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C31- to C35-hop-17(21)-enes are identified by gas chromatography-mass spectrometry (GCMS) analysis to exist as double isomers in most samples of the Aershan Formation and members 1 and 2 of the Tenggeer Formation from well SH3.Comprehensive organic geochemistry and organic petrology study indicates that algae and bacteria are the main biological source of lower Cretaceous sediments in the Saihantala Sag,and this is in accordance with the existence of hop-17(21)-enes.The similar distributions of hop-17(21)-enes and hopanes of these samples indicate that hop-17(21)-enes were transformed into hopanes through hydrogenation during diagenesis processes.The existence of hop-17(21)-enes means that not only the formation of organic matter is related to an anoxic environment and a biological source of algae and bacteria,but also hop-17(21)-enes are direct indicators of hydrocarbon rock at an immature to low-maturity stage.High hydrocarbon conversion ratio,algae and bacteria source and a high abundance of organic matter suggest that the Saihantala Sag has the potential to generate immature to low-maturity oil,which may be of great significance for oil exploration in the Erlian Basin.