Characteristics of the Rongcheng Bulge Geothermal Field and the Evolution of Geothermal Fluids, Xiong’an New Area, China

Abstract

The study of geothermal field characteristics and the mechanisms involved in the hydrogeochemical evolution of geothermal fluids is an effective means to understand the formation, reservoir conditions and circulation mechanics of geothermal resources. Documenting these parameters within the Rongcheng Bulge of Xiong’an New Area, China, is of great significance to its future development and utilization as a geothermal resource. In this paper, we investigate the hydrochemical characteristics of geothermal fluids and the rock thermal properties of the Rongcheng Bulge Reservoir and the surrounding rocks to identify the mechanisms of geothermal fluid genesis within the geothermal field. The results show that the geothermal water in the Rongcheng Bulge is mainly recharged by atmospheric rainfall. The thermal storage temperature at which the deep geothermal fluid is located ranges between 63 and 105 °C, whereas the depth of geothermal water circulation is between 1500 and 2700 m. Fluid exchange is weak during deep circulation, but as the geothermal fluid rises, the proportion of cold water mixed with the geothermal waters is 75–95%. The overall geothermal gradient of the carbonate thermal reservoir is significantly lower than that within the overburden and basement rocks, probably due to convection during the upward transport of groundwater in the reservoir. The geothermal fluid in the area of the Rongcheng Bulge is recharged by the Taihang Mountains and atmospheric precipitation. Following recharge and deep circulation, the fluids rise along fractures and by interlayer convection and are mixed with cold water to form the current, accessible geothermal water.

1. Introduction

The demand for renewable green energy is growing in response to rapid economic and social development on a global scale, a shortage of non-renewable energy resources and increasing environmental problems [1,2]. Geothermal energy is gaining attention for its unique advantages as a renewable, safe and highly utilizable energy source. For example, Iceland has a long history of geothermal resource utilization and has gained successful experiences in the exploitation and management of geothermal resources. Additionally, Iceland has managed the long-term effective development of its low-temperature geothermal resources [3,4]. Many countries are committed to the development and utilization of geothermal resources [5,6,7,8,9]. In fact, the use of geothermal energy in recent years has increased, which has led to an increase in the development and utilization of geothermal water [10,11]. The study of the mechanisms involved in the evolution of geothermal fluids and the characteristics of the geothermal field is extremely important for the development and utilization of geothermal resources. Such studies may also assist in the overall ecological protection of the area.

Medium- and low-temperature geothermal resources are distributed throughout China; a particularly large number of medium- and low-temperature hot water resources are found in the North China Plain [12], among which the Beijing–Tianjin–Hebei region is the richest in geothermal resources. Two exhaustive systematic studies have been conducted to summarize and assess the potential of geothermal resources in the North China Plain [12,13,14]. As a key area of development, Xiong’an New Area in the North China Plain has been the focus of numerous geothermal research projects in recent years [15,16,17,18,19].

The Rongcheng Bulge represents an initial area of development within Xiong’an New Area and has been a particularly important region of geothermal resource assessment. To date, the general geochemical characteristics of subsurface hot waters in the area of the Rongcheng Bulge have been documented [20,21], but the characteristics of the geothermal reservoir and the surrounding environment within the Rongcheng Bulge are poorly defined. In addition, the vertical distribution and circulation of geothermal fluids within the reservoir are not fully understood.

Isotopic signatures of geothermal fluids are often used to analyze the circulation of geothermal fluids, the environmental characteristics of the geological environment through which the fluids flow and the geochemical processes operating in the geothermal system [22,23,24,25,26,27]. Moreover, when combined with studies documenting the characteristics of the geothermal field [28,29,30,31], the distribution of different sections of the thermal reservoir and the vertical distribution of the geothermal fluids within the thermal reservoir can be further clarified.

Along with the rapid development of Xiong’an New Area, identifying the geothermal resources in the Rongcheng Bulge has become a key research direction. In this paper, the mechanisms involved in the evolution of the geothermal fluids and the characteristics of the geothermal field in the Rongcheng Bulge of Xiong’an New Area are examined to characterize the geothermal resources in the region. The objectives have been accomplished through isotopic characterization of the geothermal fluids and by documenting the circulation mixing mechanisms, the thermal properties of the rocks and the vertical distribution of the geothermal field. The obtained results are used to propose a model of geothermal fluid evolution and circulation within the Rongcheng Bulge area. The developed model can describe the transport pattern and storage characteristics of the geothermal resources more intuitively and accurately than the regional conceptual model of Xiong’an New Area.

2. Study Area

Xiong’an New Area is located in the middle of the Hebei Plain (Figure 1a), 120 km from the center of Beijing to the north, 110 km from Tianjin to the east, 70 km from Bao in the west and 100 km from Cangzhou in the southeast. The area covers 2000 km² in Xiong County, Rongcheng, Anxin and some surrounding areas in Hebei Province. The terrain is slightly higher in the west and north than in the east and south. The altitude is 5–20 m above the sea. The main tectonic units in Xiong’an New Area include three topographic highs (bulges), four depressions and two slopes. The topographic highs are referred to as the Niutuozhen Bulge, the Rongcheng Bulge and the Gaoyang Low Bulge; the depressions include the Baxian depression, the Raoyang depression, the Baoding depression and the Xushui depression. The slopes include the Niubei slope and the Lixian slope. The axis of the Rongcheng Bulge is oriented north-northeast, which is consistent with the direction of the main tectonic line fractures in the area. The genesis mechanism of the hot field in the area belongs to the deep circulation of atmospheric precipitation [14].

The Rongcheng Bulge is located in the central part of the Jizhong Plate, with the Langgu Fault Depression to the north, the Niubozhen Bulge to the east, the Baoding Fault Depression to the south and the Xushui Depression to the west. The Rongcheng Bulge is oriented in a north-northeasterly direction, in line with the orientation of the main tectonic fractures in the area. The regional geothermal background temperatures are controlled by the eastern North China hot basin, which imparts a high regional background heat flow to the region [13,32].

The Rongcheng Bulge Misty Mountains formation contains the main geothermal resource that is developed and utilized in Xiong’an New Area. It is mainly covered by Cenozoic rocks that are in contact with loose strata characterized by poor thermal conductivity, which provides good thermal insulation [11]. The east side of the Rongcheng Bulge is bound by the Rongcheng Fault, whereas the south side is bound by the Xushui Fault. The Rongcheng Fault extends for a distance of about 30 km (between the towns of Anxin and Baigou) and forms the boundary between the Niuhuozhen Fault Convexity and the Rongcheng Fault Convexity. The Rongcheng Fracture has strikes of NNE and locally E and has an inclination of about 45°. Strata are vertically offset across the fault by a distance of 3000 m. The horizontal offset distance ranges between 1000 and 3000 m.

