Study To The Formation Of Nature Gas Enriched Helium In Geothermal Water In Xi'an District

Basing on the study of main faults,basement's petrology of Xi'an depression and its geological evolution,δ¹³C_org,δ¹³C_CO2and the helium isotope of nature gas from geothermal water are analyzed together with the component and content of the gas.Gas bed,stratigraphy and its distribution of the nature gas enriched helium,relationship of distribution with the faults and basement's petrology are ascertained.Origin of the methane and the helium gas and their source rocks are discussed.This research provides much important information for gas exploration in geothermal water in Xi'an area.According to the component and content of the gas contained in geothermal water together with drilling documents,the gas bed horizon is from 1200m to 1800m,stratigraphy of gas beds are tertiary,including Lantian-Bahe formation,Gaoling formation,Bailuyuan formation and Honghe formation,The horizon of gas bed is almost the same with geothermal water.The gas in geothermal water is distributed within the Tieluzi-Huxian fault,the Chang'an-Xianyang fault and the Weihe fault,which shows that the distribution of gas are controlled by faults and the tectonic evolution in Cenozoic era.δ¹³C₁ of gas lower than -55‰in Huxian area proved that it is biochemical gas,which came from the black mudrock of the Zhangjiapo formation in Huxian area.δ¹³C₁ of gas contain in geothermal water in other area is from -27.2‰to -38.7‰,which is a kind of thermal pyrolysis gas,relate to the Paleozoci hydrocarbon source rock.³He/⁴He ratio is from(4.7±0.24)×10^-8 to(7.80±0.23)×10^-7 in the gas in geothermal water which indicates that the gas generated from the crust and the mantle. The helium gas is radiogenic with a little from the mantle,whose main source rock is the granite distributed at the basement of Weihe basin and the Muhuguan granite and the Lantian granite in Qinling Mountain area near Xi'an.