Investigations of Light Hydrocarbon Formation at Elevated Temperatures and Pressures: Generation Kinetics and Its Implication



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Hydrocarbons, as an important energy resource in the world, are associated with the past of the Earth, and are generated through different processes: abiogenic, microbial, and thermogenic. Hydrocarbons of thermogenic origin are the most common composition of fossil fuels, whereas hydrocarbons of abiogenic origin, light alkane gases in particular, have also been observed in subseafloor hydrothermal systems on mid-ocean ridges and may have a close relationship with the origin of life on Earth. To better understand the generation of light hydrocarbons and the effect of geological environment on their production, a series of laboratory experiments are conducted under different conditions, including temperature, mineralogy, and redox. The ultimate objective was to assess those controlling factors for the formation of thermogenic and abiogenic hydrocarbons. The first chapter is focused on evaluating the role of sulfate minerals on chemical and isotopic compositions of light hydrocarbon generation during the thermal degradation of kerogen. The results showed the changes in chemical and isotopic compositions of light alkanes were caused by the presence of gypsum, and the changes need to be incorporated into geochemical proxies for assessing source rocks. The second and third chapters are centered on assessing the effect of different environmental conditions on the generation kinetics of light hydrocarbons of abiogenic origin. The experiments were set up in a thermodynamically open system, allowing for adjustment of each individual factor and subsequent evaluation. Magnetite or nickel oxide was used as a mineral catalyst in experiments, where carbon dioxide reacts with hydrogen through Fischer-Tropsch type (FTT) synthesis. The results suggested that nickel oxide is more active in generating light alkanes. Furthermore, we found temperature, pH (fluid acidity), and redox conditions contribute to different generation kinetics of light alkanes. Within the whole process, the mineral surfaces, which in turn are controlled by physical and chemical properties of fluids, are the key to the generation and selectivity of alkanes. Considering the importance of hydrothermal FTT synthesis as a likely source of hydrocarbons in hydrothermal systems, this study sheds light on the wide range of physicochemical factors that control the formation of different organic compounds and corresponding reaction mechanisms.



Light hydrocarbon, Thermogenic gas, Abiogenic gas, Thermochemical sulfate reduction, Fischer-Tropsch type synthesis


Portions of this document appear in: Chen, X., Liu, Q., Meng, Q., Zhu D., Liu W., & Fu Q. (2019). Assessing effects of sulfate minerals on petroleum generation in sedimentary basins using hydrous pyrolysis: I. light alkanes, Marine and Petroleum Geology, 110, 737-746.