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.
Bibliographical noteFunding Information:
We gratefully acknowledge support from the Department of Energy (DOE) Geothermal Technologies Program under Grant Number EE000276 for this contribution and related research. Rick Knurr and Nicholas Seaton are gratefully acknowledged for assistance with ICP-OES and SEM analyses, respectively, and Anthony Runkel is thanked for his assistance in acquiring the core samples utilized in this study. XRCT data and images were produced at the X-ray Computed Tomography Laboratory in the Department of Earth Sciences, University of Minnesota (UMN), which received funding from a UMN Infrastructure Investment Initiative Grant. SEM images were acquired within UMN Characterization Facility, which receives partial support from the U.S. National Science Foundation through the MRSEC program. This manuscript uses data and analyses that formed the foundation of T.K.’s M.S. thesis at the University of Minnesota. Finally, we thank two anonymous reviewers for their thorough reading of this manuscript, which helped to improve it significantly.
- CO2 storage