Abiotic Hydrocarbons Generation Simulated by Fischer-Tropsch Synthesis under Hydrothermal Conditions in Ultradeep Basins

FTT experiments with water as a hydrogen source and three types of possible carbon sources in the subsurface(diiron nonacarbonyl,siderite and formic acid,representing CO,CO_(2)and a simple organic acid,respectively)were carried out in this study.Our experimental results showed that n-alkanes with the highest carbon number of C_(33)were produced when CO was used as a carbon source(series A);a variety of polycyclic aromatic hydrocarbons(PAHs)were detected in series B with CO_(2)as a carbon source;gaseous hydrocarbons were also detected with formic acid added(series C).The different products in the three series showed that there were different hydrocarbon generation mechanisms and reaction processes with different carbon sources.The generation of long-chain n-alkanes in series A provided experimental support for the formation of abiogenic petroleum underground,which was of significance to early membranes on the Earth.PAHs in series B provide experimental support for the possibility of an abiotic source of reduced carbon on other planets.The carbon isotopes of gaseous hydrocarbons produced by CO exhibited a partial reversed order(δ^(13)C_(1)<δ^(13)C_(2)>δ^(13)C_(3)>δ^(13)C_(4)>δ^(13)C_(5)),while the gaseous hydrocarbons produced by CO_(2)and HCOOH showed a positive order(δ^(13)C_(1)<δ^(13)C_(2)<δ^(13)C_(3)<δ^(13)C_(4)<δ^(13)C_(5)).Based on these,the alkylene mechanism and the carbonyl insertion mechanism were used to reasonably explain these characteristics.