Abstract: The oilfield A located at the southwestern end of Shijiutuo swell in the northern Bozhong depression of Bohai Bay basin, belongs to heavy-oil sandstone reservoirs featured with gentle and complicated nose-like structures. Drilling data shows that the oil-water contact (OWC) of M layer exhibits a SE-NW rising trend with the altitude difference of 5.0~22.0 m. Tracer data and production performance prove that the reservoir is continuous between wells with different OWCs. Analyses of capillary pressure and buoyance balance theory show that the differences of OWC in the connected sandbodies are mainly constrained by formation oil viscosity and reservoir physical properties; the larger the formation oil viscosity is, the poorer the reservoir physical properties are and the higher the OWC will be. Biodegradation of different levels results in the gradual rise of formation oil viscosity from southeast to northwest; the development of abandoned channel deposits and lateral accretion layers leads to worse and worse reservoir physical properties from southeast to northwest. Both the biodegradation and development of abandoned channel deposits and lateral accretion layers result in the SE-NW rising trend of the OWC.