碳酸盐岩热储隐伏型中低温热水的成因与水—岩相互作用研究
摘要
地热以其资源丰富、污染小、运营成本低等优势，正成为受人们青睐的一种新能源，具有重要的经济意义。长期以来，我国的地热研究及开发主要集中在具有地表热显示的中、高温地热田，中低温隐伏型地热资源未受到足够重视。随着传统能源日益短缺及对清洁能源需求量的增大，隐伏型中、低温地热的开发变得目益迫切。而开展隐伏型中、低温热水的成因、机理和水—岩相互作用等基础研究，对该类型地热能源的开发利用具有重要的指导意义。
在太原地区，1995年前只开展过地热资源的零星勘查工作。1995年至今，陆续钻探出17眼热水井。山西省地质工程勘察院曾于2004年和2005年分别完成《山西省太原市西边山热矿水勘查》和《太原市地热水资源调查评价》工作，但以传统的探测手段为主，未对太原岩溶地下热水的形成机理进行过系统的研究。目前为止，对太原地下热水的成因尚未形成统一的认识，对热水分布和形成机制也有待进一步的研究。
本研究对太原岩溶冷、热水进行了全面的调查，尤其是对热水进行了迄今最新最系统的一次调查、采样及测试工作，并深入分析了太原岩溶热水资源的形成机制。论文首先通过研究区地质、水文地质条件，特别是构造特征及其水文地质学意义的分析，对岩溶水系统进行了划分，分析了其水动力特征；借助物探、热红外遥感、钻探测温等技术，分析了岩溶水的温度分布特征，完成了地热水异常区的圈划；然后基于岩溶冷水与热水的温度、水化学和同位素测试数据，从水文地球化学、水—岩相互作用和同位素地球化学三个方面，分析了热水的流动模式；最后，对上述研究成果进行总结，提出太原碳酸盐岩热储陷伏型中低温热水形成的水文地质概念模型。
1．岩溶水温度分布特征
根据热红外遥感解译结果、CSMAT物探工作成果和钻探测温结果，分析了太原奥陶系中统地层岩溶水温度分布情况，发现：
(1)从边山到盆地区，整个岩溶水系统的水温呈不断增高的趋势。
(2)不同的岩溶水系统，水温的变化有所不同。①对北山岩溶水系统，从补给区→径流区→排泄区，水温一般都小于15℃，局部排泄区可达18℃。②西山岩溶水系统的水温，总体上呈西北向东南增高趋势，但不同构造区增温程度不同。在汾河以北，水温小于15℃；在镇城底—清徐向斜的东南侧，有明显的水温高值异常区；在王封地垒—三给—西铭主径流带，出现低温异常；在石相—原千峰地区，沿径流方向缓慢增温，至山前断裂带水温变高。③对东山岩溶水系统，沿东北方向至山前排泄带，水温逐渐增大。④在盆地区，亲贤地垒岩溶水温度较高，西温庄、小店区都显示出较高的水温异常。
(3)凹陷区为岩溶热水的赋存提供了良好条件，但巨厚的盖层，对岩溶热水开采的经济性有影响。
2．岩溶水水文地球化学特征
在研究区不同地点和不同深度，分4次共采集地下水(冷、热水)、地表水、大气降水样67组，进行了水化学常量元素、微量元素、气体成分等指标的测试，通过各岩溶水系统不同区段水文地球化学测试数据的对比分析，发现：
(1)从补给区→径流区→排泄区，岩溶水水化学类型按HCO_3→HCO_3·SO_4→SO_4的顺序转变，矿化度依次增大，由＜0.5 g／l增至～1 g／l，再增至＞2 g／l，SO_4~(2-)、Ca~(2+)、Mg~(2+)离子浓度不断增大，HCO_3~-含量基本不变；随岩溶水的温度增高，Sr、Si、Fe、F及一些微量元素含量不断增大。
(2)在水化学三线图上，冷水样与热水样位于不同区域，而位于两者之间的点则显示了冷、热水混合作用的存在。
(3)水化学数据的空间分布特征表明，较冷的排泄区岩溶水可能来自局部流动系统；深部温度较高的岩溶水可能来自于区域流动系统。
3．水—岩作用过程
选择地球化学温标进行热储温度的计算，并由地温梯度推算热水的最大循环深度；利用PHREEQC软件计算所有水样中各种矿物的饱和指数，结合各组分与温度间、不同组分间关系的分析，推断Ca~(2+)、Mg~(2+)、SO_4~(2-)离子及氟、硅、锶的水文地球化学过程，发现：
(1)太原热水属“未成熟水”，即水—岩相互作用尚未达到平衡；石英温标比较适于推断热储温度；水文地球化学温标的计算结果表明，太原热储温度介于35℃～68℃，岩溶热水循环深度介于898～2192 m。
(2)热水水化学特征的形成过程主要受碳酸盐溶解及冷、热水混合作用影响。①各岩溶水系统补给区的Ca~(2+)、Mg~(2+)、HCO_3主要是由碳酸盐和白云岩溶解生成，排泄区的Ca~(2+)则有其他来源，即径流过程中石膏的不断溶解而成。②岩溶水水化学演变存在白云石化作用，即石膏的溶解而引起方解石的沉淀，方解石的沉淀又促进白云石的溶解。③从补给区到排泄区，无论是岩溶冷水还是热水，过剩SO_4~(2-)始终存在且不断增高，表明除石膏的溶解外，SO_4~(2-)必然还有其他来源，可能是地层深部H_2S气体的氧化及黄铁矿的氧化形成。
(3)在太原热水中，氟、硅、锶、铁含量偏高是其共同特征。其中，F离子浓度主要受控于萤石的溶解；硅含量主要受控于石英和玉髓矿物的溶解；锶离子浓度主要受控于天青石矿物的溶解。
4．同位素地球化学特征
与上述水化学分析配套，采集了45组水样用于δD、~3H、δ~(18)O分析，28组水样用于~(14)C和δ~(13)C分析，53组水样用于~(87)Sr／~(86)Sr分析，13组水样用于硫酸盐中硫同位素分析、15组水样用于硫酸盐中氧同位素分析，18组样品用于惰性气体同位素的测试，通过同位素化学测试数据的分析，发现：
(1)氢、氧同位素
①δD—δ~(18)O图中水样点与现代大气降水线的关系表明，岩溶水起源于古大气降水，并且经受了不同程度的蒸发浓缩作用。②岩溶水D、~(18)O同位素存在分层现象，说明处于不同流动系统的地下水，其补给时期不同。③δD、~3H、δ~(18)O的空间分布都证明了冷、热水混合作用的存在。④地下水补给高程的D、~(18)O同位素计算结果表明，西山热水的补给高程为1700～2000 m，西山岩溶冷水的补给高程大约为1500～1700 m，东山热水的补给高程为1700～1900 m。⑤EC—δ~(18)O关系显示，相对于岩溶冷水，排泄区热水经历了较长的水—岩作用过程，证明存在着不同层次的流动系统。
(2)碳同位素
①~(14)C测试数据表明，西山岩溶水系统热水年龄为13000～18000 a，东山岩溶热水年龄为10000～20000 a，岩溶热水由温度较低的晚更新世大气降水补给。②δ~(13)C测试数据表明，岩溶水中CO_2起源于生物成因或碳酸盐变质作用。
