Geochemical, Geophysical Genesis of the Ranggu Geothermal Spring in Aba Prefecture, Western Sichuan: Evidence from Hydrogeochemical and Geophysical Exploration
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摘要:
地热资源是一种清洁低碳、储量丰富、安全优质的可再生能源，大力开发利用地热资源，对落实“碳达峰、碳中和”战略目标具有重要意义。壤古温泉位于青藏高原东南缘川西阿坝州壤塘县，为松潘-甘孜褶皱带地热资源空白区钻获的优质地热资源，井口水温39.5℃，自流流量1500 m3/d，属富含偏硅酸、偏硼酸、锶的氟、锂优质热矿水，具有极高的医疗价值。文章以壤古温泉为研究对象，通过水文地球化学、地球物理特征研究，探讨了地热形成机制。结果表明：壤古温泉pH值6.7～7.1，溶解性总固体2050~2760 mg/L，水化学类型为HCO3-Na型，水岩作用强烈。其氢氧同位素分布于全球大气降水方程线附近，说明热水主要为大气降水补给。Na-K-Mg平衡图表现为未成熟水，表明热水受裂隙潜水或地表冷水强烈混合作用。基于传统地热温标、硅焓混合模型、Cl−校正估算热储温度为138~183.3℃，冷水混合比例为77.9~84.3%。综合地球物理勘探、钻探揭露特征，本文构建了壤古温泉成因概念模型，可为壤古温泉的开发利用提供理论支撑。
Abstract:Geothermal resource is a clean, low-carbon, abundant, safe and high-quality renewable energy. Vigorously developing and utilizing geothermal resources is of great significance to the implementation of the strategic goal of "carbon peak, carbon neutral". Located in Rangtang County, Aba Prefecture, western Sichuan on the southeast edge of the Qinghai-Tibet Plateau, Ranggu geothermal spring is a high-quality geothermal resource drilled in the geothermal resource blank area of the Songpan-Ganzi fold zone. The wellhead water temperature is 39.5℃, and the artesian flow is 1500 m3/d. It is a high-quality thermal mineral water rich in metasilicic acid, metaborate, strontium, fluorine and lithium, and has high medical value. Based on the study of hydrogeochemical and geophysical characteristics, the paper discusses the formation mechanism of geothermal energy. The results show that the pH value of the spring is 6.7~7.1, the total dissolved solids are 2050~2760 mg/L, the hydrochemical type is HCO3-Na, and the water-rock interaction is intense. Its hydrogen and oxygen isotopes are distributed near the global atmospheric precipitation equation line, indicating that geothermal water is mainly recharged by atmospheric precipitation. The Na-K-Mg balance diagram shows immature water, indicating that the geothermal water is intensely mixed by fissure phreatic water or surface cold water. Based on the traditional geothermal temperature scale, silicon enthalpy mixing model and Cl− correction, it is estimated that the reservoir temperature is 138~183.3℃, and the mixing ratio of cold water is 77.9~84.3%. Based on the characteristics of geophysical exploration and drilling exposure, a conceptual model of the genesis of the Ranggu geothermal spring is constructed in this paper, which can provide theoretical support for the development and utilization of the Ranggu geothermal spring.
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图 3 吉布斯图解：(a) Cl−/(Cl−+HCO3−) vs. TDS, (b) Na+/(Na++Ca2+) vs. TDS（Gibbs, 1970）和阳离子交换关系判别图：(c) Na++K+-Cl− vs. Ca2++Mg2+-(SO42-+HCO3−)（Ren et al., 2021）; (d) CAI-Ⅰ vs. CAI-Ⅱ（Zhang et al., 2021）
Figure 3.
图 4 含水层岩性判别图：(a) Ca2+/Na+ vs Mg2+/Na+, (b) Ca2+/Na+ vs HCO3−/Na+（Gaillardet et al., 1999）和离子比值关系图：(c) HCO3− vs. Na++K+, (d) Ca2+ vs. Mg2+, (e) Ca2+ vs. SO42−, (f) Ca2++Mg2+ vs. HCO3−+SO42−
Figure 4.
图 6 (a) Na-K-Mg三角图（Giggenbach, 1988）和 (b) SiO2溶解判别图（Giggenbach and Glover, 1992）
Figure 6.
