Structural characteristics of Triassic carbonate geothermal reservoir and genesis of thermal water in the Tongluo mountain anticline of Guang’an City, China
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摘要:
为探讨广安市铜锣山背斜三叠纪岩溶热储特征、地热水水化学与同位素组成、热储温度及地热水循环机理,采用地热钻探、水化学与同位素取样测试、热水溶质组分图解分析等手段和方法,开展了地热水成因的研究工作。结果表明:研究区三叠纪碳酸盐岩热储结构相对完整,热储盖层、热储层和热储下部隔水层形成独立的地热水文地质单元。岩溶地热水水化学类型主要为SO4-Ca·Mg和SO4-Ca型,富含F、Sr、Li、B和SiO2物质,其水源补给为大气降水,补给区位于铜锣山以北的大巴山一带,深部地热水补给高程大于1 100 m,补给区年均温度为9 ℃。热储温度为56~76 ℃,热水循环深度为2 013~3 030 m。地热水在循环过程中,主要发生碳酸盐岩和蒸发岩溶解、冷热水混合过程,且冷水混入比例大于80%。结合区域地热地质条件,构建了研究区地热水成因概念模型。
Abstract:There are abundant geothermal resources at low-medium temperature in the fold mountain areas of eastern Sichuan, and the main geothermal reservoirs are Leikoupo and Jialingjiang formations. Generally, karst geothermal reservoirs have the advantages of large water volume, easy reinjection of tail water after exploitation, and less damage to geothermal reservoirs. Recently, a great breakthrough in geothermal exploration of the Tongluo mountain anticline in fold mountain areas of eastern Sichuan has been achieved. Two deep geothermal wells with a depth of more than 1,700 m have been consecutively drilled to obtain karst geothermal resources with high development and utilization value. However, although good results have been obtained in terms of geothermal exploration, there are relatively few studies on the characteristics of geothermal reservoirs and the mechanism of geothermal water circulation.
In the study on structure characteristics of Triassic karst geothermal reservoir, geothermal water chemistry and stable isotope composition, geothermal reservoir temperature, and mechanism of geothermal water circulation, methods including geothermal drilling, hydro-geochemistry, diagram analysis of thermal water components, and geo-thermometer assessment are used to analyze the occurrence condition of karst geothermal water and genetic model in this study area. Results show that the geothermal reservoir structure in the study area is relatively intact, and the cap layer of thermal reservoir, the thermal reservoir, and the lower water barrier layer are formed as independent geothermal hydrogeological units. Karst geothermal water is mainly SO4-Ca·Mg and SO4-Ca types and rich in contents of F, Sr, Li, B, and SiO2. The oxygen and hydrogen isotope analysis indicates that karst geothermal water is of meteoric origin, and the recharge area is located in the Daba mountainous area to the north of Tongluo mountain. The elevation of deep geothermal water recharge is more than 1,100 m; the average annual temperature of the recharge area is 9℃; the horizontal distance reaches 220 km. According to the SiO2 geo-thermometer, the karst geothermal reservoir temperature is 56-76℃; the circulation depth of geothermal water is 2,013-3,030 m, and the karst geothermal water is immature. Moreover, the dissolution of carbonate and evaporite and the mixing of cold groundwater are major hydro-geochemical processes in the geothermal water circulation path, and the mixing ratio of cold groundwater is higher than 0.8. Based on the results of geothermal geology, geothermal drilling, hydro-geochemistry, and environment isotopes, the genesis mechanism of geothermal water in this area is summarized as follows, groundwater receives water recharge from rainwater through karst depressions, sink, and pit of the Leikoupo and Jialingjiang formations in the anticline core region, heated by geothermal gradient and hot rock during the deep circulation of the anticline wing. During water circulation, major hydro-geochemical processes are water-rock interaction and thermal-cold water mixing. After then, karst geothermal water occurs along the anticline axis and wing region in forms of hot springs and geothermal wells. This study not only enriches the theory of the karst geothermal reservoirs system in the fold mountainous areas of eastern Sichuan but also provides a scientific basis for the exploration and exploitation of deep karst geothermal resources in this area.
