In the Dongying Depression, high abundance of parallel microfractures are found in the hydrocarbon source rocks of the upper unit of E 2-3 s 4 Member and the lower unit of E 2-3 s 3 Member. Most of the microfractures are filled with secondary calcite. The layers with microfractures are extensive in the basin and appear at about 2900~3000 m, which are consistent with the top depth of the maturation stage of hydrocarbon source rocks. Lithological and geochemical studies show that the occurrence of microfractures in the rocks is heterogeneous. Most of microfractures are found in the paper laminated shales with high organic matter abundance, while still none is found in the massive mudstones with lower organic matter abundance. The relations between organic matter abundance, organic matter maturation and the occurrence of microfractures indicate that hydrocarbons generation is the key reason for the parallel microfractures to produce. When maturated the source rocks with high organic matter abundance begin to generate a great deal of hydrocarbons, which quickly raise the pore pressure and lead the rocks to fracture at last. As the upper unit of E 2-3 s 4 Member and the lower unit of E 2-3 s 3 Member are interbedded with shales and mudstone, and the occurrence of microfractures is heterogeneous and only limited to shales, the layers with microfractures and layers lack of microfractures are intercalated. So, we propose that the expulsion of hydrocarbons in this section will be similar to source rocks of intercalated sands and muds in front of the delta, and can improve hydrocarbons expulsion efficiency.