The stratigraphy of the study area is mainly divided into the upper cap layer, the central bedrock thermal reservoir and the underlying basement stratigraphy of the thermal reservoir. The upper cover is mainly the Neoproterozoic Minghuazhen Formation, with a depth range of 0–1000 m, and the lithology is mainly sandstone and some mudstone. The central bedrock thermal storage is mainly the Jixian System Wushan Formation and the Jixian System Gaizhuang Formation, with a depth range of 1000–3000 m. The lithology is mainly carbonate rocks. The underlying stratigraphy is mainly Tauric strata, and the lithology is mainly gneiss. The Minghuazhen formation of the uplifting terrain directly overlies Middle and Neoproterozoic rocks. The thickness of the Neoproterozoic sedimentary strata of the descending disk reaches 2000–3000 m, before transitioning into the crystalline basement along a growth fault that controlled the development of the Neoproterozoic. The Xushui Fault is located along a line from Xushui, Anxin, to Zhao Beikou and is the bounding structure controlling the Rongcheng Bulge. The Xushui Fault has a length of about 35 km. It is a positive fracture oriented approximately east-west (and locally to the south). The vertical fracture distance ranges between 1200 and 3200 m, whereas the horizontal fracture distance is between 1000 and 2500 m.

3. Materials and Methods

3.1. Fluid Sample Collection and Test Methods

Geothermal water samples and rock samples from the Jixian strata were obtained for testing (Figure 1b). A total of 21 water samples of three fluids, namely cold surface water, shallow geothermal water and deep geothermal water, were collected from the Rongcheng Bulge area of Xiong’an New Area and the Taihang Mountains in the northwestern part of the study area. Among them, seven cold water samples, numbered SS01–SS07, were collected from the Taihang Mountains in the northwestern part of the study area, all of which were collected from the civil water wells in use. Five shallow geothermal water samples, numbered QR01–QR05, were collected from the shallow Guantao Formation sandstone thermal reserve in the study area, all of which were collected from the Guantao Formation sandstone thermal reserve geothermal wells in use (sampling depth 600–1000 m). Nine deep geothermal water samples, numbered SR01–SR09, were collected from the deep Wumishan Formation carbonate thermal reserve in the study area, all of which were collected from the Wumishan Formation carbonate thermal reserve geothermal wells in use (

Table 1. Hydrogeochemical data (mg/L).

No.	Layer	PH	TDS	K	Na	Ca	Mg	HCO3	Cl	SO4	SiO2	δ18O	δD(‰)	δ34S(‰)	87Sr/86Sr(2s)
SS01	Surface Water	7.3	785.3	5.2	28.7	134.0	31.2	257.3	56.6	153.3	18.7	−8.6	−61.9	6.1	-
SS02		7.7	331.6	5.0	11.0	51.5	13.3	162.3	23.7	22.1	17.9	−8.6	−61.7	3.4	-
SS03		7.8	467.5	0.9	3.5	65.2	34.6	300.6	8.0	30.8	7.2	−9.0	−66.5	6.5	-
SS04		7.7	458.0	0.7	9.0	70.6	25.2	276.5	11.3	17.0	12.5	−8.4	−61.6	7.6	0.71153
SS05		7.7	433.5	0.4	10.1	66.6	22.6	246.5	16.4	23.6	12.6	−8.3	−51.2	8.8	0.71158
SS06		7.5	544.1	1.9	28.8	57.9	34.3	360.7	7.3	23.1	24.3	−10.3	−74.1	-	0.71051
SS07		7.6	629.1	1.5	12.7	108.2	27.0	324.6	27.4	32.7	19.1	−8.0	−60.0	8.2	0.7108
QR01	Neoproterozoic	8.2	1497.0	3.2	478.9	8.8	2.3	459.1	514.7	2.0	23.2	−9.9	−75.7	-	-
QR02		8.3	1193.0	2.1	370.9	5.3	1.5	447.3	308.7	18.7	22.9	−9.9	−73.9	-	0.70873
QR03		8.6	1061.0	1.9	323.2	4.8	1.6	439.3	208.1	33.7	20.6	−10.0	−75.6	10.5	0.70897
QR04		8.7	1424.0	1.8	472.5	12.2	3.1	349.0	500.9	30.2	21.5	−9.9	−76.3	9.5	0.7177
QR05		8.8	1014.0	2.0	317.4	4.8	1.5	388.3	205.0	34.5	19.4	−8.7	−74.8	9.5	0.7077
SR01	Jixian system	7.2	2918.0	49.0	816.1	63.8	31.5	734.5	1138.0	2.4	36.0	-	-	-	0.71251
SR02		6.9	2862.0	50.8	826.3	51.1	22.2	651.6	1155.0	3.1	54.1	-	-	-	0.71258
SR03		7.0	2958.0	53.3	875.0	54.8	24.8	681.2	1169.0	3.0	47.9	-	-	-	0.71258
SR04		6.9	2832.0	48.2	804.8	57.5	28.2	706.6	1101.0	1.2	39.7	-	-	-	0.71257
SR05		7.2	2926.0	50.4	848.6	54.9	27.0	682.0	1164.0	8.7	50.3	−8.6	−74.2	27.0	0.71261
SR06		7.3	2960.0	52.1	857.7	58.8	27.4	694.3	1175.0	1.6	47.8	−8.6	−74.2	24.8	0.71270
SR07		8.0	1830.0	4.6	589.1	17.5	4.7	405.5	620.9	159.0	20.0	−9.7	−74.8	34.5	0.70833
SR08		7.2	2799.0	45.0	800.8	62.9	30.6	700.5	1079.0	0.0	38.9	−8.7	−74.2	18.0	0.71187
SR09		7.0	2908.0	52.6	835.0	53.8	23.9	662.4	1182.0	1.3	54.0	−8.6	−75.1	28.4	0.71265

) (sampling depth 1000–3500 m) (Table 1).

