Exploring Geochemical Characteristics of Composite Geothermal Reservoirs for Sustainable Utilization: A Case Study of the Northwestern Shandong Geothermal Area in China

Abstract

Presently, geothermal resources have been globally recognized as an indispensable component of the energy system due to their sustainability. However, previous studies on geothermal reservoirs focus primarily on single reservoirs, lacking a systematic investigation of composite geothermal reservoirs. The geothermal reservoirs in the northwestern Shandong geothermal area in China are primarily of sandstone and karst types, characterized by extensive distributions, shallow burial depths, high water temperatures, and high water abundance, holding considerable potential for exploitation. This study explored the hydrochemical, isotopic, and circulation characteristics of geothermal fluids in the composite geothermal reservoirs in the study area using methods like hydrogeochemistry and geothermal geology. The purpose is to determine the geochemical differences in geothermal fluids across the composite geothermal reservoirs and provide scientific support for subsequently efficient and sustainable exploitation and utilization of geothermal resources in the study area. The composite geothermal reservoirs in the study area are composed of porous sandstone geothermal reservoirs (also referred to as sandstone reservoirs) in the upper part and karst-fissured geothermal reservoirs (also referred to as karst reservoirs) in the lower part. The results show that the geothermal fluids in the sandstone and karst reservoirs are primarily of Na-Cl-SO₄ and Na-Ca-Cl-SO₄ types, respectively. The hydrochemical composition of geothermal fluids in the karst reservoirs is principally influenced by the precipitation–dissolution equilibrium of carbonate and sulfate minerals, while that in the sandstone reservoirs is predominantly influenced by the precipitation–dissolution equilibrium of carbonate and silicate minerals, as well as cation exchange reactions. The temperatures of the karst reservoirs were calculated at 52.9–82.09 °C using geothermometers. Given the cold-water mixing ratios range from 89% to 96%, the corrected reservoir temperatures vary from 200 to 225 °C. In contrast, the temperatures of the sandstone reservoirs were calculated at 60.54–85.88 °C using geothermometers. These reservoirs exhibit cold water mixing ratios ranging from 85% to 90%, and their corrected reservoir temperatures vary from 150 to 200 °C accordingly. The circulation depths of geothermal fluids in the karst and sandstone reservoirs range from 1107.28 to 1836.69 m and from 1366.60 to 2102.29 m, respectively. The study area is primarily recharged by meteoric water from Mount Tai and the Lushan and Yishan mountains (collectively referred to as the Tai-Lu-Yi mountains) to the southeast of the study area. Investigating the differences in geochemical characteristics of geothermal fluids in composite geothermal reservoirs in the study area is significant for balancing the exploitation and supply of geothermal resources, optimizing the exploitation and utilization modes, and promoting the efficient and sustainable exploitation and utilization of geothermal resources in the study area.

1. Introduction

With rapid economic and social advancement, the world is facing increasingly severe challenges such as environmental degradation, climate change, and shortages of renewable energy, thus showing a growing demand for green renewable energy [1,2,3,4,5]. Geothermal energy, as a clean renewable energy source, has gradually attracted global attention in recent decades. China, located at the junction of the circum-Pacific and Mediterranean-Himalayan geothermal belts, boasts abundant geothermal resources [6,7,8]. Shandong Province in China exhibits diverse and widely distributed geothermal resources. Within Shandong Province, the sedimentary-basin geothermal resources can be found primarily in the study area and the southwestern Shandong geothermal area [9,10,11]. The study area consists of the Jiyang depression, the Linqing depression, and the piedmont loose sedimentary cover area in the northern Luzhong uplift. It manifests relatively shallow Moho discontinuity, high heat flow values, and aquifers with strong water yield properties, establishing it as the area with the highest exploitation potential for geothermal resources in Shandong Province [12]. In addition to abundant geothermal resources, the study area predominantly develops stratified geothermal reservoirs, which are characterized by extensive distribution areas, shallow burial depths, high water temperatures, and high water yields from individual geothermal wells. These characteristics facilitate exploitation and are particularly suitable for decentralized heating exploitation in towns, villages, enterprises, and public institutions. Currently, the geothermal resources in the study area are primarily exploited for heating, bathing, swimming, and aquaculture, with geothermal heating reaching a certain scale. Considering various geothermal reservoirs in the study area, it is essential to balance and regulate their exploitation and replenishment to ensure a long-term stable supply of geothermal resources. Many studies delved into the hydrochemical characteristics and genetic mechanisms of geothermal fluids in the dominant sandstone and karst reservoirs in the study area [13,14]. They found that the geothermal fluids in sandstone reservoirs originated primarily from the infiltration and recharge of meteoric water and were subjected to minor influence of tectonic activity [15]. In contrast, the geothermal fluids in karst reservoirs were principally sourced from a mixture of meteoric water and deep circulating groundwater and were significantly affected by tectonic processes and water–rock interactions [16].

The hydrochemical and especially isotopic characteristics of geothermal fluids are often used to analyze the circulation characteristics, water–rock interactions, and hydrogeochemical processes of geothermal fluids [17,18,19,20,21,22]. Isotopic characteristics can identify various physicochemical processes in geothermal systems, such as the origin of geothermal fluids, water–rock interactions, circulation depths, recharge elevations, and the mixing of shallow groundwater with geothermal water [23,24,25,26,27]. Classic geothermometers based on the chemical compositions of geothermal fluids are effective in assessing whether a hydrothermal system is a high-temperature system [28,29,30,31,32,33,34]. Based on the hydrochemical and isotopic characteristics of geothermal fluids in sandstone and karst reservoirs in the study area, this study comprehensively analyzes the origin, water–rock interactions, circulation characteristics, and mixing processes of geothermal fluids in different reservoir layers in the study area. It reveals the differences in geothermal fluids among various reservoir layers in the study area, providing a theoretical basis for the balanced regulation and exploitation of various reservoir layers in the study area.

