Preliminary Study On Mercury Contents And Mercury Isotopes In Shale Gas In YS108 Block,Sichuan Basin

Traditional stable isotopes in natural gas,such as carbon isotopes(δ¹³C)hydrogen isotope（δ²H）,nitrogen isotope(δ¹⁵N),sulfur isotope(δ³⁴S),and noble gas stable isotopes（He,Ar,etc.）have made a series of achievements,but there is still a lack of research on non-traditional mercury stable isotopes in natural gas,especially the mercury stable isotopes in shale gas developed and utilized in recent years.Mercury isotopes are widely used to trace mercury sources and study the geochemical process of mercury.Natural gas（including shale gas）generally contains mercury,and the toxicity and corrosivity of mercury have a serious impact on the exploitation,processing,and environmental ecology of natural gas.Therefore,it is necessary to study mercury and mercury isotopes in shale gas.This paper uses the acid potassium permanganate solution mercury capture method to capture mercury in shale gas in YS108 block of Zhao tong national shale gas demonstration area in Sichuan Basin,and the mercury content and stable isotope composition of mercury are analyzed.The mercury content and isotope characteristics of shale gas are summarized,and the differences of mercury content and isotope composition of different types of natural gas are discussed,the similarity between the mercury isotopic composition of natural gas and that of gas forming organic matter.The conclusions are as follows:（1）The mercury content in shale gas in YS108 block of Zhao tong demonstration area in Sichuan Basin ranges from 170.55 to 2906.22 ng/m³,with an average value of 1551.08±787.08 ng/m³（n=37,1SD）.Compared with other types of natural gas（coal formed gas,oil-based gas,and coalbed methane）,the mercury content is quite different,which is mainly related to the accumulation form of Oil and gas,the distribution,source,and type of organic matter in the gas source and the occurrence state of natural gas.（2）The mercury stable isotopeΔ¹⁹⁹Hg value of shale gas in the study area range from 0.02‰to 0.39‰,with an average value of 0.22‰±0.08‰（n=37,1SD）,andδ²⁰²Hg value range from-1.68‰to-0.04‰,with an average value of-0.87‰±0.31‰（n=37,1SD）;The Hg-MIF characteristics of shale gas are similar to the MIF values of black shale in the same horizon,and there is no significant difference with the MIF values of gas forming organic matter planktonic algae（t-test,P>0.05）,which are all positive.It is speculated that the mercury isotopic composition of shale gas is likely to retain the mercury isotopic composition of hydrocarbon forming algae and lower plankton.（3）The comparative analysis of MIF characteristics of shale gas,oil-based gas and coalbed methane shows that there are significant differences in MIF characteristics between shale gas and coalbed methane,but the MIF information is similar to that of continental oil-based gas.It is speculated that the gas forming organic matter of continental oil-based gas is mainly floating organisms such as algae rich in lake or delta environment,which can indicate the search for terrestrial target strata or delineation of algae in rich the rock stratum with a positive value ofΔ¹⁹⁹Hg is the exploration target stratum.The fractionation model of mercury isotope in the formation of shale gas is preliminarily speculated.（4）The fractionation model of mercury isotope in the formation of shale gas is preliminarily speculated.Shale gas well inherits the positive value Hg MIF of algae.For continental oil-bearing gas,only the terrestrial gas source rocks with positiveΔ¹⁹⁹Hg lake algae as the main organic matter can generate large-scale oil-type gas.