(3)锶同位素
①锶同位素来源的分析表明：岩溶水~(87)Sr／~(86)Sr比值的分布是典型的碳酸盐中水—岩作用的结果，其空间差异证明存在着不同的流动系统。②~(87)Sr／~(86)Sr—1／Sr关系、~(87)Sr／~(86)Sr—Sr／Ca关系、~(87)Sr／~(86)Sr—δ~(18)O关系的分析表明：热水中可能有寒武系岩溶水的混入；不同岩溶水系统在排泄区深部存在着一定的水力联系；裂隙水与孔隙水之间有混合作用，但两者与岩溶水基本无水联系。③锶同位素冷、热水二元混合模型的计算结果表明，排泄区岩溶冷水中有较大比例的热水混入。
(4)硫酸盐中的硫、氧同位素
①硫酸盐中硫同位素来源的分析表明：岩溶水硫酸盐中的δ~(34)S值与碳酸盐岩地区常见蒸发岩石膏的δ~(34)S(SO_4~(2-))值比较接近，为石膏来源，黄铁矿氧化来源很少，没有幔源物质的混入；深部岩溶热水硫酸盐中的δ~(34)S值稍大于常见蒸发岩石膏的δ~(34)S，可能有寒武系地层水的混入。②SO_4~(2-)—硫酸盐中δ~(34)S关系及~(14)C—硫酸盐中δ~(34)S关系的分析表明，热水处于强还原环境。
(5)氦同位素
~3He／~4He比值(即R)及R／Ra比值的分析表明，岩溶热水的氦同位素主要为大气、壳源氦混合而成，无明显三元混合特征，指示了当地热水中不存在岩溶水与幔源热水的混合。
5．热水的流动模式
以岩溶水动力条件、温度场、水化学场和同位素特征研究结果为基础，对三个岩溶水系统的地下水流动模式进行了综合分析，发现：
(1)东山、西山岩溶水系统
两者具有类似的流动模型，即在补给区接受补给，随后沿几个径流带，形成局部、中间和区域三个不同层次的流动系统。①局部流动系统的补给高程较低，循环深度最浅，水流途径短，流速快，在岩溶水局部区排泄，水温多低于15℃，水化学类型主要为HCO_3·SO_4型，矿化度小，地下水年龄小，水—岩作用时间最短。②中间流动系统循环深度变深，水流途径较长，流速较缓，形成边山排泄带岩溶冷水，水温多介于15～25℃，水化学类型主要为HCO_3·SO_4或SO_4·HCO_3型，矿化度较高，地下水年龄较大，水一岩作用时间较长。③区域流动系统的岩溶水由更新世古大气降水补给，补给高程高，地下水循环深度最深，水流途径最长，流动缓慢，尤其在靠近排泄区时流动滞缓，使地下水有充足的时间加热，形成岩溶热水，水化学类型主要为SO_4·HCO_3或SO_4型，矿化度高，地下水年龄达10000多a，水—岩作用时间最长；热水在山前断裂带的上升过程中与岩溶冷水混合。
(2)北山岩溶水系统
相对于东山、西山岩溶水系统，具有如下特点：构造发育，具有较好的径流条件；接受较晚时期的降水补给，流动途径短，流速大，交替快；水温多低于15℃；从补给区→径流区→排泄区，水化学类型均为HCO_3—Ca·Mg型，矿化度较低；地下水年龄较小，水—岩作用时间短。
6．热水形成的水文地质概念模型
岩溶热水是由区域流动系统形成，其水文地质概念模型为：在高程较高处的灰岩裸露区，岩溶水系统接受降水入渗或地表水渗漏补给；补给区的岩溶冷水通过区域循环系统，经过较长的渗流途径到达边山排泄带；在流动滞缓，无幔源物质混入的情况下，受较大热流的加热，温度增高；部分岩溶热水在构造断裂带上升，与浅部冷水混合，形成低温水，另有部分岩溶热水深排入盆地隐伏型岩溶区，盆地中的巨厚砂、页岩和第四系、第三系松散堆积物为热水提供了保温盖层，形成了较高温度的岩溶热水。在重力势能和深部热能的双源作用下，区域流动系统中的岩溶水可能存在湍流运动。
Geothermal resource has become an important type of new energy resources that are rich inreserve, pollution-free, and cost-effective for exploitation. In China, high or medium temperaturegeothermal fields have been studies and exploited for decades. However, fewer works has beendone on low- medium temperature geothermal resources. This status needs to be changed as thedemand for cleaner energy keeps increasing. To investigate and exploit low or mediumtemperature hydrothermal resources in a reasonable way, the origin and water-rock interactionprocesses of the geothermal water should be understood first.
There were only several scattered surveys of karst geothermal resources in Taiyuan before1995. Seventeen hot wells have been successively constructed since 1995. Shanxi GeologicEngineering Survey Institute accomplished Hydrothermal Survey of XiShan Mountain in TaiyuanCity, Shanxi Province, and Survey & Evaluation of Hydrothermal Resources in Taiyuan City, in2004 and 2005 respectively. However, all these surveys are by means of the traditionaltechniques, and no systemical work has been done on the genesis of karst geothermal water inTaiyuan.