2. Geological Setting

Shandong Province is located on the third major step of China’s topography. Based on geomorphic characteristics and genetic types, it is zoned into three parts: the low mountain and hilly area dominated by denudation structures in eastern Shandong, the medium-low hilly area with tectonic erosion in south-central Shandong, and the accumulation plain in northwestern Shandong. The sedimentary-basin geothermal resources in Shandong Province are primarily distributed in the study area and the southwestern Shandong geothermal area. The study area consists of the Jiyang depression, the Linqing depression, and the piedmont loose sedimentary cover area in the northern Luzhong uplift. It manifests relatively shallow Moho discontinuity, high heat flow values, and aquifers with strong water yield properties, establishing it as the area with the highest exploitation potential for geothermal resources in Shandong Province. The study area, characterized by alternating uplifts and sags, resides in the North China depression in terms of tectonic unit and the accumulation plain in northwestern Shandong in terms of geomorphology. The uplift zone in the study area is primarily composed of Archean metamorphic rocks and Paleozoic limestones or sandstones. The sag zone develops a faulted basin with the Archean Taishan metamorphic rock series as the basement, on which extremely thick Paleozoic–Cenozoic sedimentary rocks were deposited. The study area primarily includes northern Ji’nan City, western Liaocheng and Heze cities, and most parts of Dezhou, Dongying, and Binzhou cities [9].

The study area is located in a Mesozoic–Cenozoic faulted basin developed in the North China platform. In terms of tectonic unit, it is situated in the North China depression within the North China plate. From the Mesozoic to the Cenozoic, the Himalayan orogeny and Yanshanian movement resulted in fault structures and concave–convex tectonic units like the Jiyang and Linqing depressions in the study area. Under the influence of the Neocathaysian tectonic system, the bedrock fault structures were intensely developed in the study area. The fault structures in the study area are all of the hidden type, with the Qihe–Guangrao, Liaocheng–Lankao, and Cangdong faults being major deep heat-controlling faults [35,36].

The study area develops stratified geothermal reservoirs and extremely thick Paleozoic and Cenozoic strata. The extremely thick Cenozoic loose sedimentary strata cover the metamorphic and magmatic rocks of the basement while providing insulation for hot water in the underlying reservoirs. Additionally, pores and fractures in the Cenozoic sandstones manifest high water-bearing properties and a certain water-retaining function. The dominant geothermal reservoirs comprise the Neogene sandstone reservoirs and the Cambrian–Ordovician karst reservoirs. The Neogene sandstone reservoirs primarily include the sandstone reservoirs of the Minghuazhen and Guantao formations. The sandstone reservoirs of the Minghuazhen formation are distributed throughout the study area, with floor burial depths generally ranging from 500 to 1100 m. Typically, they display thicknesses ranging from 30 to 120 m, with slender reservoirs in the uplift zone and thick reservoirs in the sag zone. They show single-well water yields ranging from 40 to 80 m³/d and wellhead water temperatures ranging from 30 to 48 °C. The sandstone reservoirs of the Guantao formation are distributed in most of the study area except for a local absence in northern Weifang City, with floor burial depths generally ranging from 1000 to 1700 m and up to 2300 m. They display thicknesses generally ranging from 250 to 400 m, with single-well water yields ranging from 40 to 80 m³/h and water temperatures ranging from 49 to 78 °C. The Cambrian–Ordovician geothermal limestone reservoirs for karst water are primarily distributed in the Ningjin, Yihezhuang, Chenjiazhuang, Chengdong, Qingtuozi, Guangrao, and Shouguang uplifts, and the Guantao and Gaotang uplifts in the Linqing depression. Their burial depths are principally subjected to the basement structure, with roof burial depths generally ranging from 500 to 1700 m. With thicknesses ranging from about 100 to 700 m, they are principally composed of limestones and dolomites. They show single-well water yields generally ranging from 100 to 1000 m³/d and up to over 3000 m³/d and wellhead temperatures ranging from 50 to 90 °C [37].

The study area is part of the upper mantle uplift zone in North China, with Moho depths ranging from approximately 33 to 37 km. It displays concave-downward Moho discontinuity in the uplift zone, suggesting thicker crust, and convex-upward Moho discontinuity in the sag zone, indicating thinner crust. The terrestrial heat flow in the study area shows values ranging from about 55 to 75 MW/m², exceeding the average value (47.16 mW/m²) of terrestrial heat flow in the North China region. The geothermal gradients in the study area show lower values in the inland and higher values along the coast, and lower and higher values in the west and east, respectively. The geothermal gradients range from 3.0 to 3.5 °C/100m in the southwestern part of the study area, and from 3.5 to 5.0 °C/100m in the northern and eastern parts. At a depth of 1000 m below the surface, the formation temperatures range from 40 to 50 °C in the southwestern part and 45 to 60 °C along the coast and in the eastern and northeastern parts. In terms of thermal conductivity in the study area, the Paleozoic strata show the highest values, followed by the Paleogene and Neogene strata, with the Quaternary strata showing the lowest thermal conductivity. Specifically, the Quaternary strata exhibit high insulation performance due to the low heat conduction efficiency. Despite relatively low geothermal gradients, the Paleogene and Neogene strata exhibit high heat conduction performance, facilitating the upward conduction of terrestrial heat flow. The geothermal reservoirs in the study area exhibit extensive distributions, considerable thicknesses, shallow burial depths, high temperatures, and high water yield properties, laying a solid thermal source foundation for the exploitation and utilization of geothermal resources [37]. The distributions of geothermal reservoirs in the study area are shown in Figure 1.

3. Materials and Methods

A total of 43 groups of geothermal fluid samples were collected from the study area, including 29 groups from the Neogene sandstone reservoirs and 14 groups from the karst reservoirs. The sampling sites are shown in Figure 1. The data of samples S8–S14 and S18–S19 were obtained from references [12,13], respectively.

The samples for water quality analysis from the study area were tested at the laboratory of the Groundwater, Mineral Water, and Environmental Monitoring Center of the Ministry of Land and Resources, supported by the Institute of Hydrogeology and Environmental Geology of the Chinese Academy of Geological Sciences. The concentrations of cations like K⁺, Na⁺, Ca²⁺, and Mg²⁺ in the samples were determined using an inductively coupled plasma—optical emission spectrometer (Thermo Fisher iCAP6300, Waltham, MA, USA), while the concentrations of anions like Cl⁻, SO₄²⁻, NO₃⁻, and F⁻ were tested using a liquid-phase ion chromatograph. HCO₃⁻ and CO₃²⁻ were measured using the double indicator titration method. The relative errors of all test analyses were controlled within 5%. Additionally, water temperatures and pH values were determined on site using a portable tester, the Easy Probe water quality analyzer, with resolution of 0.01. Hydrogen and oxygen isotopes were tested using a water isotope analyzer (Picaro2140-i) with a test accuracy of 0.1‰.

