摘要
文章通过水化学和同位素分析,研究天津市深部热储地热流体水化学特征、水岩相互作用及其形成的水文地球化学过程,以揭示深部地热过程和循环机理,定量评价研究区热储温度、冷水混合比例以及地热流体最大循环深度等,建立天津市深部地热流体循环概念模型。结果表明:(1)天津市地热流体主要来自北部蓟县山区大气降水入渗补给,补给高程443.34~722.7m;(2)大气降水经由入渗作用及周边深大断裂带,进入南部平原区封闭、半封闭的热储层,在径流过程中与围岩发生充分的溶滤、吸附、碳酸盐再沉淀、阳离子交换、脱碳酸等多重作用,同时地热流体发生冷水混入现象,地热流体初始温度为94.54~160.90℃,最大循环深度达2383.29~4279.29 m,冷水混合比例介于0.01~0.77之间,混合后热储温度67.06~121.38℃,热循环深度为1828.27~3150.24m,最终形成了现今高钠离子、高氯离子、高溶解性总固体(TDS)的地热资源特征。
Geothermal resources,as a pivotal renewable and environmentally benign energy source for advancing peen development and establishing a clean energy system,hold immense potential for exploitation in Tianjin.Tianjin is situated in the northern region of the North China Plain,and its geothernal resources are predominantly distributed in the southern plain area,south of the Ninghe-Baodi Fault.These resources encompass porous thermal reservoirs within the Neogene Minghuazhen Formation and the Guantao Fonnation,as well as bedrock fiacture-type thermal reservoirs in the Ordovician,Cambrian,and Mesoproterozoic Wumishan Formation(Jixian System).By integrating hydrochemical and isotope geochemical signatures,this study aims to quantitatively evaluate the mixing proportions of deep geothernal fluids and to systematically elucidate the circulation patterns of deep geothermal reservoirs,thereby providing a theoretical basis for the sustainable development of Tianjin's geothermal resources.Sampling and analytical testing of geothermal fluids indicated a pH range of 7.08 to 8.43,suggesting a weakly alkaline nature.The TDS ranged from 762.1 to 6,040.4 mg·L^(-1),averaging at 1,768.97 mg·L^(-1).Along the flow path,the anionic composition of the geothermal fluids exhibited significant shifts,transitioning from HCO,dominance to CI and SOdominance.This transition was accompanied by an increase in TDS.Both porous geothermal reservoirs and bedrock fracture-type geothermal reservoirs displayed distinct spatial zonation in their hydrochemical characteristics.An in-depth analysis using Gibbs plots and ion ratio coefficients demonstrated that water-rock interactions are the key factors influencing the chemical composition of geothermnal fluids.Specifically,CTand Na'primarily originate from the dissolution of salt rocks.In contrast,Ca^(2+)and Mg^(2+)ions are mainly affected by the dissolution of carbonate minerals.Furthermore,cation exchange processes resulted in an increase in Na^(+)concentrations and a conresponding decrease in Ca^(2+)and Mg^(2+)concentrations.Gypsum dissolution also served as a significant source of SO7in geothermal fluids.The dissolution of gysum induced a common ion effect that promoted the precipitation of CaCO_(3),further reducing the concentrations of Ca^(2+)and HCO_(3)^(-).Isotopic analysis of hydrogen and oxygen revealed that atmospheric precipitation is the primary source of recharge for geothermal fluids.However,the isotopic drift observed in most geothermal fluids indicated that they did not originate directly from local precipitation.Instead,these fluids underwent deep circulation,with lateral recharge serving as the main mode of replenishment.During circulation,these fluids exchanged onygen isotopes with the surrounding rocks.Plotting the geothermal fluids on the Na-K-Mg ternary diagram showed that all samples fell within the partially equilibrated and immature fields,indicating that either(i)the fluid-rock system has not reached cationic equilibrium,or(i1)the ascending deep fluids have been diluted by shallow cold water.Consequently,cationic geothermometers are not recommended for estimating reservoir temperatures.Calculations by PHREEQC software showed quartz and chalcedony to be in supersatunation or near saturation,suggesting that SiO_(2)geothemmometry can reliably estimate temperatures.Reservoir temperatures derived from the quartz geothermometer were generally higher than those from the chalcedony geothermometer and exceeded the measured wellhead temperatures.Therefore,we adopted the quatz geothermometer results as representative of the reservoir temperature.The estimated thermal storage temperature range in the study area was between 67.06°C and 121.38°C.Using the silicon-enthalpy hybrid model,we analyzed deep circulation temperatures and cold water mixing in geothermal fluids.The cold water mixing ratios ranged from 0.01 to 0.77,resulting in estimated deep circulation temperatures of the geothermal fluids between 94.54°C and 160.90°C.To ascertain the maximum circulation depth of the geothermal fluids,we integrated the reservoir temperatures derived from both the quartz geothermometer and the hybrid model,along with the average geothermal gradient.The quartz geothermometer results indicated that the thermal circulation depth of the middle reservoir ranged approximately from 1,828.27 m to 3,150.24 m.Conversely,the hybrid model calculations revealed a deeper maximum thermal circulation depth for the deep reservoir,ranging from 2,383.28 m to 4,279.28 m.Based on the aforementioned study,we have developed an initial conceptual model for geothermal fluid circulation.This model divides the area along the Ninghe-Baodi Fault,designating the recharge zone mainly in the bedrock-exposed region of Jixian county to the north.Atmospheric precipitation infiltrates through this and adjacent deep faults,entering enclosed and semi-enclosed thermal reservoirs in the southern plain.As the precipitation flows,it is progressively heated by underlying heat sources.Over ekstended geological periods,the dissolution of calcite and dolomite reaches equilibrium in the groundwater,maintaining a stable HCO,concentration.Meanwhile,Ca^(2+)and Mg^(2+)undergo processes such as cation exchange and adsorption,leading to their gradual reduction.In contrast,highly soluble rock salt results in significant accumulation of Na*and CI during prolonged migration.Consequently,the geothermal fluids exhibit high concentrations of Na^(+),Cl^(-),and TDS.Furthermore,the mixing of cold water with the geothermal fluids along their flow path has contributed to the current characteristics of geothermal resources in the study area.This study is of great significance for understanding the genetic mechanisms,occurrence modes,and geochemical evolution patters of underground theral water.
作者
张秋霞
刘东林
岳冬冬
杨骊
冯昭龙
李胜涛
ZHANG Qiuxia;LIU Donglin;YUE Dongdong;YANG Li;FENG Zhaolong;LI Shengtao(Center of Hydrogeology and Environmental Geology,China Geological Survey,Tianjin 300304,China;Tianjin Engineering Center of Geothermal Resources Exploration and Development,Tianjin 300304,China)
出处
《中国岩溶》
北大核心
2025年第3期445-461,共17页
Carsologica Sinica
基金
国家重点研发计划“典型干热岩系统热循环持续供热发电试验”(2021YFB1507404)
中国地质调查项目“东部地区干热岩资源调查评价”(DD20221680)
中国地质调查项目“天津东丽−河北牛驼镇地热资源调查与试验”(DD20190127)。
关键词
天津市
深部地热
水文地球化学
热储温度
硅-焓混合模型
循环模式
Tianjin
deep geothermal resources
hydrogeochemistry
geothermal reservoir temperature
silicon-enthalpy hybrid model
circulation model