The fluid samples collected in this study were collected in 2.5-L plastic bottles and sealed. The water samples were first filtered through a 0.45 μm microporous membrane and then stored in plastic bottles that were washed three times with the water samples to be collected. The geothermal water was collected at the corresponding depth of the thermal reservoir and obtained by pumping test. The fluid was relatively clear and not turbid. The main geothermal fluid chemical elements K⁺, Na⁺, Ca²⁺, Mg²⁺, Fe²⁺, Fe³⁺, HCO₃⁻, Cl⁻, SO4²⁻, H₂SiO₃, δ²H and δ¹⁸O were tested at the Key Laboratory of Groundwater Science and Engineering of the Ministry of Land and Resources. In accordance with the drinking natural mineral water testing method “GB 8538-2016” and the groundwater quality testing method “DZ/T0064-93”, the testing mode was plasma emission spectroscopy (model ICAP6300, Thermo Fisher, Waltham, MA, USA) and the test environment was 23 °C, with 48% relative humidity. Hydrogen and oxygen isotopes were detected using a water isotope analyzer (model Picarro 2140-i, Picarro, Inc., Santa Clara, CA, USA) with a testing accuracy of up to 0.1‰. Strontium and sulfur isotopes were tested by the Zhong’nan Geological Science and Technology Innovation Center, Wuhan Geological Survey Center, China Geological Survey. The sulfur isotopes were analyzed using an elemental analyzer inline with a gas isotope mass spectrometer (EA IsoLink-Delta V Advantage, Thermo Fisher, Waltham, MA, USA) with an analytical error range of ±0.2‰. Strontium isotopic compositions were analyzed using a TRITON thermal ionization mass spectrometer with an error range < ±10 × 10⁻⁶ (2σ). Strontium was separated and purified by the cationic resin (Dowex 50 × 8) exchange method. The mean value of 0.71035 ± 0.00003 (2σ) was obtained by repeating the analysis of NBS987 standard throughout the isotope analysis.

3.2. Rock Sample Collection and Test Methods

The rock samples for this study were collected in situ after coring during the construction of the geothermal wells, and a total of 12 rock samples were collected. The rock samples were obtained from four 2500-m geothermal wells at sites DR1, DR2, DR3 and DR4 (Figure 1b), at different depths and from different layers. Twelve rock samples were obtained, along with 12 samples of thermal conductivity and heat generation data from rate tests. Among the 12 samples, 8 samples were subject to variable temperature specific heat capacity tests. The rock thermal conductivity, specific heat and heat generation rate tests were performed by the State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences. Thermal conductivity scanning (TCS) was used for thermal conductivity testing; it possessed a measurement accuracy of ±3% and a range of 0.2–25 W·(m·K)⁻¹. The specific heat test utilized a Hot Disk system with a temperature range of 10–1000 K. The heat generation rate tests used an inductively coupled plasma mass spectrometer (model ICAP-RQ), with an accuracy of up to 3%.

3.3. Geothermal Well Temperature Measurement and Geothermal Gradient Calculation Method

In this study, three geothermal wells (DR1, DR6 and DR7) in the Xiong’an New Area Rongcheng geothermal field were measured (Figure 1b). The depth of the bottom hole of the DR1 well is 2500 m, the depth of the bottom hole of the DR6 well is 3000 m and the depth of the bottom hole of the DR7 well is 4000 m. Temperature measurement was carried out using the self-developed KH-3S integrated CNC logging system, and the depth of temperature measurement was 2500 m for the DR1 well, 2900 m for the DR6 well and 4000 m for the DR7 well, with a sampling interval of 0.125 m; the total error of temperature and depth did not exceed ±2%.

The geothermal gradient was calculated using an empirical formula and was calculated separately for different lithological strata according to the measured temperature data as follows:

G = (T − T₀)/(H − H₀) × 100

(1)

where G is the ground temperature gradient (°C/100 m); T is the measured deepest temperature of the corresponding stratum (°C); T₀ is the temperature of the thermostatic zone (°C), taking the value of 15.0 °C; H is the deepest depth of the corresponding stratum (m); and H₀ is the depth of the thermostatic zone (m), taking the value of 25 m.

3.4. Geothermometry

Geochemical temperature scales are often used to calculate reservoir temperatures of geothermal fluids [33], such as silica and cationic geothermal temperature scales. Based on the PHREEQC geochemical model [34] and fluid chemistry components, the mineral saturation index (SI) was calculated for the fluids in the study area using the WATEQ4F database. Based on the mineral saturation index (SI) and the actual geology of the study area, this study used the chalcedony geothermal temperature scale to estimate the thermal storage temperature of the study area.

The calculated storage temperature (T) was more in line with the actual situation. The following formula was utilized:

$$\mathrm{T}=\frac{1032}{4.69-\lg \left({\mathrm{SiO}}_2\right)}-273.15,$$

(2)

where SiO₂ is the SiO₂ concentration of the geothermal water (mg/L).

4. Results

4.1. General Water Chemistry

The water samples from the geothermal field in the area of the Rongcheng Bulge area and the Taihang Mountains were plotted on a Piper diagram (Figure 2) using the contents of K⁺, Na⁺, Ca²⁺, Mg²⁺, HCO₃⁻, Cl⁻ and SO₄²⁻. These data were also plotted on a box-and-whisker diagram (Figure 3). In addition, the data are shown with other chemical components in the water (Table 1). The samples from the mountainous area (SS) possessed cation concentrations that varied in the following order: Ca²⁺ > Mg²⁺ > Na⁺ > K⁺. The cations in the water were dominated by Ca²⁺ and Mg²⁺, whereas the dominant anion was HCO₃⁻. These data show that the samples consisted of a HCO₃-Ca·Mg water type, with SiO₂ varying between 7.2 and 24.3 mg/L, with an average of 16.1 mg/L, and a pH varying between 7.3 and 7.8, with an average of 7.61. Total dissolved solids (TDS) varied from 458.0 to 785.3 mg/L; the average value was 521.3 mg/L, indicating that the alkalinity was neutral. Cation concentrations in the water samples from the Rongcheng Bulge area (QR and SR) decreased in the following order: Na⁺ > Ca²⁺ > K⁺ > Mg²⁺. The main cation was Na⁺. The anions differed from the mountainous water samples in that their Cl⁻ content was higher, followed by HCO₃⁻. Water in the geothermal field can be classified as a Cl·HCO₃-Na type. SiO₂ varied between 19.4 and 54.0 mg/L, averaging at 35.4 mg/L. Its pH ranged between 7 and 8.8; the average value was 7.7. TDS ranged between 1014.0 and 2960.0 mg/L, with an average value of 2227.3 mg/L. The water was therefore neutral to weakly alkaline. Compared with neutral water with low TDS in the mountainous areas, the geothermal fluid in the Rongcheng geothermal field was higher in TDS and possessed more alkaline water. The Piper diagram (Figure 2) shows that the content of Na⁺, HCO₃⁻ and Cl⁻ in the geothermal fluid of the Rongcheng geothermal field is higher than that in waters from the mountains, which is presumably due to the fact that the geothermal fluid of the Rongcheng geothermal field is derived from the deep circulation system. Figure 4 shows that the geothermal water samples from the Rongcheng geothermal field all fall in the partial equilibrium area, whereas water samples from the mountainous areas plot in the range of immature water.