Concerning hydrochemical data, this study employed the Shukarev classification method to analyze the hydrochemical types of the samples [40] and software including IBM SPSS Statistics 26 (for correlation and principal component analyses), Aquachem 3.70 (for hydrochemical type analysis), and Excel to analyze the hydrochemical data.

4. Results

Table 1. Statistics of hydrochemical components in geothermal fluid samples from the study area.

Reservoir Type		pH	Depth (m)	T (°C)	TDS (mg/L)	Concentrations of Major Ions (mg/L)
						K	Na	Ca	Mg	SO4	HCO3	Cl	Sr	F
Sandstone reservoir	Minimum	7.60	1050.00	42.00	3676.03	5.20	1037.50	14.56	8.83	631.64	134.24	417.41	0.00	1.05
	Maximum	9.20	1649.02	65.00	6788.47	49.00	2400.00	280.56	65.00	2734.35	369.91	3578.68	10.88	7.50
	Average	8.03	1416.54	55.14	5079.16	12.97	1675.42	136.79	26.00	1260.83	221.74	1829.10	5.66	1.66
Karst reservoir	Minimum	7.10	576.90	33.00	3314.91	18.50	152.50	609.40	124.46	1626.94	103.94	173.40	11.75	2.60
	Maximum	8.00	2400.00	56.00	7416.26	80.00	1450.00	868.30	159.57	2337.87	176.69	2393.81	17.50	4.50
	Average	7.56	1204.85	43.98	5307.03	46.11	805.39	727.57	142.29	2030.42	150.15	1374.23	14.87	3.39

shows the statistics of hydrochemical components in geothermal fluid samples from the study area. Specifically, samples from the Neogene sandstone reservoirs exhibited water temperatures ranging from 42 to 65 °C, pH values between 7.6 and 9.2, and total dissolved solids (TDS) concentrations primarily ranging from 3676 to 6788 mg/L. In contrast, samples from karst reservoirs manifested water temperatures ranging from 33 to 82 °C, pH values between 6.9 and 8.0, and TDS concentrations primarily ranging from 3314 to 7416 mg/L.

According to the Shukarev classification method, the data of cations and anions in all samples were projected onto the Piper diagram using the Aquachem software (Figure 2). Figure 3 and Figure 4 show the concentrations and percentages of major ions in geothermal fluid samples. The results indicate that the samples from sandstone reservoirs contained Na⁺ as major cations and Cl⁻ and SO₄²⁻ as major anions, suggesting hydrochemical types of Na-Cl-SO₄ (63%), Na-SO₄-Cl, Na-Cl, and Na-SO₄. In contrast, samples from karst reservoirs contained Na⁺ and Ca²⁺ as major cations and Cl⁻ and SO₄²⁻ as major anions, suggesting hydrochemical types of Na-Ca-Cl-SO₄ (50%), Ca-Mg-SO₄, and Ca-Na-SO₄-Cl.

5. Discussion

5.1. Hydrogeochemical Processes for the Formation of Major Ions

5.1.1. Dissolution–Precipitation Processes

The hydrogeochemical processes that form major ions in geothermal fluids primarily include the dissolution–precipitation processes of carbonate, sulfate, and silicate minerals. This study analyzed the dissolution–precipitation processes in geothermal fluids in the study area based on the ion ratios describing the relationships between different ions (Figure 5) [41]. The results show that the geothermal fluid samples from sandstone reservoirs were distributed near the line of 2Ca²⁺ = HCO₃⁻, leaning towards the Ca²⁺ side, suggesting that ions Ca²⁺ and HCO₃⁻ in geothermal fluids from sandstone reservoirs originated primarily from the dissolution of carbonate minerals like calcite and dolomite. In addition, leaning towards the Ca²⁺ side indicates that in addition to carbonates, silicate minerals like feldspar also experienced dissolution. In contrast, geothermal fluid samples from karst reservoirs showed much higher Ca²⁺ concentration compared to the HCO₃⁻ concentration, suggesting other major sources for Ca²⁺. The HCO₃⁻ concentration in the geothermal fluid samples from two types of reservoirs, especially karst reservoirs, decreased with an increase in the Ca²⁺ concentration. This is because in a high-temperature environment, high-concentration Ca²⁺ ions are prone to interact with HCO₃⁻ ions to form CaCO₃ precipitates, leading to decreased HCO₃⁻ ions in geothermal fluids.

The Mg²⁺/Ca²⁺ ratios were all below 1 in the geothermal fluid samples from two types of reservoirs (Figure 5), indicating that magnesium-deficient minerals like calcite had minor impacts on the hydrochemical composition of deep geothermal fluids. Concerning the Na⁺/Ca²⁺ ratios, the geothermal fluid samples from sandstone reservoirs showed relatively high Na⁺/Ca²⁺ ratios, indicating that compared to carbonate dissolution, silicate dissolution showed more significant impacts on the hydrochemical composition of geothermal fluids in sandstone reservoirs. Moreover, the continuous increase in Na⁺/Ca²⁺ ratios suggests more intense carbonate precipitation. In contrast, the geothermal fluid samples from karst reservoirs exhibited relatively low Na⁺/Ca²⁺ ratios, near the line of Na⁺/Ca²⁺ = 1, indicating that silicate dissolution exerted minor impacts on the hydrochemical composition of geothermal fluids in karst reservoirs.

