陕西咸阳地热田地热流体成因研究

Study on the genesis of geothermal fluid in Xianyang geothermal field

陕西咸阳地热田位于关中盆地腹地,横跨两个不同的基底构造和岩性单元,南北分界线为渭北断裂.研究区地热水的水化学类型主要是Cl—Na型,渭北断裂北侧的水比南侧的水相对更成熟.氢氧同位素证据表明咸阳市区和武功县城的地热水来自于秦岭山区大气降水的补给;地热水γ(Na^+)/γ(Cl-)均大于0.85,阳离子交换指数位于-0.42~5.93之间,脱硫酸系数均大于1,推断研究区的地热水主要形成于溶滤作用,其中武功地区的热储层较封闭,可能存在沉积水;惰性气体同位素n(3 He)/n(4 He)—n(4 He)/n(20 Ne)表明流体循环仅发生在地壳深度内,没有幔源物质的补给.

Objectives: Xianyang is considered to be one of the most important geothermal fields in Guanzhong Basin.With the increasing exploitation of geothermal water year by year,it is one of the primary conditions for geothermal resource development to find out the origin of geothermal water. The Xianyang geothermal field is located in the hinterland of the Guanzhong Basin,spanning two different basement tectonic and lithologic units. The north-south boundary is the Weibei fault. Therefore,the recharge sources and geothermal conditions may show different characteristics on both sides of the Weibei fault. This paper will further study the fluid genesis of Xianyang geothermal field.Methods: We apply the analysis methods of hydrochemical characteristics,specific ion content ratios and hydrogen oxygen stable isotope characteristics of the geothermal water in Xianyang field to determine the recharge source and evolution characteristics of the geothermal water,and also apply the analysis method of noble gas isotope ratio to determine the origin of the geothermal gas.Results: The geothermal water chemistry types are mainly Cl-Na type,and the water on the north side of the Weibei fault are relatively more mature than the water on the south side. There is an obvious oxygen shift in the Xianyang geothermal water,the maximum oxygen shift could up to 10‰. The hydrochemical characteristics,hydrogen and oxygen stable isotope values of Xianyang geothermal water is similar to Xi’an geothermal water;the r Na/r Cl of geothermal water in the study area are greater than 0.85,the cation exchange index is between-0.42 and 5.93,and the desulfurization coefficient is greater than 1. The relationship between the noble gas isotope3 He/4 He-4 He/20 Ne of dissolved gases in geothermal water located on the mixing line of the crust and air.Conclusions: Xianyang geothermal water was recharged from atmospheric precipitation in the Qinling Mountains,most of the geothermal water is orignated from atmospheric precipitation or surface water,and the possibility of mantle derived material is excluded.