Evaluation of the matrix influence on the microscopic pore-throat structures of deep-water tight sandstone: A case study from the Upper Triassic Chang 6 oil group of the Yanchang Formation in the Huaqing area, Ordos Basin, China

Former studies have suggested that the matrix of clastic rocks is unfavorable for the storage-percolation of reservoirs. However, the contribution of the matrix to the microscopic pore-throat structures in deep-water tight sandstone cannot be ignored. Aiming at the deep-water tight sandstone of the Chang 6 reservoir in the Ordos Basin (China), we have evaluated the characteristics of the matrix and the secondary pores in the matrix based on a multiscale microscopic identification and testing method, to reveal the influence of the matrix on the types, distribution, and heterogeneity of the pore throats. The results show that, unlike cements, the composition of the matrix is complex, characterized by its poor crystal form with no cement generation relationship. The structure of the matrix components is not completely dense. Intercrystalline pores and dissolved matrix pores are developed in the matrix after diagenetic modification, with a pore diameter of 20–1000 nm. These pores form a complex matrix secondary pore network. The matrix controls the number and volume of 0–1 μm pore throats. The matrix is constructive to the distribution of pore throats when its content is ≤7%. This positive effect gradually decreases with the increase of the matrix content, intensifying the compaction of the reservoir. The matrix controls the heterogeneities of the pore throat structures in deep-water tight sandstone. Large pore throats are gradually separated and disintegrated into a large number of micro-/nanopores by the matrix with the increase of the matrix content. Meanwhile, the fractal dimensions approached 3, increasing the complexity of the pore structures. Therefore, the matrix is favorable and unfavorable to the microscopic pore throat structures of the reservoir. The matrix not only results in the loss of intergranular pores but also generates a large number of secondary micro-/nanopore throat networks with complex structures, constituting an effective space for hydrocarbon accumulation and percolation in deep-water tight sandstone.