Abstract:  In order to clarify the oil and gas exploration prospects in the middle and deep layers of Nanbaxian area in the northern margin of Qaidam basin and predict the distribution area of favorable reservoirs, this paper takes the Upper Member of Xiaganchaigou Formation in the Late Oligocene of Paleogene as the research object. Based on the comprehensive analysis of detailed core observation and drilling (logging) data, combined with the comprehensive analysis results of casting thin sections, clay mineral assemblages, chloride ion content of mudstone cuttings, porosity and permeability variation characteristics, the sedimentary environment of the Upper Member of Xiaganchaigou Formation in this area during the sedimentary period, the characteristics of clastic reservoir physical properties and their controlling factors were studied deeply. The results show that: (1) The sedimentary period of the Upper Member of the Xiaganchaigou Formation in the Nanbaxian area is the sedimentary environment of the braided river delta front. The lake water is mainly fresh-brackish, and the reservoir is mainly lithic feldspar sandstone and feldspar lithic sandstone. The compositional maturity of sandstone varies greatly and the structural maturity is low. The climate is dry and warm, and the clay mineral assemblage is dominated by illite and illite-montmorillonite mixed layer. The content of kaolinite and chlorite is low, and montmorillonite is absent. (2) The favorable reservoir of the Upper Member of Xiaganchaigou Formation in Nanbaxian area is the underwater distributary channel sand body and the mouth bar sand body of the braided river delta front in the Ⅳ-1-Ⅳ-4 sand group, with an average porosity of 16.3% and an average permeability of 37.4 × 10-3μm2. It is a medium porosity and low permeability reservoir and a medium porosity and medium permeability reservoir. The primary intergranular pore is the main pore type. Among them, the physical properties of the sand body of theⅣ-1-Ⅳ-4 sand group are better than those of the Ⅳ-5 sand group, and the physical properties of the underwater distributary channel sand body are better than those of the mouth bar sand body. (3) The controlling factors of reservoir physical properties in the Upper Member of Xiaganchaigou Formation in Nanbaxian area are mainly sedimentary facies and cementation, followed by compaction and dissolution. The high content of secondary calcite cement is the main reason for the poor reservoir physical properties of Ⅳ-5 sand group.

Key words: sedimentary environment, reservoir physical properties, oil gas, control factors, Upper Member of Xiaganchaigou Formation, Nanbaxian area, Qaidam basin