Changes in Molecular Biomarker and Bulk Carbon Skeletal Parameters of Vitrinite Concentrates as a Function of Rank

Abstract

A sequential extraction scheme to differentiate between molecular alkanes and those covalently-bound to the macromolecular structure has been applied to a suite of vitrinite concentrates handpicked from six UK bituminous coals covering the rank range % Ro = 0.47−1.32 (at 546 nm). The aim was to ascertain whether the biomarker maturity indices (i) were consistent with the measured vitrinite reflectance values and (ii) differ markedly for the easily extractable, clathrated, and covalently-bound phases. The quantitatively reliable single-pulse excitation (SPE) solid state 13C NMR technique has also been used to elucidate the changes in the bulk vitrinite structure across the relatively narrow rank range investigated. Although both hopane and methylphenanthrene parameters for easily extractable species are sensitive to small variations in vitrinite reflectance, significant variations have been found between the biomarker parameters for easily extractable, clathrated, and covalently-bound material. In general, the steric protection afforded to covalently-bound biomarker species by the macromolecular structure make the latter less mature, as reflected by the hopane, sterane, and methylphenanthrene parameters, than the easily extractable and clathrated molecular species. Solid state 13C NMR revealed that the variations in carbon aromaticity and the degree of condensation of the aromatic structure occurring in the % Ro range of ca. 0.45−0.80 are relatively small.