Abstract: The red beds in the lower submember of the fourth member of Shahejie Formation (Es₄) in the Bonan Subsag of the Jiyang Depression of the Bohai Bay Basin have undergone a long period of sedimentation and diagenesis. The mechanism of primary pore retention and the evolution constraints of secondary pores are not clear, which restricts exploration progress. The controlling factors of favorable reservoirs in the red beds were analyzed by means of core observation, casting thin section identification and scanning electron microscopy and combined with reservoir characteristics analysis. The formation mechanism of the favorable reservoirs was summarized, and their distribution was also predicted. There are two major sedimentary systems in the red bed sediments of the lower Es₄ submember of Bonan Subsag, including alluvial fan-braided river-braided river delta-lake and fan delta-lake. Sandstones are mainly lithic feldspars. Reservoir porosity is composed of residual primary pores, secondary dissolution pores and fractures. Reservoir physical properties are poor. The reservoir belongs to the category of ultra-low porosity and ultra-low permeability. Under the influence of mechanical compaction and the alternation of alkali and acid fluids, the reservoir porosity in the study area experienced three stages: primary pore retention, secondary pore formation and reservoir densification. Burial depth and favorable facies zones determine the preservation degree of primary pores, while organic acid, abnormally high pressure in overlying strata and fracture distribution determine the degree of secondary pore development. The favorable reservoirs with primary pore development are fan-terminal reservoirs of alluvial fan buried less than 3 000 m, which are distributed as belts on the southern basin margin. The favorable reservoirs with secondary pore development are braided river and braided river delta reservoirs with pressure coefficient greater than 1.2 in the upper submember of Es₄ and fault system develop in the lower submember of Es₄, which are distributed in sag zones and show strip-like and overlapping sheet distribution.