Exploration and Development Research Institute, Daqing Oil field Ltd Co., Daqing 163712, Heilongjiang, China

A set of volcaniclastic rocks were developed in the Lower Cretaceous Ta �� Formation of South Buir Sag in Tamtsag Basin of Mongolia. The main rock types are lithic arkose and tuffaceous sandstone, of which the reservoir physical properties and diagenesis are different from normal sedimentary rocks. Detailed studies were carried on the petrology, physical properties, diagenesis and evolution of reservoirs, using polarizing microscope, scan electron microscope and X ray diffraction. The results indicate that intergranular pore, micropore among matrix and intergranular dissolution pores are the main pore types of Ta �� Formation. The main diagenetic phase belongs to stage A of middle diagenesis phase. Compaction and cementation were the main factors reducing reservoir porosity, and the secondary porosity zone was developed because of erosion-dissolution of feldspar and tuffaceous material.