Organic petrography and biomarker proxies of subbituminous coal from the Dubrava field (Aleksinac Basin, Serbia)

A comprehensive petrographic and biomarker study has been performed to evaluate sources, paleoenvironment and maturity of organic matter (OM) in coal samples taken from the borehole BD-4 in the Dubrava block (Aleksinac Basin, Serbia). Studied samples are dominated by vitrinite group macerals, with variable content of liptinite and very low amount of inertinite. The measured random vitrinite reflectance, Rock Eval data and biomarker assemblages confirmed subbituminous rank of coal. The OM is at the late diagenetic and early catagenetic stage. Contents of total organic carbon and mineral matter vary in wide range, indicating frequent changes of depositional settings. Content of sulphur is moderate to very high. The main sources of OM in Aleksinac coal were angiosperms, ferns and aquatic macrophytes, with subordinate contribution of conifer paleovegetation. Diagenetic alteration of OM was governed by methanotrophic bacteria and chemoautotrophic bacteria. Deposition of OM occurred in a low-lying mire environment, showing decreasing concentration of sulphate ions upward the borehole BD-4. The coal deposition was terminated by rise of water level, which led to drowning of the mire and transition to lacustrine environment.