Abstract: Diagenesis plays an important role in the formation and evolution of sedimentary rocks. However, diagenesis has different characteristics in different basins due to variations in sedimentary and tectonic evolution. The diagenesis of Cretaceous sandstones in the Termit Basin in Niger was studied by experimental analysis of samples, clarifying the main types of diagenesis of the sandstones and their influence on reservoirs. The diagenetic stages of Cretaceous sandstone reservoirs in the basin were distinguished according to diagenesis appearance, clay mineral characteristics and transformation laws, and the homogenization temperature of inclusions. The results indicate that most of the Cretaceous sandstones in the study area are in phase A of middle diagenesis, and that the sandstones with shallower burial depth are in phase B of early diagenesis. The rock type of the sandstones is an important factor affecting diagenesis. The Cretaceous sandstones in the basin are dominated by quartz sandstones. Strong support by the quartz grains reduced the damage caused to the reservoir by compaction. Primary intergranular pore is the dominant pore type. Small numbers of other pore types—such as intragranular pores and intergranular dissolved pores—are also developed. There are some differences in the development of Cretaceous sandstone reservoirs in different zones of the Termit Basin. Reservoirs in the Fana low bulge and the Yogou slope are relatively developed, with good physical properties. However, reservoirs in the Soudana uplift belt and Dinga fault terrace are comparatively poor. There is good exploration potential in the Fana Low bulge, which has been proved by drilling results. This is accordingly the key area for future Cretaceous exploration.