Heterogeneity and its genesis of tight sandstone reservoir: a case study of Chang 6 reservoir in Dalugou area of Jing’an Oilfield

In order to clarify the heterogeneity characteristics and their genesis of Chang 6 tight sandstone reservoir in the Dalugou area of Jing’an oilfield, the macroscopic heterogeneity and microscopic pore structure fractal characteristics of the reservoir in the target area are analyzed by using data from casting thin sections, scanning electron microscope, physical property tests, mercury injection experiment, drilling and logging, and so on. The causes of reservoir heterogeneity are discussed. The results show that the Chang 6 reservoir is mainly composed of feldspathic sandstone. Based on the parameters from mercury injection experiments, the pore structure can be divided into three types: medium pore, medium-to-fine throat (type I); medium-to-small pore, fine throat (type II)； small pore, microfine throat (type III). The macroscopic heterogeneity of Chang 6 reservoir is investigated from three aspects: intra-layer, inter-layer and planar heterogeneity. The evaluation of permeability heterogeneity coefficient reveals strong heterogeneity across all sub-sections of Chang 6 reservoir, with increasing heterogeneity from Chang 6₁ to Chang 6₃. The study of reservoir microscopic heterogeneity based on fractal theory shows that the fractal curve of Chang 6 reservoir samples has two turning points, resulting in three fractal intervals corresponding to macropores (<italic>D</italic>₁), mesopores (<italic>D</italic>₂) and micropores (<italic>D</italic>₃). The average fractal dimensions for <italic>D</italic>₁, <italic>D</italic>₂ and <italic>D</italic>₃ are 2.989 2, 2.693 1 and 2.177 2 respectively. Macropores and mesopores are the primary contributors to permeability and microscopic pore structure heterogeneity in Chang 6 reservoir. The heterogeneity of macropores and micropores has more obvious influence on the maximum mercury saturation. The analysis of heterogeneity causes of Chang 6 reservoir show that sedimentary microfacies control the macroscopic heterogeneity factors such as sand body and physical properties. Compaction greatly reduces the primary intergranular pores of the reservoir, which has great impact on the heterogeneity of macropores and micropores. Chlorite cementation and laumontite dissolution have a strong effect on the transformation of pore structure and the heterogeneity of reservoir pore structure. These findings provide valuable insights for the efficient development of the Chang 6 reservoir in Dalugou area.