4.2. Isotopic Characteristics

The δ¹⁸O values of mountain water samples (SS) ranged from −8.0% to −9.0%, whereas the δD values ranged from −51.1% to −74.1% (Figure 5). These data indicate that the water samples from the mountains possess an isotopic signal similar to the local atmospheric precipitation line [35,36,37]. The δ¹⁸O values of the water samples from sites QR and SR ranged from −9.9% to −10.0% and −8.6% to −9.7%, respectively; the δD values ranged from −73.9% to −76.3% and −74.2% to −75.1%, respectively. It is noteworthy that the isotopic data from the deep thermal storage waters in the Rongcheng geothermal field (obtained at the SR sites) were characterized by “oxygen drift”, while shallow thermal storage waters from the QR sample sites plotted near the atmospheric rainfall line.

As a result, the water acquires a Sr isotopic composition similar to that of the rock, allowing ⁸⁷Sr/⁸⁶Sr ratios to be used as a tracer of the source of the geothermal fluid and its flow path [38]. Within China, typical ⁸⁷Sr/⁸⁶Sr ratios for different water bodies are as follows: (1) water containing dissolved aluminosilicate minerals exhibit ⁸⁷Sr/⁸⁶Sr ratios greater than 0.72; (2) new and ancient oil field waters possess ⁸⁷Sr/⁸⁶Sr ratios between 0.7112 and 0.7119; (3) river water has an ⁸⁷Sr/⁸⁶Sr ratio of 0.711; (4) rainwater has an ⁸⁷Sr/⁸⁶Sr ratio of 0.709; and (5) water from marine sediments exhibits an ⁸⁷Sr/⁸⁶Sr ratio of 0.708 [27]. All of the water samples collected in the study area exhibited ⁸⁷Sr/⁸⁶Sr ratios that differed significantly from mantle-related material (0.705) [27] (Figure 6), indicating that the source of recharge of the geothermal waters is not from the mantle. The ⁸⁷Sr/⁸⁶Sr ratios of the water samples from site SS generally fall between 0.709 and 0.712. The ⁸⁷Sr/⁸⁶Sr ratios of the geothermal water samples from site QR are widely distributed on Figure 6, indicating that they have different water flow paths and circulate faster. The ⁸⁷Sr/⁸⁶Sr ratio of the geothermal waters is mainly above 0.712.

As shown in Figure 7, the water samples from site SS were lower overall. The δ³⁴S values of shallow mountain surface water (SS) varied between 3.4 and 8.8‰, with a mean value of 6.8‰. Shallow geothermal fluids’ (QR) δ³⁴S values varied between 9.5 and 10.5‰, with a mean value of 9.8‰. The δ³⁴S values of deep thermal fluid (SR) in the study area varied between 18.0 and 34.5‰, with a mean value of 26.6‰. It can be seen that the distribution of δ³⁴S values is SR > QR > SS—that is, the δ³⁴S values increase gradually with the increase in depth.

4.3. Mineral Saturation Index and Thermal Storage Temperature

The thermal storage temperature and the degree of mixing of cold water with geothermal fluids are important parameters used to assess the geothermal resources of a region [39]. PHREEQC simulation software was used to calculate the underground hot water saturation index (SI) of the Rongcheng Bulge in Xiong’an New Area (

Table 2. Mineral saturation index.

	Quartz	Chalcedony	Calcite	Gypsum	Halite	Illite	Dolomite
QR01	0.56	0.13	0.2	−4.33	−5.22	1.38	0.17
QR02	0.55	0.12	0.11	−3.52	−5.54	1.34	0.03
QR03	0.49	0.06	0.35	−3.31	−5.76	1.22	0.6
QR04	0.5	0.07	0.7	−3.02	−5.23	0.88	1.17
QR05	0.45	0.02	0.43	−3.30	−5.77	1.15	0.72
SR01	0.76	0.33	0.23	−3.58	−4.68	2.31	0.51
SR02	0.94	0.51	−0.26	−3.54	−4.67	2.11	−0.54
SR03	0.89	0.46	−0.11	−3.54	−4.64	2.12	−0.22
SR04	0.81	0.38	−0.12	−3.92	−4.7	2.52	−0.2
SR05	0.91	0.48	0.07	−3.07	−4.65	1.87	0.18
SR06	0.89	0.46	0.24	−3.79	−4.65	1.79	0.51
SR07	0.5	0.07	0.2	−2.2	−5.06	1.68	0.17
SR08	0.8	0.37	0.17	−5.56	−4.71	2.24	0.39
SR09	0.94	0.51	−0.13	−3.89	−4.65	1.99	−0.26

Note(s): SI > 0 indicates supersaturation; SI = 0 indicates saturation; SI < 0 indicates unsaturation.

). The dissolution and precipitation patterns of minerals in water differ as a function of temperature as well as other factors (e.g., ion concentrations). When SI = 0, the water is considered saturated, and fluids and minerals are in equilibrium. By judging the equilibrium state of different minerals, a geothermal temperature scale suitable for the Rongcheng Bulge can be selected and its thermal storage temperature calculated [40]. Herein, quartz, chalcedony, calcite, gypsum, rock salt, illite and dolomite were selected to estimate temperatures based on mineral saturation. Table 2 shows that the saturation indices of quartz, chalcedony and calcite are all close to zero, but since the measured minerals in the study area contain significantly more quartz than chalcedony and calcite [20], the calculated mineral saturation indices of chalcedony were used as a geothermometer of the underground heat storage temperature. Fourteen geothermal water sampling sites in the Rongcheng Bulge area were selected for the calculations; the results are presented in

Table 3. Geothermal water storage temperature in the Rongcheng Bulge area.

No	SiO2 (mg/L)	T (°C)
QR01	23.2	69.1
QR02	22.9	68.6
QR03	20.6	64.5
QR04	21.5	66.2
QR05	19.4	62.3
SR01	36.0	87.1
SR02	54.1	105.5
SR03	47.9	99.8
SR04	39.7	91.4
SR05	50.3	102.1
SR06	47.8	99.7
SR07	20.0	63.5
SR08	38.9	90.4
SR09	54.0	105.4

. The calculated thermal storage temperature range of the geothermal fluid in the study area is 62.3–105.5 °C.