The overall objective of this study was to understand the genesis of karst geothermal waterin Taiyuan. Specifically the purposes of this paper were: (1) to divide the karst water system intoseveral sub-systems according to geologic, hydrogeologic and hydrodynamic characteristics; (2)to describe the temperature distribution of karst water and then locate hydrogeothermal anomalyzones by means of geophysical prospecting, thermal infrared remote sensing, and boretemperature logging; (3) to reveal the flow patterns of geothermal water using the evidences ofwater temperature, hydrogeochemistry, water-rock interaction, and isotope geochemistry, and (4)to develop hydrogeologic conceptual model of low-medium geothermal groundwater incarbonate bedrock in Taiyuan based on the above research findings. This study is the mostcomprehensive investigation of cold karst water, and the most systematic investigation of hotkarst water up to date in Taiyuan.
1. Temperature Distribution of Karst Groundwater
The temperature distribution of groundwater in Ordovician karst strata was characterizedusing thermal infrared remote sensing, CSMAT geophysical prospecting, and bore temperaturelogging. It is found that:
(1) Groundwater temperature tends to be higher from mountain to the basin for all karstwater systems.
(2) The courses of groundwater temperature increasing differ for different karst groundsystems.①For North Mountain karst water system, the water temperature is always less than 15℃from recharge area to runoff area, and to discharge area, except some local discharge areas.②For West Mountain karst water system, the water temperature generally increases from northwestto southeast. However, the rates of increase differ in different tectonic zones. Specifically, thetemperature is less than 15℃north from Fenhe River, high abnormally in southeastZhenchengdi-Qinxu syncline, low abnormally in Wangfeng-Sangei-Ximing primary groundwaterrunoff area, increasing gently along groundwater runoff route in Shixiang-Yuanqianfeng area,and increasing rapidly in mountain-front fracture zone.③For East Mountain karst watersystem, the water temperature increases gradually from southwest to northeast.④In the basin,Qinxian, Xiwenzhuang, and Xiaodian areas are with abnormal high water temperature.
(3) The sag in the basin adapts to reserve karst geothermal water. However the hugethickness of cover strata makes it high cost to exploit karst geothermal water there.
2. Hydrogeochemistry of Karst Groundwater
Cold/hot groundwater and surface water in different sites and depth, and precipitation, weresampled. Sixty-seven groups of water sample were gained and analyzed for hydrochemicalparameters, major and minor elements and gas composition. Then the hydrogeochemistryic datain different areas within each karst water system were compared. It is found that:
(1) From recharge area to runoff area, and to local discharge area, the karst water changesfrom HCO_3 to HCO_3·SO_4, and to SO_4 for hydrochemical type, and from less than 0.5 g/L toabout 1 g/L, and to more than 2 g/L for TDS (total dissolved solid). The concentration of SO_4~(2-),Ca~(2+) and Mg~(2+) also increases along groundwater flow path, but that of HCO_3~- keeps invariablegenerally. The concentration of Sr、Si、Fe、F and some trace elements increases as karst watertemperature increases.
(2) In hydrochemical Piper trilinear diagram, cold and hot water locate different zones.Water samples between them indicate the mixture between cold and hot water.
(3) Spatial distribution of hydrochemical data shows that cold karst water in discharge areashould come from local groundwater flow system while the hot one in deep part from theregional system.
3. Water-Rock Interaction of Karst Water System
Proper geochemical thermometers were selected to calculate the temperature of geothermalreservoir. The maximal depths of hydrothermal circulation were calculated using the localgeothermal gradient, and SI (Saturation Index) of each mineral in all water samples usingPHREEQC. The above results were combined with analysis of relationship between eachchemical component, and relationship between temperature and each component, to infer thehydrogeochemical behaviors of Ca~(2+), Mg~(2+), SO_4~(2-), F, Si, and Sr. It is found that:
(1) Geothermal water in Taiyuan is so-called "immature water", which means that ionexchange between water and rock is still under equilibrium. Consequently the quartzthermometric scales are suited to calculate temperature of geothermal reservoir. According to theresults of quartz thermometer, the temperature of geothermal reservoir in Taiyuan is between35℃and 68℃. The hydrothermal circulation depth is between 898 m and 2192 m.
(2) The hydrochemistry of geothermal water in Taiyuan is controlled mainly by carbonatedissolution and mixing between cold water and hot water.①Ca~(2+), Mg~(2+) and HCO_3~- in rechargearea of each karst water system mainly originate from the dissolution of carbonate and dolomite,while Ca~(2+) in the discharge area has one additional source, namely the dissolution of gypsumalong groundwater runoff route.②Evolution of karst water hydrochemistry is caused by bothdolomitization and calcite deposition.③From discharge area to discharge area of each karstwater system, whatever water temperature, SO_4~(2-) in water is increasing and always in excess,which indicates that SO_4~(2-) must have additional sources besides dissolution of gypsum. Theymay be oxidation of H_2S gas from deep strata or oxidation of pyrite.
(3) All geothermal water samples show high concentration of F, Si, Sr and Fe.Concentration of Fe, Si, and Sr ions is mainly controlled by solubility of fluorite, quartz andchalcedony, and celestite, respectively.
4. Isotope Geochemistry of Karst Groundwater
At the same sites/times where/when water samples for hydrochemical analysis were gained,thirty-seven, twenty-two, thirty-five, fifteen, and fifteen groups of water sample were sampledfor analysis ofδD, ~3H andδ~(18)O, ~(14)C andδ~(13)C, ~(87)Sr/~(86)Sr,δ~(34)S andδ~(18)O in sulfate, and noble gas,respectively. Then the isotope geochemical data in different areas within each karst water systemwere analyzed. The results are showed as follows:
(1) Hydrogen and Oxygen Isotope
①The relationship betweenδD andδ~(18)O values in water samples was compared withmodern MWL, which indicates that karst groundwater should originate from old precipitation,and experience concentration effect of evaporation.②Karst water shows D and ~(18)O isotopicstratification, indicating that groundwater in different hierarchies of flow system originates fromdifferent aged precipitation.③Spatial distribution ofδD, ~3H andδ~(18)O values in karst waterdemonstrates the mixture between cold water and hot water.④Groundwater rechargeelevations were calculated usingδD andδ~(18)O values in water samples. The results show that recharge elevations of hot water in West Mountain, cold karst water in West Mountain, and hotwater in East Mountain should be 1700～2000 m, 1500～1700 m, and 1700～1900 m, respectively.⑤The relationship between EC andδ~(18)O values shows that hot water in discharge areas shouldexperience the longer water-rock interaction than cold karst water at the same areas. It indicatesthat there are different hierarchies within the same karst groundwater flow system.
