Sedimentary Paleoenvironmental and Diagenetic Control Mechanism of Pores in Lacustrine Shales: A Case Study of Triassic Yanchang Formation Chang 7 ₃ Sub‐Member, Ordos Basin, <scp>NW</scp> China

ABSTRACT As a favourable important reservoir for shale oil, the pore structure characteristics and controlling mechanisms of continental shale carry vital implications for hydrocarbon resource evaluation and subsequent development; modern pore features form the natural product of shale formation and evolutionary history, jointly regulated by sedimentary‐period paleoenvironments and late‐stage diagenetic processes, yet the precise controlling pathways and differential impacts of these geological processes on pore evolution are still poorly understood and warrant further thorough research. This study focuses on the shale of the Chang 7 3 sub‐member in the Ordos Basin, employing multiple testing methods such as scanning electron microscopy (SEM), low‐field nuclear magnetic resonance (NMR) testing, low‐temperature nitrogen adsorption–desorption, mineral phase automatic identification (TIMA), X‐ray fluorescence (XRF) spectroscopy, and inductively coupled plasma mass spectrometry (ICP‐MS). It clarifies the paleoenvironment during the Chang 7 3 sedimentary period, types of diagenesis, and the evolutionary process of shale pores, and reveals the controlling mechanisms of sedimentation‐diagenesis on shale pore structure. The results show that: (1) The sedimentary paleoenvironment of the Chang 7 3 shale was characterised by a warm and humid, anoxic‐reductive paleoenvironment with low‐salinity fresh to brackish water. (2) Major diagenetic processes include compaction, cementation, dissolution, and organic matter hydrocarbon generation. Compaction reduced porosity by an average of 38.49%, cementation lowered porosity by an average of 6.24%, dissolution increased porosity by an average of 0.21%, and organic matter hydrocarbon generation raised porosity by an average of 0.16%. (3) Paleoclimate, paleoredox conditions, and paleosalinity in the sedimentary paleoenvironment significantly constrained the pore structure and complexity. Specifically, reductive water conditions and freshwater environments reduced the pore volume and specific surface area of the shale while increasing the complexity of mesopores, which is opposite to the effects of warm and humid paleoclimatic conditions. (4) Compaction and cementation predominantly took place prior to the Cretaceous with stronger compaction and cementation leading to severe macropore damage alongside elevated micropore proportion, specific surface area and pore volume, and dissolution and hydrocarbon generation of organic matter jointly acted starting in the Cretaceous to render shale pore structure more intricate and boost the growth of specific surface area and pore volume. Comprehensive research indicates that diagenesis is the dominant factor controlling pore evolution in the Chang 7 3 shale of the study area. Pore structure characteristics, including pore volume, specific surface area, and pore fractal dimension, are jointly controlled by both the sedimentary paleoenvironment and diagenesis. Specifically, the sedimentary paleoenvironment indirectly modulates pore structure by controlling primary mineral composition. Compaction acts as the dominant factor controlling pore evolution throughout diagenesis. These results offer critical geological support for predicting favourable pore development intervals and optimising sweet spots in continental shale oil exploration.