This study is a contribution to the petrographic and petrophysical characterization of the reservoir sandstones of Yogou Formation in the Termit Sedimentary Basin (Niger). It focuses on the impact of diagenetic processes on the petrophysical properties of Campanian sandstones. The pore types are generally intergranular, intragranular, and rare microcracks. The porosity varies from 0.3% to 25.3% and the permeability ranges from 0.1 mD to 470.3 mD. Diagenetic features that influenced the reservoir quality evolution include mechanical and chemical compaction, precipitation of carbonate cement, clay mineral cement, the formation of quartz overgrowths, and dissolution of feldspar grains. Compaction and cementation reduced significant volumes of primary porosity and permeability. On the other hand, feldspar dissolution and quartz corrosion contributed to an increase in the volume of primary porosity of the sandstones. The Yogou Formation reservoir was subjected to a high diagenetic overprint resulting in marked reservoir heterogeneity. This study also demonstrated the effect of diagenetic processes on the quality of hydrocarbon reservoirs and showed that good quality reservoirs are mainly concentrated in the 2545 m to 2565 m depth range of the study area.