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Article

Geochemical Characteristics of Geothermal Fluids of a Deep Ancient Buried Hill in the Xiong’an New Area of China

1
Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geological Sciences, Shijiazhuang 050061, China
2
Innovation Center of Geothermal and Dry Hot Rock Exploration and Development Technology, Ministry of Natural Resources, Shijiazhuang 050800, China
*
Author to whom correspondence should be addressed.
Water 2022, 14(19), 3182; https://doi.org/10.3390/w14193182
Submission received: 19 August 2022 / Revised: 13 September 2022 / Accepted: 27 September 2022 / Published: 10 October 2022
(This article belongs to the Special Issue Hydrochemical Characteristics of Geothermal Water)

Abstract

:
The Xiong’an New Area is one of the areas with the richest geothermal resources in the east-central part of China. However, the genesis of the geothermal water in Jixianian carbonate reservoirs in this area is still unclear. This study conducted systematical geochemical and isotopic analyses of the geothermal water in the Jixianian carbonate reservoirs in the Rongcheng geothermal field and summed up the genetic mechanisms of geothermal fluids in deep geothermal reservoirs. The results are as follows: the geothermal water in the study area has a hydrochemical type of Cl·HCO3-Na and originates from meteoric water in the Taihang Mountains. The age of the geothermal water increases from 22 ka in the west to 45 ka in the east, and its transport rate is approximately 1.02 m/a. The Sr concentration and 87Sr/86Sr ratio of the geothermal fluids increase along their runoff direction and are related to the dissolution and filtration of minerals such as dolomite and gypsum and the decay of 87Rb in the Earth’s crust. The geothermal water is involved in deep circulation and occurs in a closed system. These results are consistent with those obtained using the PHREEQC inverse model. The reverse hydrogeochemical simulation results exhibited the precipitation of gypsum and halite, the dissolution of anorthite and quartz, and desulfurization. The geothermal reservoir temperatures were estimated to be 92–113 °C using a SiO2 geothermometer, and the thermal groundwater may have undergone deep circulation, with a prolonged retention time. Moreover, the groundwater occurs in a closed environment, strong water-rock interactions occur between the groundwater and related minerals, and the groundwater absorbs the heat from the deep heat source, thus forming geothermal water.

1. Introduction

In the 21st century, mankind is faced with new challenges for sustainable development, such as energy shortage and environmental pollution. Countries all over the world have successively put peak carbon dioxide emissions and carbon neutrality on the agenda through legislation or planning and strategy formulation, which have become a new direction of green and low-carbon sustainable development. According to the latest energy statistics published by Our World in Data (2021a), geothermal energy, solar energy, water energy, wind energy, biomass energy, and ocean energy accounted for approximately 11.4% of the total global primary energy consumption in 2019 and are known as the six renewable energy sources in the world. Geothermal energy stored in the deep part of the Earth has large reserves and a low carbon footprint, and it is widely distributed and environmentally friendly. Moreover, it is easy to achieve the controllable sustainable exploitation of geothermal energy, thus providing a sustainable and stable energy supply [1,2]. As a national new district, the Xiong’an New Area is one of the regions with the richest geothermal resources and the best development and utilization conditions in central and eastern China. To build this area into an eco-friendly city, it is necessary to study the deep hydrothermal process of geothermal fluids and focus on geothermal energy in the planning and construction of this area, achieve the goals of peak carbon dioxide emissions and carbon neutrality, and build a clean, low-carbon, reliable, and safe energy system.
At present, the main methods used to study the genesis and evolution of geothermal water include isotopic analysis, hydrogeochemistry, and hydrogeochemical simulation. Among them, stable isotope ratio analysis is widely employed to explain the origin and migration path of groundwater [3,4,5,6,7], and radioisotopes are widely used to determine the age of groundwater [8]. The study of the hydrochemical characteristics of geothermal fluids began in the 1950s. Many components and ion ratios can be used as tracers in the process of groundwater circulation and evolution, such as Cl, Br, I, and Sr2+ [9], as well as Ca/Cl, Mg/Cl, Ca/Sr, and Sr/87Sr/86Sr ratios [9,10]. In addition, the traditional hydrochemical analysis can be used to determine the possible water-rock interactions in the groundwater system based on hydrochemical data. However, it cannot quantify the water-rock interactions, which can be quantified through hydrogeochemical simulation. Scholars at home and abroad frequently study groundwater by simulating the reaction paths using the PHREEQC program, which can be used to investigate the formation, distribution, and evolutionary mechanisms of groundwater [11,12,13].
During the early 1990s and the 12th Five-Year Plan period, China conducted two detailed and systematic studies on geothermal resources in the North China Basin, summarizing the regional geothermal field characteristics and genetic models of geothermal resources and evaluating the potential of the geothermal resource in the basin [14,15]. In the Xiong’an new area, which lies in the North China Basin, the Ministry of Geology and Mineral Resources of China conducted many geological, hydrogeological, and geothermal surveys and evaluations and accumulated abundant hydrogeological and geothermal geological data. In recent years, many scholars have conducted in-depth research and discussions on the geothermal resources in this area [16,17,18,19]. The Rongcheng geothermal field is less geologically explored than the Niutuozhen geothermal field to the east. Therefore, the China Geological Survey has increased the exploration activities in the Rongcheng geothermal field since 2017, aiming to promote its development process. Previous studies on the geochemical characteristics of geothermal fluids in deep thermal storage in this area essentially focused on the geothermal reservoirs of the Jixianian Wumishan Formation [20,21]. By contrast, limited studies have been conducted on the geothermal reservoirs of the Gaoyuzhuang Formation—another main Jixianian thermal reservoir discovered in recent years—due to the karst characteristics of the reservoirs [22], leading to insufficient understanding of the occurrence environment and the migration and evolution patterns of the geothermal fluids in ancient buried-hill geothermal reservoirs. In addition, it is difficult to accurately identify the material sources and complex water-rock interactions of geothermal fluids only using traditional hydrochemical methods.
This study aims to (1) understand the hydrochemical characteristics of the geothermal water in the study area through the comprehensive analysis of geothermal water samples, (2) infer the origin of the geothermal water from the δ2H–δ18O isotopic composition of the geothermal water, spring water, and rainwater, (3) obtain the water age of the geothermal water based on the 13C–14C characteristics, (4) speculate the intensity of the water-rock interactions that the geothermal water has experienced by conducting Sr–87Sr/86Sr isotopic analysis and PHREEQC reverse simulation, and (5) determine the genetic mechanisms of the geothermal water in the study area by combining all the abovementioned results with the geological and structural evolution of the study area.

