Genesis mechanism of geothermal fluid with high mineralization in karst geothermal reservoir: A case study of geothermal field of the Yanchang river, Badong county
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摘要:
岩溶热储中地热流体多为中低温低矿化度热水，而岩溶热储中出现高矿化度地热水多与岩溶含水层矿物溶解有关。然而，湖北省巴东县盐场河地热田中地热水TDS高达12 477.7 mg·L−1，水温约34 ℃，含水层矿物溶解并不足以解释其成因。文章在野外调查和地热钻探的基础上，对4个地热钻孔、1个温泉及附近4个冷泉进行了水文地球化学采样和测试。研究结果表明：盐场河地热田地热水化学类型为Cl-Na型，单位涌水量最大可达1 767 m3·d−1，出水口温度在30.2~34.5 ℃。对比钻孔测温和SiO2温度计分析，热储温度为59.1 ℃，循环深度为1 923 m。Phreeqc水化学模拟揭示含水层中的水–岩相互作用（主要是含水层矿物溶解）为地热水化学组分提供了部分贡献，主要来源于径流过程中咸化潮坪泻湖相盐岩的溶解。水文地质条件和氢氧同位素指示地热水的大气降水来源，但季节性的冷水混入控制了水−岩相互作用的平衡。可见，巴东县盐场河岩溶热储高矿化度地热水主要是径流过程中盐岩的溶解提供了水化学组分，但是出露过程中受到季节性的冷水混合影响。
Abstract:At present, China's energy development has entered a new stage of carbon reduction and energy conservation. In order to achieve the goal of "carbon peak" and "carbon neutrality", the development and utilization of geothermal energy has been ushered in unprecedented opportunities. However, not all geothermal resources can be directly exploited and utilized. Based on a funded project—Feasibility of Geothermal Resources Exploration in Yanchang river, Badong county, Hubei Province, we have built a genesis model of the geothermal field mainly by means of software analysis, traditional geological survey, drilling, sampling analysis and systematic temperature measurement. In the aspect of hydrochemistry, we also analyzed the genesis of geothermal fluid with high mineralization in karst geothermal reservoir in the Yanchang river. According to previous research, hydrogeological survey, geophysical exploration and drilling, we found out the geothermal geological conditions, regional stratigraphic distributions, lithologic characteristics and structural distributions in the study area, which can provide data for the construction of genesis model of geothermal field. In addition, by sampling, testing, and monitoring water temperatures, we compared the chemical components of cold springs and geothermal fluids in different periods, and further analyzed the hydrogeochemical characteristics of geothermal fluids and the reasons for temperature anomalies. The research findings may provide the technical and theoretical basis for the genesis mechanism of geothermal fluid with high mineralization in karst geothermal reservoir as well as the basis for the scientific development and utilization of karst geothermal reservior.
The geothermal fluid in the karst geothermal reservoir is mostly hot water with low mineralization at low temperature, while the highly mineralized geothermal water is often related to the dissolution of karst aquifer minerals. However, the TDS of geothermal water in the geothermal field of the Yanchang river is as high as 12,477.7 mg·L−1, and the water temperature is about 34 ℃. The dissolution of aquifer minerals is unlikely to explain the genesis mechanism. On the basis of field investigation and geothermal drilling, we conducted the hydrogeochemical sampling and testing in four geothermal boreholes, one hot spring and four nearby cold springs. The research shows that the study field belongs to the convection-type geothermal resource at medium-low temperatures under the control of