Abstract: To explore the diagenesis and development pattern of quality reservoirs in the Weizhou Formation in the Weixi'nan Sag, Beibu Gulf Basin, South China Sea, taking the Third Member of the Oligocene Weizhou Formation in the Weizhou X Oilfield as an example, the types and mechanisms of diagenesis, the sequence of diagenetic evolution, and the division and distribution of diagenetic facies were analyzed in depth through techniques such as thin section microscopy identification, scanning electron microscopy, cathodoluminescence, and numerical simulation of diagenesis. The main controlling factors and selection of quality reservoirs were determined. Results show that the compaction effect of the reservoir in the Third Member is moderate showing mainly point-line or line-line contact. Different types of clay minerals were involved in the cementation process, which occurred in mainly the favorable stage of A₁²-A₂¹ micro-facies of diagenesis. The research area has experienced acidic and then alkaline dissolution and the most prominent products are the acid-alkaline dissolution products of typical kaolinite and chlorite. The sedimentary diagenetic micro-facies of A₁² in distributary channel are the favorite zones of development of quality reservoirs in the study area.