Diagenetic Environment Evolution of Fracture Veins of Shale Core in Second Member of Niutitang Formation in Southern Limb of Huangling Anticline and Its Connection with Shale Gas Preservation

Core observation shows that high-angle and bed-parallel fractures of tectonic compression origin develop in the black shale of the second member of the Lower Cambrian Niutitang Formation in the southern limb of Huangling anticline,West Hubei. Typical fracture veins with different occurrences were selected,calcite vein was filled in high-angle fractures and calcite-dolomite composite vein was filled in bedding fractures under thin section microscopy. Through cathodic luminescence,fluid inclusion analysis and in-situ element determination,the evolutions of diagenetic environment of fractured veins were studied,and the effects of different occurrence fractures on local sealing of shale gas reservoirs were analyzed. The cathodic luminescence observation indicates that the high-angle calcite vein and bed-parallel calcite-dolomite composite vein were both formed by two phases of fluid activity. According to fluid inclusion analysis,the early vein-forming fluids of high-angle calcite vein were mainly deep-metamorphic stratum brine while the late vein-forming fluids were mixed with low-salinity fluids,however the two-stage vein-forming fluids of bed-parallel calcite-dolomite composite vein were not modified by low salinity fluids. The in-situ element determination results of carbonate minerals with different cathodoluminescence characteristics in fracture veins,and analyses of iron and manganese content,rare earth elements(REE) and redox discriminant parameters,show that the diagenetic environment of high-angle calcite vein has undergone a transformation from reductive environment to oxidative environment,while the diagenetic environment of bed-parallel calcite-dolomite composite vein has maintained a relatively reductive environment for a long time. Therefore,it is considered that high-angle fractures may have destructive effect on the local sealing conditions of shale gas reservoirs,resulting in the loss of shale gas to a certain extent. While bed-parallel fractures may have relatively limited destructive effect on the local sealing conditions. The study of fracture development characteristics and diagenetic evolution process can provide basis for the evaluation of self-sealing conditions of shale gas reservoirs and micro-preservation conditions of shale gas.