Identification of organic matter in lignite samples from basins in the Czech Republic: Geochemical and petrographic properties in relation to lithotype

► Five Czech Tertiary deposits studied using coal organic geochemical methods. ► Four lithotypes: xylite rich, matrix, liptobioliths, and sapropelic coals characterized. ► The n-alkane distributions vary according to coal lithotype. ► The majority of the xylite rich, matrix and sapropelic coals originate from mixed forests. ► Particular composition of liptobiolith 2 from the Sokolov basin relate to density separation in water. Five Tertiary deposits in the Czech Republic were studied using coal petrological and organic geochemical methods. Coal samples were chosen to cover four lithotypes: xylite rich coal, matrix coal, liptobiolith, and sapropelic coal. Xylite rich and matrix coals are characterized by higher ulminite and densinite contents, and lower textinite and resinite contents. Sapropelic coals from the North Bohemian and Žitava basins are formed by large amount of ulminite and densinite, and a high proportion of liptodetrinite, sporinite and alginite that prevails in sapropelic shale from the Uhelná relic. The essential difference between sapropelic coal and liptobiolith coal consists of high contents of liptodetrinite, bituminite, resinite, inertinite, and lower contents of clay minerals in liptobioliths. The biomarker composition of the aliphatic fraction includes n-alkanes, and their distribution varied according to coal lithotype: long n-alkanes were dominant in liptobioliths, sapropelic coals and shale, the mid-chain n-alkanes prevailed in xylite rich coals, and the short-chain n-alkanes were abundant in matrix coals. The prevalence of diterpenoids represented by phyllocladane type compounds indicated the presence of gymnosperm vegetation. Molecular indicators for angiosperms, present mainly as oleanenes, were also recognized in the lignite samples. Hopanoids were represented mainly by 17α(H),21β(H)-homohopane and hop-17(21)-ene. The biomarker fingerprints and petrographic based indices show that the majority of the xylite rich, matrix and sapropelic coals and shale originated from mixed forests, formed by arborescent and herbaceous communities, aquatic plants and macrophytes, under wet and slightly dry and slightly oxic–anoxic conditions.