Abstract: In order to explore the hydrocarbon accumulation mechanisms for successful exploration of Palaeogene subtle reservoirs, sequence stratigraphy, depositional characteristics and types of sedeiments as well as depositional systems of the Paleogene in the Chengbei sag of the Bohai Bay Basin are studied in this paper. The Paleogene in the sag, which consists of the 3^rd Member of the Shahejie Formation and the relatively complete Dongying Formation, is a kind of multi-cyclic deposits with breaks. Difference of depositional features is obvious between the north and the south of the sag. The Paleogene consists of six types of depositional systems, i.e. the nearshore subaqueous fan, fan delta, deep-water turbidite fan, braided stream delta, low-sinuosity channel, and lacustrine subottom fan, and all are good reservoirs for hydrocarbon.