Molecular evidence reveals the presence of hydrothermal effect on ultra-deep-preserved organic compounds

Abstract

Hydrothermal alteration, transformation and synthesis of organic compounds have long been noted in submarine hydrothermal vents, terrestrial hot springs, epithermal deposits and laboratories. However, little is documented about the influence of deep hydrothermal activities on organic compounds at the ultra-deep horizon. It remains unclear to what extent the compounds are hydrothermally (secondarily) affected. Here we report molecular and isotopic level analyses of liquid oils derived from an ultra-deep reservoir in the Tarim Craton, south of the Central Asian Orogenic Belt. The organic compounds had an unexpectedly convoluted molecular and isotopic geochemistry, suggesting that the deep hydrothermal activities impact the organic compounds to varying degrees. In the absence of severe gas washing, in-reservoir mixing and thermochemical sulfate reduction, hydrothermal alteration results in a progressive increase in the relative abundance of organosulfur compounds. The transformation of three compound classes (e.g., phenanthrene ring systems, biphenyl ring systems and dibenzothiophene ring systems) during hydrothermal alteration was observed. As the extent of the hydrothermal effect increases, dibenzothiophene ring systems are favourably formed through the oxidation of phenanthrene ring systems, followed by the incorporation of sulfur from biphenyl ring systems that were less thermally stable. The spatial variation in the hydrothermal effect was decoupled from other thermogenic effects. A new perspective regarding the hydrothermal effect on molecular and isotopic compositions may shed light on the organic–inorganic interactions of deep-sourced organic fluids with their aqueous and mineralogical environments, and the temporal and spatial variation in organic signals and environments at known hydrothermal systems on Earth, Mars and even Titan.