Abstract: A preliminary analysis of physical properties of the main oil pay (Fuyu oil pay) in southern Songliao Basin and the factors affecting them was carried out through the study of the thin-section and the regional sedimentary characteristics. It was found that in addition to the mineral constituents,the textures, and the secondary fractures of the rocks which affect the reservoir behaviour, the most important factor is: as burial depth of oil beds gets deeper, diagenesis of rocks gets stronger. As a result, primary porosity becomes lower, and reservoir behaviour becomes worse. Thus, diagenesis directly influences physical properties of the sandstone beds. For the sake of delineating areas of better reservoir properties, the variation trend of the Fuyu oil pay is investigated. Taking Qian An as a center, which is located in the central depression of the basin, the variation trend appears to be shaped as a horse-shoe. Within the horseshoe-like ring zone, the burial depth of the Fuyu oil pay is greater than 2000 m, the diagenesis of the reservoir rocks is rather strong, the porosity is lower than 10%, and the permeability is less than 1 mD. Off the horse-shoe-like ring zone, the physical properties of the rocks become better gradually. Thus, it is suggested that the area off the horse-shoe-like ring zone is a favourable zone for exploration.