Hydrogeochemical characteristics and circulation model of deep geothermal resources in Tianjin

ZHANG Qiuxia; LIU Donglin; YUE Dongdong; YANG Li; FENG Zhaolong; LI Shengtao

doi:10.11932/karst20250301

Abstract：Geothermal resources,as a pivotal renewable and environmentally benign energy source for advancing green 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 geothermal 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 Formation,as well as bedrock fracture-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 geothermal 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 HCO3-dominance to Cl-and SO42-dominance.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 geothermal fluids.Specifically,Cl-and Na+primarily originate from the dissolution of salt rocks.In contrast,Ca2+and Mg2+ions are mainly affected by the dissolution of carbonate minerals.Furthermore,cation exchange processes resulted in an increase in Na+concentrations and a corresponding decrease in Ca2+and Mg2+concentrations.Gypsum dissolution also served as a significant source of SO42-in geothermal fluids.The dissolution of gysum induced a common ion effect that promoted the precipitation of CaCO3,further reducing the concentrations of Ca2+and HCO3-.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 oxygen 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(ⅱ)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 supersaturation or near saturation,suggesting that SiO2 geothermometry 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 quartz geothermometer results as representative of the reservoir temperature.The estimated thermal storage temperature range in the study area was between 67.06 ℃ and 121.38 ℃. 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 ℃ and 160.90 ℃.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 extended geological periods,the dissolution of calcite and dolomite reaches equilibrium in the groundwater,maintaining a stable HCO3-concentration.Meanwhile,Ca2+and Mg2+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 Cl-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 patterns of underground thermal water.