delta(18)O shifts of geothermal waters in the central of Weihe Basin, NW China

The thermal-waters resources in Weihe Basin of Shaanxi province, NW China are historically classified as middle-to low-temperature thermal-waters in China. Recent exploitation of the deep thermal reservoir in the centre part of the basin (i.e. Xi'an and Xianyang) had observed plentiful supply of thermal fluid with higher measured maximum temperature (120 degrees C) and higher hydraulic pressure (80.50 MPa) in the deeper (more than 4,000 m deep) sedimentary basin. A recent isotope study shows that deep geothermal waters in the cities of Xi'an and Xianyang are characterized by an observable horizontal oxygen-18 (delta(18)O) shift and minor deuterium ((2)H) enrichment. The considerable oxygen shift is possibly due to the following four reasons: water-rock interaction at high temperature, slow circulation rate of water, low waterto- rock ratio, and old age. On the end number of the delta(18)O shift, minor delta(2)H enrichment occur when there is higher concentrations of H(2)S, CH(4), I and Br with lower rate of rSO(4)(2-)/rCl(-) and r(Na)(+)/r(Cl)(-) suggesting relatively isolated geological environment. In a few thermal waters points, rNa(+)/rCl(-) < 0.85. This shows possible presence of formation waters. Combining the results from isotopic study and chemical analysis, we can classify the types of geothermal waters into three groups, the shallow and fast circulating system, the semi-circulating system and the deep and slow circulating system.