水化学特征在恩施盆地地热资源调查中的指示意义

Indicative significance of hydrochemical characteristics in geothermal resource investigations in the Enshi Basin

地热资源是一种宝贵的清洁能源,恩施盆地蕴含丰富的地热资源,探明地热形成机理具有重要意义。从地热水化学特征和同位素特征入手,结合恩施盆地水文、地热地质条件,采用Piper三线图法、主要离子相关分析法等探讨分析了恩施盆地地下热水地球化学特征及其主要离子补给来源;选取适宜的二氧化硅温标法解决了热储温度难以准确测量的问题;利用氢氧同位素测试技术,示踪判定了地下热水的补给来源、循环深度及补给高程。研究结果显示:恩施盆地地下热水的水化学类型主要为SO_(4)·Cl^(-)Na型水,水中离子以SO_(4)^(2-),Cl^(-),Na^(+)为主,地下热水中TDS与Na+,Ca^(2+),Mg^(2+),Cl^(-),SO_(4)^(2-)有极好的正相关性,而地下热水中TDS远高于冷泉,分析认为是由于地下热水在演化过程中埋藏深度大、径流途径长、溶解—溶滤作用强烈,更容易从围岩中萃取相关离子,从而导致离子浓度远高于地表水;研究区内地下热水主要接受大气降水补给;地下热水14C,34 S同位素特征均表明盆地边缘到盆地中心地下热水储存环境逐渐封闭,地下热水滞留时间逐渐变长,水-岩反应程度逐渐变强;水-岩平衡判定结果表明,热水中SiO_(2)的浓度受石英的溶解平衡控制,利用SiO_(2)地热温标估算热储温度为55.74~58.24℃,热储埋深为1793~1906 m,热水循环深度为1823~1936 m;根据大气降水δ18 O的高程效应估算地下热水的补给高程为1022.64~1109.00 m,依据研究区高程范围确定地下热水补给区主要为盆地西侧寒武系-奥陶系碳酸盐岩低中山区。

[Objective]Geothermal resources are valuable clean resources,and the Enshi Basin contains abundant geothermal resources.It is very important to expore the origin of geothermal resources.[Methods]Starting from the chemical and isotopic characteristics of geothermal water,combined with the hydrological and geothermal geological conditions of the Enshi Basin,the following works have been performed.The geochemical characteristics and main ion recharge sources of geothermal water in the Enshi Basin are discussed and analyzed by the Piper diagram method and main ion correlation analysis method.In this study,the appropriate silica temperature scale method is used to solve the problem that it is difficult to accurately measure the heat storage temperature.Using hydrogen and oxygen isotope testing technology,the recharge source,circulation depth and recharge elevation of geothermal water are determined.[Results]Results show that the hydrochemical type of geothermal water in the Enshi Basin is mainly SO_(4)·Cl-Na water,and the main ions in the groundwater are SO_(4)^(2-)4,Cl-and Na+.There is a good positive correlation between TDS in geothermal water and Na^(+),Ca^(2+),Mg^(2+),Cl-and SO_(4)^(2-),while TDS in geothermal water is much higher than that in cold springs.It is due to the large burial depth,long runoff path and strong dissolution leaching action,which makes it easier to extract relevant ions from the surrounding rock and results in ion concentrations much higher than those of surface water.The hot water in the study area is mainly supplied by atmospheric precipitation,and the 14 C and 34 S isotopic characteristics of geothermal water show that the storage environment of geothermal water from the edge of the basin to the center of the basin is gradually closed,the retention time of geothermal water is gradually longer,and the degree of water rock reaction is gradually stronger.The results of the water-rock balance show that the concentration of SiO_(2) in hot water is controlled by the dissolution balance of quartz.Using the SiO_(2) geothermal temperature scale,the estimated thermal storage temperature is 55.74-58.24℃,the burial depth of thermal storage is 1793-1906 m,and the circulating depth of hot water is 1823-1936 m.The recharge elevation of geothermal water is estimated to be 1022.64-1109.00 m,according to the elevation effect of δO of atmospheric precipitation.[Conclusion]According to the elevation range of the study area,the geothermal water recharge area is mainly the low and middle mountain area of Cambrian Ordovician carbonate rocks in the western part of the basin.