The pore characteristics and fractal characteristics of Permian marine-continental transitional shales in the Sichuan Basin, southwestern China

Currently, researchers at home and abroad mainly study the pore space of marine shale, so the research on the pore space of marine-transitional shale is still relatively small. Based on the research findings, marine-continental transitional shale exhibits wide distribution, significant resource potential, and promising exploration and development prospects. This paper addressed this gap by analyzing the geochemical characteristics, petrological features, and pore characteristics of shale in the Upper Permian Leping Formation of the Sichuan Basin. It has been obtained through experimental analysis that the shale's reservoir storage space primarily consisted of organic pores, providing significant potential for shale gas accumulation. The gas adsorption experiments demonstrated the development of micropores, which contributed substantially to the pore specific surface area and volume in Leping shales. Additionally, the TOC content and clay minerals in shale samples have an influence on the storage space of shale. The TOC content had a more significant correlation with micropore volume and specific surface area, and clay mineral content had a better correlation with pore volume and specific surface area of mesopores and macropores. Furthermore, the calculated fractal dimensions (D₁, D₂, and D₃) for the Leping Formation ranged from 2.3616 to 2.5832 (average: 2.4835), 2.7698 to 2.8654 (average: 2.8172), and 2.3274 to 2.5161 (average: 2.4287), respectively. These values indicate that the pores have a complexity and heterogeneous. D₁ and D₂ are mainly affected by clay minerals, indicating that the complexity of mesopores is mainly controlled by clay minerals; D₃ is mainly controlled by TOC content, indicating that the complexity of micropores is mainly controlled by TOC content.D₃ has a good positive correlation with pore characterization parameter, indicating that the pore will be more complex with the increase of micropores' percentage and its volume and surface area.The findings from this study provide valuable insights into the pore characteristics of marine-continental transitional shale and offer potential implications for exploration and development prospects in the area.