2. Geological Setting

The Xiong’an New Area lies in the central part of the Jizhong Depression in the Bohai Bay Basin (Figure 1a). It includes six fourth-order tectonic units: the Langfang depression, the Niutuozhen uplift, the Baoding depression, the Gaoyang uplift, the Raoyang depression, and the Wuqing-Bazhou depression. The study area is located in the west-central part of the Xiong’an New Area and consists of the central portion of Rongcheng and the northern part of Anxin County. Regarding the tectonic location, the study area lies in the south of the Langfang depression in the Rongcheng uplift (Figure 1b), adjacent to the Niutuozhen uplift to the east and the Raoyang depression, the Gaoyang uplift, and the Baoding depression to the south [23]. The Rongcheng fault is a growth fault controlling the development of the Neoproterozoic strata and serves as the dividing line between the Niutuozhen uplift and the Rongcheng uplift. As a deep major fault active in a long term, the Xushui fault to the south was encountered by boreholes XR05 (D16) and XR21 (D19) and extends into the crystalline basement, acting as the dividing line between the Rongcheng uplift and the Baoding depression [21]. The Niudong fault, which is a syngenetic extensional fault, is the boundary between the Wuqing-Bazhou depression and the Niutuozhen uplift. As a deep major fault active in a long term, the Niunan fault, which is immediately south of the Niutuozhen uplift, is the normal fault controlling the southwestern boundary of the Niutuozhen uplift. These faults extend to different depths, and some of them even involve the crystalline basement, providing thermal and hydraulic conductivity channels for geothermal fluid transport (Figure 1c).
The strata in the study area mainly include Archean metamorphic rocks, Upper–Middle Proterozoic carbonate rocks, Ordovician–Cambrian limestones, Neogene breccias, and Quaternary sediments from bottom to top (Figure 2). The important target for geothermal resources exploration and development in this area is the carbonate reservoirs of the Jixianian Wumishan and Gaoyuzhuang formations, which are distributed throughout the study area. The deep carbonate strata have experienced erosion, weathering, and leaching for a long geological period and have well-developed dissolution vugs and fissures, which constitute storage space at favorable structural positions. The Jixianian geothermal reservoirs in the study area are characterized by a large water volume, high temperature, and strong rechargeability. The Jixianian strata have significantly different thicknesses on both sides of faults, which is small in uplifted parts and large on the flanks. According to the pumping test data of drilled boreholes, geothermal wells in the Wumishan Formation have a water yield of 60–200 m3/h·m and a water temperature of 60–80 °C, and the geothermal wells in the Gaoyuzhuang Formation have a water yield of 50–170 m3/h·m and a water temperature of 60–80 °C. Both the Wumishan and the Gaoyuzhuang Formations are marine strata mainly composed of carbonate rocks. They are thermal reservoirs featuring deep circulation and fractures and mainly comprise dolomite, quartz silica, and argillaceous dolomite. Their mineral components primarily include dolomite, calcite, gypsum, quartz, anorthite, halite, and illite.

3. Methodology and Raw Data

In this study, 24 sets of water samples were collected from 2018 to 2020, including four sets of spring water samples from the Taihang Mountains area (T01–04), one set of rainwater samples from the Rongcheng geothermal field (XR01), three sets of geothermal water samples from the geothermal wells in the Neoproterozoic Guantao Formation (XR02–03), 12 sets of geothermal water samples from the geothermal wells in the Jixianian Wumishan Formation (XR04–16), and four sets of geothermal water samples from the geothermal well in the Gaizhuang Formation (XR16–21) (Figure 1c). While sampling, information such as geographical location, coordinates, and elevation were recorded, while the pH and temperature on site were measured using portable thermometers. After rinsing twice using water from the sample source, all the geothermal water sample were filtered through 0.45 μm membranes and stored in three 200 mL polyethylene bottles. For H2SiO3 content analysis, the geothermal water samples were diluted by deionized water to 10% of their initial contents. With respect to metallic and cation elements analyses, samples were acidified to pH 1 with HNO3.
The water samples were sent to the Key Laboratory of Groundwater Science and Engineering, Ministry of Land and Resources, using the Determination Method for Underground Water published by the Chinese government (DZ/T 0064-1993) [24]. The cation contents were detected using inductively coupled plasma analyses (ICAP6300), and the anion contents were measured using chromatography (ICS1500). The charge-balance error was <5%. The δ2H and 18O isotopic compositions were obtained using the isotope analyzer L2130i based on wavelength-scanned cavity ring-down spectroscopy technology. The analytical precision was ±1‰ for δ2H and ±0.1‰ for δ18O. Carbon isotopes of the samples (XR04, 06, 07, 08, 09, 14, 15, and 17) were analyzed using the accelerator mass spectrometry (AMS) technique at the Beta Analysis Laboratory in Xiamen City, China, with analytical precision of ±0.1‰ for δ13C and ±0.1 pMC for 14C. The 87Sr/86Sr ratios of the samples (XR04, 10, 13, 14, 15, 17, 18, and 20) were analyzed at the ultra-clean laboratory of the Zhongnan Mineral Resources Supervision and Testing Center, Ministry of Land and Resources. According to DZ/T 0184.19-1997 [25], Sr was separated and purified using the cation exchange method, and then the Sr isotopic composition was analyzed using a Triton thermal ionization mass spectrometer (00682T). To ensure the high accuracy, the test results were corrected according to the anion and cation milligram equivalent percentages of the water samples. The resulting data errors of the water samples were less than 5%, which was within the normal range. The test results are shown in Table 1.
During the runoff, geothermal water’s hydrogeochemical composition changes with the reactions between the geothermal water and the surrounding rocks as minerals dissolve and precipitate [8,26]. The inverse hydrogeochemical simulation using the PHREEQC program is an effective tool to reflect the variations in water composition and water-rock interaction path [12]. By combining the reservoir minerals with hydrochemical composition of the geothermal water, the inverse model can be used to exhibit the mole transfer of major elemental ions and the behavior of the associated minerals [27]. This study constructed the inverse model using PHREEQC v3.6.2 (database: phreeqc.dat), and the flowchart is shown in Figure 3.

