Abstract: As a key research subject in karst systems, ancient underground rivers exhibit complex spatial structures and diverse filling patterns, posing significant geological challenges to the development of fracture and cavity reservoirs. Conventional seismic interpretation methods are no longer sufficient to support the research needs for these reservoirs. The unclear understanding of the spatial structure and reservoir models of these ancient underground rivers has severely limited the effective development of fracture and cavity reservoirs and well deployment in the Tahe area. Based on drilling data, well logging, conventional seismic interpretation, acoustic impedance inversion, shale content inversion, and production performance data, the study reconstructed the paleogeomor-phology as well as the water and fault systems that controlled the development of underground rivers. It further analyzed the filling characteristics and patterns of ancient underground rivers under different geomorphic settings. Additionally, by integrating data of wells intersecting underground rivers, the study explored the genesis of reservoirs in sinkholes, corridors, and channel expansion areas, and established reservoir development models under various geomorphic contexts. The results indicated that: (1) In regions of relatively high topography (e.g., highlands and gentle slopes) and in steep-slope recharge zones, large-scale caves filled with stacked breccias and sandstone are developed in sinkholes, whereas in steep-slope drainage zones, large-scale caves are predominantly filled with mudstone and breccia composites. (2) Along the geomorphic transition from highlands to steep slopes, reservoir types shift from fractured surrounding rocks to caves filled with sandstone or siltstone at geomorphic turning points, and eventually evolve into small caves filled with horizontal pores, fractures, and siltstone. (3) In the straight reaches of ancient underground rivers, reservoir development exhibits zonal differentiation: shallow recharge and drainage zones in highlands have poor reservoir physical properties, while deeper recharge and runoff zones demonstrate better properties. (4) At junctions and expansion zones of underground rivers, multi-phase collapse-filled or unfilled caves are mostly developed and exhibit favorable reservoir properties. In contrast, caves in corridors are typically filled with mudstone or mudstone-cemented breccia, resulting in poor reservoir quality.