地热水氢氧同位素分布特性

Hydrogen and oxygen isotope distribution characteristics of geothermal water

地热水的流动和运移可以带来深部地质、地层信息,可以反映地应力场的变化.对地热水的研究可以更好地了解地下结构,为地震预测提供一种方法和依据.地热水与雨水、河水等有不同的氢氧同位素分布,根据分布的不同,可以对地热水补给源进行探讨.前人研究结果表明,地热水的可能补给源有大气降水、卤水、岩浆水等,大部分地热水的补给源被认作是当地大气降水,但地热水氢氧同位素组成与当地大气降水不同,不同的可能原因有多种,如地热水的大气降水补给源不在当地、大气降水与围岩的氧同位素交换、大气降水的不平衡蒸发、大气降水与氢氧同位素组成不同的水混合等等.为了验证前人成果,本文根据前人研究,利用氢氧同位素分布特征总结地热水可能的补给源,并利用有限元方法构建台湾仁泽2号井理想渗流模型,计算得到地热水渗流速度场,再结合氢氧同位素分布值,讨论地热水的可能来源.有限元计算结果表明,当地下水源处于饱和状态时,地表水的最大下渗速度为1.3×10-8m/s,因此,地热水的补给源并非当时当地的大气降水,综合考虑氢氧同位素背景值与海拔的线性关系,水源有可能是几千年前的远距离的高山大气降水,补给过程中与围岩发生物质交换,且可能还有另一个补给源,该源为氢同位素值与高山大气降水相近,氧同位素值较之稍高的岩浆水.

The flow and transportation of geothermal water can bring deep geological strata information, and also reflects the changes of situ stress field. Research on geothermal water can be a better understanding of underground structure,provide a method and basis for earthquake prediction. Geothermal water,rain water and river water, which are different from hydrogen and oxygen isotope distribution,such different distribution can be used to study the supply sources of geothermal water. Previous studies showed that,the possible supply sources are meteoric water,brine and magmatic water. Most geothermal water supply source is considered to be the local meteoric water,while,the composition of hydrogen and oxygen isotopic in geothermal water is differ with local meteoric water,there are several reasons for the differences,for example,the meteoric water as geothermal water supply source is not local,there is oxygen exchange between the meteoric water and surrounding rock,unbalanced evaporation of meteoric water,the mixing between the meteoric water and the water with different hydrogen and oxygen isotopic composition. This paper was based on previous researches,summarized the potential supply sources of geothermal water using hydrogen and oxygen isotope distribution characteristics to verify previous results,also,an ideal percolation model of #2 Renze well in Taiwan was built by finite element to get the geothermal water seepage velocity field,along with hydrogen and oxygen isotope distribution value,discussed the potential supply sources. Finite element calculation results showed that when the underground water source was saturated,the maximum infiltration velocity was 1. 3 × 10-8 m/s,therefore,geothermal water supply source is not the instant local atmospheric precipitation,considering the linear relationship between oxygen and hydrogen isotope background value with the altitude,the source is maybe the distant mountain precipitation from thousands years ago, exchange with surrounding rock occurred during the recharge process,and there maybe another supply source,this source own a similar hydrogen isotope value, and a little higher oxygen isotope value, like magmatic water.