Using the (SO₄²⁻ + HCO₃⁻) vs. (Mg²⁺ + Ca²⁺) diagram, this study analyzed the hydrogeochemical processes experienced by the principal chemical components of geothermal fluids. Figure 6 shows that the geothermal fluid samples from karst reservoirs exhibited Mg²⁺ + Ca²⁺/SO₄²⁻ + HCO₃⁻ ratios close to 1, indicating that the precipitation–dissolution equilibrium of carbonate, silicate, and sulfate minerals in karst reservoirs jointly affected the formation of major ions in geothermal fluids. Combined with the finding that silicate dissolution exerted minor impacts on the hydrochemical composition of geothermal fluids in karst reservoirs, it further indicates that geothermal fluids in karst reservoirs were primarily affected by the precipitation and dissolution of carbonate and sulfate minerals. In contrast, the geothermal fluid samples from sandstone reservoirs manifested Mg²⁺ + Ca²⁺/SO₄²⁻ + HCO₃⁻ ratios far below 1, with relatively stable Ca²⁺ + Mg²⁺ values, indicating that geothermal fluids in sandstone reservoirs were principally affected by the dissolution of carbonate and silicate minerals. The interrelationship between Na⁺ + K⁺ and Cl⁻ concentrations can be employed to analyze the possibility of a source of silicate mineral dissolution. Figure 6 illustrates that most geothermal fluid samples from karst reservoirs showed similar Na⁺ + K⁺ concentrations to Cl⁻ concentrations, indicating that the related ions originated primarily from halite dissolution. However, the geothermal fluid samples from sandstone reservoirs showed Na⁺ + K⁺ concentrations higher than Cl⁻ concentrations, reflecting that they were also affected by silicate mineral dissolution in addition to halite dissolution. In line with the earlier findings, silicate dissolution exerted minor impacts on the hydrochemical composition of geothermal fluids in karst reservoirs but significant impacts on that of geothermal fluids in sandstone reservoirs.

5.1.2. Cation Exchange Reactions

In the study of the hydrochemical evolutionary process of geothermal fluids, it is found that the ionic equivalent ratio rNa⁺/rCl⁻ can effectively reflect the extent of cation exchange. Figure 7 shows that the rNa⁺/rCl⁻ ratios of geothermal fluid samples from sandstone reservoirs all exceeded 1. With an increase in Cl⁻ concentration, they gradually decreased and tended to stabilize, suggesting intense cation exchange reactions. In contrast, the rNa⁺/rCl⁻ ratios of geothermal fluid samples from karst reservoirs were distributed near value 1, suggesting less intense cation exchange reactions. Additionally, Mg²⁺ in geothermal fluids of sandstone reservoirs also participated in cation exchange reactions, but its exchange intensity was lower than that of Na⁺. The exchange effect of Mg²⁺ was weaker in the geothermal fluids of karst reservoirs.

Moreover, chloro-alkaline indices CAI1 and CAI2 can be employed to illustrate the likelihood of cation exchange reactions [42]. They are calculated as follows:

$$\mathrm{CAI}1=\left[{\mathrm{Cl}}^{-}-{(\mathrm{Na}}^{+}+{\mathrm{K}}^{+})\right]/{\mathrm{Cl}}^{-}$$

(1)

$$\mathrm{CAI}2=\left[{\mathrm{Cl}}^{-}-{(\mathrm{Na}}^{+}+{\mathrm{K}}^{+})\right]/({{\mathrm{SO}}_4}^{2-}+{{\mathrm{HCO}}_3}^{-}+{{\mathrm{NO}}_3}^{-}+{{\mathrm{CO}}_3}^{2-})$$

(2)

When Na⁺ and K⁺ in geothermal fluids exchange with adsorbed Ca²⁺ and Mg²⁺, the values of CAI1 and CAI2 will be positive. Conversely, when Ca²⁺ and Mg²⁺ in geothermal fluids exchange with adsorbed Na⁺ and K⁺, the values of CAI1 and CAI2 will be negative. Moreover, higher absolute values of CAI1 and CAI2 suggest more significant impacts from cation exchange reactions [43]. Figure 8 illustrates that both indices for geothermal fluids in sandstone reservoirs were negative, with higher absolute values. In contrast, the two indices for geothermal fluids in karst reservoirs were primarily distributed near zero, indicating relatively minor impacts from cation exchange reactions. The results align with the previous analysis.

5.2. Circulation Characteristics of Geothermal Fluids

Stable isotopes in geothermal fluids can identify various physicochemical processes in geothermal systems, such as the origin of geothermal fluids, water–rock interactions, circulation depths, recharge elevations, and the mixing of shallow groundwater with geothermal water. In this study, O and H stable isotopes were employed to comprehensively analyze the origin, water–rock interactions, circulation characteristics, and mixing processes of geothermal fluids in sandstone and carbonate reservoirs in the study area.

5.2.1. Estimation of Reservoir Temperatures

Reservoir temperatures, a significant parameter for geothermal systems, play a critical role in the assessment, exploitation, and utilization of geothermal resources. However, it is difficult to obtain reservoir temperatures directly due to the deep burial depths of geothermal reservoirs. Moreover, single geothermal drilling can merely reveal local reservoir temperatures. Hence, geothermometers were employed to predict the approximate temperature ranges of reservoirs. The commonly used geothermometers include cationic and SiO₂ geothermometers [28,44]. Cationic geothermometers can be used when geothermal fluids reach an equilibrium state. The Na-K-Mg equilibrium diagram can assist in determining the equilibrium state of geothermal fluids [45]. Figure 9 shows that the geothermal fluid samples from the karst reservoirs all fell within the zone of immature water at the bottom right corner of the equilibrium diagram. In contrast, the geothermal fluid samples from sandstone reservoirs all fell within the zone of partially equilibrated water. Therefore, the cationic geothermometers are inapplicable to the estimation of reservoir temperatures of geothermal fluids in karst reservoirs. Among the cationic geothermometers, the Na-K geothermometer applies only to geothermal water above 150 °C, especially in boreholes. The Na-K-Ca geothermometer is specifically used for calcium-rich geothermal water. The K-Mg geothermometer is suitable for low-temperature geothermal water, with estimated temperatures generally higher than the water temperatures at the wellheads. Therefore, this study calculated the reservoir temperatures of geothermal fluids in sandstone reservoirs based on the K-Mg geothermometer, and the reservoir temperatures of geothermal fluids in karst reservoirs based on the quartz and chalcedony geothermometers. The calculation equations are as follows [46]:

Quartz geothermometer (non-destructive):

$$\mathrm{T}={\displaystyle \frac{1309}{5.19-{\mathrm{lgSiO}}_2}}-273.15$$

(3)

Chalcedony geothermometer (non-destructive):

$$\mathrm{T}={\displaystyle \frac{1302}{4.69-{\mathrm{lgSiO}}_2}}-273.15$$

(4)

Cationic K-Mg geothermometer:

$$\mathrm{T}={\displaystyle \frac{4410}{13.95-\lg \left[{\left(\mathrm{K}\right)}^2/\mathrm{Mg}\right]}}-273.15$$

(5)

Table 2. Calculated reservoir temperatures in the study area.

