Pore Structure and Geochemical Characteristics of Marine Shale in the Xiaonan Area, Middle Yangtze Region: A Case Study of the Lower Triassic Daye Formation

This study investigates 30 core samples of the Lower Triassic Daye Formation shale from Well ZK-01 in the Xiaonan area, Middle Yangtze region. Comprehensive analyses were conducted using organic petrology, X-ray diffraction (XRD), argon ion polishing–field emission scanning electron microscopy (AIP-FESEM), gas adsorption, and high-pressure mercury intrusion porosimetry to characterize the microscopic pore structure of the under-investigated marine shale in the Yangtze Region, and herein controlling factors of its development. Results indicate that the organic matter in the study area is predominantly composed of marine amorphous sapropelinite, with solid bitumen commonly observed filling pores. The pores are mainly nanoscale organic pores ranging from 2 to 200 nm, predominantly exhibiting slit-like morphologies. The shale reservoir is generally characterized as a tight reservoir with low porosity and low permeability. The pore development characteristics are jointly controlled by total organic carbon (TOC) content, organic matter maturity, mineral composition, and sedimentary environment. The results demonstrate that organic matter pores (2–200 nm) are the dominant pore type in the studied shale, with pore volume and specific surface area positively correlated with TOC content (R2 = 0.72 and 0.68, respectively) and thermal maturity. The combined effects of moderate thermal maturity, favorable mineral composition (dominance of carbonate and quartz), and weak tectonic deformation have contributed to the preservation of nanoscale pores. These findings provide new insights into the pore systems of Mesozoic marine shales and their gas storage potential.