deep and large faults. The Ordovician limestone and dolomite are the main strata for geothermal reservoir, belonging to the karst fissure type. The chemical type of geothermal water in the Yanchang river is Cl-Na. The maximum unit water inflow can reach 1,767 m3·d−1, with the outlet temperatures from 30.2 ℃ to 34.5 ℃. Compared with the analysis of borehole temperature and SiO2 thermometer, the temperature of geothermal fluid is 59.1 ℃, and the circulation depth is 1,923 m. It is found that the geothermal fluid can complete sufficient heat exchange with heat source in the long migration path and long runoff time, the process of which may gradually increase groundwater temperatures. The sulfur isotope analysis shows that sulfate in karst water is derived from recharge water including atmospheric precipitation, surface water, and water formed by the oxidation of pyrite in rock mass. Groundwater maintains a relatively stable balance between oxidation and reduction. The aquifer is a transition between the alternation of a weak (lagging) and a strong environment, having a certain but low-degree recharge condition. Phreeqc hydrochemical simulations reveal the water-rock interactions (mainly the dissolution of aquifer minerals) in the aquifer, and further reveal that the high salinity of chemical composition in geothermal water is derived mainly from the dissolution of salt rocks in the salinized tidal flat lagoon facies during the runoff process. According to the analysis of tritium isotope, the content of tritium in geothermal fluid increases significantly, mainly because the geothermal fluid with low tritium content is mixed with shallow water or surface cold water when the geothermal fluid pours out along the fault under the influence of deep heat source and long runoff path. Hydrogeological conditions and hydrogen and oxygen isotopes can indicate the origin of atmospheric precipitation of geothermal water. The groundwater recharge height ranges from 1,261.21 m to 1,298.25 m, while the height of the Xiaoshennongjia mountain area in the north of the geothermal field ranges from 900 m to 1,300 m, which is the recharge area of the geothermal field of the Yanchang river. However, seasonal cold water addition controls the balance of water-rock interactions. It can be concluded that the high mineralization in geothermal water of the Yanchang river is mainly formed by the dissolution of salt rock during the runoff process, during which the upward flow is affected by seasonal mixing of cold water. Furthermore, F6 tensile faults and F7 water-blocking faults in geothermal fields affect not only the flow direction and velocity of groundwater, but also the increasing content of TDS in geothermal fluids.
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Key words:
- high mineralization /
- karst geothermal reservoir /
- hydrochemistry /
- isotopes /
- genesis mechanism
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表 1 地下水赋存条件统计表
Table 1. Statistics of groundwater occurrence conditions
地层年代 含水地层代号 主要岩性 赋存地下水类型 富水性 第四系 Qh 砂卵砾石 松散岩类孔隙水 弱 二叠、三叠系 P2w、P1m、P1q、 T1d 微晶灰岩 碳酸盐岩裂隙岩溶水 中等 泥盆系 D2-3y 石英砂岩 碎屑岩类裂隙水 弱 寒武、奥陶系 O1n、O1Є3l、Є2qn、Є1sl 白云岩、微晶灰岩 碳酸盐岩裂隙岩溶水 中等 志留系 / 页岩、泥岩、泥质粉砂岩 隔水层 差 
