Abstract: The Oligocene (E₃s₂) is the main producing layer of the Oilfield A, which is located in the Bohai Bay. It is significant to understand the physical characteristics of the reservoir and its controlling factors for selection of favorable producing zones in the area and for adjustment scheme in the later stage. Based on the work of mineralogical analysis, thin section identification, scanning electronic microscopic examination, particle size analysis, conventional physical properties measurement and other laboratory analysis, we gained a clear idea of the E₃s₂ reservoir characteristics. A careful study is made on controlling factors over physical properties of the E₃s₂ low permeability reservoir from the aspects of sedimentation, diagenesis and tectonics. The results indicate that the reservoirs of E₃s₂ are mainly consisting of lithic arkoses, and characterized by medium levels of maturity and cementation. The reservoir space is dominated by mixed pores. Thus the reservoirs can be put in the class of medium porosity and low permeability reservoir. The original sedimentary features, such as the size and sorting of clastic particles and the content of mud are all factors controlling over the quality of the E₃s₂ reservoir. Compaction and cementation by carbonate are the main reasons for the low permeability of the E₃s₂ reservoir, and the dissolution of organic acid to some extent improved the physical properties of the reservoir. The improvement effects of later tectonic activity on the physical properties of the E₃s₂ reservoir are very weak. The results of this study may provide a basis for the optimization of relative high permeability area and the deployment of infill wells in the E₃s₂ reservoir.