Abstract: High-quality reservoirs with high-yield oil flow are still developed in the deep saline lake basin (burial depth>4500 m) in Zahaquan area, Qaidam Basin. But there are significant differences in the physical properties of clastic rock reservoirs at similar burial depths. In order to clarify the main controlling factors for reservoir physical properties and the lower limit of effective reservoir thickness in saline lake basins, Upper Ganchaigou Formation in Zhahaquan area is studied in this paper. By comprehensively using experimental methods such as core observation, cast thin section identification, rock physical properties test, and mercury injection test, comparative analysis is conducted on reservoir performance of sand bodies with various sedimentary microfacies. The study results show that sand bodies of shallow shore lake beach bar microfacies in Zhahaquan area are subdivided into three types, i.e., bar main sand, bar edge sand, and beach sand. The reservoir space is dominated by primary pores, and the bar main sand has the best reservoir performance, followed by bar edge sand and beach sand. The compaction and cementation were the main diagenetic processes that caused pore loss in reservoirs. The compaction degree of reservoirs of the same microfacies was equivalent, while the difference in cementation strength generally led to the variation in physical properties of beach bar sand reservoirs with various thicknesses and the heterogeneity of single beach bar sand reservoir. The high-quality beach bar sand reservoirs in deep formations were jointly controlled by the original sedimentary hydrodynamic conditions, early cementation, early oil and gas charging, and early slow and late rapid burial history. After clarifying the lower limit of conventional effective reservoir porosity, which is 8%, it is predicted that the lower limit of effective thickness for silty to fine-grained beach bar sand reservoirs is 1 m, and that for medium- to finegrained beach bar sand reservoirs is 0.5 m.

Key words: Qaidam Basin, saline lake, Upper Ganchaigou Formation, beach bar sand, reservoir characteristics, diagenesis, controlling factor, lower limit of effective thickness