Abstract: The results of section identification, geophysical and well logging data from side-wall coring and debris of the borehole Y, that is located in the southwest Maichen sag of the Beibuwan Basin, suggest that the sequence penetrated by the hole are dominated by sandstone intercalating great amount of shale, siltstone and pelitic siltstone, and small quantity of conglomerate deposited near the proximity and middle part of the provenance. The Oligocene reservoirs in the Weizhou Formation, which are fine and rich in mud in genaral, are characterized by poor physical properties and weak diagenesis (A or B stage of eogenetic and A stage of middle diagenetic phase) As a whole, the effective porosity and permeability are low. However, the 2nd Member of the Liushagang Formation is a reservoir with certain amount of porosity and permeability. It is dominated by intergranular pore with intergranular dissolved pores as the subordinate. Feldspar corrosion pores are observed sometimes. The pores in the Changliu Formation are largely composed of intergranular pores, intergranular dissolved pores and small amounts of feldspar corrosion pores. The development of reservoirs is mainly controlled by the distance to provenance, sedimentation environment, and diagenesis. Diagenesis in general is weak. Compaction, dissolution and cementation are the main diagenetic processes which affect the reservoir properties.