Pore structure characteristics and controlling factors of Upper Permian marine-continental transitional shales in the central South Yellow Sea Basin

Compared to the study of marine and lacustrine shales, the exploration and development and theoretical research of marine-continental transitional shales remain exploratory due to complex depositional environments, strong reservoir heterogeneity, and unclear pore evolution mechanisms. The Upper Permian Longtan Formation transitional shales in the central South Yellow Sea Basin was studied in depth. Through multi-scale characterization techniques—including field emission scanning electron microscopy, energy-dispersive spectroscopy, low-pressure gas adsorption, high-pressure mercury intrusion porosimetry, and fractal dimension analysis, we systematically revealed full-scale pore structure characteristics and elucidated pore development mechanisms. The Longtan shales are dominated by quartz (mean 52.3%) and clay minerals (mean 28.5%), with total organic carbon (TOC) averaging 3.66%. Organic matter is type Ⅲ (humic) and in a high-maturity stage. Pores are primarily intergranular and intragranular inorganic pores, with poorly developed organic pores and microfractures. The pore system exhibits a differentiated pattern: mesopores dominated (peak: 2～64 nm), micropore contribution increased significantly in high-TOC samples (peak: 0.45～1.0 nm), and macropores were underdeveloped. Strong pore heterogeneity (fractal dimensions D₁/D₂＞2.5) was controlled by coupled organic geochemistry and diagenesis. TOC promoted micropore development and complexity but inhibited mesopores. In contrast, clay mineral interlayer pores provided mesopore space and reduced the complexity. Meanwhile, brittle minerals (e.g., quartz) suppressed micropore/mesopore development via secondary cementation while enhanced the heterogeneity. This work further clarified pore structure characteristics of the transitional shales in the study area and provided a theoretical basis for sweet-spot prediction in analogous reservoirs within the Yangtze Platform.