Abstract: Data from cores, casting thin sections, scanning electron microscopy, cathodoluminescence and X-ray diffraction are applied to study the petrological characteristics and diagenesis of the reservoirs of the 3^rd Member of the Liushagang Formation in the Weixinan Sag of Beibu Gulf Basin. Special attention is paid to diagenesis and its evolutionary stages. It is found that strong compaction is the main reason for density and tightness increase of the sediments. Cementation seems to have certain negative impact to reservoir properties, but the early carbonate cementation and authigenic kaolinite always have close connections with high-quality reservoirs. Dissolution is a basic process for enhancement of reservoir qualities, and fractures may improve the permeability of reservoir rocks. According to the intensity of diagenesis and the composition of diagenetic minerals, five types of diagenetic facies are qualitatively recognized. Among them, the diagenetic facies of weak compaction and weak cementation are the best for hydrocarbon accumulation, followed by the medium compaction and medium dissolution facies, and the strong compaction and medium to strong dissolution facies. The compaction and filling facies and the densely carbonate cemented facies are listed the worst in quality. Five types of logging curves are selected to define and describe the diagenetic facies. With the spatial distribution of diagenetic facies, it is recognized that the shallow buried deltaic reservoirs in the east of the study area, and the thick conglomerate reservoir of underwater distributary channel facies in northwest and southwest fan deltas are the sweet points of reservoirs in the study area.