4. Results and Discussion

4.1. Hydrochemical Characteristics

In this study, the water samples for hydrochemical analysis were collected from spring water in the Taihang Mountains, rainwater in the Rongcheng geothermal field, and geothermal water in the Guantao, Wumishan, and Gaoyuzhuang Formations. According to the results of the hydrochemical compositional analysis (Table 1) and Piper diagram (Figure 4), the spring water samples collected from the area of exposed carbonate rocks in the Taihang Mountains (T01–04) had an average temperature of 14 °C, pH of 7.48–7.92, and TDS content of 458–544.1 mg/L, thus belonging to fresh water. Their main cations included Ca2+ and Mg2+, their anions were HCO3, and they had a hydrochemical type of HCO3-Ca·Mg. The geothermal water samples from the Guantao Formation in the Rongcheng geothermal field (XR02–03) had a wellhead temperature of 42–43 °C, pH of 8.31–8.63, a hydrochemical type of Cl·HCO3-Na, and TDS content of 1061–1193 mg/L. The geothermal water samples from the Jixianian carbonate reservoirs (XR04–21) had a wellhead temperature of 55–72 °C and pH of 7.04–8.18, indicating weakly alkaline water. Moreover, they had a TDS content of 2445–2960 mg/L and, thus, had the characteristic of brackish water. Their main cations were Na+, which accounted for more than 80% of their total cations and had an average concentration of 677.8–893.7 mg/L. Their main anions included Cl and HCO3; thus, they had a hydrochemical type of Cl·HCO3-Na. According to the test results, the contents of chemical components and TDS content of the surface spring water samples from the Taihang Mountains area, the geothermal water samples from the Guantao Formation, and geothermal water samples from the Jixianian carbonate reservoirs (Jixianian Wumishan and Gaoyuzhuang formations) increased in turn (Figure 5), indicating that the leaching of the surrounding rocks is the material source of the geothermal water components. Moreover, the Jixianian carbonate reservoir system has a long circulation path, a closed storage space, and intense water-rock interactions between the geothermal water and the surrounding rocks.

4.2. Isotopic Characteristics

4.2.1. H and O Isotopes

According to the global meteoric water line (GMWL) [28] and local meteoric water line (LMWL) [20], the relationship between the δ2H and δ18O values of different water bodies in the study area is plotted in Figure 6a. According to this figure, the water samples from different regions and thermal reservoirs are distributed on the meteoric water lines, indicating that the recharge source of the various water bodies in this area is precipitation.
As shown in Figure 6b, different water-rock interaction processes lead to different distribution patterns of the data points of groundwater samples originating from atmospheric precipitation and can be used to reflect the changes in the hydrochemical type of the samples. The spring water samples from the Taihang Mountains area had δ18O values less than −9‰, high Ca2+ and Mg2+ millimole fractions, a low Na+ concentration, and a hydrochemical type of HCO3-Ca·Mg. The area of exposed carbonate in the Taihang Mountains is more open and has quick water circulation. The geothermal water in this area is directly recharged by atmospheric precipitation and affected by evaporation and has a short circulation path. Moreover, the geothermal water in this area features a low degree of filtration and insignificant water-rock interactions. The geothermal water samples from the reservoirs of the Wumishan Formation had higher δ18O values (greater than −9‰) than those from the deep Jixianian Gaoyuzhuang reservoirs in the study area. Moreover, they had a Na+ millimole fraction more than 80%, Ca2+ and Mg2+ millimole fractions decreasing to less than 10%, and a chemical type of Cl·HCO3-Na. Regionally, the deep carbonate reservoirs have rich geothermal water in some portions of the basin after being recharged by precipitation in the carbonate outcrops of the Taihang Mountains in the west and northwest and undergoing long-distance deep circulation and migration. As for the geothermal fluids in the study area, their hydrochemical type changes from HCO3-Ca·Mg to Cl-HCO3-Na, their salinity increases, and their water-rock interactions become stronger from northwest to southeast. Compared with the shallower Guantao Formation, the geothermal water samples from the sandstone geothermal reservoirs deviated from the meteoric water lines in the vertical direction and showed significant oxygen drift, indicating that the geothermal water in the carbonate reservoirs occurs in a closed environment and that the oxygen isotopic exchange with the surrounding rocks causes heavy isotope enrichment of the water.
In addition, Dansgaard [29] put forward the concept of the deuterium excess parameter (d = δ2H − 8δ18O), which can be used to study underground hot-water circulation. By comparison with the water samples from the Wumishan Formation in Niutuo Town in the adjacent area, the hot-water samples from the carbonate geothermal reservoirs in the study area had smaller d values. This result indicates that the portions with a large burial depth and hot water in the carbonate geothermal reservoirs feature more significant isotopic exchange.

