Abiotic CO2 reduction on transition metal minerals has been proposed to account for the synthesis of organic compounds in alkaline hydrothermal systems, but this reaction lacks experimental support, as only short-chain hydrocarbons (<C5) have been synthesized in artificial simulation. This presents a question: What particular hydrothermal conditions favor long-chain hydrocarbon synthesis? Here, we demonstrate the hydrothermal bicarbonate reduction at ∼300 °C and 30 MPa into long-chain hydrocarbons using iron (Fe) and cobalt (Co) metals as catalysts. We found the Co0 promoter responsible for synthesizing long-chain hydrocarbons to be extraordinarily stable when coupled with Fe-OH formation. Under these hydrothermal conditions, the traditional water-induced deactivation of Co is inhibited by bicarbonate-assisted CoOx reduction, leading to honeycomb-native Co nanosheets generated in situ as a new motif. The Fe-OH formation, confirmed by operando infrared spectroscopy, enhances CO adsorption on Co, thereby favoring further reduction to long-chain hydrocarbons (up to C24). These results not only advance theories for an abiogenic origin for some petroleum accumulations and the hydrothermal hypothesis of the emergence of life but also introduce an approach for synthesizing long-chain hydrocarbons by nonnoble metal catalysts for artificial CO2 utilization.