The Sources of Diagenetic Fluids and Diagenetic Evolution Model in Deep‐to‐Ultra‐Deep Tight Sandstone Reservoirs: A Case Study From Bashijiqike Formation, Kalesu Belt, Kuqa Depression, Tarim Basin, <scp>NW</scp> , China

ABSTRACT The origin and evolution of diagenetic fluids in deep to ultra‐deep reservoirs are complex and critical for understanding reservoir quality. Based on the conventional formation water analysis, this study investigates the sources and effects of diagenetic fluids in the Cretaceous reservoirs of the Kelasu structural belt, Kuqa Depression, NW China, integrating analyses of carbonate cement carbon and oxygen isotopes, rare earth elements (REEs), fluid inclusion homogenization temperatures, thin section petrography, scanning electron microscopy (SEM), and X‐ray diffraction (XRD). The results show that formation water is generally characterized by slightly enriched Na + + K + , high Ca 2+ , and low Mg 2+ and SO 4 2− concentrations. Ion ratio coefficients indicate strong water–rock interactions and good sealing conditions in both areas. The high salinity of the formation water is mainly controlled by seawater evaporation, evaporite (especially salt rock) dissolution, and water–rock interactions. Carbon and oxygen isotopic and REE data suggest multiple sources of diagenetic fluids, including meteoric water, seawater or saline lake water, low‐salinity organic acid fluids, and gypsum‐derived diagenetic water. Diagenetically, BBZ experienced early shallow burial followed by rapid deep burial, which helped preserve primary porosity. In contrast, the DDB underwent continuous deep burial, resulting in stronger compaction and lower porosity. Carbonate cementation and clay mineral transformation occurred earlier in the DDB than in the BBZ. These findings reveal significant spatial differences in fluid origin and diagenetic processes that control reservoir properties in deep tight sandstone reservoirs.