Authigenic calcite in shales: Implications for tracing burial processes and diagenetic fluid evolution in sedimentary basins

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. Calcium sources for authigenic calcite include pore water, calcium minerals dissolution, and clay mineral transformation. Organic carbon sources of authigenic calcite refer to organic matter that undergoes diagenetic thermal evolution, redox reactions, and bacterial effects. Inorganic carbon primarily arises from carbonate dissolution, magma degassing, and thermal decomposition of carbonates during metamorphism. 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. Influenced by tectonic shear and compressive stresses, fibrous and bladed crystals intersect the fracture plane obliquely at varying angles. Authigenic calcite in shale strata serves as a valuable tracer for sedimentary basin evolution, fluid evolution, and burial history due to its extensive and multi-stage formation process. However, its complex history retains characteristics from various sources and evolution stages, resulting in distinct isotope fractionation features. Calcite formed during early burial diagenesis undergoes late-stage diagenetic alteration, accumulating carbon isotope features from multiple processes. This complexity presents difficulties in retracing the formation process. Utilizing physical and numerical simulations based on burial conditions aids in analyzing authigenic calcite genesis and reconstructing its formation history. The formation history can be determined through in-situ micro-area isotope testing and analyzing fluid inclusions for temperature, pressure, and composition.