Variation in permeability with lithotype and maceral composition of Cretaceous coals of the Canadian Cordillera

Abstract

The permeability of high- and medium-volatile bituminous Cretaceous coal at the megascopic and microscopic scale is investigated utilizing a pressure-decay permeameter. The order of decreasing permeability with lithotype is: bright (average = 4.1 md), banded (0.79 md), fibrous (0.50 md), banded dull (0.14 md) and dull (0.016 md). Bright coals are the most permeable because of associated macro-fracturing (cleat). Banded dull coal permeability generally increases with increasing vitrinite content. The lowest permeabilities occur in dull coals with a high mineral and inertinite content. Fibrous coal has a higher permeability than dull coal of the same rank due to the abundance of macroporous fusinite in fibrous coal. Dull coal permeability decreases with an increase in rank, but these results are overshadowed by compositional variability between samples. Pressure-decay measurements are more reliable for dull lithotypes as these lithotypes do not fracture as easily during sample preparation. Although measured permeabilities are optimistic due to the relaxation of stress upon exposure of coal to atmospheric pressure, the results obtained are representative of the range of permeabilities of coal lithotypes and provide insight into the variability in permeability that may exist at high effective stress in the subsurface.