4.2.2. C Isotopes

14C is a major radioactive carbon isotope, and the quantification of geothermal water retention time by 14C dating is the most simple and reliable method used to determine the age of groundwater [30]. 14C dating is based on the measurement of the reduction in the radioactive parent nuclei (14C) in a sample. The essence of 14C dating is to determine the age of dissolved inorganic carbon in water rather than the age of water [10]. Therefore, it is necessary to determine the carbon sources when 14C dating is used to determine the age of groundwater.
On the basis of the δ13C values of different sources, the source and migration of carbon in groundwater can be traced. The inorganic carbon in groundwater is mainly derived from HCO3 formed by the dissolution of external CO2 and carbonate rocks, and the external CO2 mainly originates from atmospheric input, soils, and deep Earth. According to Mauna Loa Observatory in Hawaii, the highest CO2 concentration in the atmosphere is no more than 410 μg/g. The content of inorganic carbon dissolved in rainwater is very low; thus, the influence of atmospheric CO2 on the inorganic carbon in underground hot water can be ignored. Since there is no tectonic activity (e.g., magmatic intrusion and volcanic activity) in the study area, the CO2 input from deep rock structural faults was not considered in this study. Dissolved carbonate rocks generally have δ13C values of 0‰, while CO2 in soils has a δ13C value of approximately −23‰ [5]. The geothermal fluids in the deep thermal reservoirs of the study area had δ13C values of −16.6‰ to −2.2‰, indicating that the 13C of the geothermal water originates from both soils and carbonate rocks.
Overall, the dissolved inorganic carbon in the geothermal fluids of the Jixianian carbonate reservoirs mainly originates from the balanced CO2 in soils and the carbon from carbonate rocks. Considering the effects of the source of dissolved inorganic carbon in the geothermal fluids on the 14C dating of geothermal water, this study calculated the age of the hot groundwater in the study area using the groundwater dating decay equation corrected with the Pearson model [31]:
t = 1 λ ln qA 0 A t = 8267 ln qA 0 A t ,
q = δ 13 C DIC δ 13 C carb δ 13 C soil δ 13 C carb ,
where t is the apparent age of hot groundwater (a), λ is the decay constant of 14C (12.1 × 10−6), δ 13 C DIC is the δ13C value of groundwater, δ 13 C soil is the δ13C value of CO2 in soil (−23‰), δ 13 C carb is the δ13C value of carbon from carbonate rocks (0‰), A0 is the initial radioactivity concentration of parent nucleus (100 pMC), and At is the radioactive 14C concentration in groundwater.
The age of geothermal water in deep carbonate reservoirs in the study area was estimated to be 20–41 ka (Table 2). The corrected ages of geothermal water samples from well XR04 in the west and well XR15 in the east were 21 ka and 42 ka, respectively. Therefore, the geothermal water in the Rongcheng Uplift has a significantly smaller age than that in the Wacao area of Dahe Town in the east. In the Rongcheng geothermal field, the geothermal fluids from the Jixianian carbonate reservoirs at the locations of wells XR04 to XR15 have a flow migration rate of approximately 1.02 m/a, and they circulate slowly from west to east.

4.2.3. Sr Isotopes

Sr does not show any significant fractionation in natural processes and is not affected by evaporation and other processes. Presently, many scholars use Sr to study and solve the practical problems related to underground hot water [32,33]. The cold-water samples from the Taihang Mountains area had Sr concentrations of 0.1–0.378 mg/L and 87Sr/86Sr ratios of 0.7105–0.71153. The geothermal fluid samples from the Jixianian carbonate reservoirs in the study area had high Sr concentrations of 1.466–2.922 mg/L and 87Sr/86Sr ratios of 0.7115–0.7126. The Sr concentration in atmospheric precipitation is generally very low (the local rainwater samples collected in this study had a Sr concentration of 0.006 mg/L), and the Sr concentration in groundwater largely depends on the water-rock interactions between groundwater and water-bearing media [29]. The Sr in groundwater mainly originates from wind-carried sediments, minerals in weathering products (e.g., silicate, carbonate, and sulfate in soils), and water-bearing media. The 87Sr/86Sr ratios of the water samples were compared with those of carbonate sources (0.7088) and aluminum-silicate sources (0.7160–0.7200; Figure 7a) [34,35]. As revealed by the comparison results, the cold-water samples from the carbonate outcrops of the Taihang Mountains had similar 87Sr/86Sr ratios to the weathering source of carbonate rocks. However, there was a poor correlation between the TDS content and Sr2+ and Ca2+ concentrations, and the Sr in groundwater may originate from the dissolution of Sr in carbonate rocks. The 87Sr/86Sr ratios of the geothermal fluid samples from the Jixianian carbonate reservoirs in the study area were higher than those of the carbonate source but lower than those of the aluminosilicate source. By combining the positive linear relationships between the TDS content and Sr2+ and Ca2+ concentrations (Figure 7b,c) as well as the lithologic composition of water-bearing media, it can be concluded that the Sr concentration in the geothermal fluids of the Jixianian carbonate reservoirs may be related to the filtration of Ca-bearing minerals (e.g., carbonates), which is further discussed below.
The relationship between the Sr/Ca and 87Sr/86Sr ratios and the relationship between the δ18O values and the 87Sr/86Sr ratio indicate the enrichment process of Sr isotopes from northwest to southeast and from the Taihang Mountains area to the water sampling points near the Rongxi fault zone and to the water sampling points near the Rongdong fault zone (Figure 8). During the long evolution of the crust, 87Rb decayed into the radioactive element 87Sr through β decay. With the decay of 87Rb, the 87Sr concentration in Rb-bearing rocks (gneiss) and minerals (muscovite) increases continuously as a function of time, showing the cumulative effect of time. The contour maps of the Sr concentration and 87Sr/86Sr ratio of the geothermal water of the Jixianian strata in the study area were plotted as a function of the accumulation effect of Sr isotope age and the characteristics of the groundwater hydrodynamic field (Figure 9a,b). They show a consistent upward trend from northwest to southeast.