4.4. Rock Thermal Properties

The thermal properties of 12 in situ rock samples from four strata at different depths, namely the Cambrian strata, Jixian system strata, Great Wall System strata and Pacific Ocean strata, in the Rongcheng Bulge area were tested to determine their thermal conductivity, specific heat capacity and radiogenic heat rate. As shown in Figure 8 and

Table 4. Rock thermal properties results.

No.	Well	Depth	Stratigraphic	Thermal Conductivity					Average Value	Relative	Th	U	K	ρ	Radioactive Heat Generation Rate
				(W/mK)						Error (%)	(μg/g)	(μg/g)	(%)	(g/cm3)	(μW/m3)
1	DR1	1299–1301	Jixian System	5.73	5.77	5.78	5.75	5.76	5.76	0.40	0.12	0.16	0.03	2.80	0.05
2	DR1	1661–1663	Jixian System	4.06	4.08	4.09	4.09	4.09	4.08	0.30	2.55	0.76	1.09	2.90	0.51
3	DR1	2403–2404	Jixian System	5.87	5.82	5.88	5.88	5.88	5.87	0.40	0.15	0.20	0.11	2.56	0.07
4	DR2	1603–1608	Cambrian System	2.97	2.98	2.97	3.00	2.98	2.98	0.40	1.65	0.78	1.04	2.72	0.42
5	DR2	2496–2501	Cambrian System	2.95	2.96	2.95	2.95	2.96	2.95	0.20	6.62	2.79	3.08	2.68	1.45
6	DR2	2604–2608	Jixian System	4.82	4.85	4.88	4.85	4.91	4.86	0.70	0.28	0.32	0.14	2.81	0.12
7	DR3	900–1400	Jixian System	5.74	5.68	5.72	5.70	5.71	5.71	0.40	0.13	0.12	0.12	2.73	0.05
8	DR3	1501–1507	Great Wall System	2.00	2.02	2.02	2.01	2.01	2.01	0.50	1.53	0.49	8.49	2.56	0.98
9	DR3	2168–2172	Archaic	2.76	2.78	2.77	2.77	2.76	2.77	0.30	4.85	0.26	7.58	2.36	0.98
10	DR4	882–888	Jixian System	3.84	3.84	3.85	3.88	3.86	3.85	0.40	3.27	2.86	1.45	2.77	1.13
11	DR4	785.7–790	Jixian System	6.61	6.67	6.64	6.63	6.59	6.63	0.40	0.37	0.26	0.09	2.75	0.10
12	DR4	1212–1218	Jixian System	5.53	5.54	5.51	5.51	5.51	5.52	0.30	1.98	1.75	0.83	2.22	0.55

, the thermal conductivity of the Cambrian strata in the Rongcheng Bulge ranged from 2.95 to 2.98 W·(m·K)⁻¹ and exhibited an average of 2.97 W·(m·K)⁻¹. The thermal conductivity of Jixian system strata ranged from 4.08 to 6.63 W·(m·K)⁻¹; the average conductivity was 5.28 W·(m·K)⁻¹. The measured thermal conductivity of the Great Wall strata was 2.01 W·(m·K)⁻¹, whereas the measured thermal conductivity of the Swire strata was 2.77 W·(m·K)⁻¹. Based on the thermal conductivity tests, specific heat capacity tests were conducted on 8 of the 12 rock samples from the study area. A total of eight sets of Jixian system rock samples were collected and analyzed at temperatures between 25 and 190 °C to determine their specific heat capacity (

Table 5. Rock specific heat capacity test results.

Well	DR1	DR1	DR1	DR2	DR3	DR4	DR4	DR4
T/°C	Specific Heat Capacity (J/(g·K))
25	0.66	0.7	0.66	0.57	0.54	0.51	0.91	0.58
30	0.74	0.77	0.65	0.6	0.65	0.45	1.04	0.68
35	0.79	0.82	0.66	0.62	0.71	0.45	1.13	0.74
40	0.81	0.84	0.67	0.64	0.74	0.47	1.17	0.77
45	0.82	0.86	0.68	0.65	0.75	0.48	1.2	0.79
50	0.82	0.87	0.69	0.65	0.76	0.5	1.23	0.81
55	0.83	0.88	0.7	0.66	0.77	0.52	1.25	0.82
60	0.83	0.9	0.71	0.67	0.77	0.54	1.28	0.83
65	0.83	0.91	0.72	0.68	0.78	0.55	1.31	0.84
70	0.84	0.91	0.73	0.68	0.78	0.57	1.35	0.85
75	0.84	0.92	0.74	0.69	0.79	0.59	1.39	0.86
80	0.84	0.93	0.75	0.69	0.79	0.6	1.44	0.87
85	0.85	0.93	0.76	0.7	0.8	0.62	1.49	0.87
90	0.85	0.93	0.77	0.7	0.8	0.64	1.54	0.88
95	0.85	0.94	0.79	0.71	0.81	0.65	1.6	0.88
100	0.86	0.94	0.8	0.72	0.81	0.67	1.66	0.89
105	0.86	0.94	0.81	0.72	0.82	0.69	1.72	0.9
110	0.86	0.95	0.82	0.73	0.82	0.71	1.78	0.9
115	0.87	0.95	0.83	0.73	0.83	0.73	1.83	0.91
120	0.87	0.96	0.84	0.74	0.83	0.75	1.87	0.92
125	0.87	0.96	0.85	0.74	0.83	0.76	1.89	0.92
130	0.87	0.96	0.86	0.75	0.84	0.78	1.89	0.93
135	0.87	0.97	0.87	0.75	0.84	0.8	1.88	0.94
140	0.88	0.97	0.88	0.76	0.85	0.82	1.85	0.94
145	0.88	0.98	0.89	0.77	0.85	0.83	1.81	0.95
150	0.88	0.98	0.9	0.77	0.85	0.85	1.77	0.95
155	0.88	0.99	0.91	0.78	0.86	0.87	1.72	0.96
160	0.88	0.99	0.91	0.78	0.86	0.89	1.66	0.96
165	0.88	1	0.92	0.79	0.86	0.91	1.6	0.97
170	0.89	1	0.93	0.79	0.87	0.93	1.55	0.98
175	0.89	1.01	0.94	0.8	0.87	0.95	1.52	0.98
180	0.89	1.01	0.95	0.8	0.87	0.96	1.49	0.99
185	0.89	1.01	0.96	0.81	0.88	0.98	1.47	0.99
190	0.89	1.02	0.97	0.81	0.88	1	1.46	1

). The specific heat capacity of the Jixian system rocks at 25–190 °C was obtained as 0.45–1.89 J·(g·K)⁻¹.

