Comparison of Pore Size Distribution, Heterogeneity and Occurrence Characteristics of Movable Fluids of Tight Oil Reservoirs Formed in Different Sedimentary Environments: A Case Study of the Chang 7 Member of Ordos Basin, China

The complexity and diversity of reservoir characteristics, including strong heterogeneity, complex pore structures, distinct pore size distributions, and various fluid occurrences, in different environments lead to differential oil recovery from the Chang 7 reservoir. In this research, several experiments such as nuclear magnetic resonance, high-pressure mercury intrusion, scanning electron microscopy, thin section, and X-ray diffraction were utilized to analyze PSD, heterogeneity, and movable fluid occurrence in the Chang 7 sandstone samples from the Ansai and Heshui Blocks. A 3-stage construction method is proposed to transform the T-2 spectrum to pore-throat radius distribution to realize large-scale and continuous PSD characterization. The results indicate that the sandstone in the Heshui Block is mainly feldspar lithic fragment sandstone, while the sandstone in the Ansai Block is mainly lithic fragment feldspar sandstone and feldspar sandstone. The storage spaces in the Chang 7 reservoirs were comprised primarily of nanoscale pores, followed by submicron pores. The Chang 7 reservoirs in different environments differed greatly in PSD, fractal feature, and movable fluid occurrence. Sandstone in the Heshui Block, compared to that in the Ansai Block, had better physical properties generally characterized by more submicron pores, weak heterogeneity, and more movable fluid in submicron pores. The movable fluid occurrence could be affected by various factors such as physical properties, mineral composition, pore size and distribution, and heterogeneity. The influence of these factors on movable fluid distribution varied in reservoirs of different areas. Generally, reservoirs with good physical properties and higher availability of submicron pores are often associated with high movable fluid content. The influence of carbonatite on the movable fluid content in the two areas was opposite. The relationship between the movable fluid content in different pores and pore size parameters varied in different sedimentary environments. The influence of heterogeneity on the movable fluid occurrence in submicron pores was stronger than nanoscale pores in the Heshui Block, while it was the opposite in the Ansai Block.