Abstract: Based on core observation, the origins of the lower member in the first submember of the Shahejie formation in Madongdong area of Qikou sag are analyzed by using the data including rock types and their combinations, sedimentary structures, grain size, the results of seismic and logging, and reservoir properties.The reservoir in this area represents gravity flow deposits in the deep lake environment.Deposits consist of channel, overflow, interchannel and lake mud deposits.Gravity flow channel sediments in the study area mainly come from north east area.Channel position overall does not change too much. Gravity flow deposits show band distribution and the thicker sand bodys located near Well Gs24-26 area and Madongnan area.Overall, channel facies are most widely distributed in the BinⅠoil group, and bifurcated more.In the Ban Ⅱ, Ⅲ and Ⅳ oil group, gravity flow channels are mainly joined by channel downstream.There is close correspondence between microfacies types and thickness of sandstone, and the thicker sand body, the greater porosity.Waterways microfacies have the best reservoir properties, and is the most favorable oil and gas-rich facies unit.