5. Discussion

5.1. Hydrogeochemical Processes in Geothermal Fluids

The hydrochemical components of geothermal fluids are often closely related to the processes encountered during the circulation of geothermal fluids and the surrounding nature of the geological materials. If the circulation path of the geothermal fluid is longer and the circulation process is relatively slow, then the geothermal fluid has more time to interact with the surrounding rock. The geochemical reactions are likely to produce water that more closely reflects the hydrochemical characteristics of the rock (i.e., the water has a higher hydrochemical fraction). In contrast, if the circulation path of the geothermal fluid is shorter and the circulation rate is faster, then the geothermal fluid has less time to interact with the surrounding rock, and the corresponding hydrochemical fraction in the geothermal fluid will be relatively low. Based on the characteristics and distribution patterns of the main elements and trace elements of the three water bodies in the study area, when comparing geothermal fluid (QR and SR) with neutral water with low TDS in the mountainous areas (SS), the geothermal fluid in the Rongcheng geothermal field was higher in TDS and possessed more alkaline water, which is presumably due to the influence of water–rock interactions during the deep transport of the geothermal fluid to the Rongcheng Bulge area after receiving recharge from the mountainous area. Thus, the ability of the water to exchange constituents with waters in the surrounding rocks is more limited as the renewal rate is slow. As the geothermal fluids rise toward the surface, CO₂ gas in the fluid becomes more active because of lower temperatures and pressures, and more water–rock interactions occur, leading to an increase in the relevant ion concentrations in the geothermal fluid.

According to the geothermal fluid water–rock balance diagram in the study area (Figure 4), these data indicate that the water–rock interactions between the geothermal fluid and the surrounding rocks during deep circulation in the Rongcheng geothermal field were limited, and the water–rock system failed to reach mineral equilibrium.

Fluid hydrogen and oxygen isotopes and sulfur isotopes also play an indicative role. As shown in Figure 5, the geothermal fluid in the study area showed “oxygen drift”. The occurrence of “oxygen drift” indicates that the geothermal waters collected from deep thermal storage at the SR sampling sites undergo water–rock interactions during deep circulation and therefore have a relatively long retention time. However, the high thermal storage temperature of geothermal water at site SR may affect the δ¹⁸O enrichment of the geothermal waters during deep circulation. In addition, the collected SS water samples’ δD values and geothermal water samples have an overlapping data range, mainly because the SS, QR and SR water sample points are from atmospheric precipitation recharge, infiltrated from the higher altitude northwestern mountains, and formed QR and SR geothermal water sample points through deep circulation. The fluids’ ⁸⁷Sr/⁸⁶Sr ratios indicated that the source of recharge of the geothermal waters is not from the mantle. The ⁸⁷Sr/⁸⁶Sr ratios of water samples from site SS generally fall between 0.709 and 0.712, revealing that the water from SS sites is predominantly derived from a mixture of atmospheric precipitation and river water. The ⁸⁷Sr/⁸⁶Sr ratio of the geothermal water samples from site QR indicates that they have different water flow paths and circulate faster, and it is farther from the ratios associated with silica-aluminate minerals. Since the main thermal reservoir of the Rongcheng geothermal field is composed of carbonate rocks, the influence of carbonate rock dissolution must be considered. In other words, the water is less likely to be affected by silica-aluminate dissolution. In addition, the ⁸⁷Sr/⁸⁶Sr ratio of the geothermal water samples from site SR is more concentrated in Na than waters from the SS sampling sites (located in the mountains) or the geothermal waters from site QR, indicating that their source paths are more consistent and the circulation rate is slower and more closed.

Sulfur-containing components in geothermal fluids will exchange sulfur isotopes with other sulfides in their thermal storage environment during water–rock interactions. In doing so, the geothermal fluids will become enriched in ³⁴S as a result of the strong fractionation that occurs in the reducing environment. Thus, the ³⁴S content of the geothermal fluid can be used to assess the degree of confinement of the thermal storage environment.

By correlating the range of δ³⁴S values from each water sampling site with δ¹⁸O values, within the hydrogeological environment, geothermal water samples from site SR were located in the most closed environment. Moreover, the δ³⁴S values of geothermal water samples from the SR sites were significantly higher than those from the other two types of water sampling sites, indicating that the fluid stored in the rock units possessed a higher degree of thermal storage closure. In addition, the desulfurization effect was stronger.

5.2. Geothermal Fluid Circulation Characteristics and Cold Water Mixing

The circulation depth of the geothermal water in the Rongcheng Bulge area was estimated using quartz as a geothermometer, which is a more accurate method of determining thermal storage temperature. The formula used is as follows:

T = T₀ + (H − h) × R,

(3)

where T is the thermal storage temperature (°C), T₀ is the temperature in the ambient temperature zone (°C), R is the geothermal gradient (°C/100 m), h is the depth of the normal temperature zone (m) and H is the maximum circulation depth of the geothermal water (m). According to the geothermal resource exploration results for data obtained from the Rongcheng Bulge area, T₀ is 11.9 °C, h is 26 m and R is 3.5 °C/100 m.

As shown in Table 3 and

Table 6. Geothermal water circulation depth in the Rongcheng Bulge area.

No	T (°C)	H (m)
QR01	69.1	1658.9
QR02	68.6	1646.7
QR03	64.5	1528.9
QR04	66.2	1576.6
QR05	62.3	1465.2
SR01	87.1	2175.1
SR02	105.5	2701.5
SR03	99.8	2537.4
SR04	91.4	2296.5
SR05	102.1	2601.6
SR06	99.7	2534.2
SR07	63.5	1499.9
SR08	90.4	2270.0
SR09	105.4	2698.6

, the thermal storage temperatures at which the geothermal water was located at the QR sites varied between 62.3 and 69.1 °C; the average temperature was 66.1 °C. The geothermal water circulation depth was between 1465.2 and 1658.9 m, with an average circulation depth of 1575.3 m. The thermal storage temperatures at which geothermal water was located at the SR sites ranged between 63.5 and 105.5 °C, with an average temperature of 93.9 °C. The depths of geothermal water circulation were between 1499.9 m and 2698.6 m; the average depth of circulation was 2368.3 m.

