorific value determinations.

The coals range in rank from subbituminous C to high volatile bituminous C. They have variable ash content and low sulfur content The relationships between calorific value (dry basis, kJ/kg) and ash (dry basis, percent), and calorific value (moist mineral matter free, kJ/kg) and equilibrium moisture (MEQ) percent were determined to be: CV(D) = 29262- 286(Ash[Dl) and CV(MMMF) = 31816 - 442 (MEQ).

Near surface coals in the Alberta plains increase in rank toward the west-southwest (that is, towards the foothills/ mountains region). A new model to explain this coal rank distribution is proposed. This model relates a coal seam's rank to its maximum (paleo)depth of burial. This maximum depth was reconstructed on the basis of established relationships between equilibrium moisture loss and depth of coal seam burial. A least squares regression analysis of calorific value (MMMF, kJ/kg) on the calculated depth of burial (DOB, m) yielded the equation: CV(MMMF) = 14748 + 6.25(DOB).

This equation indicates a coalification gradient, determined on the reconstructed overburden, of 6.25 kJ/kg per metre (0.82 Btu/lb/ft) of depth (or overburden). Progressively greater amounts of overburden existed in a direction toward the mountains at the time of coalification. Erosion since middle Tertiary time has removed between 900 and 1900 m (3000 and 6200 ft) of sediment; the greatest amount of removal is in the west southwest area, where coals of higher ranks are exposed.