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Ternary Diagram of Fluorenes, Dibenzothiophenes and Dibenzofurans: Indicating Depositional Environment of Crude Oil Source Rocks

Fluorene (FL), dibenzothiophene (DBT) and dibenzofuran (DBF) and their alkylated homologues are important polycyclic aromatic compounds in crude oils and source rock extracts. They share the similar chemical structure and may have genetic relationships. The parent compounds (unsubstituted), methyl-, dimethyl- and ethyl-substituted isomers are identified by comparison with retention indices reported in literatures and correlation with co-injection of internal standards in gas chromatography-mass spectrometry (GC-MS) analysis. Therefore, their relative concentrations can be determined. Based on the large data sets, this paper preliminarily establishes a ternary diagram to interpret the depositional environment/lithology for oils and related source rocks. Five typical depositional environments/lithologies are distinguished on this ternary diagram. Data points from typical marine carbonate, fluvial/deltaic/fresh lacustrine shale and swamp fall into the DBTs, FLs and DBFs end-members, respectively. Oils from marine shale have subequal contents of FLs and DBFs and lie between the zones of fluvial/deltaic/fresh lacustrine shale and swamp, but have a little higher abundance of DBTs. The oils from brackish/saline lacustrine have intermediate contents of DBTs with a wide variety and some degree overlap with that of marine shale. On the basis of the ternary diagrams, two petroleum systems in the cratonic region of the Tarim basis are defined. The majority of discovered oils were derived from the Middle-Upper Ordovician carbonate source rocks. Whereas, some oils from wells TD2, T904, LK1 and TZ62 originated from the Cambrian and Lower Ordovician source rocks. Therefore, the ternary diagram of FLs, DBTs and DBFs is a useful method to interpret depositional environment/lithology for oils and related source rocks and to definite petroleum systems as well.