Synthesis of hydrocarbon gases from four different carbon sources and hydrogen gas using a gold-tube system by Fischer–Tropsch method

• The heating experiment of FTS could not interpret the formation of abiogenic gas in nature.
• The gases cracking in FTS cause the discrepancy between natural and synthetic gas.
• The abiogenic gases should form rapidly under geological setting.

Several series of Fischer–Tropsch synthesis (FTS) experiments were conducted in a gold tube system with montmorillonite K-10 loaded with Fe3 + and Ni3 + as catalysts. Four different carbon sources: graphite, Na2CO3 solution (20%) and two types of CO2 gases with distinctive isotopic compositions were reduced in pure hydrogen gas at 400 °C and 50 MPa for 2–60 h. The experimental results showed that the FTS reaction between liquid carbon and H2 could hardly occur. However, the reaction between gaseous phase carbon (CO2) and H2 gas was easier than that between solid phase carbon (graphite) and H2 gas; and the 13C depleted CO2 is more reactive than the 13C enriched CO2. Our results also show that the production of synthetic hydrocarbon gases from different carbon sources with H2 depends largely on the phases, and structural and thermal stability of carbon sources. In a relatively short reaction time at 400 °C, the carbon isotope values of the synthesized alkane gases showed a full reversal trend with their molecular carbon numbers (δ13C1 > δ13C2 > δ13C3 > δ13C4). However, with increasing reaction time, such a reversed isotopic distribution pattern disappeared. Our interpretation is that the final products were gradually replaced by cracking of the hydrocarbon products formed at the earlier stage of the synthesis process. Thus the 13C depleted gas from the thermal cracking was mixed with the 13C enriched residual gas, leading to the occurrence of a partial reversal or a normal isotopic distribution among C1–C4 alkane series (δ13C1 < δ13C2 < δ13C3 < δ13C4), similar to the thermogenic alkane gases in nature. Under longer reaction time or/and higher reaction temperature (700 °C), hydrocarbon gases would crack and generate monatomic carbon.The observed great discrepancy between the natural abiogenic gas and synthetic gas is likely due to the big difference in temperatures between geological settings and the laboratory experiment process. FTS experiments conducted under laboratory experimental condition are usually from low to high temperature and differ significantly from the abiogenic synthesis process for hydrocarbons in real geological settings, which is perceived as a cooling process from high to low temperature either under aqueous hydrothermal or volcanic intrusion conditions. Under certain (stable) geological temperature/pressure conditions, hydrocarbon gases generated might never suffer further decomposition, and thus might preserve a fixed “inverse” molecular isotopic fingerprinting as we observed in the laboratory. This has also been proven by a cooling FTS experiment.