Stable Carbon Isotopic Fractionation And Hydrocarbon Generation Mechanism Of Co2 Fischer-Tropsch-Type Synthesis Under Hydrothermal Conditions

A series of gold-tube isothermal laboratory experiments at 370 degrees C and 50 MPa were carried out to investigate the stable carbon isotope fractionation during the formation of hydrocarbon gases from siderite reacting with the iron-bearing minerals, which simulates CO2-H-2-rich fluids under geological settings. Only negligible amounts of hydrocarbons were generated in the pure siderite + H2O series. However, with the addition of Fe or FeO, a large amount of methane and some amounts of C-2-C-3 gaseous hydrocarbons were produced. The generated gaseous hydrocarbons exhibit increasing delta C-13 values with carbon number, opposite to the usual decreasing trend observed in the classical Fischer-Tropsch (FT) synthesis with CO as the carbon source. These two opposite trends in carbon isotopic distributions of hydrocarbon gases indicate different chain growth mechanisms for CO2 versus CO as carbon sources. A carbonyl mechanism is introduced in this study to expound the rationality of the increasing trend of delta C-13 values. Our experimental observation provides a perspective that in the Earth system where CO2 is the main carbon source the stable carbon isotopic distributions of abiogenic hydrocarbons may display an increasing delta C-13 distribution pattern with carbon number.