Reservoir Type	Sample No.	Area	T (Measured) (°C)	T (Quartz) (°C)	T (Chalcedony) (°C)	T (K-Mg) (°C)	T (Na-K) (°C)	T (Na-K-Ca) (°C)	Selected Temperature (°C)
Karst reservoir	TR1	Liaocheng	56.00	82.09	50.88	76.72	180.78	105.08	82.09
	TR2	Liaocheng	50.20	82.09	50.88	79.07	190.41	106.05	82.09
	TR3	Liaocheng	52.00	78.01	46.59	76.79	186.73	102.65	78.01
	TR4	Dezhou	/	76.58	45.09	52.59	234.12	50.44	76.58
	TR5	Dezhou	27.00	67.06	35.13	55.54	226.66	56.58	67.06
	TR6	Ji’nan	/	52.97	20.54	66.49	159.78	83.74	52.97
	TR7	Ji’nan	41.80	76.58	45.09	85.23	170.80	117.24	76.58
	TR8	Ji’nan	36.20	61.53	29.38	63.93	159.66	80.81	61.53
	TR9	Ji’nan	33.00	59.53	27.31	62.22	154.01	79.49	59.53
	TR10	Ji’nan	40.60	68.78	36.92	79.22	156.39	108.88	68.78
	TR11	Ji’nan	42.00	73.60	41.96	82.18	163.99	113.05	73.60
	TR12	Ji’nan	55.00	75.11	43.55	58.81	217.88	62.59	75.11
	TR13	Ji’nan	43.00	75.11	43.55	55.43	220.20	56.49	75.11
	TR14	Ji’nan	39.90	73.60	41.96	81.47	163.99	111.64	73.60
Sandstone reservoir	SR1	Liaocheng	/	75.11	43.55	57.37	60.72	80.05	75.11
	SR2	Liaocheng	/	75.11	43.55	51.75	70.18	72.87	75.11
	SR3	Liaocheng	52.00	67.06	35.13	53.02	63.85	75.60	67.06
	SR4	Liaocheng	50.00	82.09	50.88	51.10	59.28	79.68	82.09
	SR5	Liaocheng	55.00	82.09	50.88	56.33	61.90	76.63	82.09
	SR6	Liaocheng	53.00	85.88	54.89	72.97	69.29	94.09	85.88
	SR7	Liaocheng	54.00	/	/	64.45	68.17	78.39	/
	SR8	Dezhou	62.50	84.64	53.59	70.30	72.02	94.28	84.64
	SR9	Dezhou	/	84.64	53.59	67.31	71.20	93.25	84.64
	SR10	Dezhou	55.00	79.41	48.06	62.73	62.31	87.72	79.41
	SR11	Dezhou	58.50	79.41	48.06	63.99	65.19	89.76	79.41
	SR12	Dezhou	56.00	80.76	49.49	64.36	64.57	89.85	80.76
	SR13	Dezhou	58.00	82.09	50.88	65.52	58.76	84.77	82.09
	SR14	Dezhou	56.00	79.41	48.06	65.97	59.66	86.68	79.41
	SR15	Dezhou	/	84.64	53.59	55.81	52.93	79.94	84.64
	SR16	Dezhou	51.00	79.41	48.06	63.66	64.78	91.83	79.41
	SR17	Dezhou	47.00	78.01	46.59	55.10	48.65	77.35	78.01
	SR18	Dezhou	58.00	/	/	61.93	68.49	91.46	/
	SR19	Dezhou	59.00	/	/	63.75	63.09	88.50	/
	SR20	Binzhou	/	76.58	45.09	51.21	42.97	71.76	76.58
	SR21	Binzhou	/	76.58	45.09	54.59	45.00	74.24	76.58
	SR22	Binzhou	/	60.54	28.36	35.85	38.48	63.50	60.54
	SR23	Ji’nan	54.00	81.08	49.83	64.82	67.52	89.79	81.08
	SR24	Ji’nan	55.00	85.88	54.89	66.03	68.00	90.25	85.88
	SR25	Ji’nan	56.00	72.04	40.33	52.07	49.28	76.42	72.04
	SR26	Dongying	65.00	82.09	50.88	65.24	63.77	87.63	82.09
	SR27	Dongying	60.00	76.58	45.09	65.01	60.76	86.33	76.58
	SR28	Dongying	54.00	78.01	46.59	67.17	66.80	90.10	78.01

shows the estimation results of reservoir temperatures. The calculation results of SiO₂ geothermometers are recommended for karst reservoirs. The measured temperatures of geothermal fluid samples from karst reservoirs ranged from 27 to 56 °C. However, the calculation results of the chalcedony geothermometer were generally lower than the measured temperatures at the wellheads, failing to conform to the actual conditions. Therefore, the calculation results (52.97–82.09 °C) of the quartz geothermometer were regarded as the reservoir temperatures of geothermal fluids in karst reservoirs. For sandstone reservoirs, the SiO₂ (quartz and chalcedony) and cationic (K-Mg) geothermometers were used to calculate the reservoir temperatures of geothermal fluids. The measured temperatures of geothermal fluid samples from sandstone reservoirs ranged from 42 to 65 °C. The chalcedony geothermometer yielded too low calculated values, which are unrealistic. Since geothermal fluid samples from sandstone reservoirs were partially equilibrated water, major cations were in a relatively active state and failed to fully reflect the final temperatures of geothermal fluids. As a result, the cationic K-Mg geothermometer is inapplicable to sandstone reservoirs. Therefore, it is more appropriate to select the calculation results (60.54–85.88 °C) of the quartz geothermometer as the reservoir temperatures of geothermal fluids in sandstone reservoirs [41]. Table 2 shows the final temperature selection results, involving the measured temperatures (T (Measured)), quartz thermometer calculated temperatures (T(Quartz)), chalcedony geothermometer calculated temperatures (T(Chalcedony)), K-Mg geothermometer calculated temperatures (T(K-Mg)), Na-K geothermometer calculated temperatures (T(Na-K)), and Na-K-Ca geothermometer calculated temperatures (T(Na-K-Ca)).

