Analysis of the Influence of Micro-Pore Structure on Oil Occurrence Using Nano-CT Scanning and Nuclear Magnetic Resonance Technology: An Example from Chang 8 Tight Sandstone Reservoir, Jiyuan, Ordos Basin

The micro-pore structure of a tight sandstone reservoir remarkably impacts the occurrence characteristics of the tight oil. The micro-pore structure of the Jiyuan Chang 8 tight sandstone reservoir in the Ordos Basin was examined in this research using a core physical property test, an environmental scanning electron microscope, thin section identification, and high-pressure mercury intrusion. Using nano-CT scanning and nuclear magnetic resonance technologies, representative core samples were chosen for studies evaluating the tight oil occurrence statically and dynamically. The micro-pore structure effect of a tight sandstone reservoir on the occurrence of tight oil was investigated, and the occurrence of tight oil in the reservoir forming process was discussed. It was significant to the study of tight oil in the reservoir forming process in Ordos Basin. Findings indicated that the Chang 8 reservoir in Jiyuan, Ordos Basin has poor physical properties and exhibits a high degree of heterogeneity. In addition, the oil charging simulation experiment (oil charging) can be separated into the following three stages: fast growth, gradual growth, and stability. In the process of crude oil charging, oil always preferentially entered into medium pores and large pores. These pores were the primary areas of tight oil distribution. Furthermore, the occurrence of tight oil was affected by pore type, pore structure parameters, throat parameters, and combination mode of pore and throat. First, substantially large and medium pores lead to effective pore connectivity and generate a considerable amount of tight oil. The occurrence morphology includes oil film, cluster, porous, and isolated. Second, the greater the degree of intergranular pore growth and soluble feldspar pore development, the thicker the throat, the more developed the effective throat, and the greater the quantity of tight oil. Finally, oil saturation was negatively correlated with median pressure and displacement pressure and positively correlated with sorting factors, median radius, maximum pore throat radius, and efficiency of inverted mercury.