Pore Structure Characteristics and Comparisons of Upper Permian Longtan and Dalong Formation Transitional Facies Shale in Xiangzhong-Lianyuan Depression

Taking transitional facies shale of Permian Longtan and Dalong Formations as the research object, 12 typical drilling core shale samples in Xiangzhong-Lianyuan depression were selected for tests including organic carbon content, rock-eval, X-ray diffraction, density porosity, high-pressure mercury intrusion, CO₂/N₂ adsorption while using field emission scanning electron microscopy(FE-SEM) to observe pore characteristics. In this study it focuses on the development of nano-microscale pore, pore structure characteristics and main control factors of the transitional facies shale through the combinations of qualitative description and quantitative measure method. The results show that the organic carbon content of Longtan and Dalong Formation shales is high, the degree of thermal evolution is in the early stage of generating condensate oil and wet gas, corresponding to R_o of 1.22%-1.43%; pore types of shale are mainly intergranular pores, intragranular pores, organic pores and micro-nanoscale cracks. There are differences in the pore morphology, pore size and control factors between Longtan and Dalong Formations. In Longtan Formation, N₂ adsorption hysteresis loop is wide and the shapes of organic poresare mostly round and elliptic with larger pore diameter. The N₂ adsorption hysteresis loop of the Dalong Formation is narrow and the shapes of the organic poresare irregular with small pore diameter. There is a positive correlation between TOC and clay mineral in both Longtan and Dalong Formation shales. The volume of micropore in both Longtan and Dalong Formation shales is positively correlated with TOC and clay mineral. The mesopore+macropore pore volume of Longtan Formation shale has a position linear relationship with TOC and clay mineral, but is negatively correlated with quartz+feldspar. Carbonate mineral in Dalong Formation has a significant influence on its pore structure, and the relationship of mesopore+macropore pore volume with TOC-clay mineral and quartz+feldspar in Dalong Formation shale is not obvious.