According to the analyses of sporo-pollen assemblage, petrologic characteristics and basin evolution, it is proved that there is absolute difference between the Ch8-Ch10 oil bearing formations and Ch1-Ch7 oil bearing formations. Ch8-Ch10 oil bearing formations are featured by even landform and low compositional maturity of sandstone. At the time, fluvial delta and coastal lake facies developed extensively. The sporo-pollen identified from the sample of Ch8-Ch10 strata are mainly composed of pteridophyta spore, and Punctatisporites –one of the significant members of sporo-pollen living in middle Triassic adds up to 50.5%. While the deep lake widened expeditiously during the Ch7 depositional period. The quartz composition in Cha7 sandstone developed increased tremendously which resulted in the corresponding change of rock types in the west and the southwest. As far as the sedimentary system is concerned, alluvial fan and fan delta controlled the deposition at the period when Ch7 strata depositing. The biotic community in Ch7 strata is characterized by the coming forth of Duplexisporites-an important member of sporo-pollen assemblage in late Triassic. In addition, a set of tuffs, which is the result of the syn-depositional volcanism and magmatic activity and has been found through drilling, lies steadily at the base of Ch7 oil bearing formation. Consequently, such a conclusion can be drawn that stratigraphic boundary between Ch7 and Ch8 oil bearing formations could be regarded as the boundary between mid-Triassic and upper Triassic and it can be also regarded as the representative of the first episode of the Indosinian Movement.