Full-scale pore structure characterization and main controlling factors of marine–continental transitional shale and coal reservoirs in the Shanxi Formation, Ordos Basin, China

Pore structure heterogeneity between marine–continental transitional shale and coal reservoirs plays a crucial role in exploration and development of unconventional oil and gas resources. This study uses the Shan 2 3 sub-member in the Danning–Jixian area of the Ordos Basin as a case study, systematically characterizing and comparing the full-scale pore structures of shale and coal reservoirs. Using field emission scanning electron microscopy (FE-SEM), mercury intrusion porosimetry (MIP), N 2 and CO 2 adsorption experiments, along with total organic carbon (TOC) and x-ray diffraction (XRD) analyses, the study investigates the influence of organic matter and inorganic minerals on pore structures at different scales. The results show that the average TOC value of the shale is 4.69% and exhibit well-developed organic matter pores, inorganic pores, and microfractures, with organic matter pores being the most abundant and often densely and clustered. In contrast, the coal has a significantly higher average TOC value of 74.22%, with organic matter pores being the dominant pore type. The pore diameter in coal is also significantly larger than that in marine–continental transitional shale and marine shale. For shale, micropores, mesopores, and macropores all contribute to the total pore volume, with organic matter serving as the primary material foundation for micropore development. Meanwhile, clay mineral diagenesis plays an important role in promoting mesopore and macropore development. For coal, micropores and macropores are the main types, with organic matter being the most significant factor influencing pore development; A higher TOC content supports the formation of larger organic pores. Overall, this study provides a comprehensive look at the similarities and differences in the pore structures of marine–continental transitional shale and coal reservoirs at the micro scale, providing a scientific basis for the precise evaluation and development of unconventional oil and gas resources.