Origin of structurally controlled hydrothermal dolomite in epigenetic karst system during shallow burial: An example from Middle Permian Maokou Formation, central Sichuan Basin, SW China

摘要

Based on the outcrops, drilling cores, thin sections observation and geochemical analysis, studies were done on the genesis of dolomite in the Middle Permian of central Sichuan Basin. Mosaic fine-crystalline dolomite and subhedral-euhedral siliceous fine-crystalline dolomite can be identified microscopically, which have different geochemical characteristics. Genetic analysis indicates insufficient mantle-derived fluids and marine-derived fluids entered karst system through basement faults, and then were mixed with previously-preserved crust-derived fluids in varied degrees during early Longtan period; in the relatively porous karst system, carbonate sands were dolomitized to subhedral-euhedral dolomites; insufficient mantle-derived hydrothermal fluids were mixed with previously-preserved crust-derived fluids, with coincidence reconstruction on the regions where there were preexisting karst systems but short of hydrothermal fluids, resulting in precursor limestone breccias and host rocks dolomitized to form anhedral mosaic dolomite; during late Longtan period, the overlying sediments sealed the epigenetic karst systems, and the insufficient deep mantle-derived fluids were mixed with previous fluids within the karst systems, with coincidence reconstruction on the relatively porous dolomites, while the tight anhedral mosaic fine-crystalline dolomites didn't change much, forming two types of dolomites with different petrological and geochemical characteristics. Thus, the formation of preferable dolomite reservoir is possibly related to inland facies-controlled epigenetic karst, and therefore the exploration of Maokou Formation in Middle Permian should pay more attention to the karst-related reservoirs.