5.2.2. Circulation Depths of Geothermal Fluids

The circulation depths of geothermal fluids are positively correlated with reservoir temperatures. This correlation can be described by the following equation [47]

H = (t₁ − t₂)/G + h

(6)

where H is the circulation depth of geothermal fluids; t₁ is the reservoir temperature (°C); t₂ is the average local temperature (°C), and h is the thickness of the constant temperature zone (m).

Table 3. Estimated circulation depths of geothermal fluids in the reservoirs.

Reservoir Type	Area	Sample No.	H (m)	t1 (°C)	t2 (°C)	G (°C/100 m)	h (m)
Karst reservoir	Liaocheng	TR1	1742.21	82.09	13.2	4	20
	Liaocheng	TR2	1742.21	82.09	13.2	4	20
	Liaocheng	TR3	1640.35	78.01	13.2	4	20
	Dezhou	TR4	1297.68	76.58	12.7	5	20
	Dezhou	TR5	1107.28	67.06	12.7	5	20
	Ji’nan	TR6	1161.89	52.97	13	3.5	20
	Ji’nan	TR7	1836.69	76.58	13	3.5	20
	Ji’nan	TR8	1406.45	61.53	13	3.5	20
	Ji’nan	TR9	1349.39	59.53	13	3.5	20
	Ji’nan	TR10	1613.71	68.78	13	3.5	20
	Ji’nan	TR11	1751.50	73.60	13	3.5	20
	Ji’nan	TR12	1794.70	75.11	13	3.5	20
	Ji’nan	TR13	1794.70	75.11	13	3.5	20
	Ji’nan	TR14	1751.50	73.60	13	3.5	20
Sandstone reservoir	Liaocheng	SR1	1567.87	75.11	13.2	4	20
	Liaocheng	SR2	1567.87	75.11	13.2	4	20
	Liaocheng	SR3	1366.60	67.06	13.2	4	20
	Liaocheng	SR4	1742.21	82.09	13.2	4	20
	Liaocheng	SR5	1742.21	82.09	13.2	4	20
	Liaocheng	SR6	1837.01	85.88	13.2	4	20
	Liaocheng	SR7	/	/	13.2	4	20
	Dezhou	SR8	2075.57	84.64	12.7	3.5	20
	Dezhou	SR9	2075.57	84.64	12.7	3.5	20
	Dezhou	SR10	1925.90	79.41	12.7	3.5	20
	Dezhou	SR11	1925.90	79.41	12.7	3.5	20
	Dezhou	SR12	1964.69	80.76	12.7	3.5	20
	Dezhou	SR13	2002.52	82.09	12.7	3.5	20
	Dezhou	SR14	1925.90	79.41	12.7	3.5	20
	Dezhou	SR15	2075.57	84.64	12.7	3.5	20
	Dezhou	SR16	1925.90	79.41	12.7	3.5	20
	Dezhou	SR17	1886.12	78.01	12.7	3.5	20
	Dezhou	SR18	/	/	12.7	3.5	20
	Dezhou	SR19	/	/	12.7	3.5	20
	Binzhou	SR20	1853.84	76.58	12.4	3.5	20
	Binzhou	SR21	1853.84	76.58	12.4	3.5	20
	Binzhou	SR22	1395.37	60.54	12.4	3.5	20
	Ji’nan	SR23	1965.28	81.08	13	3.5	20
	Ji’nan	SR24	2102.29	85.88	13	3.5	20
	Ji’nan	SR25	1707.00	72.04	13	3.5	20
	Dongying	SR26	2016.81	82.09	12.2	3.5	20
	Dongying	SR27	1859.55	76.58	12.2	3.5	20
	Dongying	SR28	1900.40	78.01	12.2	3.5	20

shows that the circulation depths of geothermal fluids in karst and sandstone reservoirs ranged from 1107.28 to 1836.69 m and 1366.60 to 2102.29 m, respectively.

5.2.3. Fluid Sources

δ¹⁸O and δD values can serve as effective tracers to determine the recharge sources and elevations of geothermal fluids. Figure 10 shows the δ¹⁸O vs. δD plot of geothermal fluids in the study area. The geothermal fluid samples from sandstone reservoirs yielded δ¹⁸O values ranging from −7.4‰ to −10.2‰ and δD values ranging from −59‰ to −80.94‰, while those from karst reservoirs yielded δ¹⁸O values ranging from −8.0‰ to −10.6‰ and δD values ranging from −62.8‰ to −76‰. The δ¹⁸O and δD values of geothermal fluids in the study area were correlated with the global, Chinese, and local meteoric water lines (i.e., GMWL, CMWL, and LMWL). The equations for the GMWL and CMWL are δD = 8δ¹⁸O + 10 and δD = 7.9δ¹⁸O + 8.2, respectively. For LMWLs, the meteoric water lines of the East China monsoon region and the Yiyuan area were selected for a comparative study [48]. Figure 10 illustrates that the geothermal fluid samples from karst reservoirs fell near the MWLs, with some exhibiting oxygen isotope shifts, indicating that the geothermal fluids in karst reservoirs in the study area originated principally from meteoric water. However, most geothermal fluid samples from sandstone reservoirs manifested significant δ¹⁸O shifts, with some showing severe shifts. The dissolution–precipitation processes reveal that the significant impacts of silicate mineral dissolution on geothermal fluids in sandstone reservoirs might be due to isotopic exchange between geothermal fluids and oxygen-bearing rocks (limestones or silicate rocks), which increased the ¹⁸O concentration in groundwater, causing δ¹⁸O values to deviate from the MWLs.