4.2.4. PHREEQC Inverse Model

To determine the water–mineral reactions of a range of groundwater from that with simple composition and short-path circulation to that with complex composition and longer-path circulation, this study adopted the PHREEQC software to conduct inverse simulations of the mineral dissolution and precipitation reactions in the underground hot water of the deep carbonate reservoirs in the study area [8]. During the simulations, the starting point of the water flow path was set to the sampling point (T01) of cold spring water in the Taihang Mountains area, and its endpoint was set to the sampling point (XR08) of the geothermal well of the Jixianian thermal reservoirs in the Rongcheng geothermal field at a low altitude. As demonstrated in Section 2, the reservoir minerals in the study area mainly include dolomite, calcite, gypsum, quartz, anorthite, halite, and illite. Considering the cation exchange and adsorption during the formation of spring water, CaX2 was added to the model as an exchange reaction entity. Indicators such as selected typical ion concentration, pH, and temperature were input into the corresponding modules of the PHREEQC software, and the uncertainty of the model was set to 0.05. The calculation results are listed in Table 3.
As revealed by the simulation results (Table 3) of the reverse hydrogeochemical reaction paths, gypsum, rock salt, calcite, dolomite, and illite all undergo dissolution reactions in the paths of the study area, and their dissolved components enter the underground hot water, increasing the K+, Na+, HCO3, and Cl concentrations. According to the geological background and lithologic characteristics of the study area, it is considered that the dissolution of minerals such as dolomite and gypsum in carbonate rocks is the main source of Sr in the geothermal fluids of the deep carbonate reservoirs.
In addition, the 87Sr/86Sr ratio of the fluids leaching through rocks of different compositions is directly related to specific rocks. The geothermal fluids in the deep thermal reservoirs of the Rongcheng geothermal field have higher 87Sr/86Sr ratios than the crust-derived Sr in carbonate rocks (global average: 0.7119) [36,37], suggesting that the Sr in the geothermal fluids may originate from crust-derived materials in addition to related mineral dissolution.

4.2.5. Estimation of the Temperature of Geothermal Reservoirs

Geothermometers are frequently used to estimate the temperature of geothermal fluids in deep reservoirs. At present, the cationic and SiO2 geothermometers are most commonly used.
Cationic geothermometers (K–Na–Ca, K–Na, and Na–Li) are based on the association among cations [38]. Before applying a cationic geothermometer, a Na–K–Mg ternary diagram is used to determine the degree of equilibrium of the samples. If the samples are not in equilibrium (or at least partial equilibrium), cationic geothermometers cannot be applied. As shown in Figure 10, the water samples from the study area were all immature. This finding indicates that the water-rock interactions between the underground hot water and aluminum feldspar, potassium feldspar, and magnesium feldspar minerals have proceeded for a short time and have not reached the equilibrium state. Therefore, the cationic geothermometers are not suitable for this study.
The SiO2 geothermometer was developed on the basis of the solution–precipitation equilibrium theory of silica-bearing minerals in hydrothermal solution [39,40]. In other words, the solubility of SiO2 increases with an increase in the temperature. The SiO2 dissolved in water is not prone to be affected by the ion effect, volatilization loss, and the dilution of other complexes and is only controlled by the solubility of polycrystalline substances of silica, such as quartz. Therefore, the SiO2 geothermometer is the most widely applied [40,41]. It can provide more reliable results than a cationic geothermometer. In this study, four SiO2 geothermometers, namely, quartz, quartz (no steam loss), quartz (maximum steam loss), and chalcedony geothermometers (Table 4), were used to calculate the underground hot-water temperature of the deep thermal reservoirs in the study area. The chalcedony geothermometer should yield a discharge temperature of the geothermal reservoirs close to its actual value; however, the water temperatures at the locations of wells XR05 and XR08 estimated using the chalcedony geothermometer were lower than the measured temperatures, indicating that the chalcedony geothermometer yielded inaccurate results. Therefore, this study used the quartz geothermometer to estimate the average temperature of the deep thermal storage in the study area, and the estimated values were 92–113 °C (Table 5).

