Abstract: To provide more information for exploration and development of the subtle sandstone oil and gas reservoirs in Zhou 311 area of Northern Songliao Basin,it is necessary to describe the sedimentary characteristics of reservoir sand bodies in the 3^rd and 4^th members of Quantou Formation.Six short-term base-level cycles were identified under the guidance of high resolution sequence stratigraphy and reservoir sedimentology theories and through integration of core,logging and regional seismic data,and were used to establish a sequence stratigraphic division scheme.The types and characteristics of reservoir sand bodies were analyzed within the established sequence framework.The 3^rd and 4^th members of Quantou Formation can be divided into two long-term base-level cycles (Q3-LSC1-Q4-LSC1),five medium-term base-level cycles (Q3-MSC1-Q4-MSC3),and twelve short-term base-level cycles (Q3-SSC1-Q4-SSC7).The short-term base-level cycles mainly develop asymmetric structure dominated by ascending half cycles (A₁、A₂、A₃),almost complete-complete symmetrical structure with ascending and descending half cycles being nearly equal (C2),incomplete symmetrical structure dominated by either ascending half cycles or descending half cycles (C1,C3).Vertically,the evolution of the 3^rd and 4^th members of Quantou Formation shows a pattern ofbase level rising,falling,and then rising.The vertical sedimentary evolution sequence is distributary channel-meandering channel-distributary channel-underwater distributary channel.The channel sand body types develop from less overlap to more vertical or lateral overlap,and then gradually change to isolated type.Characteristics of sedimentary section and internal sedimentary cycle structure of the meandering channel sand bodies,distributary channel sand bodies,underwater distributary channel sand bodies are analyzed on the basis of high-resolution sequence stratigraphy division and correlation and under the control of short-term base-level cycles.The sand bodies were mostly oil-immersed and thick during the deposition of Q4-SSC1 and Q3-SSC5,thus they are most favorable for quality sand body development.In the process of fine exploration and development,fine description of channel sand bodies is imperative due to the complex heterogeneity within the reservoir.

Key words: channel sand body, base level cycle, high resolution sequence stratigraphy, Quantou Formation, Zhou 311 area, Songliao Basin