(2) Carbon Isotope
①According to ~(14)C data, hot waters in the karst water systems of West Mountain and EastMountain should be 13000～18000 and 10000～20000 years old respectively, and should originatefrom the colder precipitation in late Pleistocene.②According toδ~(13)C data, CO_2 gas in karstwater should originate from biologic effect or carbonate metamorphism.
(3) Strontium Isotope
①Through the analysis for sources of Sr isotope, it is found that spatial distribution of~(87)Sr/~(86)Sr ratio for karst water is the result of typical water-rock interaction in carbonate strata.Spatial variation of the ~(87)Sr/~(86)Sr ratio indicates there are different hierarchies within the samekarst groundwater flow system.②Relationships between ~(87)Sr/~(86)Sr ratios and 1/Sr values,between ~(87)Sr/~(86)Sr ratios and Sr/Ca ratios, and between ~(87)Sr/~(86)Sr ratios andδ~(18)O values wereanalyzed for all groundwater samples. It is found that karst water in Cambrian strata may bemixed into geothermal water, the karst water systems may have hydraulic connections with eachother in deeper part of discharge areas, and fissure water may mix with pore water. But neither ofthem has hydraulic connection with karst water.③According to the results of Sr isotopemixing model with hot water and cold water as end members, a large proportion of hot water ismixed with cold karst water in the discharge areas.
(4) Sulfur and Oxygen Isotope in Sulfate
①δ~(34)S values of sulfate in karst water are close to those of evaporite gypsum found incarbonate rock area, which implies that sulfate in karst water comes from gypsum dissolution,not from mantle.δ~(34)S values of sulfate in hot karst water from deep part are a little larger thanthose of evaporite gypsum found in carbonate rock area, which implies that hot water is inreducing environment and may be mixed with water from Cambrian strata.②According torelationships between SO_4~(2-) concentration andδ~(34)S values of sulfate, and between ~(14)C andδ~(34)Svalues of sulfate, hot water should be in strong reducing environment and few originates frompyrite oxidation.
(5) Helium Isotope
~3He/~4He ratios (namely R) and R/Ra ratios in water samples show that helium isotope in hot karst water should be a mixture of that from atmosphere and crust, and the local geothermalwater is not the mixture of karst water and hot water from mantle.
5. Flow Model of Geothermal Water
Based on the above studies of hydrogeology, temperature distribution, hydrochemistry andisotope hydrogeochemistry of karst water, the groundwater flow models in three karst watersystem were analyzed comprehensively. The results are summarized as follows:
(1) East Mountain Karst Water System and West Mountain Karst Water System
The flow models of them are similar. Each is supplied in recharge area, then groundwaterflows through three runoff zones at different depths, and thereby forms three hierarchies of flowsystem, namely local groundwater flow system, intermediate groundwater flow system, andregional groundwater flow system.
①For the local flow system, recharge elevation is low, circulation depth is shallow, flowpath is short, flow velocity is large, water temperature is low (mostly less than 15℃) and waterdrainages as cold water, TDS is small, groundwater is young, water-rock interaction duration isshort, and hydrochemical type is HCO_3·SO_4 in the main.②For the intermediate flow system,recharge elevation is higher, circulation depth is deeper, flow route is longer, flow velocity issmaller, water temperature is higher (mostly from 15℃to 25℃) but water still drainages as coldwater near mountain, TDS is larger, groundwater is older, water-rock interaction duration islonger, and hydrochemical type is HCO_3·SO_4 or SO_4·HCO_3 in the main.③For the regionalflow system, karst water was supplied by precipitation in Pleistocene, with the highest rechargeelevation, circulation depth is the deepest, flow route is the longest, TDS is the largest,groundwater is more than 10000 years old, water-rock interaction duration is the longest, andhydrochemical type is SO_4·HCO_3 or SO_4 in the main. In the regional flow system, groundwaterflows very slowly, and drainages as hot karst water with high temperature, even though it ismixed with cold karst water during upward flow.
(2) North Mountain Karst Water System
Compared with East Mountain karst water system and West Mountain karst system, itshows the following characteristics: better permeation because of developed structures;groundwater recharged by recent precipitation; shorter flow path; higherflow velocity; lowerwater temperature (mostly less than 15℃) and TDS; young groundwater; and shorterwater-rock interaction duration. The hydrochemical type is always HCO_3—Ca·Mg alonggroundwater flow path.
