Differences in bitumen and kerogen-bound fatty acid fractions during diagenesis and early catagenesis in a maturity series of New Zealand coals

Abstract

Oxygen-bearing functional groups, in particular the carboxylic groups of acids and esters, are mainly responsible for the chemical reactivity of sedimentary organic matter. We have studied kerogen and bitumen fractions from a coalification series from the New Zealand Coal Band covering the rank range from 0.28% to 0.80% vitrinite reflectance. We investigated the composition of fatty acids separated from the bitumen, and compared this to the distribution of kerogen-bound fatty acids (esters) obtained after selective chemical degradation of the macromolecular organic matter. We found remarkable differences in the fatty acid composition between bitumen and kerogen-bound acids, both in the short (<C20) and long chain (≥C20) fatty acid range. The compositions of these two acid fractions changed independently as a function of maturation. This points to the long and short chain fatty acids in bitumen and kerogen having different susceptibilities towards degradation, possibly as a result of their different solubilities in the pore water. The compositional differences may further reflect the occurrence of different types of bitumen e.g. if bitumen is trapped in organic micropores of the macromolecular network. Application of the carbon preference index for fatty acids (CPIFA) of bitumen and kerogen-bound acids revealed a linear correlation to the Tmax maturity parameter. This shows that the CPIFA has a clear relation to thermal stability and, thus, reactivity of the buried organic matter. The difference in slopes of CPIFA vs. Tmax for short and long chain as well as bitumen and kerogen-bound acids may indicate their different degradation susceptibilities. The short chain fatty acids of the bitumen show the highest susceptibility whereas the kerogen-bound long chain fatty acids seem to be most resistant towards diagenetic degradation.