MOVABLE FLUID TRAITS AND ITS MAIN CONTROLLING FACTORS IN TIGHT SANDSTONE RESERVOIRS: TAKING THE CHANG 63 OF HUAQING AREA IN ORDOS BASIN, CHINA AS AN INSTANCE

The microscopic pore-throat structure of tight sandstone reservoirs is complex, and the characteristics of fluid occurrence and seepage characteristics in different specimens are obvious. It is the key and difficult point in the study of tight sandstone reservoirs, and it is also the key to restrict the development of tight sandstone reservoirs. Taking the Chang 6₃ tight sandstone reservoir in the Huaqing area of the Ordos Basin as an example, various advanced microscopic methods such as nuclear magnetic resonance, rate-controlled mercury intrusion, pressure-controlled mercury intrusion, scanning electron microscopy, and casting thin sections were combined with comparative analysis to analyze the reservoir qualities. The quantitative evaluation of the movable fluid and the the dertermination of main controlling factors have been conducted. The classification of microscopic pore-throat characteristic parameters of tight sandstone reservoirs is obvious. Combined with physical properties, mercury intrusion and nuclear magnetic resonance experiments, the reservoirs can be divided into four categories, and the classification evaluation criteria for tight sandstone reservoirs are established. The results show that the T₂ spectrum of tight sandstone reservoirs is complex, mainly characterized by bimodal structure, followed by multi-peak and single-peak structures, and the amplitudes of the left and right peaks are different. The medium-large pores have high propotion of bound fluid. These results show that the heterogeneity of microscopic pore-throat structure is the main controlling factor causing the difference of movable fluid in various specimens. Again, the composition, content, the heterogeneity and features of clay minerals are also important factors affecting the movable fluid traits.