Origin and Evolution of Paleofluid of the Middle Permian Maokou Formation in Southern Sichuan Basin

Abstract

Summary Based on petrology, rare earth elements, carbon and oxygen isotopes and fluid inclusions analysis, paleofluid origin and evolution of carbonate rocks in diffenert diagenetic stages of Maokou Formation are discussed. The results show that: (1) Maokou Formation was occurred “penecontemporaneous seawater cementation→ -eogenetic mixed water cementation→ epigenetic atmospheric freshwater dissolution→phyllomorphic formation water cementation, metasomatism and acidic-fluid dissolution”; (2) origins of sedimentary and diagenetic fluid include seawater, atmospheric freshwater, hydrocarbon fluids and thermal fluids: the oxidized seawater is characterized by the left-lead limestone REE, and similar the δ13C characteristics with the global paleo-seawater, the acidic hydrothermal fluid is mainfested Eu positive anomaly and obvious negative δ13C, atmospheric freshwater is conducted obviously negative δ18O in carbonate cement; (3) Maokou Formation was experienced multi-phase oil and gas charge: the Late Permian-Early Triassic Maokou Formation was exposed to leaching, and the anomalous thermal of the Emei mantle plume caused dolomitization and hydrocarbon charge. Meteoric water , organic acid dissolution, dolomitization and fracture play constructive roles in Maokou Formation reservoir. Dissolution and coarse-grained calcite cementation damaged the Maokou Formation since the Jurassic, karst reservoirs associated with structural fractures are more conducive to later gas accumulation.