鲁西北平原地热热源机制的气体同位素约束

Gas Isotope Constraints on the Geothermal Heat Source Mechanism in Northwest Shandong Plain

利用地热系统中气体的组分、丰度及同位素组成,可以识别地热流体的来源,追溯其热源形成机制。鲁西北平原地热资源自1998年以来大规模开发利用,但对其热源形成机制的认识尚不明确。本文利用溶解性气体组分和同位素特征对鲁西北平原热源成生条件进行了研究。溶解气成分结果表明,地热气体组分均以N2为主,主要为大气降水入渗成因,在相对封闭的地质环境中深循环,混合一定量的地壳深部或地幔来源的气体。利用^(3)He/^(4)He比值,定量计算出研究区壳幔热源配分比为0.90~1.57,热源主要为地壳岩石放射性生热与来自上地幔沿深大断裂的对流热。其中,砂岩热储受构造活动影响较小,地幔热流占45%左右。而岩溶热储受构造控热明显:处于深大断裂或济南岩体与灰岩接触带附近的岩溶热储地幔热流占比相对较大,达48.77%~52.75%,显示除了正常的大地热流传导产热,沿深大断裂、岩体与灰岩接触带存在来自上地幔的对流聚热;处于齐广断裂与济南岩体中间的稳定地块地幔热流相对较小,仅占38.87%~40.97%,指示构造相对稳定区获得深部热流较少。

Gas composition,abundance,and isotopic compositions of geothermal systems can not only help identify the source of geothermal fluids but also trace the formation mechanism of heat sources.The geothermal resources in northwest Shandong Plain have been exploited and utilized on a large scale since 1998,but the formation mechanism of their heat sources is unclear.In this study,the characteristics of dissolved gas components and isotopes were used to explore the heat source generation conditions in northern Shandong Plain.The results of dissolved gas composition showed that the geothermal gas is mainly composed of N2,which results from atmospheric precipitation infiltration.It circulates deeply in a relatively closed geological environment,mixing with a certain amount of gas from the deep crust or mantle.The crust-mantle heat source distribution ratio in the study area was calculated to be 0.90–1.57 based on the ^(3)He/^(4)He ratio.The crust heat flow accounts for a high proportion,and the heat source is mainly radioactive heat generated by crust rocks.Among them,the sandstone geothermal reservoir is less affected by tectonic activities,and mantle heat flow accounts for approximately 45%.The karst geothermal reservoir is obviously controlled by structures:the mantle heat flow located near the deep fault or the contact zone between Jinan intrusive rock mass and limestone accounts for a relatively large proportion,up to 48.77%–52.75%,indicating that convective heat flow accumulation occurs from the upper mantle along the deep fault or the contact zone between rock mass and limestone,in addition to heat generation from normal terrestrial heat flow conduction.The mantle heat flow in the stable block between the Qiguang deep fault and the Jinan intrusive rock mass is relatively small,accounting for only 38.87%–40.97%,indicating that the deep heat flow in the relatively stable tectonic area is less.