5. Geological Genetic Model of the Geothermal Fluids

Owing to the breakage of the North China Craton, the lithosphere has thinned, the deep Moho has been uplifted to an average depth of 33 km, and the temperature at the bottom boundary of the upper crust has increased from 144 °C to 387 °C from the outside to the center of the basin. The measurements of heat flux in the Xiong’an New Area [42] show that the Rongcheng uplift has geothermal heat flow of 54–90 W·m−2, with an average of 74.2 W·m−2. As revealed by these results combined with the data on well temperature and the thermal conductivity tests of core samples [43], the Rongcheng uplift has geothermal gradients of 32–40.9 °C·km−1. In addition, granulocyte basalts with a thickness of approximately 120 m are exposed at a depth of 3000 m at the location of well XR21 (D19) in the study area, which is evidence of shallow intrusions. Moreover, the geothermal fluids in the deep geothermal reservoirs in the study area have higher 87Sr/86Sr ratios than the crust-derived Sr in the carbonate rocks (global average: 0.7119). All these findings reflect strong tectonic activities in the study area, thereby indicating a high thermal background. However, according to the theoretical calculations, the fault activities generate too little heat to serve as a heat source. Moreover, there is no residual additional heat source of magmas in the study area. Therefore, the heat from the deep mantle and the heat from the decay of radioactive elements in the upper crust constitute the heat source of the geothermal system in the study area.
After several tectonic movements in this area, a series of NNE- and NNE-trending secondary structures and large tensile faults have been formed on the Sino-Korean paraplatform and have created uplift and subsidence phases. Faults control the overall pattern of karst development and the karst paleo-landform in the study area [1,44]. The rocks on both sides of the fault zone are affected by the presence of structural fractures and intrusions. Meanwhile, diagenetic fractures develop in the fault zone and provide favorable conditions for hot water circulation. In addition, favorable factors such as fractures form reservoir space and seepage channels for thermal reservoirs. According to the analysis of hydrogeological conditions, the study area receives atmospheric precipitation, which penetrates the ground and provides a long period of lateral groundwater runoff recharge along the fault zone for the deep carbonate reservoirs. Owing to various favorable factors, the underground water rises from deep carbonate rocks after being heated through circulation convection, and the hydraulic connection between deep and shallow parts is established.
In summary, the geothermal fluids of the deep buried-hill thermal reservoirs in the study area evolve as follows: after the rainwater provides lateral runoff recharge in the northwestern Taihang Mountains followed by precipitation and infiltration, the groundwater migrates to deep strata along the fractures generated by the fault movements. Afterward, the groundwater is heated by the heat from the deep mantle and from the radioactive element decay in the upper crust and then rises along faults. With a prolonged retention time, the groundwater occurs in a closed environment. Strong water-rock interactions occur between water and related minerals such as crust-derived materials, dolomite, and gypsum, carrying crust-derived Sr and absorbing heat provided by crust-derived materials. As a result, underground hot water is formed. Along the direction of groundwater flow, the combined effects of the cumulative effect of time and rock–water interactions lead to an increase in the Sr concentration and 87Sr/86Sr ratio of the underground hot water (Figure 11).

6. Conclusions

(1) The geothermal fluids of Jixianian carbonate reservoirs have a hydrochemical type of Cl·HCO3-Na, pH of 7.04–8.18, and TDS content of 2445–2960 mg/L. Stable isotopes δ2H and δ18O indicate that the geothermal water of the Jixianian carbonate reservoirs is recharged by atmospheric precipitation. According to this and the relationship between isotope and hydrochemical characteristics, the geothermal water in the Jixianian carbonate geothermal reservoirs in the study area has experienced long circulation, and its storage space is relatively closed. The water-rock interactions between the geothermal water and surrounding rocks are more intense, and the contents of the hydrochemical components and TDS in the geothermal water are relatively high.
(2) 14C can be used to effectively calculate the age of groundwater of the Jixianian geothermal reservoirs in the study area. According to 14C dating results, the age of the geothermal water in the study area is 20–41 ka overall. In Rongcheng geothermal field, the age of the geothermal water increases from west (XR04) to east (XR15), and the water flow rate is approximately 1.02 m/a. The water in the geothermal field features slow circulation and a long runoff path, and it is characterized by deep circulating water. The formation of geothermal water is controlled by structures.
(3) Sr2+ and Sr isotopes can be used to effectively trace the migration path of geothermal fluids of the Jixianian geothermal reservoirs and the degree of water-rock interactions. As revealed by the simulation results of the reverse hydrogeochemical reaction paths, gypsum, rock salt, calcite, dolomite, and illite all undergo dissolution reactions in the study area, leading to an increase in the K+, Na+, HCO3, and Cl concentrations. Along the groundwater flow direction, the Sr2+ concentration and the 87Sr/86Sr ratio in the geothermal water increase due to the dissolution of minerals such as dolomite and gypsum in carbonate rocks, as well as the cumulative effect of ages.
(4) The geothermal water of the Jixianian carbonate reservoirs in the study area is recharged by atmospheric precipitation, and their temperature is estimated to be 92–113 °C using a SiO2 geothermometer. The geothermal fluids of the deep buried-hill thermal reservoirs in the study area evolve as follows: after the rainwater provides lateral runoff recharge in the northwestern Taihang Mountains followed by precipitation and infiltration, the groundwater migrates to deep strata along the fractures generated by fault movements. With a prolonged retention time, the groundwater occurs in a closed environment, strong water-rock interactions occur between the groundwater and related minerals such as crust-derived materials, dolomite, and gypsum, and the groundwater absorbs heat from the deep mantle and from the radioactive element decay in the upper crust, thus forming geothermal water.

Author Contributions

Conceptualization, M.Y. and G.W.; methodology, M.Y. and W.Z.; formal analysis, M.Y.; investigation, M.Y. and F.M.; resources, G.W.; data curation, M.Y.; writing—original draft preparation, M.Y. and G.W.; writing—review and editing, G.W., X.Z., W.L. and H.Z.; supervision, G.W.; project administration, G.W.; funding acquisition, G.W. and W.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was financially supported by the National Key R&D Program of China (No. 2019YFB1504101) and the Geothermal Survey Project of China (No. DD20189112 and No. DD20221676). The APC was funded by the Geothermal Survey Project of China (No. DD20221676).

Data Availability Statement

All the data have been presented in the tables.

