Organic geochemical characteristics and organic matter accumulation of the Eocene lacustrine source rock in the Yingxi area, Western Qaidam Basin, China

The lacustrine source rocks of the upper member of the lower Ganchaigou Formation ( E_3^2 ) in the Yingxi area are considered to be important source rocks in the Qaidam Basin in northwestern China. In this study, we investigated the organic geochemical characteristics of the source rocks by selecting representative samples of the Eocene lacustrine succession spanning 200 m. The results show that the source rock quality is at a fair to good level, and the organic matter (OM) is in the immature–low-maturity stage. The OM originated from plankton/algae and bacteria, with a fluctuating input of terrigenous plants. The source rocks were deposited in a reducing saline-hypersaline environment, with water-column stratification associated with a variable chemocline related to a fluctuating salinity. The vertical variability of the OM abundance is accompanied by simultaneous changes in the source of the OM and the lake water environment parameters, which is caused by the oscillations between drier and wetter conditions in inland Asia in the Late Eocene. The drier periods had strong evaporation and low fluvial input, which facilitated the formation of widespread water anoxia and enhanced OM preservation, while the wetter periods had higher fluvial input and moderate OM preservation conditions because the higher sedimentation rate led to a strong terrestrial dilution effect, which in turn limited the accumulation of OM.