The sandstone reservoirs of the braided river delta facies are developed in the 2nd Member of the Paleogene Shahejie Formation (Sha- 2 Member) in the steep slope zone of the Shanan Sag, the Bohai Bay Basin. The reservoir is influenced by structural location, burial rate, paleotemperature changes, and faulting, and its physical properties vary significantly in different structural areas, especially in the middle—deep layers. In this paper, the sedimentary facies, diagenesis, and pore evolution characteristics of the middle—deep sandstone reservoirs are studied by using the data of drilling, logging, seismic, and laboratory analysis, and the main controlling factors and evolution regularities of the high- quality reservoirs are proposed. The results show that the middle—deep sandstones in the study area are mainly lithic arkose, and the pore spaces are mainly intergranular dissolved pores. There are four types of reservoirs, i.e., the medium porosity—medium permeability type, the medium porosity—low permeability type, the low porosity—low permeability type, and the low porosity—ultra- low permeability type. The medium porosity—medium permeability type is the main one in the area. The pore space of the reservoir is generally well developed, being characterized by a medium porosity and a medium permeability, supporting a good physical property. Further investigation has revealed the major controlling factors of the high- quality reservoirs. The provenance rock rich in quartz and feldspar is the material foundation, the delta plain distributary channel is the favorable sedimentary facies type, and the strong dissolution of calcite cement during the late diagenesis provides major pore space, all of which play the key role in the development of the high- quality reservoir. The analysis of the spatio—temporal distribution of the high quality reservoirs shows that, in the Caofeidian 14- e structure area in the southern sub- sag of the Shanan Sag, the braid river delta facies sandstones are developed and these sandstones are surrounded by the effective source rocks. These conditions are superior to the hydrocarbon migration and accumulation, demonstrating a great potential of the lithologic reservoir exploration potential in the Caofeidian 14- e structure area. The presence of thick oil layer has already been proved by drilling, which indicates that this area is most favorable for exploration of lithologic reservoirs in the Shanan Sag.