The concentration of mercury vapor (elemental mercury, Hg-0) in natural gases has been used to differentiate between coalbed gas and oil-type gas. However, the concentrations of Hg-0 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 delta Hg-202) and the mass-independent fractionation (MIF, measured with Delta Hg-199, Delta Hg-200 and Delta Hg-201) of Hg-0 isotopes between coalbed gas and oil-type gas. Coalbed gas has an extremely negative delta Hg-202 (-5.8 parts per thousand to -3.08 parts per thousand) and negative Delta Hg-199 (-0.19 parts per thousand to -0.01 parts per thousand) relative to the assumed source, which is Permian coal from the Henan Province, China. Oil-type gas is characterized by less negative Delta Hg-202 (-3.08 parts per thousand to -0.77 parts per thousand) and positive Delta Hg-199 (0.06 parts per thousand-0.30 parts per thousand) relative to its E(2-3)s source rocks in the Liaohe oilfield (dark mudstones: delta Hg-202 = -1.96 parts per thousand to -2.48 parts per thousand, Delta Hg-199 = 0.09 parts per thousand-0.17 parts per thousand). This significant difference in the MIF can be used to distinguish between coalbed gas and oil-type gas. Notably, the Delta Hg-200 values of our samples are as high as 0.15 parts per thousand. We suggest that significant Hg-0 MDF might occur during the formation of coalbed gas, but not during the formation of oil-type gas. This study highlights the potential of Hg-0 isotopes as proxies for the source of natural gases.