The mercury isotope signatures of coalbed gas and oil-type gas: Implications for the origins of the gases

The concentration of mercury vapor (elemental mercury, Hg0) in natural gases has been used to differentiate between coalbed gas and oil-type gas. However, the concentrations of Hg0 in coalbed gas and oil-type gas are highly variable, and its use as a genetic indicator is problematic. Here, for the first time, we report significant differences in the mass-dependent fractionation (MDF, measured with δ202Hg) and the mass-independent fractionation (MIF, measured with Δ199Hg, Δ200Hg and Δ201Hg) of Hg0 isotopes between coalbed gas and oil-type gas. Coalbed gas has an extremely negative δ202Hg (−5.8‰ to −3.08‰) and negative Δ199Hg (−0.19‰ to −0.01‰) relative to the assumed source, which is Permian coal from the Henan Province, China. Oil-type gas is characterized by less negative δ202Hg (−3.08‰ to −0.77‰) and positive Δ199Hg (0.06‰–0.30‰) relative to its E2-3s source rocks in the Liaohe oilfield (dark mudstones: δ202Hg = −1.96‰ to −2.48‰, Δ199Hg = 0.09‰–0.17‰). This significant difference in the MIF can be used to distinguish between coalbed gas and oil-type gas. Notably, the Δ200Hg values of our samples are as high as 0.15‰. We suggest that significant Hg0 MDF might occur during the formation of coalbed gas, but not during the formation of oil-type gas. This study highlights the potential of Hg0 isotopes as proxies for the source of natural gases. •The Hg0 isotope composition in natural gases is presented for the first time.•The Hg0 isotope composition of coalbed gas is different from that of oil-type gas.•The MIF of Hg0 isotopes might reflect the origin of coalbed gas and oil-type gas.•Oil-type gases have a similar Hg0 isotopic composition to their Eocene source rocks.•It might be possible to use Hg0 isotopes to determine the origin of natural gases.