湖南省热水圩地热田干热岩形成的热源机制与成因模式

Heat source-related mechanisms and genetic modes for the formation of hot dry rocks in the Reshuiwei geothermal field,Hunan Province

湖南省热水圩地热田90℃以上的高温温泉指示该地热田具备良好的地热地质条件,是潜在的干热岩勘查有利地段。为合理评价热水圩干热岩的储层温度与勘查开发前景,对汝城地区热水圩地热田的深部地质构造、地球化学和重磁电震地球物理特征、地温场特征等进行综合分析,自深至浅揭示热水圩地热田深部热结构,探讨干热岩形成的热源机制与地球动力学过程。结果表明:(1)利用SiO_(2)地热温标估算的热水圩地热田深部热储温度为79.4~143.9℃;(2)热水圩附近中棚岩体、鱼王岩体等花岗岩体平均生热率为7.07~8.44μW/m^(3),明显大于中国大陆主要地质构造单元的地壳平均生热率;(3)重磁特征反映出热水圩地区岩石圈厚度相对较薄,大地电磁与地震波速解释的区域内壳内高导低速体与深大断裂带相吻合,指示这些深大断裂有可能构成深部热物质上侵的通道。在此基础上,总结归纳了湖南省热水圩地热田干热岩的成因模式:太平洋板块俯冲与回撤,导致板块前缘形成强烈的热扰动,造成软流圈的隆起和幔源热物质的上侵,形成相对较高的幔源热源;生热率较高的花岗岩体与铀矿体放射性产热形成了良好的地壳热源;深大断裂构成深部热物质上侵的通道,同时为浅部干热岩的形成提供重要热源。有利的热源条件及沿深大断裂的深部热物质上侵,使湖南省东南部形成以热水圩周边岩体为代表的干热岩有利靶区。根据综合确定的地温梯度推算,热水圩地热田呈隐伏状态的、致密的、裂隙不发育的埋深在4000~6000 m的花岗岩体温度可达176.8~256.6℃,是良好的、近期可勘查开发的干热岩体。

Hot springs with temperatures exceeding 90℃indicate that the Reshuiwei geothermal field in Hunan Province enjoys sound geothermal geological conditions,making this geothermal field a potential favorable area for hot dry rock(HDR)exploration.To carry out a reasonable evaluation of the geothermal reservoir and prospects for explora-tion and exploitation of HDRs in the Reshuiwei geothermal field in the Rucheng area,this study comprehensively ana-lyzed the deep geological structures,geochemical characteristics,geophysical characteristics including gravity,magnet-ic,electrical,and seismic properties,and geothermal field characteristics of the geothermal field.Furthermore,it re-vealed the deep thermal structures of the geothermal field from deep to shallow and explored the heat source-related mechanisms and geodynamic process for the formation of HDRs in the geothermal field.Primary results are as follows.(1)The Reshuiwei geothermal field exhibits deep reservoir temperatures ranging from 79.4 to 143.9℃,as estimated us-ing a silica geothermometer.(2)The Zhongpeng and Yuwang granite plutons near the Reshuiwei area display average heat production rates between 7.07 and 8.44μW/m^(3),which are significantly higher than the average heat production rates of the crust in the major geological units in Chinese continent.(3)The gravity and magnetic characteristics reveal relatively thin lithosphere in the Reshuiwei area.Furthermore,the interpretations of the magnetotelluric sounding and seismic wave velocities reveal that the crust hosts geobodies with high conductivity and low velocity,which coincide with deep-seated fault zones.This indicates that these faults might serve as pathways for the upward intrusion of deep thermal materials.Based on these results,this study summarized the genesis mode of HDRs in the Reshuiwei geotherm-al field.Specifically,the subduction and retreat of the Pacific Plate led to intensive thermal disturbances to the plate front,further resulting in the uplift of the asthenosphere and the intrusion of mantle-derived thermal materials.As a res-ult,a relatively highly located mantle-derived heat source was formed.Granite plutons with high heat production rates and uranium deposits featuring radioactive heat production form a favorable crustal heat source.Furthermore,deep-seated faults serve as pathways for the upward intrusion of deep thermal materials while supplying heat sources for the formation of shallow HDRs.The favorable thermal source conditions and the upward intrusion of deep thermal materi-als along deep-seated faults jointly lead to the formation of a favorable HDR target area represented by plutons around Reshuiwei in southeastern Hunan.As inferred from the comprehensively determined geothermal gradients,the con-cealed and tight granite plutons with scarce fractures and burial depths ranging from 4000-6000 m in the Reshuiwei geothermal field exhibit temperatures reaching 176.8-256.6°C,serving as favorable HDRs to be explored and exploited in the near future.