Abstract Greensands formations are globally abundant sedimentary rocks rich in Fe clays (typically glauconite) that commonly contain natural hydrocarbon accumulations and may be important reservoirs for geologic storage of anthropogenic CO2. Diagenesis in greensands is commonly accompanied by the conversion of primary glauconite to siderite (FeCO3), a process that could be exploited for the permanent trapping of CO2. Importantly, siderite formation after glauconite requires that the mostly oxidized Fe in the primary Fe clay minerals is reduced during diagenetic interactions. Here, we explore the effect of solution redox state on the stability of glauconite in sandstones with implications for the diagenetic and/or engineered formation of siderite. We performed two flow-through experiments on intact, glauconite-rich sandstone cores at 150 °C and 150 bar. Both experiments employed a 1 mol NaCl/kg, 0.1 mol NaHCO3/kg solution charged with ~0.58 mol CO2/kg solution, but the redox state of the injected fluid was manipulated between experiments in order to compare glauconite reactivity and siderite saturation state at oxidizing and reducing end-member conditions. After reaction with the oxidizing (O2 (aq) ≈ 6 μmol/kg) fluid, chemical and Mӧssbauer spectroscopic analyses indicate the production of Fe(III)-oxy/hydroxide minerals from glauconite, whereas, in the reducing (H2(aq) ≈ 5–40 mmol/kg) experiment, thermodynamic calculations and coupled chemical, mineralogical, and Mӧssbauer analyses suggest glauconite dissolution and precipitation of an Fe(II) mineral, likely siderite, and minor magnetite formation. These experimental results, along with thermodynamic calculations, confirm that solution redox state is the master variable dictating siderite formation in greensands.

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氧化还原在海绿石-CO2-盐水相互作用中的作用的实验评价

摘要 绿砂地层是全球丰富的富含铁粘土（通常为海绿石）的沉积岩，通常含有天然碳氢化合物堆积，可能是人为二氧化碳地质储存的重要储层。绿砂中的成岩作用通常伴随着原生海绿石向菱铁矿 (FeCO3) 的转化，这一过程可用于永久捕获 CO2。重要的是，海绿石之后菱铁矿的形成要求原生铁粘土矿物中大部分氧化的铁在成岩相互作用过程中被减少。在这里，我们探讨了溶液氧化还原状态对砂岩中海绿石稳定性的影响，对菱铁矿的成岩和/或工程形成有影响。我们对完整的、150 °C 和 150 bar 下富含海绿石的砂岩岩心。两个实验都使用了 1 mol NaCl/kg、0.1 mol NaHCO3/kg 溶液，其中加入了 ~0.58 mol CO2/kg 溶液，但在实验之间控制了注入流体的氧化还原状态，以比较氧化时海绿石反应性和菱铁矿饱和状态并减少终端成员条件。与氧化性 (O2 (aq) ≈ 6 μmol/kg) 流体反应后，化学和 Mӧssbauer 光谱分析表明从海绿石中生成 Fe(III)-氧/氢氧化物矿物，而在还原性 (H2(aq) ≈ 5–40 mmol/kg) 实验、热力学计算和耦合化学、矿物学和 Mӧssbauer 分析表明海绿石溶解和沉淀的 Fe(II) 矿物，可能是菱铁矿和少量磁铁矿形成。这些实验结果，