The relationship between aromaticity, vitrinite reflectance and maceral composition of coals: Implications for the use of vitrinite reflectance as a maturation parameter

Abstract

Previous studies on the aromaticity ( f a) of coals have either limited themselves to the effects of rank or to differences between the macerals and generally conclude that f a and rank increase simultaneously although not necessarily linearly. This study contains 13C CP/MAS NMR analysis data of more than 70 coals obtained from different coal basins. These are combined with published data to produce a data base with more than 130 analyses. The aim is to determine the influence of rank and maceral composition on the measured f a of coals. A plot of f avs R o (vitrinite reflectance) shows a non-linear relationship that can be described by a polynomial equation. The shape of the curve produced by this equation is determined by the processes that occur during maturation. Only approx. 50% of the coals in the data base have f a values that are accurately predicted by the vitrinite reflectance; this is an indication of the large spread of values observed in the plot of f a vs R o. As a result of the processes that occur during maturation, the influence of the maceral composition on the f a was restricted to coals with R omax values > 0.70% R o. The equation generated shows that more than 80% of the coals tested with this level of rank have f a values that can be accurately predicted by an equation containing the R omax value, the total vitrinite content and the contents of fusinite, semifusinite and macrinite in the coals. Comparison of the f a values obtained from vitrinitic (Type III) kerogen with the f a values obtained from liptinitic (sporinites and Type II) kerogens, reveals a pattern which illustrates the structural changes that occur during maturation. Both vintrinites and liptinites show a change in the slope in an f avs R o plot, although the reflectance level at which the change in slope occurs differs between the two kerogen types. The changes in molecular structure that occur within the two kerogen types during maturation are discussed. The implications of the f a− R o relationship of vitrinites on the use of vitrinite reflectance as a maturation parameter are also discussed.