Abstract
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/m3, with an average of 1,551.08 ± 787.08 ng/m3 (n = 37, 1 SD). The Δ199Hg values of mercury stable isotopes range from 02‰ to 0.39‰, with an average of 22‰ ± 0.08‰ (n = 37, 1 SD); the δ202Hg 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 Δ199Hg and δ202Hg information of coalbed gas, but similar to the Δ199Hg and δ202Hg information of terrestrial oil-type gas and the Δ199Hg 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 Δ199Hg are the target strata for the exploration of terrestrial oil and gas.
Highlights
As proposed by Ozerova (1983), the concentration of mercury reflects the origin of natural gas. Dai (1992) showed that Hg0 concentrations are greater than 700 ng/m3 in coalbed gas and less than 500 ng/m3 in oil-type gas
The mercury content in shale gas detected by Han et al (2021) in the southern Sichuan Basin 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)
According to the lithology of the reservoir, natural gas can be divided into coal-formed gas, shale gas, and tight sandstone gas (Liu, 2018)
Summary
As proposed by Ozerova (1983), the concentration of mercury (elemental mercury, Hg0) reflects the origin of natural gas. Dai (1992) showed that Hg0 concentrations are greater than 700 ng/m3 in coalbed gas and less than 500 ng/m3 in oil-type gas. 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 Hg0 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. The acid potassium permanganate solution was recovered as a sample for mercury content and mercury isotope analysis in shale gas. The mass fractionation of mercury isotopes is usually expressed by the thousandth fractionation (‰) of δXXXHg, where δ refers to the deviation between the sample mercury isotope ratio and FIGURE 3 | Mercury content in different types of natural gas. It is calculated according to the following formula (Blum and Bergquist, 2007): Δ199Hg δ199Hg − δ202Hg p 0.2520
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