Stratigraphic sequence and sedimentary facies distribution of the Permian-Middle Triassic in the northwestern margin of the Yangtze block

The Permian-Middle Triassic is mainly composed of carbonate rocks in the northwestern margin of the Yangtze block. In this paper, based on the application of theories and methods of carbonate sequence stratigraphy and the analysis of seismic data, outcrop, drill core and log information, we identify in the Permian-Middle Triassic strata four depositional sequence surfaces, i.e., erosional unconformity, transgression onlap sequence unconformity, subaqueous depositional break sequence unconformity and local exposure sequence unconformity. We also divide the strata into 11 third-level sequences and 23 system tracts. By the contrast of stratigraphic sequence, the sequence stratigraphic framework is established, within which characteristics of sedimentary facies of different system tracts are analyzed. And it’s concluded that platform edge reef and beach facies and platform inside beach facies are the best reservoir facies belts, and the former is mainly located in highstand systems tract of sequence I (Qixia Formation), and transgressive systems tract of sequence V (Changxing Formation) and sequence VI (Feixianguan Formation).