Abstract: Good oil and gas shows have been discovered in E2s4 Member below 3 600 m in LD21-A structure, Bohai sea area and DST was done successfully. E2s4 Member is a new oil & gas exploration layer in this structure, with good exploration prospects. In order to investigate the reservoirs in this structure, the reservoir characteristics have been analyzed systematically with core data, sidewall core data, cast thin section data and the scanning electronic microscope data. It is believed through study that lithologically, the sandstone reservoirs in this structure consist mainly of feldspathic litharenite, secondarily lithic arkose. The reservoir physical properties are medium, with moderate to good porosity, ultra-low to moderate permeability. The main influencing factors on the reservoir physical properties are sedimentation, diagenesis and tectonism. These three factors have influenced together the change of reservoir physical properties.