Impacts of Pore‐Throat Combination Types and Pore‐Throat Structure on the Seepage Capacity in Tight Sandstone Reservoirs: A Case Study of the Yanchang Formation in the Southeastern Yishan Slope of Ordos Basin

ABSTRACT The physical properties of throats, including the size, shape and combination types with pores, are essential for hydrocarbon migration pathways within the tight sandstone reservoirs; therefore significantly affecting the seepage capacity of tight oil. In the Ordos Basin, the Chang 6 member of the Triassic Yanchang Formation developed typical tight sandstones, which underwent various complex diagenetic processes following sedimentation, resulting in a complex pore‐throat structure. To analyse the mechanism by which pore‐throat combination types control the production and development of tight oil, systematic experiments were conducted on the Chang 6 3 sublayer samples in the southeastern Yishan Slope. According to the results of cast thin section (CTS), scanning electron microscopy (SEM), we suggest that the main pore types are feldspar intergranular dissolution pores and feldspar intragranular dissolution pores. Primary throat types include sheet‐like, curved sheet‐like and clay‐mineral‐filled bundle‐like throats. The tight samples of the study area were classified into three typical types based on the results of high‐pressure mercury injection (HPMI) and constant rate mercury injection (CRMI). For all samples (Type I–III), mercury injection saturation of the throat exceeds the pore, indicating that the throat controls the reservoir quality. From Type I to III samples, the pore volume progressively decreases while the throat volume increases correspondingly. Notably, Type III samples exhibit the exclusive development of throat networks, with the complete absence of pores. Meanwhile, by applying fractal theory, the pore‐throat structures exhibit three‐stage fractal characteristics with inflexion point radius r t1 and r t2 : large pores (P L ), > r t2 , resembling bead‐string‐network structures; medium pores (P M ), between r t1 and r t2 , transitional between bead‐string‐network structures and capillary‐tree‐like structures; and small pores (P S ), < r t1 , similar to capillary‐tree‐like structures. Samples with smaller r t1 and larger r t2 have better‐connected pore‐throat networks, and higher P M proportions contribute to better reservoir quality. The bead‐string model has the greatest impact on permeability and mercury saturation, while P L and P M dominate porosity, P M controls permeability and P S (nanoscale, mercury‐impenetrable) pores barely impact permeability. Our results could provide theoretical guidance for improving the recovery rate of tight oil reservoirs.