We use thin section microscope observation, fluorescence analysis in fluid inclusions, the scanning electron microscope (SEM), quantitative grain fluorescence on extract (QGF-E), and X-ray computed tomography (X-CT) to identify and estimate the hydrocarbon primary migration paths in the lower Es3 source rock within the Niuzhuang-Liuhu sag, Dongying Depression, China. The microfractures in the oil source rock, including the microfractures paralleled layers and vertical or almost vertical microfissures are mostly filled with solid bitumen and calcite veins, and a lot of oil fluid inclusions with light-yellow fluorescence discovered in calcite veins. The QGF-E fluorescence intensity of the samples collected form Niu 24, Niu 83 in 4 wells, which are folium sandstones within lower Es3 source rock, is almost much more than 120 pc and a few less than 40 pc. The result of SEM and X-CT indicts that the kerogen network in the lower Es3 source rock is continuous. According to the lower threshold of hydrocarbon in the organic matter (2.5%~10%), the hydrocarbon expulsed from the kerogen network existed in the lower Es3 source rock. Abundant evidence indicates that the subtle migration paths, for example fracture and microfracture, folium sandstone and the organic kerogen network, are the main route for a great deal hydrocarbon expulsion.