Abstract: Mixed shale oil reservoirs are characterized by diverse rock fabric and lithofacies types, significant differentiation in diagenetic evolution, strong heterogeneity in micro-pore structure, complex genetic mechanism of micro-pore and fracture system and effective storage and permeability space. Taking the mixed shale oil reservoir of Lucaogou Formation in Jimsal Depression as the research object, the diagenetic types and diagenetic facies types of the shale layer of Lucaogou Formation were defined and the differential rock formation and reservoir formation mechanism of the mixed shale oil reservoir was revealed by comprehensive analysis and testing methods such as cast thin section, field emission scanning electron microscopy, X-ray diffraction, high pressure mercury injection and nitrogen adsorption. The results show that the diagenesis types of mixed shale oil reservoirs in Lusaogou Formation are diverse, including compaction, cementation and dissolution. According to the key diagenetic types and characteristic fabric, diagenetic facies can be divided into tuffe-feldspar dissolution phase, mixed cementation dissolution phase, chlorite film dissolution phase, carbonate junction phase and mixed cementation compact phase. The dissolution phase of tuffaceous feldspar is dominated by the dissolution pores of feldspar and tuffaceous, which are mainly in the range of 50-800 nm, and the total pore volume is the largest, which is the result of partial or complete dissolution of feldspar particles. The combination of solution pores and residual intergranular pores is developed in the mixed cementation phase, mainly in the range of 50-400 nm. It is the result of the superposition of carbonate, siliceous cementation and feldspar dissolution, and the total pore volume is the largest. The cement phase of chlorite film is dominated by residual intergranular pores with small pore size, and pores less than 50 nm are dominant. The heterogeneity is the weakest in all diagenetic phases, and the total pore volume is in the middle in all diagenetic phases, which is the result of corrosion and anti-compaction. The development of intergranular solution pores in carbonate cementation phase is dominated by pores in the range of 20-50 nm, which is the result of dissolution and carbonate cementation. All kinds of pores in the dense phase of mixed cementation are not developed, mainly in the range of less than 50 nm, which is the result of the comprehensive failure of compaction and cementation. Tuffaceous feldspar dissolution phase, mixed cementation dissolution phase and chlorite film phase are the dominant diagenetic facies types, while carbonate cementation phase and mixed cementation dense phase are not conducive to forming good reservoir conditions. This understanding is conducive to further understanding the differential formation process and mechanism of mixed shale oil formations. This finding helps deepen the understanding of the differentiated reservoir-forming processes and mechanisms of mixed shale oil layers, serving the precise prediction and efficient exploration and development of favorable shale oil production areas in the Jimusar Depression.

Key words: Jimsar sag of Junggar Basin, Lucaogou Formation, Mixed shale oil reservoir, Diagenetic facies, Microscopic heterogeneity;Storage mechanism