Comparative geochemical and pyrolytic study of coals, associated kerogens, and isolated vitrinites at the limit between subbituminous and bituminous coal

Abstract

A sample set consisting of coal, overlying and intercalating siliciclastic rocks and kerogen concentrated from these rocks of nine Pennsylvanian subbituminous to bituminous coal seam intervals from the German Ruhr Basin was analysed in order to detect chemical structural differences of the organic matter. Vitrinites from the coal seams and the kerogen concentrates were isolated to study effects of maceral group composition on chemical properties of typical type III kerogen. Rock-Eval Tmax values of sediment samples are higher than both those of the kerogen concentrated from them and associated coal samples. This is a result of the mineral matrix effect. Higher OI values of the sediment and kerogen samples as compared to those of the bulk coal samples cannot be explained by the abundance of carbonyl/carboxyl groups as reflected by results obtained by attenuated total reflectance (ATR) FT-IR spectroscopy. Ratios of the areas of the aromatic γCH out of plane, the aliphatic νCHx stretching, and the aromatic νC=C ring stretch regions imply a lower degree of aromaticity and at the same time higher degree of condensation of vitrinites originated from the sedimentary layers, respectively. Upon Curie Point (CP) pyrolysis at 590 °C, these vitrinite samples generate higher amounts of long-chained normal alkanes and n-alkenes than the bulk samples or the vitrinite originating from the coals. While reflectance values of vitrinites from coals and sedimentary layers are within the standard deviation of values expected for the narrow maturity window, only the vitrinites of the coal seams show some correlation with depth. Clearly, the relative abundance of phenols is decreasing with increasing maturity, as well as the ratio of benzenes over higher condensed aromatic moieties as revealed by their pyrolysis products generated at 764 °C.