Pore structure characteristics of deep Upper Paleozoic coal measure tight sandstone reservoirs in northeast Qinshui Basin and its reservoir geological significance

Reservoir pore structure is a key factor affecting oil and gas productivity. Accurately characterizing the microscopic pore structure of tight sandstone reservoirs is very important for oil and gas exploration and development. The deep coal measure tight sandstone of Upper Paleozoic in northeast Qinshui Basin is taken as the research object, the pore structure characteristics of the deep coal-bearing tight sandstone reservoir were analyzed by physical property testing, rock slice observation, scanning electron microscopy observation, and high-pressure mercury injection, combined with fractal geometry method, and the geological significance of the reservoir pore structure characteristics was discussed. The results showed that the tight sandstones of Taiyuan Formation and Shanxi Formation in the northeast of Qinshui Basin were mainly fine grained lithic quartz gritstone and fine grained quartz gritstone, the porosity ranged from 0.933% to 6.569%, the permeability ranged from 0.001 5×10⁻³ to 0.086 4×10⁻³ μm². The pore types were mainly intergranular dissolved pores, intra granular dissolved pores and intergranular pores, and the size was most large with transition pores, the pore connectivity was poor. The reservoirs were divided into three types, the type II reservoirs dominated by transition pores with a few micropores were the main types, and the type I reservoirs dominated by transition pores and followed by medium pores were the potential favorable exploration targets. The pore throat structure is complex and heterogeneous, and the fractal dimension was distributed from 2.457 3 to 2.709 6, and its value was controlled by the complexity of the main pore throat. The segmented fractal dimension was characterized by the low proportion of medium and large pores but the most complex, and the opposite of transition pores. The fractal dimension of pore is strongly correlated with the parameters of porosity, permeability and pore structure. The correlation is controlled by transition pores, and influenced by micropores, little effect by medium and large pores. Sandstone with large fractal dimension of transition pores is deposited in a reductive environment, and diagenesis has obvious influence on pore development in different stages.