Coalification paths of exinite, vitrinite and inertite

Abstract

Coalification is a burial metamorphic process fundamental to understanding the origin and nature of coals and petroleum. Accurate assessment of Coalification requires removal of the variation in coal properties owing to differences in coal type. This is best achieved by assessing the coal rank of the maceral groups exinite, vitrinite and inertinite (and ultimately the macerals within each group) in terms of maximum reflectance. The mean maximum reflectances of exinite (R̄ E max), vitrinite (R̄ v max) and inertinite (R̄ I max) are highly correlated over the rank range, soft brown coals to low-volatile bituminous coals. A single exinite Coalification break is identified at 0.4 to 0.5% R̄ V max. Thereafter, exinite coalification occurs at an increasing rate (R̄ E max relative to R̄ V max) over the rank range 0.5–2.0% R̄ V max, crossing the vitrinite Coalification path at 1.55% R̄ V max. A phase of rapid inertinite Coalification (0.7–2.0% R̄ I max) occurs between 0.2 and 0.9% R̄ V max. Since exinite is a major land-plant source of hydrocarbons, and R̄ E max is a sensitive indicator of organic maturation over the range 0.2–1.1% R̄ E max, which corresponds to the main phase of oil generation, R̄ E max can be a most useful indicator of petroleum generation. The inertinite lnkohlungsprung represents a dramatic compositional change which should be taken into account when considering the origin of inertinite, its utilization properties and the general processes of rank change and organic maturation.