冷热混合作用下的岩溶地下热水形成机制新认识

A NEW VIEWPOINT CONCERNING GENESIS OF KARST THERMAL GROUNDWATER UNDER COLD-HOT WATER MIXING

岩溶型地热是地热系统的重要类型,岩溶介质的特殊性使得其形成过程中易与浅循环水系统发生更多的相互作用,而有别于其他介质地热系统。本文以四川盆地边缘的铜锣山背斜岩溶地下热水系统为研究对象,通过野外调查、水文地球化学、环境同位素等方法和手段,结合区内地热地质和岩溶地质条件,系统探讨了岩溶地热系统热水出露过程中的冷、热水混合作用,揭示了冷热混合作用对岩溶地热系统形成模式的遮蔽性,并建立了铜锣山岩溶地下热水成因模式。结果表明,铜锣山岩溶地下热水出露过程中冷热水混合作用极为强烈,冷水混入比高达73%~89%;冷热水混合会破坏热水携带的地热系统信息,使得补给高程和热储温度分别被低估约78%和45%。铜锣山背斜岩溶地下热水形成模式为大气降水在北部2100~2400 m高程的岩溶裸露山区渗入地下,在深循环过程中被地温增温加热,岩溶热储层温度为128~172℃,热水受岩溶和构造控制由北东向南西径流,并沿裂隙和岩溶通道向上径流,在此过程中与岩溶槽谷背斜局部浅循环冷水大量混合,混合后的热水以38~62℃的温度自然出露于深切河谷地段或被人工揭露。

Karst geothermal system is one of the most important geothermal types.The particularity of karst media makes geothermal groundwater easier to be influenced by the shallow circulating cold groundwater during its formation,as a result,differs the genetic mechanism of karst geothermal system from others.In present research,the karst geothermal system of Tongluoshan anticline at the margin of Sichuan Basin is taken as an example to reveal the potential misunderstanding of geothermal groundwater genesis caused by the mixing of cold and hot water.A comprehensive approach using of hydrogeochemistry and isotope coupled with field survey is performed to get insight into the mixing process of cold and hot water at the discharge zone of geothermal system.The effects of the mixing on the understanding of geothermal system are deeply discussed.A conceptual model of karst geothermal groundwater system with cold-hot water mixing was established.The results show that the mixing effect of cold and hot water is extremely strong at the discharge zone of geothermal system.The mixing ratio of cold water can be up to 73%~89%in Tongluoshan geothermal system.The mixing of cold and hot water would potentially destroy the information of thermal groundwater for geothermal system,resulting in underestimating the recharge elevation and reservoir temperature.This underestimation reaches to 78%and 45%for recharge elevation and reservoir temperature,respectively,in Tongluoshan geothermal system.The true model of Tongluoshan karst geothermal groundwater system is recognized by eliminating the effects of cold and hot water mixing.The geothermal system is recharged by precipitation in the exposed karst mountain area at an elevation of 2100~2400 m at the north.Recharged water is heated by the terrestrial heat flow during its deep circulation.The reservoir temperature of carbonate aquifers of lower Triassic Jialingjiang formation(T1j)and middle Triassic Leikoupo formation(T2l)is estimated ranging from 128~172℃.Thermal groundwater is controlled by the karst and geological structure of anticline,and flows from the northeast to the southwest.Thermal water mixes with the local shallow circulating cold water during its discharge.As a result,the temperature of exposed thermal water declines to 38~62℃.