Mercury Isotopes in Shale Gas From Wufeng-Longmaxi Shale Formation of Sichuan Basin, Southern China: A Preliminary Investigation

A series of investigations have been conducted concerning the study of traditional stable isotopes and rare gas stable isotopes in natural gas. However, little is known regarding non-traditional stable isotopes of mercury in natural gas, especially in the development and utilization of shale gas in recent years. In fact, the presence of mercury in natural gas (including shale gas) provides a basis for research on mercury isotopes. Mercury was extracted from shale gas at the Wufeng-Longmaxi Formation in the YS108 block of the Zhaotong National shale gas demonstration area in the Sichuan Basin by using an acid potassium permanganate solution, followed by the analysis of mercury content and stable isotope composition. The mercury content in the marine shale gas at the Wufeng-Longmaxi Formation ranged from 171 to 2,906 ng/m 3 , with an average of 1,551.08 ± 787.08 ng/m 3 ( n = 37, 1 SD). The Δ 199 Hg values of mercury stable isotopes range from 02‰ to 0.39‰, with an average of 22‰ ± 0.08‰ (n = 37, 1 SD); the δ 202 Hg values range from −1.68‰ to −0.04‰, with an average of −0.87‰ ± 0.31‰ (n = 37, 1 SD), which are significantly different from the Δ 199 Hg and δ 202 Hg information of coalbed gas, but similar to the Δ 199 Hg and δ 202 Hg information of terrestrial oil-type gas and the Δ 199 Hg in the main hydrocarbon-forming organic matter of lower organisms such as algae ( t -test, p > 0.05). This indicates that terrestrial target strata with abundant algae or strata with positive Δ 199 Hg are the target strata for the exploration of terrestrial oil and gas.