Sequence lithofacies paleogeography evolution of the Middle Permian Maokou Formation in the northwest margin of the Sichuan Basin

Due to the present tectonic and stratigraphic distribution characteristics in the northwest margin of the Sichuan Basin are complex, which restricts the understanding of sequence lithofacies paleogeography of the Middle Permian Maokou Formation. To investigate the process of tectonic-sedimentary evolution, basin-margin outcrop and intra-basin well and seismic data were used. The results show two structural sequences, namely, SSQ1 and SSQ2, and five third-order sequences, namely, SQ1-SQ5, in the Middle Permian Maokou Formation. SSQ1, with stable formation thickness, shows a slow transgression-fast regression sequence structure. SSQ2 shows a fast transgression-slow regression sequence structure in the area with large thickness and a sustained transgression-fast regression sequence structure in the area with small thickness owing to thickness differentiation caused by tectonic subsidence. Sedimentological analysis shows that sequences SQ1-SQ2 mainly consist of carbonate-ramp sediments, the sedimentary facies of which change from intermediate ramp to outer ramp as the area depressed topographically from south to north. Sequences SQ3-SQ5 mainly consist of rimmed carbonate platform sediments; the platform margin turned up along Shuangyushi-Jian'ge-Yuanba, and the region to the north subsided rapidly to form slope-basin facies. Lithofacies paleogeography in the Middle Permian epoch shows that the northwest margin of the Sichuan Basin was dominated by extensional stresses in northeast and northwest directions. Tectonic activities have a dominant control on the distribution of two structural sequences and the evolution of carbonate platform types in the Maokou Formation, and the sea-level change has a dominant control on third-order sequence boundaries and sedimentary facies migration. Thus, the Middle Permian tectono-sedimentary evolution model of the northwest margin of the Sichuan Basin is established. Research findings may offer new ideas and theoretical support to promising facies exploration in the Maokou Formation in the northwest margin of the Sichuan Basin.