Block B of South Buir Oilfield is located in the Eastern Province of the People’s Republic of Mongolia. Main oil reservoir is pay zoneⅡof L.L.Z (K1n1 II), followed by pay zone Ⅲ of L.L.Z (K1n1III) and pay zone I of L.L.Z (K1n1I). For the purpose of studying the petrological characteristics, physical properties and pore structures of the reservoir of pay zone Ⅱ and Ⅲ of Lower part of Lower ZuunBayan, a large numbers of scanning electron microscopes, thin sections, casting thin sections and well logging data were analyzed. Pyroclastic rock is the main lithology of this area, pores in the L.L.Z are mainly complete intergranular pores, dissolved pores, the formation is a reservoir of lower porosity and permeability to intermediate porosity and medium permeability. Classified evaluation shall be carried out to reservoirs, according to standard of reservoirs classified evaluation, porosity and permeability data of ordinary physical property analysis and mercury injection data analysis, and corresponding mercury injection parameters. Comprehensive evaluation of reservoirs at pay zone Ⅲ is type Ⅲ, reservoirs of pay zoneⅡ is type Ⅱ.