Organic petrological and geochemical properties of jet from the middle Triassic Mogila Formation, West Bulgaria

Abstract The paper presents the results of the petrographic and organic geochemical studies of a jet sample recovered from a Mid-Triassic carbonate succession from the West Balkan tectonic zone in Bulgaria. Total organic carbon contents (TOC=92 % daf) and high vitrinite reflectance (Ro=1.9 %) indicate semi-anthracite coalification rank. Very high Tmax (577 °C) and low HI (~10 mg HC/g TOC) further support the overmature organic matter. Extractable organic matter is characterized by high portions of NSO compounds and asphaltenes (>75 %). Hydrocarbons constitute about 20 % and are characterized by the predominance of the saturated hydrocarbons over the aromatics. n-Alkanes distribution, dominated by short-chain compounds (n-C17–18), is consistent with the woody origin of the jet and the thermal maturity of the organic matter. The predominance of PAHs with condensed structure over their alkylated isomers is considered to be a result of the complex reaction occurring within the organic matrix during the catagenesis, rather than to the presence of combustion-derived organic matter. Based on the distribution of the diterpenoids, a tentative identification of a possible Voltziales conifer family source is identified. Low Pr/Ph ratio (0.88) and aryl isoprenoids outline anoxic conditions of jet formation, whereas the presence of organic sulfur compounds and tri-MTTchroman suggest marine depositional environment with normal salinity.