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表 2 地热田地层岩性和水温基本特征一览表
Table 2. Basic characteristics of lithology and water temperature of the geothermal field
孔号 孔深/m 含水层/m 岩性特征 温度特征 ZK01 203.59 74.0~80.60 白云岩：黑色、灰黑色，可见方解石晶簇及小溶孔 该孔地热流体温度21.93~28.45 ℃。其中，在第一段含水层平均水温24.1 ℃，第二段含水层平均水温23.7 ℃。在18.00 m处温度达最高，而后逐渐降低，在134.00 m处温度达最低，孔底温度升至23.97 ℃ 101.3~105.2 白云岩：黑色、灰黑色，可见溶蚀现象，多为溶孔，见方解石晶体 ZK02 111.29 12.30~13.10 白云岩：灰白色，可见溶蚀现象，溶孔连续呈线状发育，可见方解石晶体充填 该孔地热流体温度23.70~32.36 ℃。其中，在第一段含水层平均水温30.9 ℃，第二段含水层平均水温30.4 ℃。在10.00 m处温度达最高，而后逐渐降低，在107.07 m处温度达最低，孔底温度升至24.30 ℃ 21.50-21.70 白云岩夹灰岩角砾：灰白色、深灰色，岩石节理裂隙发育，可见溶蚀现象，可见方解石晶体充填 ZK03 141.61 22.30~26.90 碎裂白云岩：灰白色、深灰色，可见溶蚀现象，节理裂隙较发育。 该孔地热流体温度21.62~35.34 ℃。其中，在第一段含水层平均水温35.2 ℃，第二段含水层平均水温29.9 ℃，第三段含水层平均水温25.6 ℃。在19.00 m处温度达最高，而后逐渐降低，在98.08 m处温度达最低，孔底温度升至23.62 ℃ 34.05~41.35 白云岩：灰白色，可见溶蚀现象，见方解石晶体充填 54.75~60.10 角砾岩：灰黑色，受断层挤压，节理裂隙较发育，可见溶蚀现象，在58.1 m处见硅化现象，59.1-59.15 m处岩芯可见黄铁矿富集 ZK04 199.6 18.45~27.80 碎裂白云岩：灰白色，节理裂隙较发育，可见溶蚀现象，多为溶孔 该孔地热流体温度22.96~29.42 ℃。其中，在第一段含水层平均水温29.3 ℃，第二段含水层平均水温29.0 ℃。在40.00 m处温度达最高，而后逐渐降低，在169.01 m处温度达最低，孔底温度升至24.96 ℃ 32.40~36.60 碎裂白云岩：灰白色，节理裂隙较发育，可见溶蚀现象，多为溶孔、溶槽
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表 3 盐场河地热田水化学及同位素数据
Table 3. Hydrochemistry and isotope data of the geothermal field of the Yanchang river
样品 采样
时间温度 pH TDS HCO3− Cl− NO3− SO42− K+ Na+ Ca2+ Mg2+ SiO2 Li Ba Sr F Si B δD
‰δ18O
‰δ34SCDT‰ 水化学
类型神农
温泉BD-
QS-0012017.08 31 7.86 8 402.7 2 003.23 3 754.82 0.12 283.5 49.7 3 090.55 130.81 27.96 15.21 0.19 0.09 2.68 − 7.09 0.27 −60.37 −8.97 −6.07 Cl-Na BD-
QS-0022017.11 25 7.92 11 193.5 606.65 6 052.44 2.94 470.76 45.12 4 109.26 155.37 32.42 8.53 0.56 0.10 3.92 − 8.53 0.35 −54.48 −7.72 Cl-Na BD-
QS-0032018.04 31 7.89 11 545.4 597.92 6 252.55 3.76 489.03 46.07 4 245.12 155.61 31.75 18.79 0.58 0.12 4.02 − 8.76 0.36 −63.7 −9.5 −6.83 Cl-Na 盐场
河地
热田ZK01 2017.12 26.5 7.66 9 379.1 531.90 5 003.53 16.3 451.63 53.31 3 397.11 149.52 34.23 5.33 0.00 0.06 2.95 1.14 2.48 0.30 −62.34 −9.31 Cl-Na ZK02 2018.04 32 7.79 12 477.7 655.25 6 773.64 5 504.18 49.51 4 598.92 163.19 33 18.87 0.62 0.10 4.21 − 8.80 0.37 −63.1 −9.6 −8.11 Cl-Na ZK03 2018.05 34.5 7.52 10 968.7 577.63 5 933.04 9.55 454.68 44.42 4 028.19 151.04 31.82 18.53 0.55 0.11 3.81 4.93 8.64 0.34 −62.9 −9.4 −8.13 Cl-Na ZK04 2018.04 30.5 7.62 10 839.6 604.5 5 852.33 2.12 452.48 44.17 3 973.4 155.13 33.09 17.79 0.55 0.09 3.8 3.39 8.30 0.34 −62.9 −9.5 −8.06 Cl−Na 冷泉 SW006 2017.12 16.6 7.49 345 384.97 4.96 1.15 17.5 6.53 1.59 80.45 31.05 7.54 0.00 0.05 0.14 0.26 3.52 0.00 − − HCO3-
Ca+MgSW007 2017.12 18 7.83 271.3 307.02 5.01 3.49 9.27 6.23 1.47 52.87 30.73 7.2 0.01 0.01 0.04 0.09 3.36 0.00 − − HCO3-
Ca+MgSW023 2017.12 16.3 7.45 155.4 119.01 5.55 13.99 11.12 0.14 0.15 2.03 0.22 10.24 0.00 0.00 0.14 0.09 4.77 0.00 − − HCO3-
CaSW027 2018.04 18.5 7.84 237.4 254.43 4.7 52.38 11.03 5.73 1.2 47.9 24.6 7.06 0.00 0.04 0.10 0.17 3.29 0.00 − − HCO3−
Ca+Mg河水 BD-
HL0012017.08 − − − − − − − − − − − − − − − − − − −59.98 −9.06 −11.5 − 雨水 BD-
YS0012018.04 − − − − − − − − − − − − − − − − − − −8.00 −1.40 − − 注：单位水温为℃，pH为无量纲，其他为mg·L −1。钻孔监测为出水口温度。
Note: The unit of water temperature is ℃; pH is dimensionless; others are mg·L−1. Borehole monitoring is the temperature of water outlet.
