Genetic Mechanism And Indicating Significance Of Fracture Calcite Veins In The Longmaxi Shales Of Southern Sichuan Area

Calcite veins are widely developed in organic-rich source rocks,and their formation process and mechanism are of great significance to the formation and evolution of hydrocarbon reservoirs and fluids.Compared to shallow oil-layers with low thermal evolution,In gas-bearing formations with high thermal evolution and organic-rich matter,the source of diagenetic fluid of calcite veins,the mechanism of formation and its influence on shale reservoirs are still unclear,which has become one of the important factors restricting the deep petroleum exploration.In this paper,taking the Longmaxi Formation in the southern Sichuan area as an example,The fluid source and genetic mechanism of calcite veins are analyzed by field survey,petrographic observation,cathodoluminescence,in-situ micro elements,isotope geochemistry,fluid inclusions and whole rock testing,At the same time,based on the geological background of the southern Sichuan,the significance of calcite vein diagenetic evolution to hydrocarbon accumulation is discussed.The results show that the growth path of calcite veins is complete and the growth process is multi-stage;the homogenization temperature of the inclusions in calcite veins is 132?227?,the salinity is 5.2%?12.6%.The characteristics of high temperature and high salinity of inclusions record the correlation between diagenetic fluid and deep heat fluid;the surrounding rock is rich in Al and poor in Mn,while the vein is rich in Mn and poor in Al,and the content of Mn element in vein is 1600×10^-6,The content of Mn element in surrounding rock is 160×10^-6.The enrichment of Mn²⁺indicates that the element composition of veins has the characteristics of volcanic material.and the high value of⁸⁷Sr/⁸⁶Sr reflects that the diagenesis process of the veins is affected by the strontium from the crust.According to the comprehensive analysis,the source of diagenetic fluids of calcite veins is related to hydrocarbon organic fluid,deep strontium-rich fluid and inorganic diagenetic fluid.Three stages of calcite veins are mainly developed,corresponding to three genetic mechanisms:the first stage calcite vein(Cal-1)is the product of biological oxidation and microbial reduction,Its?¹³C_V-PDB=0.76‰?3.48‰??¹⁸O_V-PDB=-11.25‰?-7.13‰?⁸⁷Sr/⁸⁶Sr=0.719378?0.720688;the second stage(cal-2)is the product of oil source transformation and organic matter decarboxylation,Its?¹³C_V-PDB=-7.01‰?-0.83‰,?¹⁸O_V-PDB=-15.69‰?-10.23‰,⁸⁷Sr/⁸⁶Sr=0.719366?0.721225;the third stage(cal-3)is the product of methane thermochemistry and thermooxidation,Its?¹³C_V-PDB=-17.21‰?-13.49‰,?¹⁸O_V-PDB=-9.55‰?-5.26‰,⁸⁷Sr/⁸⁶Sr=0.720644?0.721342.The genesis of calcite veins has organic-inorganic characteristics,and the source of diagenetic fluids is closely related to the generation and expulsion of hydrocarbons from the source rocks.The diagenetic evolution of calcite veins fills the pore-fracture system of reservoir.However,the impact of the calcites on the physical properties of shale reservoirs has a critical effect(?5%),Overall,Calcite fillings within a certain content range may be beneficial to improving the spatial structure of shale,enhance the adsorption and storage capacity of shale gas and increase the gas content of shale.