在太原地区，1995年前只开展过地热资源的零星勘查工作。1995年至今，陆续钻探出17眼热水井。山西省地质工程勘察院曾于2004年和2005年分别完成《山西省太原市西边山热矿水勘查》和《太原市地热水资源调查评价》工作，但以传统的探测手段为主，未对太原岩溶地下热水的形成机理进行过系统的研究。目前为止，对太原地下热水的成因尚未形成统一的认识，对热水分布和形成机制也有待进一步的研究。
本研究对太原岩溶冷、热水进行了全面的调查，尤其是对热水进行了迄今最新最系统的一次调查、采样及测试工作，并深入分析了太原岩溶热水资源的形成机制。论文首先通过研究区地质、水文地质条件，特别是构造特征及其水文地质学意义的分析，对岩溶水系统进行了划分，分析了其水动力特征；借助物探、热红外遥感、钻探测温等技术，分析了岩溶水的温度分布特征，完成了地热水异常区的圈划；然后基于岩溶冷水与热水的温度、水化学和同位素测试数据，从水文地球化学、水—岩相互作用和同位素地球化学三个方面，分析了热水的流动模式；最后，对上述研究成果进行总结，提出太原碳酸盐岩热储陷伏型中低温热水形成的水文地质概念模型。
1．岩溶水温度分布特征
根据热红外遥感解译结果、CSMAT物探工作成果和钻探测温结果，分析了太原奥陶系中统地层岩溶水温度分布情况，发现：
(1)从边山到盆地区，整个岩溶水系统的水温呈不断增高的趋势。
(2)不同的岩溶水系统，水温的变化有所不同。①对北山岩溶水系统，从补给区→径流区→排泄区，水温一般都小于15℃，局部排泄区可达18℃。②西山岩溶水系统的水温，总体上呈西北向东南增高趋势，但不同构造区增温程度不同。在汾河以北，水温小于15℃；在镇城底—清徐向斜的东南侧，有明显的水温高值异常区；在王封地垒—三给—西铭主径流带，出现低温异常；在石相—原千峰地区，沿径流方向缓慢增温，至山前断裂带水温变高。③对东山岩溶水系统，沿东北方向至山前排泄带，水温逐渐增大。④在盆地区，亲贤地垒岩溶水温度较高，西温庄、小店区都显示出较高的水温异常。
(3)凹陷区为岩溶热水的赋存提供了良好条件，但巨厚的盖层，对岩溶热水开采的经济性有影响。
2．岩溶水水文地球化学特征
在研究区不同地点和不同深度，分4次共采集地下水(冷、热水)、地表水、大气降水样67组，进行了水化学常量元素、微量元素、气体成分等指标的测试，通过各岩溶水系统不同区段水文地球化学测试数据的对比分析，发现：
(1)从补给区→径流区→排泄区，岩溶水水化学类型按HCO_3→HCO_3·SO_4→SO_4的顺序转变，矿化度依次增大，由＜0.5 g／l增至～1 g／l，再增至＞2 g／l，SO_4~(2-)、Ca~(2+)、Mg~(2+)离子浓度不断增大，HCO_3~-含量基本不变；随岩溶水的温度增高，Sr、Si、Fe、F及一些微量元素含量不断增大。
(2)在水化学三线图上，冷水样与热水样位于不同区域，而位于两者之间的点则显示了冷、热水混合作用的存在。
(3)水化学数据的空间分布特征表明，较冷的排泄区岩溶水可能来自局部流动系统；深部温度较高的岩溶水可能来自于区域流动系统。
3．水—岩作用过程
选择地球化学温标进行热储温度的计算，并由地温梯度推算热水的最大循环深度；利用PHREEQC软件计算所有水样中各种矿物的饱和指数，结合各组分与温度间、不同组分间关系的分析，推断Ca~(2+)、Mg~(2+)、SO_4~(2-)离子及氟、硅、锶的水文地球化学过程，发现：
(1)太原热水属“未成熟水”，即水—岩相互作用尚未达到平衡；石英温标比较适于推断热储温度；水文地球化学温标的计算结果表明，太原热储温度介于35℃～68℃，岩溶热水循环深度介于898～2192 m。
(2)热水水化学特征的形成过程主要受碳酸盐溶解及冷、热水混合作用影响。①各岩溶水系统补给区的Ca~(2+)、Mg~(2+)、HCO_3主要是由碳酸盐和白云岩溶解生成，排泄区的Ca~(2+)则有其他来源，即径流过程中石膏的不断溶解而成。②岩溶水水化学演变存在白云石化作用，即石膏的溶解而引起方解石的沉淀，方解石的沉淀又促进白云石的溶解。③从补给区到排泄区，无论是岩溶冷水还是热水，过剩SO_4~(2-)始终存在且不断增高，表明除石膏的溶解外，SO_4~(2-)必然还有其他来源，可能是地层深部H_2S气体的氧化及黄铁矿的氧化形成。
(3)在太原热水中，氟、硅、锶、铁含量偏高是其共同特征。其中，F离子浓度主要受控于萤石的溶解；硅含量主要受控于石英和玉髓矿物的溶解；锶离子浓度主要受控于天青石矿物的溶解。
4．同位素地球化学特征
与上述水化学分析配套，采集了45组水样用于δD、~3H、δ~(18)O分析，28组水样用于~(14)C和δ~(13)C分析，53组水样用于~(87)Sr／~(86)Sr分析，13组水样用于硫酸盐中硫同位素分析、15组水样用于硫酸盐中氧同位素分析，18组样品用于惰性气体同位素的测试，通过同位素化学测试数据的分析，发现：
(1)氢、氧同位素
①δD—δ~(18)O图中水样点与现代大气降水线的关系表明，岩溶水起源于古大气降水，并且经受了不同程度的蒸发浓缩作用。②岩溶水D、~(18)O同位素存在分层现象，说明处于不同流动系统的地下水，其补给时期不同。③δD、~3H、δ~(18)O的空间分布都证明了冷、热水混合作用的存在。④地下水补给高程的D、~(18)O同位素计算结果表明，西山热水的补给高程为1700～2000 m，西山岩溶冷水的补给高程大约为1500～1700 m，东山热水的补给高程为1700～1900 m。⑤EC—δ~(18)O关系显示，相对于岩溶冷水，排泄区热水经历了较长的水—岩作用过程，证明存在着不同层次的流动系统。
(2)碳同位素
①~(14)C测试数据表明，西山岩溶水系统热水年龄为13000～18000 a，东山岩溶热水年龄为10000～20000 a，岩溶热水由温度较低的晚更新世大气降水补给。②δ~(13)C测试数据表明，岩溶水中CO_2起源于生物成因或碳酸盐变质作用。
(3)锶同位素
①锶同位素来源的分析表明：岩溶水~(87)Sr／~(86)Sr比值的分布是典型的碳酸盐中水—岩作用的结果，其空间差异证明存在着不同的流动系统。②~(87)Sr／~(86)Sr—1／Sr关系、~(87)Sr／~(86)Sr—Sr／Ca关系、~(87)Sr／~(86)Sr—δ~(18)O关系的分析表明：热水中可能有寒武系岩溶水的混入；不同岩溶水系统在排泄区深部存在着一定的水力联系；裂隙水与孔隙水之间有混合作用，但两者与岩溶水基本无水联系。③锶同位素冷、热水二元混合模型的计算结果表明，排泄区岩溶冷水中有较大比例的热水混入。
(4)硫酸盐中的硫、氧同位素
①硫酸盐中硫同位素来源的分析表明：岩溶水硫酸盐中的δ~(34)S值与碳酸盐岩地区常见蒸发岩石膏的δ~(34)S(SO_4~(2-))值比较接近，为石膏来源，黄铁矿氧化来源很少，没有幔源物质的混入；深部岩溶热水硫酸盐中的δ~(34)S值稍大于常见蒸发岩石膏的δ~(34)S，可能有寒武系地层水的混入。②SO_4~(2-)—硫酸盐中δ~(34)S关系及~(14)C—硫酸盐中δ~(34)S关系的分析表明，热水处于强还原环境。
(5)氦同位素
~3He／~4He比值(即R)及R／Ra比值的分析表明，岩溶热水的氦同位素主要为大气、壳源氦混合而成，无明显三元混合特征，指示了当地热水中不存在岩溶水与幔源热水的混合。
5．热水的流动模式
以岩溶水动力条件、温度场、水化学场和同位素特征研究结果为基础，对三个岩溶水系统的地下水流动模式进行了综合分析，发现：
(1)东山、西山岩溶水系统
两者具有类似的流动模型，即在补给区接受补给，随后沿几个径流带，形成局部、中间和区域三个不同层次的流动系统。①局部流动系统的补给高程较低，循环深度最浅，水流途径短，流速快，在岩溶水局部区排泄，水温多低于15℃，水化学类型主要为HCO_3·SO_4型，矿化度小，地下水年龄小，水—岩作用时间最短。②中间流动系统循环深度变深，水流途径较长，流速较缓，形成边山排泄带岩溶冷水，水温多介于15～25℃，水化学类型主要为HCO_3·SO_4或SO_4·HCO_3型，矿化度较高，地下水年龄较大，水一岩作用时间较长。③区域流动系统的岩溶水由更新世古大气降水补给，补给高程高，地下水循环深度最深，水流途径最长，流动缓慢，尤其在靠近排泄区时流动滞缓，使地下水有充足的时间加热，形成岩溶热水，水化学类型主要为SO_4·HCO_3或SO_4型，矿化度高，地下水年龄达10000多a，水—岩作用时间最长；热水在山前断裂带的上升过程中与岩溶冷水混合。
(2)北山岩溶水系统
相对于东山、西山岩溶水系统，具有如下特点：构造发育，具有较好的径流条件；接受较晚时期的降水补给，流动途径短，流速大，交替快；水温多低于15℃；从补给区→径流区→排泄区，水化学类型均为HCO_3—Ca·Mg型，矿化度较低；地下水年龄较小，水—岩作用时间短。
