Research on the Microscopic Occurrence State Characterization and Influencing Factors of Tight Oil: A Case Study of the Sand and Gravel Reservoir in the Hailar Basin

The pore structure of tight reservoirs exhibits significant heterogeneity. Understanding the microscopic distribution and occurrence characteristics of tight oil is crucial for optimizing tight oil resource development. This paper focuses on the Copper Bowl Temple Formation gravel reservoir in the Uersun Sag of the Hailar Basin. Through the use of ultra-thin sections and laser scanning confocal microscopy, the distribution and occurrence characteristics of tight oil components in the reservoir were revealed. By integrating thin-section analysis, SEM, and micro-CT, we systematically analyzed how pore networks influence tight oil occurrence. The results show that the gravel reservoir can be classified into two types: grain-supported and matrix-supported frameworks. The occurrence of tight oil in the reservoir is diverse, mainly manifesting as bound-state star-shaped, particle-adsorbed, semi-bound-state fissure-shaped, and free-state cluster-shaped forms. The pore network in the grain-supported framework reservoir is relatively uniform, with good pore connectivity. Intergranular pores are the primary space for oil and gas accumulation, where tight oil primarily occurs in the free-state cluster-shaped and particle-adsorbed forms. In contrast, the matrix-supported framework reservoir has uneven pore distribution, with numerous isolated pores and poor connectivity. In this type of reservoir, tight oil primarily occurs in the bound-state star-shaped and particle-adsorbed forms, with a small amount occurring in the semi-bound-state fissure-shaped form, restricting the development of tight oil.Cluster-shaped oil in intergranular pores exhibits the highest mobility, spanning the broadest pore size distribution (0.33–7.29 μm), followed by the fissure-shaped and particle-adsorbed forms. Star-shaped oil predominantly occurs in isolated pores with the narrowest pore size range (0.28–4.37 μm).