Biomarker characteristics of hydrocarbon source rocks in saline lacustrine basin: a case study of Paleogene-Neogene strata in the Western Qaidam Basin, western China

摘要

During the Paleogene-Neogene, the Qaidam Basin was a saline lacustrine basin so the biomarkers of hydrocarbon source rocks exhibited unique characteristics under the salinization conditions. Saturated hydrocarbons extracted from a large number of source-rock samples were analyzed using gas chromatography and mass spectrometry. The results showed that n-alkanes in the source rocks were evenly distributed between even and odd carbon numbers, isoprenoid alkenes were predominantly phytanes, and ratios of pristane to phytane (Pr/Ph) were low, mostly in the range from 0.2 to 0.8. Terpanes typically showed high contents of C-35 homohopane and gammacerane. Overall, the ratios of gammacerane to C-30 hopane (gammacerane/C-30 hopane) and C-35-C-34 homohopane (C-35/C-34 homohopane) exhibited a positive correlation. Differing concentrations of oleanane were detected. The relative content of C-27 and C-29 steranes in the hydrocarbon source rocks varied from 25 to 75%, and the relative content of C-28 sterane in a considerable number of samples was higher than 30%. C-27-C-29 steranes generally showed an asymmetric V-shaped distribution, and the distribution of sterane/hopane and Pr/Ph ratios showed negative correlations. The salinity and reducibility indexes of the southern area of the Western Qaidam Basin were significantly higher than those of the northern area of the basin during the Oligocene-Miocene. Source rocks in the northern area were formed in a low-salinity and weakly reductive environment with type II2-III kerogen, whereas the hydrocarbon source rocks in the southern area formed in a high salinity and strongly reductive environment. The sedimentary water salinity and reducibility of the different areas were reduced in the following order: Hongliuquan, Yuejin, Shizigou, and Wunan-Lvcaotan. The best source rocks in this basin were developed in the Hongliuquan, Yuejin, and Shizigou areas with type II1-II2 kerogen. This study lays a foundation for further understanding the geochemical distribution of the hydrocarbon source rocks and crude oil in addition to the characteristics of the oil source in Paleogene and Neogene.