关中盆地腹部地下热水成因类型的同位素水文地球化学证据

ISOTOPE AND HYDROGEOCHEMISTRY EVIDENCE OF GENETIC TYPES FOR GEOTHERMAL WATER IN THE CENTER PART OF GUANZHONG BASIN

关中盆地腹部深层地下热水的成因类型呈多样化特征,不同构造单元具有不同的成因类型,除少量现代入渗型水外,大多为古入渗型水、残余沉积水以及二者的混合水。其中,固市凹陷华阴051井地下热水的δD和δ^(18)O发生显著漂移,氘盈余(d)值低至-41‰,严重偏离大气降水线,几乎完全丢失了入渗时的大气环境的信号,表现为封存条件下同位素水文地球化学演化的极端过程,其氢氧稳定同位素δD、δ^(18)O值落在沉积水的δD、δ^(18)O值范围内;其水化学类型以Na·Ca-Cl型水为主,rNa/rCl<0.85,Cl/Br>293,rSO_4^(2-)/rCl低至1.09以及rCa/rMg高达12.26,符合沉积成因地下热水的水文地球化学特征,可能为地质历史时期被埋藏的残余蒸发湖水,其成因类型为残余陆相沉积水。固市凹陷渭南城区和咸礼断阶东部深层地下热水的成因类型以古入渗成因为主,不排除混有少量的残余陆相沉积水;西安凹陷和咸礼断阶西地下热水成因类型为入渗混合型,即现代入渗水和古入渗水的混合水。

The study area is the center part of Guanzhong basin （coordinates between 33°00′ ～ 35°20′N and 106°30′～ 110°30′E）,which is located in the middle of Shaanxi Province NW China and consists of Xi＇an depression （Xi＇an City）,Xianli terrace （Xiangyang City） and Gushi depression.The main exploitation geothermal aquifer in the basin is Lantian-Bahe Group （N2l＋bh）,which is also the main research object in the deep geothermal aquifer.In order to obtain evidence for genetic types of geothermal water environmental isotopes （δD,δ18OH2o,δ34S,δ18Oso4,14C,δ13C,8TSr/86Sr,3He/4He） 47 water samples in the study area were collected and tested between 2008 and 2013 in the study area.Among them,41 water samples came from the deep geothermal water,2 water samples were collected from the shallow geothermal water and 4 water samples were taken from meteoric water.Genetic types of the deep geothermal water in Guanzhong basin are diverse and different tectonic units have different genetic types.Besides a little modern meteoric infiltrating water,most geothermal water genetic types in deep aquifers are ancient infiltrating water,residual sedimentary water and the mixed water between them.δD and δ18O of the core Huayin 051 in Gushi depression had significant drift,d value was as low as-41‰,which obviously deviated from previously that of local meteoric water line and showed a extreme evolution process of isotope hydrogeochemistry under the closed conditions.Moreover,hydrogen and oxygen stable isotopes δ value fell on the range of sedimentary water δ values.Besides,water chemistry type was Na· Ca-Cl,rNa/rCl＜0.85,Cl/Br＞293,rSO24-/rCl was as low as 1.09,rCa/rMg was as high as 12.26,which was hydrogeochemical characteristics of sedimentary water.It offers the evidence that genetic type of the core Huayin 051 geothermal water is residual continental sedimentary water.The genetic type of geothermal water in Weinan City of Gushi depression and east of Xianli terrace are ancient infiltrating water mixed with a small amount of continental sedimentary water.The genetic type of geothermal water in the Xi＇an depression and west of Xianli terrace are ancient infiltrating water mixed with modern meteoric water.