Abstract: Fuyu Oil Play belongs to the 4th Member (K₁q⁴) of Quantou Formation, Songhuajiang River Group of Cretaceous. It is one of the major plays in the southern Songliao Basin, After the examination on numerous thin sections of rocks, the following understandings on the diagenesis of the oil play have been achieved. 1. Fuyu Oil Play is a suit of rhythmic sediments composed of sandstones, siltsands, claystones and the transitional types between the above rocks with a total thickness of 100 meters. The sandstones of this area rich in feldspar lithic debris, characterized by low maturation of mineral composition. 2. The compositions and the textures of the sandstones control the physical and chemical diagenesis. 3. After its deposition, the oil play has experienced multiple diagenetic changes. According to the metasomatic alterations of diagenetic changes observed on thin sections, the conditions essential to physical and chemical diagenetic changes, the control of depositional environment on diagenetic environment and the general trend of diagenesis, the sequence of the various diagenetic changes can be determined. 4. The diagenesis of this area can be divided into three stages, i.e. the stage prior to effective bural, shallow burial stage and deep burial stage. 5. The authigenic kaolinites, quartz and calcites are the three kinds of major cements which extensively influenced the reservoir petrophysics.They are especially well developed in the sandstones which are somewhat coarse-grained with higher primary porosity and good pore interconnection. 6. The SiO₂ of the authigenic quartz cements is principally derived from the slow dissolution and kaolinization of feldspar and starting from the early stage of diagenesis. The late overgrowth and mosaicism of quartz are less important.