Study Of Isotopic And Hydrochemical Features Of Geothermal Waters In The Central Guanzhong Basin

During our past research, it was found that features of Xian Yang and part of Xi'an geothermal waters in deep Guanzhong Basin (2600-4100m) were different from normal-pressured geothermal waters. The Moho surface in the deep Basin was haunching up and was shallowly buried(32km), and the water hight of geothermal waters from the ground here could be up to 57.5¹20m. The water temperatures at the mouth of wells was as high as 120℃. The maxium evaluated reservoir temperature was more than 150℃. TheδD values of these geothermal waters were smaller than sea water, and was not a const comparing with naomal-pressured geothermal waters. These unnomally pressured geothermal waters might be geopressured geothermal waters.Using isotopic and hydro-chemical methods, features of geothermal waters in the central Guanzhong Basin, Xi'an and Xian Yang, were studied, and these geothermal waters were defined as geopressured geothermal waters. The range of geopressured geothermal waters in Guanzhong Basin can be defined as the central Xi'an Pit, that is Xian Yang and northwest Xi'an, adjacent to Wei River. By analyzing isotopic and hydro-chemical features, it was also found that characteristics of geothermal waters in Xian Yang and northern Xi'an were similar, conferring Wei River Fault might have some effects on geothermal waters, and these geothermal waters might have hydraulic connections to some extent. However, between 1500m and 4000m, theδ¹⁸O values of some geothermal water samples in Xi'an remained the same or even decreased a little. Temperature,δ¹³C content, r_Na/r_Cl and r_SO4×100/rCl showed similar trend, suggesting the deeper thermal water reservoirs half open and half closed.The sthdy results ofδ¹⁸O andδD data indicated that significant oxygen exchange had taken place, as well as hydrogen exchange in the end of oxygen exchange, suggesting that thermal water reservoir in the central basin had gone though significant water-rock reaction, while there was hardly any ¹⁸O and ²H exchange in the south and north of the basin. Controlling factors of ¹⁸O shifts included temperature, water-rock ratio, water-rock interaction, residence time and buried depth.Geothermal waters in the south and north of the basin were along the local meteoric water line, indicating that they were recharged by modern precipitation or shallow surface waters. TheδD values at the point of intersection ofδ¹⁸O shift line and local meteoric water line, in Xi'an and Xian Yang respectively, were accordant with those in geothermal waters of Qinling Mountain and Northern Mountains, indicating that their recharge areas were possibly precipitation of Qinling Mountain and Morthern Mountains respectively. According to the calculation of Na-K-Mg ternary diagram, the reservoir temperature of geopressured geothermal waters in the central Basin was the highest, up to 158℃; geothermal waters in south and north of the basin located in the unmatured area, calculated reservoir temperatures were the lowest, and were unreasonable samples; geothermal water samples in Xi'an located in partly matured area, indicating mixture with cold waters or low extent of water-rock equivalence, calculated reservoir temperatures could only be a reference.The geological environment of geothermal waters in Xian and northern Xi'an was relatively the most enclosed, and were defined as relatively enclosed geothermal waters; the rest in Xi'an was defined as half enclosed half open geothermal waters; the geological environment of geothermal waters in south and north of the basin were relatively open, and were defined as relatively open geothermal waters.