图 7 壤古温泉的(a, b, c) 硅-焓方程图解（图中括号里左为初始热储温度，右为冷水混合比例）, (d, e, f) 硅-焓图解，(g, h, l) 主要铝硅酸盐矿物的lg(Q/K)-T图
Figure 7.
表 1 水化学参数测试结果
Table 1. Experimental results of hydrochemical parameters
样品编号 采样位置 样品类型 采样高程 井深 Sr F B Li δD δ18O RG01 壤古温泉（6月） 温泉水 3083 231.6 2.05 3.03 1.1 6.12 −120.3 −16.5 RG02 壤古温泉（9月） 温泉水 3083 231.6 1.95 2.59 9.6 5.55 \ \ RG03 壤古温泉（12月） 温泉水 3083 231.6 1.66 2.82 8.1 6.28 \ \ DB01 俄尔柯溪水 地表水 3086 \ 0.188 \ 0.007 0.003 −120.1 −16.31 S01 壤塘泉水S01 冷泉水 3185 \ 0.417 \ 0.020 0.008 −112.7 −14.85 S02 壤塘泉水S02 冷泉水 3662 \ 0.169 \ 0.013 0.002 −112 −15.22 S03 吾伊泉水 冷泉水 3121 \ 1.550 \ 0.115 0.089 −122.9 −15.96 S04 马来泉水 冷泉水 3046 \ 0.926 \ 0.230 0.072 −123.9 −16.16 样品编号 pH TDS SiO2 K+ Na+ Mg2+ Ca2+ HCO3− SO42- Cl− 平衡误差 水化学类型 RG01 7.1 2760 39.23 81.4 737 43.0 271 3101 21.8 16.8 +1.3 HCO3-Na RG02 6.7 2050 27.85 65.2 484 36.1 232 2353 14.3 16.0 −4.6 HCO3-Na RG03 6.8 2320 31.77 62.0 579 39.1 266 2591 16.5 16.8 −0.5 HCO3-Na DB01 8.3 251 5.82 0.41 22.5 15.9 36.5 161.5 69.3 0.936 0.3 HCO3·SO4-Ca·Mg S01 7.9 268 12.69 0.35 24.8 20.9 35.1 222.4 56.6 1.97 −3.1 HCO3-Ca·Mg S02 7.7 274 6.38 0.53 21.2 20.1 43.5 245.4 51.9 0.939 −3.5 HCO3-Ca·Mg S03 7.0 842 5.45 6.14 121 69.4 69.5 782.7 149 4.32 −4.5 HCO3-Na·Mg S04 6.3 531 6.55 3.37 45 39.4 83.3 560.3 45.4 4.07 −3.8 HCO3-Ca·Mg 注：采样高程和井深的单位为m；TDS和水化学组分的单位为mg/L；平衡误差的单位为%；“\”表示未测量。 
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表 2 δ2H-δ18O同位素补给高程计算结果
Table 2. Calculation results of recharge elevation by δ2H- δ18O isotopes
取样点名称 采样高程（m） δD（V-SMOW）‰ δ18O（V-SMOW）‰ 补给高程（m） 壤古温泉 3083 −120.3 −16.05 4067 壤塘泉水（S01） 3185 −112.7 −14.85 3877 壤塘泉水（S02） 3662 −112.0 −15.22 4327 吾依泉水（S03） 3121 −122.9 −15.96 4205 马来泉水（S04） 3046 −123.9 −16.16 4169
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表 3 SiO2（石英）温标热储温度计算结果
Table 3. Calculation results of reservoir temperature by SiO2(Quartz) thermometer
样品编号 温度（℃） 无蒸汽损失（℃） 最大蒸汽损失（℃） RG01 39.6 90.8 93.0 RG02 39.3 76.4 80.4 RG03 39.5 81.8 85.1
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表 4 Cl−校后的SiO2温标结果
Table 4. Results of SiO2 temperature scale after Cl− calibration
样品编号 校正前 校正后 无蒸汽损失（℃） 最大蒸汽损失（℃） 无蒸汽损失（℃） 最大蒸汽损失（℃） RG01 90.8 93.0 155.5 148.1 RG02 76.4 80.4 153.8 146.6 RG03 81.8 85.1 155.5 148.1
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