6．热水形成的水文地质概念模型
岩溶热水是由区域流动系统形成，其水文地质概念模型为：在高程较高处的灰岩裸露区，岩溶水系统接受降水入渗或地表水渗漏补给；补给区的岩溶冷水通过区域循环系统，经过较长的渗流途径到达边山排泄带；在流动滞缓，无幔源物质混入的情况下，受较大热流的加热，温度增高；部分岩溶热水在构造断裂带上升，与浅部冷水混合，形成低温水，另有部分岩溶热水深排入盆地隐伏型岩溶区，盆地中的巨厚砂、页岩和第四系、第三系松散堆积物为热水提供了保温盖层，形成了较高温度的岩溶热水。在重力势能和深部热能的双源作用下，区域流动系统中的岩溶水可能存在湍流运动。
Geothermal resource has become an important type of new energy resources that are rich inreserve, pollution-free, and cost-effective for exploitation. In China, high or medium temperaturegeothermal fields have been studies and exploited for decades. However, fewer works has beendone on low- medium temperature geothermal resources. This status needs to be changed as thedemand for cleaner energy keeps increasing. To investigate and exploit low or mediumtemperature hydrothermal resources in a reasonable way, the origin and water-rock interactionprocesses of the geothermal water should be understood first.
There were only several scattered surveys of karst geothermal resources in Taiyuan before1995. Seventeen hot wells have been successively constructed since 1995. Shanxi GeologicEngineering Survey Institute accomplished Hydrothermal Survey of XiShan Mountain in TaiyuanCity, Shanxi Province, and Survey & Evaluation of Hydrothermal Resources in Taiyuan City, in2004 and 2005 respectively. However, all these surveys are by means of the traditionaltechniques, and no systemical work has been done on the genesis of karst geothermal water inTaiyuan.
The overall objective of this study was to understand the genesis of karst geothermal waterin Taiyuan. Specifically the purposes of this paper were: (1) to divide the karst water system intoseveral sub-systems according to geologic, hydrogeologic and hydrodynamic characteristics; (2)to describe the temperature distribution of karst water and then locate hydrogeothermal anomalyzones by means of geophysical prospecting, thermal infrared remote sensing, and boretemperature logging; (3) to reveal the flow patterns of geothermal water using the evidences ofwater temperature, hydrogeochemistry, water-rock interaction, and isotope geochemistry, and (4)to develop hydrogeologic conceptual model of low-medium geothermal groundwater incarbonate bedrock in Taiyuan based on the above research findings. This study is the mostcomprehensive investigation of cold karst water, and the most systematic investigation of hotkarst water up to date in Taiyuan.
1. Temperature Distribution of Karst Groundwater
The temperature distribution of groundwater in Ordovician karst strata was characterizedusing thermal infrared remote sensing, CSMAT geophysical prospecting, and bore temperaturelogging. It is found that:
(1) Groundwater temperature tends to be higher from mountain to the basin for all karstwater systems.
(2) The courses of groundwater temperature increasing differ for different karst groundsystems.①For North Mountain karst water system, the water temperature is always less than 15℃from recharge area to runoff area, and to discharge area, except some local discharge areas.②For West Mountain karst water system, the water temperature generally increases from northwestto southeast. However, the rates of increase differ in different tectonic zones. Specifically, thetemperature is less than 15℃north from Fenhe River, high abnormally in southeastZhenchengdi-Qinxu syncline, low abnormally in Wangfeng-Sangei-Ximing primary groundwaterrunoff area, increasing gently along groundwater runoff route in Shixiang-Yuanqianfeng area,and increasing rapidly in mountain-front fracture zone.③For East Mountain karst watersystem, the water temperature increases gradually from southwest to northeast.④In the basin,Qinxian, Xiwenzhuang, and Xiaodian areas are with abnormal high water temperature.
