Characterization and genesis of authigenic calcite in multiple diagenetic stages of shales

  Authigenic calcite abundantly forms during various diagenesis stages of shales. It meticulously records information on diagenetic fluid (organic/inorganic) migration and fluid-rock interactions, is important for understanding the burial diagenetic evolution, tectonic history, burial history, hydrocarbon generation and accumulation in sedimentary basins. During early burial diagenesis, the sulfate-methane transition zone maintains high porewater alkalinity through anaerobic oxidation of methane, promoting calcite nodule formation. Upon entering the hydrocarbon generation window, periodic opening and closing of fractures occur at lamina interfaces due to overpressure from hydrocarbon phase transitions and crystallization forces. In these fractures, calcite solubility decreases with fluid pressure reduction, leading to fibrous vein precipitation under strong overpressure conditions and bladed or equant crystal formation under weak overpressure conditions. Recrystallization processes controlled by the Ostwald mechanism. Additionally, authigenic calcite preserves characteristics reflecting multiple sources and distinct evolution stages, resulting in notable isotope fractionation features, the carbon isotope features represent cumulative results of various processes. Utilizing physical and numerical simulations based on burial conditions aids in analyzing authigenic calcite genesis and reconstructing the diagenetic environment in which it formed.