Abstract: The sedimentary characteristics of transitional facies in saline lacustrine basins (delta fronts and shallow lake subfacies) differ significantly from those of freshwater lacustrine basins, especially with regard to the distribution of skeletal sand body microfacies. Research into the control mechanisms of sedimentation over hydrocarbon reserves leads to important developments of sedimentological theories regarding lacustrine basins and practical guidance for oil and gas exploration and exploitation in terrestrial basin clastic rocks in China. Core, logging and cast thin section data are used to analyze the major controlling factors of sedimentation in the delta front-shallow lake and high-quality reservoir formation of the Lower (N₂¹) and Upper (N₂²) Youshashan Formations in the Yingdong region of the Qaidam Basin. The results show that, there are primarily two types of reservoir microfacies for the delta front-shallow lake regions in the saline lacustrine basins, namely underwater distributary channels and beach sandbars. They are considerably different in terms of their sedimentary structures, particle size distributions, microscopic structures, and superposition of logging curves. Sedimentary microfacies provide the primary material for high-quality reservoir formation. Meanwhile, microfacies and salinization of water bodies in lacustrine basins jointly control the distribution of high-quality reservoirs. In general, saline lacustrine basin clastic rock reservoirs are relatively immature. Primary pores and relatively weak corrosion were observed, while secondary pores were less developed. Intensive carbonate cementation in the early diagenetic stages enhances the compression resistance of rocks. The reservoir properties are primarily determined by the particle sizes and cement content of rocks.

Key words: saline lacustrine basin, delta front, shallow lake, sedimentary microfacies, reservoir characteristics, Yingdong Region