Combining pore structure types and reservoir forming limits to determine the grading evaluation criteria of Chang 7 tight oil reservoirs in Jiyuan Area, Ordos Basin.

In view of the common problems of difficult in characterizing pore structure, weak research on reservoir forming limit, and ambiguous meaning of grading evaluation boundary in Chang 7 tight oil reservoirs of the Jiyuan area, Ordos Basin. The pore structure types and reservoir forming limits of tight sandstone were determined by high-pressure mercury injection test and other methods, and the grading criteria were established to evaluate the reservoir quality. The results showed that the Chang 7 tight oil reservoirs have poor petrophysical properties and four types of pore structures, which correspond to different reservoir types respectively. There are three types of the reservoir forming limits in tight sandstone reservoirs, namely the theoretical lower limit, lower limit of oil accumulation, and effective seepage lower limit, with the corresponding porosity limits of 3.5%, 5.5%, and 8.0%, respectively, and the corresponding permeability limits of 0.02 × 10-3 µm2, 0.05 × 10-3 µm2, and 0.20 × 10-3 µm2, respectively. These reservoir forming limits were in good agreement with the classification boundaries of pore structure. Combining the pore structure types with the reservoir forming limits, the Chang 7 tight oil reservoirs can be divided into four classes: I, II, III, and IV, representing easily movable tight reservoir, movable tight reservoir, oil-bearing tight reservoir, and ineffective tight reservoir, respectively. Among them, the classes I and II reservoirs were the sweet spot areas for increasing the reserves and production of tight oil. The method of combining various reservoir forming limits and pore structure types can effectively evaluate reservoir quality, and provide theoretical basis for increasing geological reserves and rolling expansion of oil fields. [ABSTRACT FROM AUTHOR]