Unusual short chain lanostanes (C_(24) and C_(25)) and C_(30) lanostane were identified in sulfur rich crude oils from the Jinxian Sag, Bohai Bay Basin, northern China. Besides the regular steranes (C_(27-30)), a series of 4-methyl steranes (C_(22-23), C_(27-30)), 4,4-dimethyl steranes (C_(22-24), C_(28-30)), short chain steranes (C_(23-26)), abundant pregnanes (C_(21-22)) and androstanes (C_(19-20)), together with sulfur containing steroids (20-thienylpregnanes and thienylandrostanes) were detected in the aliphatic and branched-cyclic hydrocarbon fraction of these crude oils. A literature survey of some long chain sterane analogues (e.g., A-nor-steranes, norcholestanes, C_(30) steranes, lanostanes) and pregnanes seems to point to a sponge and/or dinoflagellate source. 4-Methyl, 4,4-dimethyl steroids and lanosterols (4,4,14-trimethyl steroids as the basic skeleton of lanostanes) can be derived from methanotrophic bacteria. Thus, a biological origin from a prokaryotic methylotroph can be used to explain the common source of abundant short chain steranes (C_(23-26)), 4-methyl (C_(22-23)) and 4,4-dimethyl steranes (C_(22-24)), as well as lanostanes (C_(24-25) and C_(30) analogues) in our oil samples. Generally, the steroids appear to have been extensively sulfurized with sulfur substitution at the C-22 position in the side chain during the early stage of diagenesis, which was readily subject to attack by bacterial degradation (enzymatic cleavage) and/or abiotic oxidation. As a consequence, short chain sterane analogues (e.g., abundant pregnanes and androstanes in this study) and short chain lanostanes (C_(24)-C_(25)) might later be released through cleavage of weak C-S bonds at the C-22 carbon in the sulfurized steroids and lanostane sulfides. Finally, the formation of the short chain C_(24)-C_(25) lanostanes and distinctive occurrence of short chain steranes in this study can be well explained by microbial biodegradation of sulfurized lanostanoids and steroids in the reservoir.
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