During deep circulation, the temperature of the geothermal water decreases due to mixing with cold water. The silicon enthalpy mixing model [41] can eliminate the effect(s) of cold water mixing and allow for the analysis of the mixing proportions between cold water and the storage temperature prior to mixing. Mathematically, the model can be expressed as follows:

$$\{\begin{array}{c}{\mathrm{S}}_{\mathrm{c}}\mathrm{x}+{\mathrm{S}}_{\mathrm{h}}\left(1-\mathrm{x}\right)={\mathrm{S}}_{\mathrm{s}}\\ {\mathrm{SiO}}_{2\mathrm{c}}\mathrm{x}+{\mathrm{SiO}}_{2\mathrm{h}}\left(1-\mathrm{x}\right)={\mathrm{SiO}}_{2\mathrm{s}}\end{array},$$

(4)

where S_c is the enthalpy of the cold water near the surface (J g⁻¹), S_h is the initial enthalpy of the hot water (J g⁻¹), S_s is the final enthalpy (J g⁻¹), SiO_2c is the SiO₂ content of the near-surface cold water (mg L⁻¹), SiO_2h is the initial SiO₂ content of the geothermal water (mg L⁻¹) and SiO_2s is the final SiO₂ content (mg L⁻¹). The variable x is the mixing ratio of the underground cold water.

Equation (4) was used to calculate the mixing ratio of the underground cold water. The calculated mixing ratio was then selected as the final mixing ratio. Moreover, the temperature used to obtain the results was the geothermal water temperature before mixing.

The saturated water enthalpy and the relationship between SiO₂ and temperature are shown in

Table 7. Relationships between temperature, enthalpy and SiO₂ content.

T (°C)	Enthalpy (J/g)	SiO2 (mg/L)
50	50	14
75	75	27
100	100	48
125	125	80
150	151	125
175	177	185
200	204	265
225	231	365
250	259	486
275	289	614
300	321	692

. When the water temperature was lower than 100 °C, the saturated water enthalpy and the temperature were equal. The SiO₂ concentration of the cold water was 17.2 mg L⁻¹, and the temperature of the water was 17.5 °C.

According to Equation (4) and Figure 9, the mixing ratios of the geothermal water in the Rongcheng Bulge area at the QR sites ranged between 90% and 95%. The temperature ranged between 210 °C and 235 °C prior to mixing, whereas after mixing, the temperature ranged from 62.3 to 69.1 °C. The mixing ratio of the geothermal water at the SR sites varied between 75% and 95%. The temperatures ranged from 180 to 225 °C prior to mixing and between 63.5 °C and 105.5 °C after mixing. Since the cold water mixing ratio of the geothermal water at the SR sites was lower than that at QR sites, the temperature at the SR sites dropped less than at the QR sites during circulation. Thus, the heat storage temperature of the geothermal water was significantly higher at the SR than at the QR sites.

5.3. Radioactivity of Thermal Storage Rocks

The thermal properties of in situ rock samples from four strata at different depths, namely the Cambrian strata, Jixian system strata, Great Wall System strata and Pacific Ocean strata, in the Rongcheng Bulge area were tested to determine their thermal conductivity, specific heat capacity and radiogenic heat rate. As shown in Figure 8 and Table 4, the thermal conductivity of the Cambrian strata in the Rongcheng Bulge ranged from 2.95 to 2.98 W·(m·K)⁻¹ and exhibited an average of 2.97 W·(m·K)⁻¹. The thermal conductivity of Jixian system strata ranged from 4.08 to 6.63 W·(m·K)⁻¹; the average conductivity was 5.28 W·(m·K)⁻¹. The measured thermal conductivity of the Great Wall strata was 2.01 W·(m·K)⁻¹, whereas the measured thermal conductivity of the Swire strata was 2.77 W·(m·K)⁻¹. The thermal conductivity of the Jixian system strata was relatively high compared to the other rock units. In addition, the range of thermal conductivity of the Jixian system strata was large, presumably because it was influenced by the high mud content within the dolomite, a primary component of the Jixian system rocks [42]. Muddy dolomites as well as dolomitic mudstones have relatively low thermal conductivity. Moreover, mudstones have a lower thermal conductivity than dolomite.

A total of eight sets of Jixian system rock samples were collected and analyzed at temperatures between 25 and 190 °C to determine their specific heat capacity (Table 5). The specific heat capacity of carbonate formations averaged 0.640 J/g·K at room temperature, 0.82 J/g·K at 60 °C, 0.97 J/g·K at 120 °C and 1 J/g·K at 190 °C. As the temperature increased, the specific heat capacity of the carbonate rocks gradually increased.

A statistical model was fit to a plot of temperature vs. specific heat capacity over the temperature range of 25 to 200 °C (Figure 10). The following polynomial equation was used to describe the relationship:

$$\mathrm{y}=\mathrm{int}+\mathrm{b}1\times \mathrm{x}+\mathrm{b}2\times {\mathrm{x}}^2$$

(5)

where y represents the specific heat capacity at different temperatures (J/g) and x represents the temperature (°C). The model’s parameters were as follows: intercept = 0.55796, b1 = 0.00519 and b2 = −1.523475 × 10⁻⁵; the regression coefficient (R²) was 0.98. As seen from the curve, the relation can represent the temperature–specific heat capacity relationship up to 200 °C.

The radioactive heat generation rate (A) of a rock is the energy produced per unit time by radioactive decay of the radioactive elements contained in a unit volume of rock. The formula is based on modified natural radioactive nuclear parameters [43], was used herein:

$$\mathrm{A}=0.01\cdot \mathsf{\rho }\cdot \left(9.52{\mathrm{C}}_{\mathrm{U}}+2.56{\mathrm{C}}_{\mathrm{Th}}+3.48{\mathrm{C}}_{\mathrm{K}}\right)$$

(6)

where A is the radiogenic heat generation rate of the rock in μW/m³, ρ is the density of the rock in g/cm³, U is the uranium content of the rock in μg/g, Th is the thorium content of the rock in μg/g and K is the potassium content of the rock in %. Based on the above equations, the radiogenic heat generation rate of the samples was calculated by combining the test data (Table 4).

The results show that the maximum radioactive heat generation rate for the Cambrian rock strata was 1.45 μW/m³, whereas the average rate was 0.93 μW/m³; for the Jixian system strata, the maximum radioactive heat generation rate was 1.13 μW/m³ and the average was 0.32 μW/m³. The radioactive heat generation rate for the Great Wall rocks was 0.98 μW/m³, and the radioactive heat generation rate of the Swale-Boundary strata was 0.98 μW/m³. The radiogenic heat generation rate of the rocks of the Jixian system strata was relatively low.