Acknowledgments

We would like to thank the professional reviewers and the editors.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) Basic tectonic map of the Bohai Bay Basin; (b) Basic tectonic map of the Jizhong depression; (c) Distributions of the major faults, geothermal fields, and boreholes in the Rongcheng [20,22].
Figure 1. (a) Basic tectonic map of the Bohai Bay Basin; (b) Basic tectonic map of the Jizhong depression; (c) Distributions of the major faults, geothermal fields, and boreholes in the Rongcheng [20,22].
Water 14 03182 g001
Figure 2. Geological profiles of the study area. The locations of profiles (A-A’ and B-B’) refer to Figure 1c.
Figure 2. Geological profiles of the study area. The locations of profiles (A-A’ and B-B’) refer to Figure 1c.
Water 14 03182 g002
Figure 3. Flowchart of the research.
Figure 3. Flowchart of the research.
Water 14 03182 g003
Figure 4. Piper diagram of the study area.
Figure 4. Piper diagram of the study area.
Water 14 03182 g004
Figure 5. Schoeller diagram showing the water composition of the study area.
Figure 5. Schoeller diagram showing the water composition of the study area.
Water 14 03182 g005
Figure 6. Plot of δ2H–δ18O (a) and δ18O–major ions (b) of water samples of the study area.
Figure 6. Plot of δ2H–δ18O (a) and δ18O–major ions (b) of water samples of the study area.
Water 14 03182 g006
Figure 7. Plots of Sr versus 87Sr/86Sr ratio (a), Ca2+ (b), and TDS (c) content of the water samples.
Figure 7. Plots of Sr versus 87Sr/86Sr ratio (a), Ca2+ (b), and TDS (c) content of the water samples.
Water 14 03182 g007
Figure 8. Plots of the Sr/Ca-87Sr/86Sr (a) and δ18O–87Sr/86Sr ratios (b) of the water samples.
Figure 8. Plots of the Sr/Ca-87Sr/86Sr (a) and δ18O–87Sr/86Sr ratios (b) of the water samples.
Water 14 03182 g008
Figure 9. Contour map showing the distribution of the Sr2+ content (a) and 87Sr/86Sr ratios (b) of the water samples from the Jixianian carbonate reservoirs.
Figure 9. Contour map showing the distribution of the Sr2+ content (a) and 87Sr/86Sr ratios (b) of the water samples from the Jixianian carbonate reservoirs.
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Figure 10. Na–K–Mg ternary diagram of geothermal water in the study area [38].
Figure 10. Na–K–Mg ternary diagram of geothermal water in the study area [38].
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Figure 11. Geological genetic model of the geothermal water in the study area.
Figure 11. Geological genetic model of the geothermal water in the study area.
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Table 1. Hydrochemical analysis data of the geothermal water from different aquifers in the study area and cold water from Taihang Mountains (mg/L).
Table 1. Hydrochemical analysis data of the geothermal water from different aquifers in the study area and cold water from Taihang Mountains (mg/L).
Sample No.StratigraphicT
(°C)
pHδ2H
(‰)
δ18O
(‰)
TDSK+Na+Ca2+Mg2+HCO3ClSO42−H2SiO3Sr2+87Sr/86Sr(2s)
T01 */147.6961.648.44580.79 70.6 25.2 276.5 11.3 17.0 /0.10.71153 ± 0.00005
T02 */147.92−67−9.35022.315.6 80.8 23.5 261.1 18.0 68.1 /0.38 0.70926 ± 0.00002
T03 */147.48−74.09−10.275441.928.8 57.9 34.3 360.7 7.3 23.1 /0.28 0.71051 ± 0.00004
T04 */147.85−75−10.53511.24.4 54.3 19.1 240.6 4.9 9.0 /0.15 0.71070 ± 0.00003
XR01//7.15−68−9.4270.20.6 2.4 0.1 17.9 1.7 1.6 /0.01 /
XR02Ng428.63−73.85−9.9110611.9323 4.8 1.6 439.3 208.1 33.7 /0.33 /
XR03Ng438.31−75.71−9.9411932.1371 5.3 1.5 447.3 308.7 18.7 /0.37 /
XR04Jxw657−74.14−8.98278442 833 53.1 24.4 669.7 1040 21.7 /2.40 0.711525 ± 0.000008
XR05Jxw727.4//278743 783 55.4 28.9 704.3 1085 2.2 53.2 //
XR06 *Jxw556.94−74.79−8.66283248 805 57.5 28.2 706.6 1101 1.2 51.6 2.92 0.71257 ± 0.00004
XR07 *Jxw687.17−74.21−8.57292650 849 54.9 27.0 682.0 1164 8.7 65.3 2.70 0.71261 ± 0.00003
XR08 *Jxw717.31−74.21−8.64296052 858 58.8 27.4 694.3 1175 1.6 62.1 2.88 0.71270 ± 0.00003
XR09 *Jxw557.2−74.22−8.7227994580162.930.6700.51079<0.02650.62.580.71187 ± 0.00001
XR10Jxw716.99−75.12−8.61290853 835 53.8 23.9 662.4 1182 1.3 /2.92 0.71265 ± 0.00002
XR11Jxw627.1−75−9291447 827 62.6 32.0 735.0 1118 4.2 60.2 2.65 /
XR12Jxw568.18−72.54−8.62232530 678 27.7 16.5 706.8 784 23.8 /1.47 /
