Hydrothermal energy is obtained by utilizing the heat or energy from the groundwater within the geothermal systems (Byrappa & Yoshimura, 2012). It is recognized that hydrothermal energy is clean, environmentally friendly, stable, and reliable when people use the heat or energy from hydrothermal water (Rybach, 2003; Marrasso et al., 2018). Besides, hydrothermal energy demonstrates the characteristic of large reservoirs and wide distribution (Wang, 2015a; Shortall et al., 2015; Moya et al., 2018; Lund & Toth, 2021). In northern China, hydrothermal energy is widely used because of its unique advantages as a heating supply (Wang, 2009; Duan et al., 2011; Kong et al., 2014; An et al., 2016; Wang et al., 2018). However, to realize the rational exploitation and utilization of hydrothermal geothermal resources, it is necessary to consider the sustainability of long-term exploitation as well as the environmental and geological consequences of exploitation (Hahnlein, Bayer, Ferguson, & Blum, 2013). To determine the exploitation potential of a hydrothermal system, it is imperative to document the water and heat sources, the thermal reservoir, the water transporting channels, and the thermal caprock of the geothermal field (Pang et al., 2018; Huang et al., 2021a). Although one can easily monitor the temperature and thermal reservoir characteristics of hydrothermal systems, it is difficult to determine the water sources, water flow paths, and heat sources for complicated geochemical processes. Precipitation generally recharges into hydrothermal reservoirs through deep, subsurface circulations. Within the circulation period, the geothermal water is heated from the comparative hot surrounding rocks with which it interacts (Lister, 1980; Le Gal et al., 2018). Therefore, the geochemical characteristics and isotopic composition of geothermal water will change accordingly (Yang et al., 2017). Thus, the combination of geochemical and isotopic characteristics in hydrothermal systems is of great value for determining the origin of the hydrothermal water and characterizing the water-rock-gas interaction process.

In northern China, a typical hydrothermal system known as the Caofeidian has been developed and utilized for many years as a heating supply (Chang et al., 2016; Dong et al., 2021; Wang et al., 2021). However, temperature anomalies have been detected in the geothermal development process at the Matouying, located less than 20 km east of the Caofeidian. The temperature of the geothermal boreholes at the Matouying was found to be much higher than those of the Caofeidian. To distinguish the temperature anomalies and realize the sustainable utilization of hydrothermal energy, it is necessary to investigate the patterns of deep groundwater circulation and geothermal reservoir conditions. Although some geophysical studies have been conducted to investigate the causes of the temperature anomalies within this area, no research has been conducted to compare the hydrothermal waters at the Caofeidian and the Matouying from the geochemical and isotopic information.

In this study, we conducted detailed geochemical and isotopic analyses of geothermal waters at the Caofeidian and the Matouying to distinguish the differences between the two sites and investigate the hydrothermal system characteristics. ¹⁴C dating was used to assess the age of deep geothermal groundwater. We also determined the heat source and circulation patterns of different geothermal waters based on their geochemical and isotopic characteristics. The principal research objectives of this study are: 1) to investigate the geochemical and isotopic characteristics of the geothermal groundwater at the Caofeidian and the Matouying; 2) to determine the causes of geothermal water differences between the two areas; and 3) to propose a conceptual model of geothermal water circulation in the research areas.

Caofeidian is located 80 km south of Tangshan and 120 km east of Tianjin in the north-central part of the Bohai Bay Basin which locates in the northern China (Figure 1A and Figure 2). In terms of geological structure, the Caofeidian is located in the north of the Nanpu Sag, one of the major oil and gas fields in China (Figure 2). The Nanpu depression is located in the north of the Huanghua Depression, and its northwest boundary is the Xinanzhuang fault, separated from the Xinanzhuang Uplift and Laowangzhuang Uplift (Figure 2) (Dong et al., 2021). The northeast boundary is the Baigezhuang fault, which is adjacent to the Baigezhuang Uplift and the Matouying Uplift (Figure 2). The southern Nanpu depression is in a fault-superposition relationship with the Shaleitian Uplift. Geothermal anomaly zones were found in the Matouying Uplift area near the Baigezhuang fault. The Caofeidian and Matouying areas are located on the west and east sides of the Baigezhuang fault, respectively (Figure 2).