Plots of the thermal contributions of U and Th relative to K are often used to determine the specific contributions of the three radioactive elements to the radiogenic heat rate (Figure 11). Most of the samples had a low K radiogenic contribution; only two samples from borehole DR3 had a dominant K contribution. The U contribution is generally the largest. When there are equal amounts of U and Th, the contribution of U to the heat generation rate is approximately four times that of Th because U has two decay regimes and a faster decay rate. Since the average Th/U ratio on Earth is 3.7, the contribution of U and Th to the heat generation rate is approximately equal. Figure 12 shows that the Th/U ratio deviates from the global mean for most of the samples, which may be related to the processes of rock formation and subsequent hydrothermal alteration.

5.4. Vertical Characteristics of the Geothermal Field

Three drilled wells (DR1, DR6 and DR7) in Xiong’an New Area’s Rongcheng Bulge were analyzed to reveal the thermal state of the strata at different depths. The part above 800 m in the DR1 well is the sediment cover layer with a high geothermal gradient, and the temperature increases significantly with depth; the geothermal gradient is 74.75 °C/km. At 800–2500 m is the carbonate thermal reservoir; the temperature in this part changes less with depth, and the geothermal gradient is lower compared with the sedimentary cover layer at only 4.06 °C/km. The temperature profile of the DR6 well also changes more obviously, and the change pattern is basically the same as that of the DR1 well. Additionally, the geothermal gradient of the overburden layer is obviously higher than that of the thermal reservoir; with the increase in depth, the temperature firstly shows an obvious increase, and after entering the carbonate thermal reservoir, the temperature increase rate tends to slow down, with a geothermal gradient of only 7.94 °C/km. The temperature curve of well DR7 shows an obvious linear pattern, and the exposed strata are sedimentary cover, carbonate thermal reservoir and granite strata; the ground temperature gradients are 23.69, 15.13 and 19.04 °C/km, respectively. The thickness of the carbonate thermal reservoir drilled at the location of well DR7 is only 100 m, and then it enters the granite strata, so it causes the temperature curve of well DR7 to be different from that of wells DR1 and DR6.

The combined downhole temperature profiles of wells DR1, DR6 and DR7 show that the overall geothermal temperature gradient of the carbonate thermal reservoir is significantly lower than the sedimentary cover and granite geothermal gradients, probably because of convection during the upward transport of groundwater in the reservoir. Moreover, the borehole logging temperature curves demonstrate that the heat source and the aquifer are unevenly distributed. The variations in temperature may also reflect variability in the degree of closure in the reservoir, including the frequency and size of rock fractures. Figure 13 shows that there are multiple aquifer sections in the thermal reservoir as the temperature of each aquifer section rises gradually before reaching a stable (constant) temperature.

The vertical variation in ground temperature is controlled by the lithology of the strata, basement relief and deep tectonics. The stratigraphic and geothermal gradients in the Rongcheng Bulge area are different; however, the geothermal gradients of the Middle and Cenozoic Boundary are greater than those of the Paleozoic and Metasedimentary Boundary strata. Therefore, as the depth increases and the stratigraphy increases in age, the geothermal gradient decreases, and the vertical rate of increase in geothermal temperature decelerates, but the overall temperature is still increasing with depth.

5.5. Genetic Model of Geothermal Water

The Rongcheng Bulge area is mainly recharged by meteoric precipitation, and the deep, underground hot water originates from the Taihang Mountains, which are located 40 km to the northwest of the study area at 500–1800 m above sea level. Meteoric precipitation combined with some shallow cold water is transported laterally from the Baoding Mountains and the plains in front of the mountains (piedmont) and transported downward along regional fractures in front of the mountains, where it recharges deep thermal storage in the study area through lateral runoff. In the process of long-distance deep circulation, circulating groundwater absorbs deep mantle heat, and in the process, the water interacts with the surrounding rocks. The fractures in the study area provide effective channels for geothermal fluids to rise, as well as for convection and mixing with cold water, thereby forming the medium- and low-temperature geothermal system in the Rongcheng Bulge (Figure 14).

6. Conclusions

At present, scholars are increasingly studying geothermal resources in Xiong’an New Area, particularly the spatial structure of thermal storage, the conditions of thermal storage formation and the development and utilization of Xiong’an New Area [44,45,46,47,48]. However, there are relatively few studies of the characteristics of geothermal water circulation and the geothermal field characteristics of thermal storage. In this paper, the mechanisms associated with the evolution of long-term deep-circulating geothermal fluids and trends in the characteristics of the geothermal flow field are used to determine the characteristics of the low- and medium-temperature geothermal system in the Rongcheng Bulge. These data provide a scientific basis for the future development and utilization of geothermal resources in Xiong’an New Area.

The geothermal water in Rongcheng Bulge is recharged by atmospheric rainfall and is mainly composed of Cl·HCO₃-Na-type waters, with an average pH of 7.7 and an average TDS content of 2227.3 mg/L. The water is therefore neutral and weakly alkaline. The deep circulation of geothermal fluids in the Rongcheng Bulge results in high concentrations of Na⁺, HCO₃⁻ and Cl⁻ in the geothermal fluids. Although water–rock interactions occur, these interactions are minor and do not allow the system to reach mineral equilibrium.

The geothermal fluid thermal storage temperature was calculated using the silica geothermal temperature scale in conjunction with mineral saturation in the study area. The thermal storage temperature with the shallow geothermal water at the QR sites ranged between 62.3 and 69.1 °C, and the depth of geothermal water circulation was between 1465.2 and 1658.9 m. The thermal storage temperature in the deep geothermal water at the SR sampling sites varied between 63.5 and 105.5 °C, and the depth of geothermal water circulation was between 1499.9 and 2698.6 m. The silicon enthalpy model suggests that the cold water mixing ratio of shallow geothermal water at the QR sites was significantly higher than that of the deep geothermal water at the SR sites, resulting in a rapid temperature decrease in the shallow geothermal water.

By testing the thermal properties of rocks and measuring the temperatures of geothermal wells, it was possible to obtain the vertical characteristics of the geothermal field in the Rongcheng Bulge area. The overall geothermal gradient of the carbonate thermal reservoir was significantly lower than the overburden and basement geothermal gradients, likely due to convection during the upward transport of groundwater in the reservoir. In addition, due to the uneven distribution of heat sources and aquifers, there was a gradual increase in temperature in each aquifer section, with a more constant final stabilization temperature.

Overall, it appears that the Rongcheng Bulge in Xiong’an New Area is mainly recharged by atmospheric rainfall in the northwestern Taihang Mountains which is transported deeply through fractures. During the long-distance deep circulation, the fluid absorbs deep mantle heat. During this process, water–rock interactions and cold water mixing occur with the surrounding rocks. Finally, convection between different reservoir layers leads to a medium–low-temperature geothermal system in the Rongcheng Bulge area.