The ninth member of Yan'an Formation in Jiyuan Region was studied to understand the sedimentary environment. The researchers found that a braided river-braided delta system was deposited under a moist climate. Different features like distributary channels, interdistributary bays, crevasse fans, and swamps were developed in the braided delta plain. Underwater distributary channels and interdistributary bays were found in the braided delta front. Various stratification and corrosive structures were abundant, with channel bars forming reservoirs with favorable physical properties.