Abstract: In the southern slope of the Huimin Sag of the Jiyang Depression in the Bohai Bay Basin,3 types of unconformity were found on the top of Mesozoic,including truncation,onlap and parallel ones.The unconformities were divided into 3 layers vertically,including basal conglomerate or transgressed sand body,weathered clay and eluvial zone.Each layer has its own logging response characteristics and identification marks.Due to the different lithology of each layer,7 patterns of lithology configuration were formed.According to core observation and sample test,secondary pores generated in the eluvial zone of unconformity in Qudi area.The unconformity served as the pathway for petroleum migration,and also provided the space for accumulation.The studies of the relationship between unconformity and petroleum accumulation have important academic and practical significances for subtle reservoir exploration in the Huimin Sag.