Petrographical and organic geochemical study of the Kovin lignite deposit, Serbia

Abstract

The origin of the organic matter (OM) and the characteristics of the depositional environment of lignites from the Upper Miocene Kovin deposit (hosting three coal seams) of Serbia were evaluated based on petrographic data, bulk organic geochemical parameters, biomarker analysis and stable isotope geochemistry (δ13C of individual biomarkers). Samples were collected from four boreholes, GD-601 and GD-603 (the “A” field), and KB-79 and KB-91 (the “B” field), representing different parts of coal seams I, II, and III.Investigated lignites are typical humic coals. The OM of lignites is derived from woody vegetation and herbaceous peat-forming plants, with prevalence of the former in most samples. Peat-forming vegetation is characterized by abundant decay resistant gymnosperm (coniferous) plants, followed by a low amount of angiosperms. Lignite forming plants mostly belonged to the gymnosperm families Cupressaceae, Taxodiaceae, Phyllocladaceae and Pinaceae. Slight input of ferns, fungi and aquatic macrophyta to lignite OM is also evident. Distributions of hopanoids and isotopic compositions of these biomarkers reflect the activity of various microbial populations represented by methanotrophic-, chemoautotrophic- and heterotrophic bacteria during diagenesis. Prominent C28 28,30-bisnorneohop-13(18)-ene was observed for the first time in the lignite extracts. According to the δ13C value it was probably derived from chemoautotrophic bacteria. C28 28,30-bisnorneohop-13(18)-ene could be one of the possible precursors of a series of orphan aromatic hopanoids bearing an ethyl group at C-21, via progressive aromatization.Peatification proceeded in a fresh water environment under variable redox conditions, from anoxic to slightly oxic. All three coal seams are heterogeneous. The lower parts of the coal seams represent a topogenous fresh water peat mire with open water areas, which was subjected to inundations. The upper parts of coal seams represent a wet forest swamp with relatively stable conditions. Despite the observed variations in all three coal seams, the mean values of petrographic and organic geochemical parameters suggest general increasing of wetness and establishment of more stable conditions from seam III to seam I. Maceral and biomarker data indicate that rapid flooding of the bogs stopped peat growth in all three coal seams.Relatively frequent variations in all three coal seams, followed by repetition of lignite characteristics depict well the pronounced seasonality e.g. short term cycles of the climate during Late Miocene in investigated area.