Abstract
Thermochemical oxidation of methane (TOM) by high-valence metal oxides in geological systems and its potential role as a methane sink remain poorly understood. Here we present evidence of TOM induced by high-valence metal oxides in the Junggar Basin, located in northwestern China. During diagenesis, methane from deeper source strata is abiotically oxidized by high-valence Mn(Fe) oxides at 90 to 135 °C, releasing 13C-depleted CO2, soluble Mn2+ and Fe2+. Mn generally plays the dominant role compared to Fe, due to its lower Gibbs free energy increment during oxidation. Both CO2 and metal ions are then incorporated into authigenic calcites, which are characterized by extremely negative δ13C values (−70 to −22.5‰) and high Mn content (average MnO = 5 wt.%). We estimate that as much as 1224 Tg of methane could be oxidized in the study area. TOM is unfavorable for gas accumulation but may act as a major methane sink in the deep crustal carbon cycle.
Highlights
Thermochemical oxidation of methane (TOM) by high-valence metal oxides in geological systems and its potential role as a methane sink remain poorly understood
Natural gas reservoirs in sedimentary basins are typically buried to depths of several kilometers, and experience temperatures from 60 to 150 °C17, while the metabolism of the archaeal groups responsible for anaerobic oxidation of methane (AOM) are mainly active at temperatures below 80 °C18,19, limiting the depth at which archaea dominate the conversion of CH4 to CO2
Electron probe micro-analysis (EPMA) reveals that these authigenic calcites are enriched in manganese, with MnO content ranging from 0.79 to 14.67 wt.%
Summary
Thermochemical oxidation of methane (TOM) by high-valence metal oxides in geological systems and its potential role as a methane sink remain poorly understood. Mn generally plays the dominant role compared to Fe, due to its lower Gibbs free energy increment during oxidation Both CO2 and metal ions are incorporated into authigenic calcites, which are characterized by extremely negative δ13C values (−70 to −22.5‰) and high Mn content (average MnO = 5 wt.%). The second is thermochemical oxidation of methane (TOM) at high temperatures by sulfate or high-valence metal oxides. The process has only been simulated in the laboratory setting at high temperatures (350 to 650 °C) 24–27 Both the extent of TOM by high-valence metal oxides in geological systems and its role in the methane sink remain poorly understood
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