In this work, the potential influences of grain-coating clays on water-CO₂-rock interactions in sandstones and subsequent ramifications for CO₂ storage were investigated using reactive transport simulations. The results indicated that, compared to pore-filling smectite, grain-coating smectite leads to significant pH decrease, increases in the CO₂-species concentrations, and decreases in smectite dissolution and the precipitation of secondary minerals. Moreover, it was revealed that smectite and chlorite coats dissolve preferentially over detrital K-feldspar being covered, while K-feldspar is dissolved preferentially over illite and kaolinite coats. While the mineral trapping mechanism is only important for smectite and chlorite coats, sandstone porosity is significantly reduced for chlorite coat but increased for the other three clay coats. The main causes of the differences between pore-filling and grain-coating scenarios for smectite and chlorite coats are ascribed to the inhibitory effect of clay coats on the growth of secondary quartz and the dissolution of clay. In addition to the above two factors, the decelerating effect of clay coats on the dissolution of K-feldspar is also important for illite coat; meanwhile, for the kaolinite coat, the dissolution of clay is less important and the other two factors are more critical. Furthermore, the coverage and thickness of clay coats, fluid flow rate, detrital grain size, detrital lithology, partial pressure of CO₂, and temperature may all impact the role of clay coats. Document Type: Original article Cited as: Li, H., Hu, Q., Zhu, R., Liu, B., Mishara, A., Ansah, E. O. Reactive transport modeling of water-CO₂-rock interactions in clay-coated sandstones and implications for CO₂ storage. Advances in Geo-Energy Research, 2025, 17(2): 121-134. https://doi.org/10.46690/ager.2025.08.04

As hydrocarbon exploration continues to advance into ultra‑deep formations, conventional theories of reservoir formation based primarily on compaction, pore preservation, and dissolution are increasingly being challenged. Growing exploration practice indicates that the formation of high‑quality ultra‑deep clastic reservoirs cannot be explained simply as a deeper continuation of conventional deep reservoir evolution. Instead, ultra‑deep reservoirs may represent a distinct geological regime, where reservoir evolution is governed by physical and chemical principles fundamentally different from those operating under conventional burial conditions. This work reviews the evolution of scientific understanding of reservoir formation in deep and ultra‑deep clastic reservoirs and identifies several fundamental scientific issues that remain unresolved. It is suggested that the formation of high‑quality ultra‑deep reservoirs should be interpreted as the evolution of an open reservoir‑forming system driven by the coupled interactions among the mass, thermal, pressure, stress, fluid, and chemical fields. Accordingly, the research paradigm is shifting from pore evolution toward the reservoir‑forming system. More importantly, this paradigm shift may provide the conceptual foundation for an emerging discipline of Ultra‑deep Reservoir Geology, offering a new theoretical framework for understanding, predicting, and evaluating high‑quality reservoirs in the ultra‑deep subsurface. Document Type: Perspective Cited as: Li, H., Qu, X., Lin, J., Wang, W., Mishra, A., Ansah, E.O. Formation mechanisms of high‑quality ultra‑deep clastic reservoirs: Progress, scientific challenges and future directions. Advances in Geo‑Energy Research, 2026, 21(3): 201‑204. https://doi.org/10.46690/ager.2026.09.01