According to the elevation effect principle of stable hydrogen and oxygen isotopes, δD and δ¹⁸O values decrease with an increase in groundwater recharge elevation [47]. Based on this, the recharge areas and elevations of geothermal fluids can be determined. Considering oxygen isotope shifts in geothermal fluids in the study area, δD was selected to calculate the recharge elevations of geothermal fluids [47].

H = H_r + (D − D_r)/grad D

(7)

where H is the elevation of the geothermal fluid recharge area (m); H_r is the surface elevation of the geothermal fluid sampling site (m); D is the δD value of meteoric water (−55.02‰); D_r is the δD value of geothermal fluids (‰), and grad D is the decreasing gradient with elevation (3‰/100 m).

The temperatures of geothermal fluid recharge sources can be estimated using hydrogen and oxygen isotopes. Due to oxygen isotope shifts in geothermal fluids in the study area, δD was also employed to calculate the temperatures of geothermal fluid recharge sources. The specific calculation equations are as follows [49]:

δD = 5.6T − 100

(8)

δD = 3T − 92

(9)

where T is the recharge source temperature (°C).

Table 4. Estimation results of δD and δ¹⁸O values and elevations for geothermal fluid samples from the study area.

Reservoir Type	Area	δDVSMOW (‰)	δ18OVSMOW (‰)	Elevation of the Sampling Site	Elevation (D)	Elevation (O)	Recharge Source Temperature
Karst reservoir	Liaocheng	−68	−9.4	34	634.00	649.38	6.86
	Ji’nan	−72	−9.8	34	834.00	803.23	5.83
		−74	−9.6	25	925.00	717.31	5.32
		−67.4	−9	29	1099.00	1067.46	7.01
		−63.4	−8.8	21	891.00	982.54	8.03
		−62.8	−9.2	22	862.00	1137.38	8.19
		−68.1	−9.6	25	1130.00	1294.23	6.83
		−76	−10.5	26	1526.00	1641.38	4.81
		−71.3	−10.1	22	1287.00	1483.54	6.01
		−68.2	−9.7	21	1131.00	1328.69	6.81
		−69.4	−9.8	25	1195.00	1371.15	6.50
		−68.3	−8.7	30	1145.00	953.08	6.78
		−63.4	−8	34	904.00	687.85	8.03
		−72	−9.8	34	1334.00	1380.15	5.83
		−70	−9.6	34	1234.00	1303.23	6.35
		−70	−9.6	25	1225.00	1294.23	6.35
		−71	−9.3	25	1275.00	1178.85	6.09
		−68	−9.7	34	1134.00	1341.69	6.86
		−69	−9.3	25	1175.00	1178.85	6.60
		−73	−9.8	25	1375.00	1371.15	5.58
		−73	−10.6	25	1375.00	1678.85	5.58
		−76	−10.1	25	1525.00	1486.54	4.81
Sandstone reservoir	Ji’nan	−73	−10.1	28	1378.00	1489.54	5.58
		−68	−9.2	17	1117.00	1132.38	6.86
		−70.39	−8.53	17	1236.50	874.69	6.25
		−66	−8.9	21	1021.00	1021.00	7.37
		−70.39	−8.53	15	1234.50	872.69	6.25
	Liaocheng	−71	−9.4	36	1286.00	1228.31	6.09
		−69	−9.4	32	1182.00	1224.31	6.60
		−70	−9.4	34	1234.00	1226.31	6.35
		−71	−9.7	33	1283.00	1340.69	6.09
		−73	−9.4	42	1392.00	1234.31	5.58
		−66	−8.8	42	1042.00	1003.54	7.37
		−70	−9.5	34	1234.00	1264.77	6.35
		−76	−9.9	41	1541.00	1425.62	4.81
		−72	−9.4	40	1340.00	1232.31	5.83
		−73	−9.7	33	1383.00	1340.69	5.58
		−72	−9.5	31	1331.00	1261.77	5.83
		−71	−10.2	36	1286.00	1536.00	6.09
		−70.37	−8.03	37	1255.50	702.38	6.25
	Dezhou	−75.05	−9.59	22	1474.50	1287.38	5.05
		−73	−10	24	1374.00	1447.08	5.58
		−74	−10	22	1422.00	1445.08	5.32
		−75	−10	26	1476.00	1449.08	5.07
		−74	−9.9	26	1426.00	1410.62	5.32
		−68	−8.6	27	1127.00	911.62	6.86
		−67	−9.2	11	1061.00	1126.38	7.11
		−75	−9.7	26	1476.00	1333.69	5.07
		−70	−8.8	22	1222.00	983.54	6.35
		−76	−9.7	22	1522.00	1329.69	4.81
		−71	−9.3	11	1261.00	1164.85	6.09
		−72	−9.4	26	1326.00	1218.31	5.83
		−74	−9.4	20	1420.00	1212.31	5.32
		−70	−8.5	13	1213.00	859.15	6.35
		−73	−8.7	18	1368.00	941.08	5.58
		−70	−8.7	23	1223.00	946.08	6.35
	Dongying	−59	−8.7	1	651.00	924.08	9.16
		−66	−8	3	1003.00	656.85	7.37
		−65	−8.2	1	951.00	731.77	7.63
		−68	−7.4	4	1104.00	427.08	6.86
		−64	−8.1	1	901.00	693.31	7.88
		−68	−8.2	1	1101.00	731.77	6.86
		−73	−9.4	3	1353.00	1195.31	5.58
		−67	−7.4	6	1056.00	429.08	7.11
	Binzhou	−71	−9.3	12	1262.00	1165.85	6.09
		−64	−7.5	12	912.00	473.54	7.88
		−66	−9.1	13	1013.00	1089.92	7.37
		−61	−7.8	10	760.00	586.92	8.65
	Weifang	−68	−9	53	1153.00	1091.46	6.86

