LAUSR

Abstract The paper presents the results of the organic geochemical studies of Bulgarian jet samples of early Jurassic and early Cretaceous age from two deposits located within the Moesian platform and the Balkan tectonic zone. Total organic carbon contents (71 and 78% daf) indicate sub-bituminous coalification rank. Moderately high to high sulfur contents (0.8–1.7% daf) support activity of sulfate reducing bacteria. High amounts of volatile matter (53–59% daf) and slightly enhanced HI values (185–249 mg HC/g TOC) argue for organic matter bituminization. The latter is also confirmed by the very low T max values (~ 400 °C), which are interpreted as a result of the release of bituminous substances during the early stages of pyrolysis. Extractable organic matter is consistent with the low maturity and is characterized by high portions of NSO compounds and asphaltenes (> 80%). Hydrocarbons constitute about 10% and are characterized by the predominance of the saturated over the aromatics. The strongly short-chain homologs dominated n -alkanes distributions, with expressed maxima at n -C 17 and n -C 18 , and CPI in the range 1–2, is consistent with the woody origin of the jet. In addition, the low C 29 /C 27 (~ 1) sterane ratios denote possible impregnation of the drift woods with phytoplankton-derived lipids from the host rocks. Low Pr/Ph (0.42–0.65) and Pr/ n -C 17 (0.28–0.43) ratios, as well as high Ph/ n -C 18 (0.4–0.78) ratio outline anoxic conditions of jet formation. Furthermore, the absence of chromans in the jet extracts from the Moesian platform suggests depositional environment with reduced salinity, whereas low di-MTTC/tri-MTTS (0.32) in the sample from the Balkan tectonic zone points to normal marine conditions. The occurrence of pimaranes and α-phyllocladane in the extracts from Nikolaevo jet points to Taxodiaceae or Araucariaceae conifers as possible precursors, whereas the presence of α-cedrane and cuparene in the sample from Lesidren deposit argues for Cupressaceae origin. In addition, triterpenoid biomarkers with lupane and ursane skeletal structures in the Lesidren jet is tentatively interpreted as an impregnation of the conifer drift wood with lipids from pre-angiosperm plants. Results from the bulk and molecular analyses revealed that Bulgarian jet originates from conifer drift wood, deposited under anoxic environmental settings, and subsequently subjected to hydrogenation and biodegradation.