(3) The sag in the basin adapts to reserve karst geothermal water. However the hugethickness of cover strata makes it high cost to exploit karst geothermal water there.
2. Hydrogeochemistry of Karst Groundwater
Cold/hot groundwater and surface water in different sites and depth, and precipitation, weresampled. Sixty-seven groups of water sample were gained and analyzed for hydrochemicalparameters, major and minor elements and gas composition. Then the hydrogeochemistryic datain different areas within each karst water system were compared. It is found that:
(1) From recharge area to runoff area, and to local discharge area, the karst water changesfrom HCO_3 to HCO_3·SO_4, and to SO_4 for hydrochemical type, and from less than 0.5 g/L toabout 1 g/L, and to more than 2 g/L for TDS (total dissolved solid). The concentration of SO_4~(2-),Ca~(2+) and Mg~(2+) also increases along groundwater flow path, but that of HCO_3~- keeps invariablegenerally. The concentration of Sr、Si、Fe、F and some trace elements increases as karst watertemperature increases.
(2) In hydrochemical Piper trilinear diagram, cold and hot water locate different zones.Water samples between them indicate the mixture between cold and hot water.
(3) Spatial distribution of hydrochemical data shows that cold karst water in discharge areashould come from local groundwater flow system while the hot one in deep part from theregional system.
3. Water-Rock Interaction of Karst Water System
Proper geochemical thermometers were selected to calculate the temperature of geothermalreservoir. The maximal depths of hydrothermal circulation were calculated using the localgeothermal gradient, and SI (Saturation Index) of each mineral in all water samples usingPHREEQC. The above results were combined with analysis of relationship between eachchemical component, and relationship between temperature and each component, to infer thehydrogeochemical behaviors of Ca~(2+), Mg~(2+), SO_4~(2-), F, Si, and Sr. It is found that:
(1) Geothermal water in Taiyuan is so-called "immature water", which means that ionexchange between water and rock is still under equilibrium. Consequently the quartzthermometric scales are suited to calculate temperature of geothermal reservoir. According to theresults of quartz thermometer, the temperature of geothermal reservoir in Taiyuan is between35℃and 68℃. The hydrothermal circulation depth is between 898 m and 2192 m.
(2) The hydrochemistry of geothermal water in Taiyuan is controlled mainly by carbonatedissolution and mixing between cold water and hot water.①Ca~(2+), Mg~(2+) and HCO_3~- in rechargearea of each karst water system mainly originate from the dissolution of carbonate and dolomite,while Ca~(2+) in the discharge area has one additional source, namely the dissolution of gypsumalong groundwater runoff route.②Evolution of karst water hydrochemistry is caused by bothdolomitization and calcite deposition.③From discharge area to discharge area of each karstwater system, whatever water temperature, SO_4~(2-) in water is increasing and always in excess,which indicates that SO_4~(2-) must have additional sources besides dissolution of gypsum. Theymay be oxidation of H_2S gas from deep strata or oxidation of pyrite.
(3) All geothermal water samples show high concentration of F, Si, Sr and Fe.Concentration of Fe, Si, and Sr ions is mainly controlled by solubility of fluorite, quartz andchalcedony, and celestite, respectively.
4. Isotope Geochemistry of Karst Groundwater
At the same sites/times where/when water samples for hydrochemical analysis were gained,thirty-seven, twenty-two, thirty-five, fifteen, and fifteen groups of water sample were sampledfor analysis ofδD, ~3H andδ~(18)O, ~(14)C andδ~(13)C, ~(87)Sr/~(86)Sr,δ~(34)S andδ~(18)O in sulfate, and noble gas,respectively. Then the isotope geochemical data in different areas within each karst water systemwere analyzed. The results are showed as follows:
(1) Hydrogen and Oxygen Isotope
①The relationship betweenδD andδ~(18)O values in water samples was compared withmodern MWL, which indicates that karst groundwater should originate from old precipitation,and experience concentration effect of evaporation.②Karst water shows D and ~(18)O isotopicstratification, indicating that groundwater in different hierarchies of flow system originates fromdifferent aged precipitation.③Spatial distribution ofδD, ~3H andδ~(18)O values in karst waterdemonstrates the mixture between cold water and hot water.④Groundwater rechargeelevations were calculated usingδD andδ~(18)O values in water samples. The results show that recharge elevations of hot water in West Mountain, cold karst water in West Mountain, and hotwater in East Mountain should be 1700～2000 m, 1500～1700 m, and 1700～1900 m, respectively.⑤The relationship between EC andδ~(18)O values shows that hot water in discharge areas shouldexperience the longer water-rock interaction than cold karst water at the same areas. It indicatesthat there are different hierarchies within the same karst groundwater flow system.
(2) Carbon Isotope
①According to ~(14)C data, hot waters in the karst water systems of West Mountain and EastMountain should be 13000～18000 and 10000～20000 years old respectively, and should originatefrom the colder precipitation in late Pleistocene.②According toδ~(13)C data, CO_2 gas in karstwater should originate from biologic effect or carbonate metamorphism.
(3) Strontium Isotope
①Through the analysis for sources of Sr isotope, it is found that spatial distribution of~(87)Sr/~(86)Sr ratio for karst water is the result of typical water-rock interaction in carbonate strata.Spatial variation of the ~(87)Sr/~(86)Sr ratio indicates there are different hierarchies within the samekarst groundwater flow system.②Relationships between ~(87)Sr/~(86)Sr ratios and 1/Sr values,between ~(87)Sr/~(86)Sr ratios and Sr/Ca ratios, and between ~(87)Sr/~(86)Sr ratios andδ~(18)O values wereanalyzed for all groundwater samples. It is found that karst water in Cambrian strata may bemixed into geothermal water, the karst water systems may have hydraulic connections with eachother in deeper part of discharge areas, and fissure water may mix with pore water. But neither ofthem has hydraulic connection with karst water.③According to the results of Sr isotopemixing model with hot water and cold water as end members, a large proportion of hot water ismixed with cold karst water in the discharge areas.
