Unconventional generation of hydrocarbon gases in the Solimões basin, Brazil: the role of the siderite – water interaction

Understanding the processes at the origin of "abiotic organic matter", i.e., of reduced carbon compounds made without any biological intervention at any step, is of importance for the global carbon cycle, the origin of life building blocks, and in an economic prospect. In the Solimões petroleum basin (Brazil) which underwent magmatic intrusions, we detected hydrocarbon gases with unconventional isotopic signatures inconsistent with a thermal evolution of fossil organic matter. Concurrently, the siderite FeCO3) content of up to 7% wt. in the source rock of Solimões petroleum was shown to decrease meanwhile the gas maturity and the occurrence of isotopic anomalies increased. We thus decided to explore possible connections between siderite reactivity and abiotic production of reduced carbon species. Synthetic siderite and water were thus reacted in inert gold containers at 200 – 300°C and 50 MPa. Iron (II) from siderite and water reacted to yield magnetite and highly reducing fluids containing CO2 and H2. While the generation of CH4 is kinetically hindered, the produced H2 and CO2 reached equilibrium with solid reduced carbon after ~ 150 h of experiment. This was confirmed by transmission electron microscopy observations of carbonaceous material coating the magnetite and siderite surfaces. The H2 fugacity produced by the water-siderite interaction was thus controlled by the formation of a solid reduced carbon compound. This prompted us to examine whether similar controls could occur in the more general and classical context of H2 generation by serpentinization of basic and ultrabasic rocks. Thermodynamic calculations were thus performed to examine the composition of fluids along serpentinization reactions and to investigate the conditions of formation of solid reduced carbon compounds in these reactions. In all the calculations performed, solid carbonaceous material, thermodynamically approximated as graphite, was indeed shown to precipitate, buffering the H2 and CO2 concentrations. Experimental and natural (Lost City – Rainbow) fluid compositions were shown to be consistent with this model, which predicts that carbonaceous material is the dominant carbon product of CO2 reduction in ultrabasic and basic contexts. Formation of carbonaceous material from CO2- and H2-rich hydrothermal fluids, not only in basic and ultrabasic contexts but also in continental sedimentary basins, would have implications for the global geological carbon cycle, and for the general understanding of reaction mechanisms leading to abiotic reduced carbon compounds. In the Solimões basin, we estimated that the siderite-water interaction may account for ~ 20% of the hydrocarbon gas generated from the Famenian source rock of the basin and we showed that siderite is kinetically more reactive with water for H2 production than ultrabasic minerals such as olivine.