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

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

“…The mercury content of 37 shale gas samples 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 mercury content in shale gas detected by Han et al (2021) in the southern Sichuan Basin (mean: 1,420 ng/m 3 ) is consistent with that detected by Han et al (2021), which is significantly lower than the average mercury content in the coal-formed gas of the Shanshan oilfield (Li, 2020) and the oil-type gas of the Liaohe oilfield, and higher than that in the coalbed gas of the Hebi Coal Mine (Tang et al, 2019). The overall mercury content demonstrates coal-formed gas > oil-type gas > shale gas > coalbed gas, but the mercury content varies by one to several orders of magnitude.…”

“…Mercury in shale gas is captured by potassium permanganate (KMnO 4 ) and sulfuric acid (H 2 SO 4 ) mixture: Hg 0 in shale gas was oxidized to ionic form mercury and captured and retained in acidic potassium permanganate solution (Figure 2) by taking advantage of the strong oxidation and super absorption of mercury in acidic potassium permanganate solution, such that the acidic potassium permanganate solution was recovered as a sample for mercury content and mercury isotope analysis. In previous studies (Tang et al, 2019), we used three series of acid potassium permanganate cylinders to capture mercury in oil-type gas and coalbed gas, but laboratory simulations and mercury isotope tests found that Hg 0 is mainly enriched in the first impinger (the percentage of mercury content is 76%-97%), and the isotopic composition of total mercury is consistent with that of the solution of the first impinger. Therefore, we simplified the mercury capture device for shale gas, and the main body of the mercury capture device was composed of three impingers (Environmental Supply Company, United States, 500 ml for each sampling bottle) in series.…”

“…The mercury isotopic composition in the mercury capture solution was determined using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS, Neptune Plus) at the Institute of Geochemistry, Chinese Academy of Sciences according to previous methods (Tang et al, 2017;Tang et al, 2019). The mercury concentration of the sample of mercury capture liquid to be diluted before the determination of mercury isotope was 0.5 ppb.…”

“…The Δ 199 Hg values (relative to NIST 3133) are positive (0.02‰-0.39‰), with an average of 22‰ ± 0.08‰ (n = 37,1 SD), which is consistent with the Hg-MIF values (Δ 199 Hg: +0.05‰-+0.15‰, with an average of 0.12‰ ± 0.03‰) of the Wufeng-Longmaxi black shale in the Sichuan Basin reported by Shen et al (2019), indicating that there is no evident mass-independent fractionation of mercury isotopes during gas formation from gas source rock to shale gas owning to the lack of exposure to light in the underground gas formation process leading to the MIF of the mercury stable isotope. Although some scholars have reported that the stable isotopes of mercury produce very small Hg-MIF under dark conditions (Zheng and Hintelmann, 2010) (Blum et al, 2014;Tang et al, 2019), photochemical reduction is believed to be the most important process causing Hg-MIF. Therefore, it is considered that the mass-independent fractionation of the stable isotope of mercury does not occur in the underground dark et al, 2019), but relatively positive, indicating that shale gas is more enriched in heavy isotopes of mercury than its source rock.…”

“…The Hg isotopic ratios show promise as a method for distinguishing different sources of natural gas. Stable isotopes of mercury in oil-type gas and coalbed gas have been studied (Washburn et al, 2018;Tang et al, 2019); however, stable isotopes of mercury in shale gas have not yet been reported.…”