(4) Sulfur and Oxygen Isotope in Sulfate
①δ~(34)S values of sulfate in karst water are close to those of evaporite gypsum found incarbonate rock area, which implies that sulfate in karst water comes from gypsum dissolution,not from mantle.δ~(34)S values of sulfate in hot karst water from deep part are a little larger thanthose of evaporite gypsum found in carbonate rock area, which implies that hot water is inreducing environment and may be mixed with water from Cambrian strata.②According torelationships between SO_4~(2-) concentration andδ~(34)S values of sulfate, and between ~(14)C andδ~(34)Svalues of sulfate, hot water should be in strong reducing environment and few originates frompyrite oxidation.
(5) Helium Isotope
~3He/~4He ratios (namely R) and R/Ra ratios in water samples show that helium isotope in hot karst water should be a mixture of that from atmosphere and crust, and the local geothermalwater is not the mixture of karst water and hot water from mantle.
5. Flow Model of Geothermal Water
Based on the above studies of hydrogeology, temperature distribution, hydrochemistry andisotope hydrogeochemistry of karst water, the groundwater flow models in three karst watersystem were analyzed comprehensively. The results are summarized as follows:
(1) East Mountain Karst Water System and West Mountain Karst Water System
The flow models of them are similar. Each is supplied in recharge area, then groundwaterflows through three runoff zones at different depths, and thereby forms three hierarchies of flowsystem, namely local groundwater flow system, intermediate groundwater flow system, andregional groundwater flow system.
①For the local flow system, recharge elevation is low, circulation depth is shallow, flowpath is short, flow velocity is large, water temperature is low (mostly less than 15℃) and waterdrainages as cold water, TDS is small, groundwater is young, water-rock interaction duration isshort, and hydrochemical type is HCO_3·SO_4 in the main.②For the intermediate flow system,recharge elevation is higher, circulation depth is deeper, flow route is longer, flow velocity issmaller, water temperature is higher (mostly from 15℃to 25℃) but water still drainages as coldwater near mountain, TDS is larger, groundwater is older, water-rock interaction duration islonger, and hydrochemical type is HCO_3·SO_4 or SO_4·HCO_3 in the main.③For the regionalflow system, karst water was supplied by precipitation in Pleistocene, with the highest rechargeelevation, circulation depth is the deepest, flow route is the longest, TDS is the largest,groundwater is more than 10000 years old, water-rock interaction duration is the longest, andhydrochemical type is SO_4·HCO_3 or SO_4 in the main. In the regional flow system, groundwaterflows very slowly, and drainages as hot karst water with high temperature, even though it ismixed with cold karst water during upward flow.
(2) North Mountain Karst Water System
Compared with East Mountain karst water system and West Mountain karst system, itshows the following characteristics: better permeation because of developed structures;groundwater recharged by recent precipitation; shorter flow path; higherflow velocity; lowerwater temperature (mostly less than 15℃) and TDS; young groundwater; and shorterwater-rock interaction duration. The hydrochemical type is always HCO_3—Ca·Mg alonggroundwater flow path.
引文
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2．毕思文，地球系统科学．北京：科学出版社，2002
3．陈墨香，邓孝．中国地下热水分布之特点及属性．第四纪研究，1996，2：131-138
4．陈墨香，汪集吻，邓孝．中国地热资源——形成特点和潜力评估．北京：科学出版社．1994
5．耿莉萍．中国地热资源的地理分布与勘探．地质与勘探，1998，34(1)：50-54
6．顾慰祖，林曾平，费光灿，等．环境同位素硫在大同南寒武—奥陶系地下水资源研究中的应用．水科学进展，2000，11(1)：14-20
7．何满潮，刘斌，姚磊华，等．地热水井回灌渗流场理论研究．中国矿业大学学报，2004，33(3)：245-248
8．李学礼，孙占学，周文斌．古水热系统与铀成矿作用．北京：地质出版社．2000
9．李学伦，李桂群，崔承琦，等．山东半岛地质环境与温泉分布．青岛海洋大学学报，1994，专辑：10-15
10．刘东生．黄土与环境．北京：科学出版社．1986．
11．刘时彬．地热资源及其开发利用和保护．北京：化学工业出版社．2005
12．刘延忠．中国地热资源开发与利用的思考．中国矿业，2001，10(5)：5-9
13．卢予北，吴烨，张秋冬，等．地热资源开发与问题研究．郑州：黄河水利出版社．2005
14．马致远，范基娇，苏艳，等．关中南部地下热水氢氧同位素组成的水文地质意义．地球科学与环境学报，2006，28(1)：41-46
15．申建梅，陈宗宇．地热开发利用过程中的环境效应及环境保护．地球学报，1998，19(4)：402-408
16．水文地质工程地质选辑——地下热水·环境地质．北京：地质出版社．1975
17．陶勇，陈亚妍，王华敏，等．地热水的开发利用对环境和健康的影响．中国环境科学，1994，14(1)：37-40
18．Tath J(胡济世译)，重力穿层地下水流动与油气运移聚集．胜利油田地质科学研究院，1984
19．汪集旸，熊亮萍，庞忠和．中低温对流型地热系统．北京：科学出版社．1993
20．汪集旸．中国大陆地区大地热流值数据汇编．地震地质，1990，12(4)：3511
21．王广才，张作辰，汪民，等．延怀盆地地下热水与稀有气体的地球化学特征．地震地质，2003，25(3)：421-429
22．王宏伟，李亚峰，林豹，等．地热能在我国的应用．可再生能源，2002，5：32-34
23．王钧，周家平．华北平原中低温地热资源及其利用的环境影响——以河间地区为例．北京：地震出版社．1992
24．王佟，王莹．陕西消河盆地地热资源赋存特征研究．西安科技学院学报，2004，24(1)：82-85
25．王心义，聂卫良，赵卫东，等．开封凹陷区地温场特征及成因机制探析．煤田地质与勘探，2001，(5)：4-7
26．王焰新，葛·马·斯贝泽尔．东亚大陆裂谷医疗矿水水文地球化学研究——以山西和贝加尔裂谷系为例．北京：中国环境科学出版社．2000
27．邢忠信，杨凤仪，翟立勤．沧州地区地热开发利用规划．地球学报，2000，21(2)：168-170
28．邢作云，赵斌，涂美义，等．汾渭裂谷系与造山带耦合关系及其形成机制研究．地学前缘，2005，12(2)：247-262
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