甘肃省地热流体化学及环境同位素特征和形成年龄分析

Hydrochemistry and environmental isotopic characteristics and formation ages analysis of geothermal fluids in Gansu Province

利用大量的地热流体化学成分及环境同位素(δD、δ18O、14C)测试数据,深入分析了隆起山地构造对流型、沉降盆地传导型地热流体的化学特征及分布规律,进而对地热流体的环境同位素分布特征及形成年龄进行了梳理总结,得出了明确的结论。结果表明:隆起山地构造对流型地热田地热流体主要以断裂上升泉的形式出露,分布于西秦岭-祁连造山带,补给来源为当地及周边大气降水入渗,地热流体形成年龄一般小于5000~30000 a,水质较好,属“开启型”的地热系统;热储层岩性、断裂规模及水热循环方式和深度等明显控制着地热流体化学类型、环境同位素特征和形成年龄。沉降盆地传导型地热田地热流体主要以管井开采的方式出露;热储埋藏深度小于1600m的地热井,地热流体补给来源为当地及周边大气降水入渗,形成年龄一般小于5000~30000 a,水质相对较好,属“半开启-半封闭型”的地热系统;热储埋藏深度介于1600~2600 m之间的地热开采井,主要为地质历史时期逐步形成的“古水”,水化学类型复杂且水质较差,地热流体形成年龄介于30000~50000a,属“封闭型”的地热系统;热储层岩性、埋藏深度、地下水在岩层中的滞留时间与循环深度等明显控制着地热流体化学类型、环境同位素富集程度和形成年龄。

Based on a large number of chemical composition and environmental isotope(δD,δ18O,14C)test data of geothermal fluid,this paper analyzes the chemical characteristics and distribution rules of the convective and conductive geothermal fluid in the uplifted mountainous area,Gansu Province,China.The environmental isotope distribution characteristics and formation ages of the geothermal fluid are then then analyzed.Results show that the geothermal fluid in the convective geothermal field in the uplifted mountainous area is mainly exposed in the form of fracture rising spring,which is distributed in the west Qinling and Qilian orogenic belt.The source of supply is the infiltration of local and surrounding atmospheric precipitation.The formation age of the geothermal fluid is generally less than 5000-30000 a and good water quality is observed.Hence,the geothermal fluid belongs to the“open”geothermal system.The geothermal fluid in the conductive geothermal field in the subsidence basin is mainly exposed by tube well exploitation.Exploitation wells with a buried depth of less than 1600 m are supplied by the infiltration of local and surrounding atmospheric precipitation.Here,the formation ages of the geothermal fluid are generally less than 30000 a and the water quality is relatively good.Hence,this belongs to the“semi-open semiclosed”geothermal system.The geothermal open exhibits a buried depth of 1600-2600 m.The production wells are mainly the“ancient water”formed gradually in the geological historical period with a complex hydrochemical type and with poor water quality.This then belongs to the“closed”geothermal system with the formation ages of geothermal fluid between 30000-50000 a.Generally speaking,the lithology of the thermal reservoir,burial depth,retention time,and circulation alternation of groundwater in the rock layer obviously control the chemical type,environmental isotope,and formation ages of geothermal fluid.Based on the geothermal occurrence law,if the buried depth of the thermal reservoir is deeper,the geological conditions are more closed,andδD andδ18O are more enriched,indicating that the formation age of the fluid is older.