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表 4 同位素补给高程统计表
Table 4. Statistics of isotope recharge elevation
名称 采样点 类型 δD/‰ 补给高程/m 盐场河
地热田ZK01 地热流体 −62.34 1 261.21 ZK02 地热流体 −63.1 1 281.71 ZK03 地热流体 −62.9 1 274.30 ZK04 地热流体 −62.9 1 298.25
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表 5 各矿物组分饱和指数一览表
Table 5. Saturation index of mineral components
神农温泉 ZK04 ZK02 ZK03 矿物
组分2017.08取样 2018.04取样 2017.11取样 2018.04取样 2018.04取样 2018.05取样 模拟反应 模拟反应 模拟反应 模拟反应 模拟反应 模拟反应 前 后 前 后 前 后 前 后 前 后 前 后 硬石膏 −1.79 0 −1.54 0 −1.54 0 −1.56 0 −1.53 0 −1.57 0 文石 0.37 −0.14 −0.1 −0.14 −0.1 −0.14 −0.09 −0.14 −0.06 −0.14 −0.12 −0.14 方解石 0.51 0 0.04 0 0.04 0 0.05 0 0.09 0 0.02 0 玉髓 −0.35 −0.43 −0.25 −0.43 −0.25 −0.43 −0.28 −0.43 −0.25 −0.43 −0.26 −0.43 温石棉 −8.37 −11.59 −8.01 −11.59 −8.01 −11.59 −7.99 −11.59 −7.98 −11.59 −8.01 −11.59 白云石 0.72 0 −0.24 0 −0.24 0 −0.19 0 0.15 0 −0.26 0 萤石 / / / / / / −0.2 0 / / 0.12 0 石膏 −1.57 0.17 −1.33 0.17 −1.33 0.17 −1.34 0.17 −1.32 0.17 −1.36 0.17 石盐 −3.68 −1.91 −3.34 −1.86 −3.34 −1.86 −3.39 −1.87 −3.28 −1.87 −3.38 −1.87 石英 0.07 0 0.18 0 0.18 0 0.15 0 0.18 0 0.17 0 菱锰矿 −0.73 −2.56 −1.10 −2.41 −4.82 −8.73 −1.10 −2.43 −1.05 −2.39 −1.12 −2.43 滑石 −5.37 −8.73 −4.82 −8.73 −1.1 −2.42 −4.85 −8.73 −4.77 −8.73 −4.83 −8.73 注：以上数据均为无量纲。
Note: Above data are all dimensionless.
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表 6 水化学温标计算表
Table 6. Calculation of hydrochemistry temperature scales
项目（℃） 神农温泉 ZK04 ZK02 ZK03 2017.08 2018.04 2018.04 2018.04 2018.05 二氧化硅温标 53.45 61.11 61.28 59.10 60.59 Na-K温标 69.02 48.85 50.02 48.47 49.63 K-Mg温标 93.98 90.32 88.70 91.70 89.35 Na-K-Ca温标 113.29 98.41 99.28 98.12 98.99 热储层温度 59.11
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