兰州市城区深层地热水的水化学成因及其指示意义

兰州市城区蕴藏有丰富的地热资源，属于沉积盆地大型水热地热田，水质具有独特的理疗价值。从地热水化学和热储矿物成份入手，与地热地质条件紧密结合，首次采用了 Piper 三线图法、Schoeller 图法、主要离子比例系数法和相关分析法，全面系统地揭示了西北黄土高原半干旱区典型山间盆地兰州地热田水文地球化学特征及其主要离子的来源。选取适宜的 K-Mg 地热温标法，解决了热储温度难以准确测量的问题。利用氢氧稳定同位素测试技术，示踪判定了地热水的补给来源、补给区域和循环深度。结果显示: 兰州地热田钻孔揭露出水温度为 35. 5 ～ 74 ℃，总溶解性固体含量( TDS)为 3. 89～22. 04 g /L，阴、阳离子分别以 Cl－和 Na+、Ca2+为主，高矿化水质及特殊组分源于地热流体比较充分的溶滤作用。推测地热水中 Na+、Cl－主要来源于岩盐的溶解，Ca2+、Mg2+除了来源于方解石、白云石、硬石膏等矿物的溶解外，还来源于长石等矿物的溶解。地热水中不存在阳离子交换作用。确定地热田热储温度为 58～90 ℃，属于中低温水热资源。结果表明: 地热水的补给主要来源于城区南部基岩山区的大气降水，经深部循环的加热而成，形成一个 “源、通、储、盖”明了和补径排条件清晰的地热水系统。

Hydrochemical origin and indicative significance of deep geothermal water in Lanzhou City

The geothermal resources are rich in the urban area of Lanzhou City，which belong to a large hydrothermal geothermal field of sedimentary basin，of which the water quality has a unique physical therapy value． Starting from both the geothermal hydrochemistry and the reservoir mineral composition and closely combining with the geothermal geological conditions therein，the hydrogeochemical characteristics and the sources of the main ions of Lanzhou Geothermal Field—a typical intermountain basin in the semiarid region of Northwest Loess Plateau，are comprehensively and systematically revealed for the first time by means of both the methods of Piper trilinear diagram and Schoeller diagram as well as the main ion ratio method and the correlation analysis method，and then the problem of the reservoir temperature is difficult to be accurately measured is solved through selecting suitable method of K-Mg geothermometer，while the recharge source，area and circulating depth of the geothermal water are determined with the hydrogen and oxygen stable isotope testing technique． The result shows that the outgoing water temperature of Lanzhou Geothermal Field exposed by drilling is 35. 5 ～ 74 ℃ with the total dissolved solids ( TDS) of 3. 89 ～ 22. 04 g /L，of which the main anions and cations are Cl － ，Na + and Ca2+ ，respectively and the high mineralized water quality and special components therein come from the sufficient leaching effect of the geothermal fluid． It is supposed that Na + and Cl-in the geothermal water mainly come from the dissolution of rock salt，while Ca2+ and Mg2+ are not only from the dissolutions of the minerals such as calcite，dolomite and anhydrite，etc．，but also from the dissolution of the minerals of feldspar and the others． No cation exchange effect is there in the geothermal water． It is determined that the reservoir temperature of the Lanzhou Geothermal Field is 58～90 ℃， which belongs to middle-low temperature hydrothermal resources． The study result shows that the geothermal water is mainly recharged by the atmospheric precipitation in the bedrock mountain area in the south of the Lanzhou city，and then is heated through deep circulation，thus a geothermal water system with distinct source，channel，reservior，cover and clear conditions of recharge，runoff and drainage is finally formed．