Abstract:Abstract: By systematic investigation on the sedimentary phase through core sample observations, sedimentary structures, sedimentary sequences, analytical chemical tests, well logging, well testing and study of the regional tectonic background, we first found a delta fan phase in the A Lei Si Ku Mu depression of the Tu Er Gai Basin in the Doshan Group of Jurassic System. We can identify three sub-phases including delta plain, delta front and prodelta as well six microfacies including clastic flow, submerged distributary channel, tributary bay, mouth bar, distal bar and prodelta mud. Among them, submerged distributary and clastic flow micofacies were well developed.It’s a series evolution of Sedimentary that with the lake level rise slowly, Southeast and northeast two major source areas gradually retreat, the fan delta plain subfacies area is gradually reduced,and replaced by the fan delta front subfacies. We know that the fan delta front of underwater distributary channel and mouth bar sand body is the most favorable oil and gas reservoirs, and also front fan delta dark mudstone can be used as an important hydrocarbon source rock and caprock.