In general, Nanpu Sag is a Tertiary sedimentary lake basin (Wang et al., 2022). From the surface downwards, the Nanpu Sag and its surrounding areas are characterized by Cenozoic, Mesozoic, Paleozoic, Neoproterozoic, and Archean strata (Zhu et al., 2014; Wang et al., 2020). The stratigraphic distribution and thickness of different structural units of the Nanpu Sag vary greatly (Wang et al., 2021). Baigezhuang Fault is a border fracture separating Matouying and Nanpu, which has controlling effect on the formation and evolution of Nanpu Sag. Because of the movement of Baigezhaung Fault, the Archean granite was uplifted at Matouying. Besides, the destruction of the eastern North China Craton and the thinning of the regional lithosphere also contributed to the shallow depth of granite at Matouying (Qiu et al., 2014; Qiu et al., 2016; Zhang et al., 2020b; Dong et al., 2021). According to the geophysical work by Dong et al. (2008), the depth of Baigezhuang Fault was estimated to be more than 5 km downwards, which reached the basement of Archean granite. The main heat reservoirs in the Nanpu Sag are the Neogene Guantao Formation and the Neogene Minghuazhen Formation (Figure 3), followed by the Cambrian and the Ordovician. The Neogene strata is principally driven by the uplift and sag of the Paleogene. Except for the absence of the Laowangzhuang uplift, the Guantao Formation contains a thickness of 300–900 m in our study area except for the Laowangzhuang Uplift. The thermal reservoirs of the Guantao Formation are mainly fine sandstone and gravel sandstone with an average porosity of 30–35%. The bottom boundary of the Guantao Formation thermal reservoir is 2,100–2,600 m deep, gradually increasing in depth from north to south. The Minghuazhen Formation has a thickness of 1,000–2000 m throughout the study region. The bottom boundary of the Minghuazhen Formation’s thermal reservoir is 1700–2,200 m deep, gradually deepening from north to south. The Quaternary in this region belongs to the Pleistocene-Holocene strata, which are not integrated with the underlying Minghuazhen Formation. Quaternary in this area is a set of undiagenetic interbedded yellow glutenite and clay, the bottom of which are alluvial and alluvial glutenite layers. In this study, we focus on the Neogene sandstone aquifer because it is the principal geological formation exploited for geothermal energy production in the Caofeidian and the Matouying. Our study objective is the Neogene heat reservoir. The Neogene strata in the Caofeidian, and the Matouying is mainly porous sandstone, which has the characteristics of high porosity and permeability.

Field sampling was performed at the Nanpu Sag in December and November 2019. A total of 35 water samples were collected, including 12 from geothermal wells, 11 from shallow wells, and one from deep wells in the Dongying Formation. To compare the hydrochemistry and isotopic characteristics of groundwater and surface water, we also collected four river samples, five pond samples, and 2 seawater sample in different directions of the geothermal wells. All sampling locations are shown in Figure 1A. The sampling time was within the heat supply period, consequently, the water samples from the geothermal boreholes are representative of the water within the aquifer. All the samples were filtered in situ with a 0.23 μm membrane prior to bottling and sealing with Parafilm. Samples for cation and trace element analysis were acidified with ultra-purified HNO₃ to adjust the pH of each sample to less than 2. The acidified process was not added to the samples for anion and water isotope analyses.

Temperature, pH, electrical conductivity (EC), and total dissolved solids (TDS) were measured in the field using a multi-parameter device (Hach HQ40D). HCO_3⁻ and CO₃ ²⁻ were measured in the field using a digital titrator (16,900 Digital Titrator, Hach) with indicators including phenolphthalein and methyl orange colorants. The samples were sent for analysis at the Water Isotopes and Water–Rock Interaction Laboratory, Institute of Geology and Geophysics, Chinese Academy of Sciences.

Cations and anions were identified according to the National Analysis standard DZ/T0064.28–93 and DZ/T0064.51–93, respectively. The detection limit was 0.1 mg/L. The trace elements were determined using ICP-MS (7500C, Agilent) with an analytical precision of less than 0.5%. Stable isotopes (δ²H and δ¹⁸O) were measured using a laser absorption water isotope spectrometer analyzer (Picarro L2120-i). All δ²H and δ¹⁸O values are expressed in δ notation per mil relative to the Vienna Standard Mean Ocean Water (V-SMOW). The measurement precisions of δ²H and δ¹⁸O were ±0.5‰ and ±0.1‰, respectively. The trace elements were determined using ICP-MS (7500C, Agilent) at the Analytical Laboratory of Beijing Research Institute of Uranium Geology with an analytical precision of less than 0.5%. The carbon isotopic compositions (i.e., ¹⁴C and ¹³C) were determined by Beta Analytic, Inc. using accelerator mass spectrometry (AMS) and isotope ratio mass spectrometry (IR-MS), respectively. The analytical precision of the AMS ¹⁴C results was ±0.1%. The ¹⁴C ages were corrected using the δ¹³C mixing model (Clark and Fritz, 2013): t = − 8267 ln ( C t q C 0 ) (1) q =   δ 13 C t − δ 13 C d δ 13 C s − δ 13 C d (2) where t is the groundwater age, C t is the measured ¹⁴C activity, q is the correction factor, and δ 13 C t , δ 13 C d , and δ 13 C s represent the measured δ 13 C values for the groundwater, the dissolved calcite (0‰), and soil (-23‰), respectively.

We found different hydrothermal patterns at the Caofeidian and the Matouying geothermal systems through water temperature and pressure monitoring. By conducting detailed geochemical and isotopic analyses of the geothermal water, we investigated the hydrothermal system characteristics of the two geothermal systems and distinguished the recharge source and circulation pattern of geothermal water at the two sites. Although geothermal water originates from precipitation at both systems, the circulation pattern of groundwater is distinct for tthem. The geothermal water at Caofeidian has experienced the process of infiltration, interaction with sandstone, BSR, and mixing with geothermal water from the underlying Dongying Formation. The infiltrated water is able to circulate deeply into the granite underlying the Matouying system. Application of Na-K geothermometers indicated reservoir temperatures to be in the interval of 83–92°C at the Caofeidian and 107–137°C at the Matouying, respectively. The corrected ¹⁴C age of geothermal water indicates a lower circulation rate and rare renewability for geothermal water at the Caofeidian system. This study provides multiple information on the geothermal systems in our study area, and the results are instructive and significant with respect to the sustainable exploitation of geothermal resources in the Caofeidian and the Matouying geothermal systems.