Microscopic Characteristics and Controlling Factors of Pore Structures in Shale Reservoirs: A Case Study of the Chang 7 Member in the Longdong Area of the Ordos Basin

The Ordos Basin is renowned for its development of continental and transitional marine-continental shale formations, particularly within the Triassic Yanchang Formation's Chang 7 member, which functions as a primary stratum for shale oil reservoirs. These reservoirs are characterized by pronounced heterogeneity, posing substantial challenges to their exploitation. In order to facilitate the assessment and potential evaluation of these shale oil reservoirs during exploratory phases, the present study concentrates on the Lower Triassic Chang 7 shale located in the southwestern region of the Ordos Basin. Employing a diverse array of analytical techniquessuch as X-ray diffraction (XRD), total organic carbon (TOC) measurement, nitrogen adsorption (N₂), high-pressure mercury intrusion (HPMI), microscopic thin sections, cast thin sections, and argon ion polished scanning electron microscopythis research meticulously examines the reservoir spaces within silty shales. The lithological composition is predominantly composed of siliceous minerals, with the reservoir spaces largely consisting of mineral matrix pores that include both intergranular and intragranular porosity. The distribution of pore volume is primarily occupied by macropores and mesopores, while the structure of these pores is significantly influenced by the TOC content and the prevalence of siliceous minerals. The volumetric presence and specific surface area of macropores and mesopores, alongside pore radius, demonstrate a negative correlation with TOC levels and a robust positive correlation with felsic content, although they exhibit a pronounced negative correlation with clay minerals. Conversely, the correlation with carbonate rock minerals appears to be negligible. Collectively, the pore structure is influenced by the content of organic matter, felsic, and clay minerals, with felsic exerting the most substantial impact. The research results indicate that pore development in the Chang 7 Member shale reservoirs of the Longdong area is jointly controlled by mineral composition and the degree of OM evolution. This suggests that, in practical exploration, siliceous-rich, clay-poor silty shales generally offer more favorable reservoir conditions. Conversely, intervals with relatively high OM content that have not yet reached the hydrocarbon generation threshold require integrated evaluation alongside their thermal evolution stage to more accurately assess their reservoir potential.