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.

中文翻译：

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页岩中的自生方解石：对沉积盆地埋藏过程和成岩流体演化的意义


自生方解石在页岩的各个成岩作用阶段大量形成。它详细记录了成岩流体（有机/无机）迁移和流体-岩石相互作用的信息，对于了解沉积盆地的埋藏成岩演化、构造历史、埋藏历史、碳氢化合物生成和成藏具有重要意义。自生方解石的钙来源包括孔隙水、钙矿物溶解和粘土矿物转化。自生方解石的有机碳来源是指经历成岩热进化、氧化还原反应和细菌作用的有机物。无机碳主要来自碳酸盐的溶解、岩浆脱气和碳酸盐在变质过程中的热分解。在早期埋藏成岩作用中，硫酸盐-甲烷过渡区通过甲烷的厌氧氧化保持较高的孔隙水碱度，促进方解石结核的形成。进入碳氢化合物生成窗口后，由于碳氢化合物相变和结晶力的超压，层状界面处会周期性地打开和关闭裂缝。在这些裂缝中，方解石溶解度随着流体压力的降低而降低，导致在强超压条件下纤维脉沉淀，在弱超压条件下形成叶片状或离子状晶体。受构造剪切应力和压缩应力的影响，纤维状和叶片状晶体以不同的角度斜着与裂隙面相交。页岩层中的自生方解石由于其广泛和多阶段的形成过程，是沉积盆地演化、流体演化和埋藏历史的宝贵示踪剂。 然而，其复杂的历史保留了各种来源和进化阶段的特征，从而产生了不同的同位素分馏特征。在早期埋藏成岩作用中形成的方解石经历了晚期成岩作用的改变，从多个过程中积累了碳同位素特征。这种复杂性给追溯形成过程带来了困难。利用基于埋藏条件的物理和数值模拟有助于分析自生方解石成因并重建其形成历史。通过原位微区同位素测试和分析流体包裹体的温度、压力和成分，可以确定形成历史。