Relationship between petrographic and geochemical characterisation of selected South African coals

Abstract Fifteen samples of South African coals from the Waterberg, Soutpansberg (Northern Province) and Main Karoo (Mpulanga and KwaZulu-Natal) provinces were investigated by petrographic and geochemical methods. The aim of this work was to establish the relationship between these two methods for evaluation of coal maturity in the case of South African coals. The petrographic analyses included maceral and microlithotype evaluation by routine quantitative methods as well as random reflectance measurements. Maceral analysis included identification of vitrinite, liptinite and inertinite groups. The same samples were used for microlithotype analysis. Vitrite, liptite, inertite, clarite, durite,vitroinertite, duroclarite, clarodurite and carbominerite were identified. To assess the facies trends, liptinite-poor and liptinite-rich durite and clarodurite were counted separately. Sporoclarite and cutinodetrite were also treated independently. Both maceral and microlithotype analyses were performed to confirm the geochemical and petrographic compatibility in interpretation of the primary organic material which was deposited in the biochemical stage of coal formation. The reflectance measurements were carried out in incident light using a Zeiss Universal microscope. The 546 nm interference filter was applied. Distributions of aliphatic and aromatic hydrocarbons present in coal extracts were analysed by gas chromatography-mass spectrometry (GC-MS) for assessment of both source/environment of deposition and coal thermal maturity. Normal alkane distribution shows odd-over-even carbon number predominance in the case of coals in the range of 0.63–0.80% vitrinite reflectance. Several groups of compounds related to conifer resin were identified indicating predominant source of organic matter of the coals, for example tetracyclic diterpanes, retene and simonellite. The coal rank was assessed by 13 thermal maturity parameters based on isomerisation reactions of sterane and triterpane diastereomers together with ratios and indices of aromatic hydrocarbons and methyldibenzothiophenes based on different thermal resistance of their α- and β-alkyl isomers. The correlation of reflectance ( R rv %) with two sterane diasteromer parameters: C 29 αββ/(ααα+αββ), C 29 ααα20S/(20S+20R) and aromatic hydrocarbon parameters of thermal maturity such as: MDR (Methydibenzothiophene ratio), MPI-1 (MethylPhenanthrene Index-1) revealed good agreement. Lower correlation coefficients were obtained for TMR-2 (Trimethylnaphthalene ratio), TeNR (Tetramethylnaphthalene ratio) and DMPR (Dimethylphenenthrene ratio) with respect to R rv %. Other correlations of biomarker and aromatic hydrocarbon parameters with R rv % were poor.