Anomalous distribution of steranes in deep lacustrine facies low maturity-maturity source rocks and oil of Funing formation in Subei Basin

Test analyses such as pyrolysis, soluble organic matter extraction, group component separation and GC-MS were conducted to 22 source rock samples and 28 crude oil samples from deep lacustrine facies of Funing Formation in Subei Basin. Source rocks stayed at the low maturity-mature stage (Ro:0.58%-0.71%), while crude oil stayed at the mature stage (Rc:0.71%-0.88%). The Pr/Ph values of samples in the research area ranged between 0.16 and 0.62. These samples could be classified as Sterane/Hopane > 1 and Sterane/Hopane < 1. For all the samples, the C-29 Steranes content was high; had inverse L distribution in alpha alpha alpha 20RC(27)-alpha alpha alpha 20RC(28)-alpha alpha alpha 20RC(29) regular sterane; alpha alpha 20R-Sterane played a dominant role; abundances of beta beta- and 20S-Sterane were low. As for samples of Sterane/Hopane> 1, G/C30H ranged between 0.63 and 2.56, and the Sterane isomerization was very low. As for samples with Sterane/Hopane < 1, G/C30H ranged between 0.04 and 0.46 and the Sterane isomerization was higher than the former. Abnormal distribution of Sterane isomerization was rarely influenced by thermal dynamic effects and sources, but was mainly influenced by the sedimentary environment. A lot of references reported Sterane isomerization with abnormally high abundance under the high-salinity environment. However, the finding obtained by the research that the higher water salinity corresponded to the lower degree of Sterane isomerization was discovered for the first time. The C-29 Sterane abundance was high and the C-29/C-27 regular sterane ratio was constant and would not vary with changes of environmental parameters and biological source parameters. Sterane content was not correlated with tricyclic terpene of algae sources, but was positively correlated with ETR of the aquatic organism source, while it had very good positive correlation with Gammacerane. In addition, samples with the high Sterane content had high abundance in Carotene, C-24+alkyl-cyclohexane and C-21+isoprenoid alkanes. Through profound analysis and reference survey, it is found that the abnormally high abundance of C-29 Sterane of samples in the research area may be correlated with halophilic protozoon in salinized deepwater lakes.