shows that the geothermal fluid samples from sandstone reservoirs exhibited overall recharge elevations ranging from 651 to 1522 m and recharge source temperatures ranging from 4.81 to 9.16 °C. Specifically, their recharge elevations ranged from 1021 to 1378 m in the Ji’nan area, 1042 to 1541 m in the Liaocheng area, 1061 to 1522 m in the Dezhou area, 651 to 1353 m in the Dongying area, and 760 to 1262 m in the Binzhou area. Their recharge source temperatures ranged from 5.58 to 7.37 °C in the Ji’nan area, 4.81 to 7.37 °C in the Liaocheng area, 4.81 to 7.11 °C in the Dezhou area, 5.58 to 9.16 °C in the Dongying area, and 6.09 to 8.65 °C in the Binzhou area. In contrast, the geothermal fluid samples from karst reservoirs manifested overall recharge elevations ranging from 634 to 1526 m and recharge source temperatures ranging from 4.81 to 8.03 °C. Specifically, they showed a recharge elevation of 634 m and a recharge source temperature of 6.86 °C in the Ji’nan area. They showed recharge elevations ranging from 862 to 1526 m and recharge source temperatures ranging from 4.81 to 8.03 °C in the Ji’nan area. The topography of Shandong Province is centered around the Tai-Lu-Yi mountains, gradually descending towards the surrounding areas. The Tai-Lu-Yi mountains show elevations of 1545, 1108, and 1032 m, respectively, with the main peak of Mount Tai being the highest point in Shandong Province. The study area is primarily located to the northwest of the Tai-Lu-Yi mountains, highly coinciding with the recharge elevations of geothermal fluids in the study area, indicating that the estimation results are relatively reasonable. In summary, the primary recharge source for geothermal fluids in the study area is distributed near the Tai-Lu-Yi mountains to the southeast of the study area.

5.2.4. Mixing Processes of Geothermal Fluids

Using the silica-enthalpy mixing model and equations can eliminate the influence of cold water mixing and analyze the proportion of cold water mixing and the reservoir temperature before mixing [50].

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

(10)

where S and SiO₂ are the enthalpy (J/g) and SiO₂ concentration (mg/L) of water, respectively; c is the initial content for near-surface cold water; h is the initial content for hot water; s is the final content for spring water; and x is the mixing ratio of underground cold water.

Table 5. Relationships between temperature, enthalpy, and SiO₂ concentration.

T (°C)	Enthalpy (J/g)	SiO2 (mg/L)	T (°C)	Enthalpy (J/g)	SiO2 (mg/L)	T (°C)	Enthalpy (J/g)	SiO2 (mg/L)
50	50.0	13.5	150	151.0	125.0	250	259.2	486.0
75	75.0	26.6	175	177.0	185.0	275	289.0	614.0
100	100.1	48.0	200	203.6	265.0	300	321.0	692.0
125	125.1	80.0	225	230.9	365.0	─	─	─

shows the relationships between temperature, enthalpy, and SiO₂ concentration.

The silica-enthalpy mixing model was employed to calculate the mixing ratios of cold water in geothermal fluids in sandstone and karst reservoirs in the study area. Figure 11 shows that the cold water mixing ratios in geothermal fluids of sandstone reservoirs ranged from 85% to 90%, resulting in corrected reservoir temperatures ranging from 150 to 200 °C. In contrast, Figure 12 shows that the cold water mixing ratios in geothermal fluids of karst reservoirs ranged from 89% to 96%, resulting in corrected reservoir temperatures ranging from 200 to 225 °C.

6. Conclusions

The geothermal fluids in the Neogene sandstone reservoirs show measured temperatures ranging from 42 to 65 °C, pH values from 7.6 to 9.2, and TDS concentrations from 3676 to 6788 mg/L, with a predominant hydrochemical type of Na-Cl-SO₄. In contrast, the geothermal fluids in the karst reservoirs show measured temperatures ranging from 33 to 82 °C, pH values from 6.9 to 8.0, and TDS concentrations from 3314 to 7416 mg/L, with a primary hydrochemical type of Na-Ca-Cl-SO₄. The hydrochemical composition of geothermal fluids in the karst reservoirs is primarily influenced by the precipitation–dissolution equilibrium of carbonate and sulfate minerals, with minor impacts of silicate dissolution. The hydrochemical composition of geothermal fluids in the sandstone reservoirs is primarily affected by the precipitation–dissolution processes of carbonate minerals and especially silicate minerals. Compared to those in the karst reservoirs, geothermal fluids in the sandstone reservoirs exhibit more intense cation exchange reactions. Hydrogen and oxygen isotope analyses indicate that geothermal fluids in the sandstone reservoirs have experienced significant oxygen isotope shifts. This aligns with the finding that the hydrochemical composition of geothermal fluids in the sandstone reservoirs is significantly influenced by the dissolution and precipitation of silicates. The isotopic exchange between geothermal fluids and oxygen-bearing rocks (limestones or silicate rocks) increases the ¹⁸O concentration in the geothermal fluids, leading to the deviation between δ¹⁸O values and the MWLs. In contrast, the hydrochemical composition of geothermal fluids in the karst reservoirs is less affected by the dissolution and precipitation of silicates, resulting in slight oxygen isotope shifts.

The calculations of reservoir temperatures using geothermometers reveal that the quartz geothermometer is more effective in calculating the temperatures of the karst and sandstone reservoirs, obtaining temperatures ranging from 52.97 to 82.09 °C and 60.54 to 85.88 °C, respectively. The geothermal fluids in the sandstone reservoirs manifest circulation depths ranging from 1366.60 to 2102.29 m and cold water mixing ratios from 85% to 90%. Accordingly, the reservoir temperatures corrected using the silica-enthalpy mixing model range from 150 to 200 °C. The geothermal fluids in the karst reservoirs display circulation depths ranging from 1107.28 to 1836.69 m and cold water mixing ratios ranging from 89% to 96%. Accordingly, the corrected reservoir temperatures range from 200 to 225 °C. The geothermal fluids in the sandstone reservoirs feature overall recharge elevations ranging from 651 to 1522 m and recharge source temperatures from 4.81 to 9.16 °C. In contrast, the geothermal fluids in karst reservoirs manifest overall recharge elevations ranging from 634 to 1526 m and recharge source temperatures from 4.81 to 8.03 °C. The primary recharge source for geothermal fluids in the study area is distributed near the Tai-Lu-Yi mountains to the southeast of the study area.

To exploit and utilize geothermal resources in the study area efficiently and sustainably, it is recommended to consider the geochemical differences in geothermal fluids across the composite geothermal reservoirs, ecological characteristics, and anthropogenic factors in the subsequent exploitation and utilization.