Objectives：The Zhanhua Sag developed three sets of source rocks, which include the upper Es4(the 4th Member of the Eocene Shahejie Formation) , the lower Es3 and the lower Es1(i.e. the 4th, 3rd and 1st Member of the Eocene Shahejie Formation). There have been a few studies on source rocks of the lower Es1 before. Methods：Analyzed source rocks qualities of the lower Es1 in the Zhanhua Sag by using organic geochemical methods. Results:the result shows that source rocks of the lower Es1 had high abundance of organic matter, which kerogen are mainly typeⅠ, are high- quality hydrocarbon source rocks. Source rocks deposited center is located in the Bonan Subsag, but best quality source rocks situated in the Gunan Subsag. Part of the hydrocarbon source rocks are in mature stage in the Gubei Subsag and the Gunan Subsag，so exploration and development research should pay attention to contribution of the lower Es1 hydrocarbon source rocks. Depositional environments analysis showed that tectonic subsidence rate was smaller and the climate was the north subtropical to warm temperate middle arid climate during sedimentary period of the lower Es1 in the Zhanhua Sag, didn't support the formation of high- quality hydrocarbon source rocks. High productivity and good water column caused by high salinity was the main reason of the formation of high- quality hydrocarbon source rocks in the lower Es1. High salinity was due to marine transgression in Lupen. Conclusions:Marine transgressived along the direction of the Gunan Subsag—Gubei Subsag—Bonan Subsag，caused the Gunan Subsag had best quality source rocks though it was not sedimentary center and sedimentation center.