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Due to the present tectonic and stratigraphie 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. [ABSTRACT FROM AUTHOR]

[Display omitted] • The deformation of the Huayingshan belt is featured by NW compression and NE transpression. • The two stages of deformation occurred at ∼105–78 Ma and ∼38–19 Ma, respectively. • Tectonic regime of the Huayingshan was dominated by the Xuefengshan belt in the Cretaceous and Tibetan Plateau in the Neogene. The Huayingshan tectonic belt in South China marks the tectonic boundary between the Xuefengshan intracontinental orogenic belt and the Sichuan Basin, and thus records interactions between the two domains. However, the processes of these interactions are still unclear due to the lack of knowledge about deformation patterns and timings. In this study, we present geometry and kinematics of the central Huayingshan tectonic belt in the Laogongqiao area (a critical location in the eastern Sichuan Basin) using field mapping, paleostress analysis and calcite U-Pb dating. Structural analysis indicates this belt is mainly featured by a southwest-plunging anticlinorium and syntectonic northwest-verging imbricated thrust faults (D1) that are cut by a left-lateral transpressional fault (D2). Results of paleostress analysis and calcite U-Pb dating from fault zones further demonstrate two stages of deformation of the Huayingshan tectonic belt, in the Late Cretaceous (105–78 Ma) and Late Eocene-Early Miocene (38–19 Ma), respectively. The deformation scenarios are correspondingly interpreted to be associated with a) final termination of the northwestward propagation of the Xuefengshan intracontinental orogenic belt during the Cretaceous; and b) eastward extrusion, uplift and southeastward escape of the Tibetan Plateau during the Neogene. Thus, the Sichuan Basin, which lies between these two active areas, underwent compression and counterclockwise rotation during the Late Eocene-Early Miocene time. As a result of the conjugation, movement along the right-lateral Leibo fault led to partial division of the southern Huayingshan tectonic belt into a system of large folds, that in map view forms a splay diverging to southwest, by 19 Ma. [ABSTRACT FROM AUTHOR]

The formation mechanisms of multi-stage dolomites and hydrocarbon reservoirs associated with volcanic activity are complex. This study investigated the Permian Chihsia Formation in the Sichuan Basin, China, which is the location of recent discoveries of giant natural gas reserves. We undertook a petrological and geochemical study, including trace element and C–O–Sr isotopic analysis. The quality of the dolomite reservoirs associated with volcanic activity was controlled by different diagenetic pathways associated with the sediment types and fault systems. Outcrop, core, and thin-section observations identified three types of replacement dolomite: very finely to finely crystalline dolomite (Rd1), medium crystalline patchy dolomite (Rd2), and coarsely to medium crystalline dolomite (Rd3). The tidal flat sediments only experienced limited seepage reflux dolomitization, which resulted in the formation of the dense very finely to finely crystalline dolomite (Rd1). The bioclastic grain shoal sediments deposited in a high-energy shallow-water environment contain abundant intergranular and dissolution pores, and vugs, which allowed convection of seawater and dolomitization during the initial phases of volcanic activity of the late early Permian. Subsequently, the bioclastic grain shoal sediments were mostly replaced by coarsely to medium crystalline dolomite (Rd3), which formed the highest-quality reservoirs in the study area. In addition, the incompletely dolomitized medium crystalline patchy dolomite (Rd2) was formed due to the low original porosity of micritic, peloidal, grain shoal sediments during a transgression in the middle and late depositional stages. During the peak of volcanism at the end-Guadalupian, deep faults allowed seawater to mix with ascending hydrothermal fluids, which resulted in the precipitation of dolomite cement in pores near the fault fracture zones, which blocked the pores. Therefore, these high-energy shoal sediments deposited in the slope break zone far from strike-slip faults have a relatively high porosity. Our results provide new insights into the origin of dolomite reservoirs formed in association with a volcanic setting, and are applicable to deep dolomite reservoirs worldwide. • Four dolomite phases were identified, including replacement and cement dolomites. • In-situ element data reveal dolomitizing fluids are mainly derived from seawater. • A model of seawater convection dolomitization in a thermal setting • Dolomite reservoir quality under a volcanic setting is still controlled by facies. [ABSTRACT FROM AUTHOR]