EVIDENCE FOR METASTABLE EQUILIBRIUM BETWEEN HYDROCARBONS UNDER HYDROTHERMAL CONDITIONS

THERMAL maturation of organic matter in sedimentary basins results in the generation of numerous organic alteration products. These products influence the chemical and physical alteration processes of sediments (1-3), and in some instances their accumulation results in the formati on of oil and natural-gas deposits(4,5). Much uncertainty exists about the factors that control the relative abundances of individual organi c species during maturation. Although it is clear that kinetic barrier s allow thermodynamically unstable species to persist in a metastable state for geologically significant periods of time, local equilibrium between more-reactive species may substantially influence their abunda nce. Here I report the results of redox-buffered hydrothermal experime nts designed to investigate reactions that may occur in geological env ironments between ethane, ethene, water and inorganic redox-sensitive minerals. I find that reversible metastable thermodynamic equilibrium is attained between these species. Because water participates directly in this equilibrium, it may represent a reactive and abundant source of hydrogen for hydrocarbon generation in sedimentary basins. This dem onstration that metastable equilibrium is attained implies that some o rganic geochemical reactions can be thermodynamically modelled and pre dicted despite the absence of total thermodynamic equilibrium.