ZHANG Quanpei¹, WU Wenrui², LIU Liping³, YANG Hongmei⁴, WANG Lianguo⁵, XIE Yuhang⁶, ZHU Yushuang¹

1. State Key Laboratory of Continental Dynamics/Department of Geology, Northwest University, Xi’an 710069, Shaanxi Province, China;

2. No. 5 Oil Production Plant, Changqing Oilfield Company, PetroChina, Xi’an 710200, Shaanxi Province, China;

3. No. 7 Oil Production Plant, Changqing Oilfield Company, PetroChina, Xi’an 710200, Shaanxi Province, China;

4. No. 2 Oil Production Plant, Changqing Oilfield Company, PetroChina, Qingyang 745100, Gansu Province, China;

5. No. 11 Oil Production Plant, Changqing Oilfield Company, PetroChina, Qingyang 745000, Gansu Province, China;

6. Research Institute of Huabei Oilfield Branch, PetroChina, Renqiu 062550, Hebei Province, China

Abstract: Seven ultra-low permeability core samples of Yanchang Formation in Zhenbei area, Ordos Basin are selected to carry out casting thin section, scanning electron microscope, X-ray diffraction, high-pressure mercury intrusion, and nuclear magnetic resonance (NMR) experiments, respectively. The characteristics of pore throat distribution, pore type, mineral composition and contents of ultra-low permeability reservoirs are clarified. The fractal theory is used to study the fractal characteristics of reservoir pore throat, and the relationships between the fractal dimension and the reservoir physical properties, pore structure parameters as well as mineral composition and its content are systematically analyzed. The results show that the reservoir pore types are mainly residual intergranular pores, dissolution pores, and intercrystalline pores. The mineral composition of the reservoir is dominated by quartz and feldspar, and the content of chlorite in clay mineral is the highest. According to the morphology of capillary pressure curve and displacement pressure, the reservoir pore structure can be divided into three types: Type I, Type II, and Type III. The corresponding reservoir performance and seepage capacity become worse in turn, and the heterogeneity of pore structure gradually increases. The pore throat size distribution obtained by high-pressure mercury intrusion has multi-fractal features. There are two obvious turning points in the fractal feature curve, which divides the pore space into macro-pores, meso-pores and micro-pores. Compared with macro-pores and meso-pores, the pore size distribution of micro-pores is relatively uniform and regular, and the corresponding fractal dimension is the lowest. NMR can provide a good characterization of the reservoir pore space. The pore throat distribution of the T₂ < T_2cutoff segment does not have fractal features. The movable fluid pore space and effective pore of the T₂ > T_2cutoffsegment conform to the fractal structure, and the corresponding fractal dimension reflects the complexity of interconnected pores. The fractal dimension of reservoir pore throat has a good negative correlation with porosity and permeability as well as pore structure parameters. The mineral composition and contents of reservoir are the internal factors that determine the size of fractal dimension, and then affect the reservoir quality and pore structure characteristics.