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表 1 铜锣山背斜水样的物理—化学和同位素组分分析结果
Table 1. Physical-chemical and isotopic compositions of water samples in the Tongluo mountain anticline
编号 类型 水温 pH TDS HCO3− Cl− SO42− NO3− K+ Na+ Ca2+ Mg2+ F− Sr SiO2 Li B δD δ18O 补给
高程补给区
温度GW01 地热井 42 7.20 3242 208.00 18.30 2091.00 2.68 12.80 28.60 678.00 177.00 3.00 12.80 26.32 0.27 0.72 −64.53 −9.58 1251 9 GW02 地热井 29 7.16 3064 223.00 12.00 2004.00 — 9.65 18.20 609.00 157.00 2.95 13.40 24.85 0.02 0.13 −62.94 −9.61 1198 10 GW03 地热井 42 7.06 3230 208.00 20.90 2067.00 — 12.80 33.80 702.00 156.00 2.40 12.20 26.47 0.24 0.63 −64.38 −9.52 1246 9 GW04 地热井 26 7.24 1002 335.00 2.80 401.00 — 2.28 4.32 186.00 48.20 0.96 4.70 15.23 0.03 0.02 −50.32 −8.14 777 14 HW01a 温泉 30 7.33 1800 265.00 264.00 858.00 — 94.60 100.00 293.00 70.64 — 12.05 — — 0.86 −50.1 −8.18 770 14 HW02b 温泉 45 7.12 1680 183.00 34.00 1643.00 0.56 30.12 28.75 578.00 120.32 12.46 12.11 — — — −59.5 −8.84 1083 11 GW05 浅孔 26 7.33 1733 198.00 1.38 1044.00 — 3.59 2.53 388.00 72.70 1.84 8.44 16.39 0.06 0.19 −56.05 −8.53 968 12 KW01 岩溶泉 18 7.76 317 349.50 7.49 38.30 37.63 4.69 5.67 102.81 28.72 0.26 0.12 9.69 0.00 0.04 −44.76 −7.24 592 16 KW02 岩溶泉 18 6.74 232 333.40 1.08 44.40 1.13 1.03 2.24 89.10 28.60 0.17 0.37 11.04 — 0.02 −44.15 −7.20 572 16 KW03 暗河 16 7.13 455 298.00 2.50 22.50 12.34 1.64 1.47 100.00 7.68 0.13 0.44 7.54 0.00 0.00 −43.29 −7.33 543 16 KW04 暗河 17 7.29 450 281.00 2.50 28.20 11.59 1.61 2.70 98.70 8.77 0.10 1.80 7.85 0.00 0.01 −44.38 −7.52 579 16 R01 河水 18 6.24 140 186.10 2.16 33.50 1.37 2.47 2.84 49.10 14.50 0.11 0.14 10.01 — 0.02 −41.09 −7.33 — — R02 河水 18 6.74 121 109.80 6.92 38.30 7.94 3.31 8.94 40.50 6.42 0.07 0.43 12.41 — 0.03 −40.74 −6.93 — — 注:水温和补给区温度单位为℃;pH无量纲;δD和δ18O单位为‰;补给高程单位为m;化学组分单位为mg·L−1;“—”为未检出或未分析;a引自[25],b引自[26]等文献数据。 表 2 铜锣山背斜地热水热储温标计算结果
Table 2. Calculated results of geothermal reservoirs of the Tongluo mountain anticline
编号 t T1 T2 T3 T4 GW01 42 74 78 75 76 GW02 29 72 76 73 74 GW03 42 74 79 75 76 GW04 26 54 60 53 56 GW05 26 56 62 56 58 注:t为地热井或泉口水温;T1=1309/(5.19−lgSiO2)-273.15,为无蒸汽损失的石英温度计算式;T2=1522/(5.75-lgSiO2)−273.15,为最大蒸汽损失的石英温度计算式;T3=−44.119+0.24469S−1.7414×10−4×S2+79.305lgS,为改进的玉髓温标计算式;T4为T1、T2和T3的平均值;上述温度单位为℃。 表 3 深部热水温度、焓值和SiO2含量[34]
Table 3. Values of temperature, enthalpy and SiO2 in deep thermal water
温度/
℃焓值/
J·g−1SiO2/
mg·L−1温度/
℃焓值/
J·g−1SiO2/
mg·L−1温度/
℃焓值/
J·g−1SiO2/
mg·L−150 50.0 13.5 150 151.0 125.0 250 259.2 486.0 75 75.0 26.6 175 177.0 185.0 275 289.0 614.0 100 100.1 48.0 200 203.6 265.0 300 321.0 692.0 125 125.1 80.0 225 230.9 365.0 -
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