XR13Jxw706.98−73.57−8.86282054 855 55.4 22.2 666.1 1080 1.6 /2.88 0.712649 ± 0.000011
XR14Jxw718.63−73.14−8.61287256 854 57.5 25.4 595.3 1136 3.1 /2.92 0.712621 ± 0.000008
XR15Jxw/7.27//295954 893 58.5 22.6 671.2 1167 2.5 67.1 2.70 0.71273 ± 0.00001
XR16Jxw717.32//270442 774 48.9 26.0 645.2 1076 4.1 60.3 2.51 /
XR17Jxg587.25−73.89−8.73280946 815 59.3 26.5 701.7 1061 5.9 67.9 2.53 0.71209 ± 0.00004
XR18Jxg667.22−74−8.8290149 828 61.0 31.3 705.9 1109 18.5 /2.46 0.712504 ± 0.000008
XR19Jxg/7.09//292748 858 62.6 26.7 687.1 1141 9.3 64.6 2.87
XR20Jxg647.48−75−9262044 721 59.2 30.4 703.5 985 4.3 68.3 2.41 0.71153 ± 0.00005
XR21Jxg/7.99//244550 813 52.8 22.3 639.7 1068 11.4 82.8 0.38 /
Note: “*” indicates data collected from [21]. Ng denotes Neogene Guantao Formation; Jxw denotes Jixianian Wumishan Formation; Jxg denotes Jixianian Gaoyuzhuang Formation.
Table 2. 14C corrected model age of geothermal water in the study area.
Table 2. 14C corrected model age of geothermal water in the study area.
Sample No.Zoning14C (pMC)13C (‰)Pearson Model Corrected Age (ka)
XR04Uplift2.41−6.220
XR06Uplift<0.44−3.535
XR07Uplift<0.44−3.836
XR08Uplift<0.44−3.435
XR09Uplift1.1−7.528
XR14Uplift<0.44−2.833
XR17Uplift1.3−3.821
XR15Wacao area0.5−16.641
Table 3. Results of possible reaction equations and the simulation path.
Table 3. Results of possible reaction equations and the simulation path.
MineralsPossible ReactionMole Transfer (mmol/kg)
Anorthite CaAlSi 2 O 8 + 1.5 H 2 O Ca 2 + + 0.5 Al 2 Si 2 O 5 ( OH ) 2 + H 4 SiO 4 + 2 OH −2.561 × 10−3
Gypsum Ca SO 4 Ca 2 + +   SO 4 2 1.639 × 10−5
Halite NaCl Na + + Cl   3.324 × 10−2
Quartz SiO 2 + 2 H 2 O H 4 SiO 4 −2.301 × 10−3
Dolomite CaMg ( CO 3 ) 3 + 2 H 2 O Ca 2 +
+ Mg 2 + + 2 HCO 3 + 2 OH
5.731 × 10−4
Illite KAlSi 2 O 8 + 5.5 H 2 O K + + 0.5 Al 2 Si 2 O 5 ( OH ) 2
+ 3 H 4 SiO 4 + OH
2.227 × 10−3
Calcite CaCO 3 +   H 2 O Ca 2 + + HCO 3 5.896 × 10−3
CaX2 CaX 2 + H 2 SO 4   CaXO 4 + 2 HX −2.638 × 10−3
CO2/5.073 × 10−3
Note: Positive numbers in mole transfer indicate dissolution, while negative numbers indicate precipitation.
Table 4. Geothermometer formulas for hot water [41].
Table 4. Geothermometer formulas for hot water [41].
GeothermometerFormulas
Quartz T ° C = 42.198 + 0.288   31 SiO 2 3.6686 × 10 4   SiO 2 2
+ 3.1665 × 10 7 SiO 2 3 + 77.0341 lg SiO 2
Quartz (no steam loss) T ° C = 1309 5.19 lg SiO 2 273.15
Quartz (maximum steam loss) T ° C = 1522 5.75 lg SiO 2 273.1
Chalcedony T ° C = 1032 4.69 lg SiO 2 273.1
Table 5. Estimated temperature of geothermal reservoirs in the study area (℃).
Table 5. Estimated temperature of geothermal reservoirs in the study area (℃).
Sample No.TEstimated Geothermal Reservoir Temperature
QuartzQuartz (No Steam Loss)Quartz (Maximum Steam Loss) ChalcedonyAverage of Thermal Storage Temperature
XR056293.18 92.69 94.71 62.18 93.53
XR065591.87 91.37 93.55 60.77 92.26
XR0768102.45 102.05 102.81 72.19 102.44
XR0871100.12 99.69 100.77 69.65 100.19
XR095590.94 90.44 92.75 59.78 91.38
XR116298.72 98.28 99.56 68.15 98.85
XR1556103.71 103.31 103.90 73.55 103.64
XR165898.79 98.35 99.61 68.22 98.92
XR1771104.23 103.84 104.36 74.12 104.15
XR19/101.91 101.49 102.33 71.59 101.91
XR2064104.53 104.15 104.62 74.45 104.44
XR21/113.76 113.48 112.64 84.53 113.29
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Yu, M.; Wang, G.; Ma, F.; Zhang, W.; Lin, W.; Zhu, X.; Zhang, H. Geochemical Characteristics of Geothermal Fluids of a Deep Ancient Buried Hill in the Xiong’an New Area of China. Water 2022, 14, 3182. https://doi.org/10.3390/w14193182

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Yu M, Wang G, Ma F, Zhang W, Lin W, Zhu X, Zhang H. Geochemical Characteristics of Geothermal Fluids of a Deep Ancient Buried Hill in the Xiong’an New Area of China. Water. 2022; 14(19):3182. https://doi.org/10.3390/w14193182

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Yu, Mingxiao, Guiling Wang, Feng Ma, Wei Zhang, Wenjing Lin, Xi Zhu, and Hanxiong Zhang. 2022. "Geochemical Characteristics of Geothermal Fluids of a Deep Ancient Buried Hill in the Xiong’an New Area of China" Water 14, no. 19: 3182. https://doi.org/10.3390/w14193182

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