Objectives: With abundant occurrence in organicrich shales，calcite veins carry important information about hydrocarbon migration and accumulation. This study aims to elaborate their formation process，reveal the behind mechanism，and further reconstruct the hydrocarbon migration and accumulation history，with a case study in the Lower Silurian Longmaxi Formation in the Southern Sichuan Basin，China.Methods: The origin of calcite veins is analyzed by microlithofacies identification，cathodoluminescence observation，insitu microelectron probe analysis，isotope geochemistry characterization and fluid inclusion measurement，accompanied by discussion on dynamic evolution of diagenetic fluids and its geological significance to shale gas exploration and exploitation based on understanding of the overall geological background.Results: There are mainly three generations of calcite veins，denoted as Cal1，Cal2 and Cal3，respectively. Specifically，Cal1 is featured by its growth in the vicinity of surrounding rocks or independent growth，abundance in Fe，Mg，and Al and deficiency in Mn，its δ13CV-PDB range of 0.06‰ to 4.53‰，δ18OV-PDB range of -13.21‰ to -10.79‰，and strontium isotope range of 0.719366 to 0.719689. In comparison，Cal2 is characterized by its abundance in Fe，Mg，Mn and Al，δ13CV-PDB range of -6.93‰ to -0.08‰，δ18OV-PDB range of -1328‰ to -10.05‰，and strontium isotope range of 0.719378 to 0.720688. Cal3 is featured by its growth far away from the surrounding rocks，abundance in Fe，Mg，and Mn and deficiency in Al，δ13CV-PDB range of -19.00‰ to -12.64‰，δ18OV-PDB range of -9.08‰ to -6.65‰，and strontium isotope range of 0.719855 to 0.721342.Conclusions: Cal1 is formed in the syngenetic to penesyngenetic stage as product of microbial reduction； Cal2 is formed in the eogenetic stage as product of organic matter decarboxylation； and Cal3 is formed in the telogenetic stage as product of methane thermal oxidation. Through above investigation into origin and fluid source of these three generations of calcite veins，their diagenetic evolution models are built in conjunction with knowledge on the thermal evolution history. In general，presence of these calcite veins improves shale reservoir performance，and thus is conducive to exploration and exploitation of shale gas.