Natural Hydrogen Generation from Siderite–Brine Interactions: Experimental Insights into Temperature and Pressure Effects

Iron carbonates and oxides are considered potential contributors to subsurface hydrogen generation alongside the well-established serpentinization process. Among iron carbonates, siderite (FeCO3) has attracted considerable attention because of the presence of Fe2+ in its structure. However, previous studies have focused mostly on synthetic siderite, and the influence of temperature and pressure on hydrogen generation from natural siderite remains poorly constrained. In this study, hydrothermal experiments were conducted using a natural siderite-rich sample from the Georgina Basin, Australia, to quantify hydrogen generation under representative subsurface conditions. The experimental results revealed that temperature strongly controls gas generation, with no detectable H2 or CO2 at 70 °C after 60 days and substantial production of both gases at 200 °C. Under atmospheric pressure at 200 °C, H2 and CO2 yields reached 5.6 and 9.1 mmol kg–1 rock, respectively, increasing to 8.7 and 12.9 mmol kg–1 rock at 100 bar. XPS, XRD, and SEM analyses confirm partial oxidation of Fe2+, dissolution of siderite, and precipitation of secondary magnetite and iron oxides. These findings demonstrate that elevated temperature is essential for H2 generation, while increased pressure further enhances gas production from natural siderite-rich systems, supporting the evaluation of siderite-rich basins as natural hydrogen resources.