Distribution and characterization of C-31 sterane from cretaceous sediments and oils, Songliao Basin, China

Relatively high abundance of C-31 steranes has been identified from cretace ous source rocks and crude oils in the Songliao Basin, China. C-31 steranes which occur as ten isomers were tentatively identified as 4, 22, 23, 23-te trametyhlcholestane based on their retention time and mass spectrum. They b ear a lower thermal stability and occur as important components in the imma ture oils of Chaochang region because they have three-methyl groups in thei r side chain. The relatively high abundance of C-31 steranes in the samples can indicate the source input of some special algae in the brackish and sa line environment. Therefore they can be used as very useful biomarkers for organic facies analysis and oil correlation. Their biological source perhap s related to the dinoflagellate such as G. foliaceum and P. foliaceum. The gorgostanol are considered as possible precursors for the putative C-31 4-m ethyl sterane which formed in the stage of diagenesis.