The lower member of Dalazi Formation is an important oil reservoir in the Songjiang Basin. Based on the research on its field-measured geological profiles, lithological combination characteristics and grain size distribution characteristics, combined with the analysis of the spatial distribution characteristics, sedimentary structural characteristics and hydrodynamic conditions of the sediments in this member, nine sedimentary microfacies of the fan delta plain subfacies, fan delta front subfacies and littoral-shallow lacustrine subfacies have been identified.The study reveals that lower member of Dalazi Formation in the research area follows a fan delta–shallow lacustrine depositional model in the steep slope zone of a rift lake basin. The sediments primarily originate from the Pre-Mesozoic strata in the steep southeastern and eastern margins. The basin center migrated from the early Xiaoshahe area to the Yangmucun–Shenglicun area, and the sedimentary system gradually transitioned from fan delta to littoral-shallow lacustrine facies.