Mercury isotope variation during organic matter maturation to petroleum

Abstract

Although both sedimentary rocks and petroleum generally contain detectable levels of Hg, the characteristics of Hg isotope fractionation and its transformation from biomass to sedimentary rocks to petroleum have not been sufficiently investigated to date. Here, we report Hg contents and isotope compositions in source rocks and crude oils from the Paleogene of the Bohai Bay Basin and in source rocks from the Permian and Jurassic of the Sichuan Basin in China. Our analysis made use of an improved crude oil Hg pre-concentration system involving pyrolytic cracking and KMnO4/H2SO4 Hg trapping to analyze Hg fractionation during organic matter maturation to petroleum. Positive mass-independent fractionation (MIF) signatures of Hg isotopes were detected in the source rocks, with Δ199Hg ranging between −0.04‰ and + 0.48‰ (mean + 0.12 ± 0.12‰), which is significantly different from the negative Δ199Hg of terrestrial non-source rocks. Positive Hg-MIF signatures were detected in the crude oil, with Δ199Hg ranging between +0.04‰ and + 0.21‰ (mean + 0.13 ± 0.06‰). We propose a model of Hg isotopic fractionation related to organic matter history. From organic matter to source rock, Hg system variations are attributed to endogenic (i.e., photoreduction) and exogenic processes (i.e., terrestrial, magmatic and hydrothermal Hg inputs). From source rock to petroleum, Hg is mobilized during hydrocarbon migration, a process that occurs with MDF but without significant MIF.