Abstract: The types, characteristics, origin and parameters of the microfractures in Es3 reservoirs of Dongpu depression were studied by such means as microscopic thin section analysis, physical property analysis and fluid-inclusion analysis. The results show that three types of microfractures, i.e., transgranular fractures, grain edge and intragranular fractures, are developed in Es3 reservoirs of Dongpu depression, and tectonism played a dominant role in the formation of microfractures, followed by diagenesis and overpressure. The scale of microfractures is medium, but they are developed intensively in Es3 reservoirs of Dongpu depression. The average length of the microfractures is 5.6mm with average density of 0.66cm/cm² and average opening of 8.6μm. The microfractures are the major reservoir space in the study area and play an important role in improving the porosity characteristics of the ultralow permeability sandstone reservoirs.