Abnormal geothermal gradient in the northern Jiyang geothermal field and its implications for the genesis of geothermal resources

Abstract

This study investigates the abnormal geothermal gradient and geothermal resource genesis in the northern Jiyang geothermal field via an integrated approach of borehole monitoring, Wide-field Electromagnetic Sounding (WFEM), Magnetotelluric Sounding (MT), well logging, hydrochemistry and isotope testing. Results show that the downthrown block of NW-dipping fault F1 in the northwest of the northern Jiyang geothermal field has a prominent high geothermal gradient anomaly (average 3.52 °C/100 m), driven by the deeply buried Guantao Formation, the NW-dipping normal fault F1 develops a highly permeable fracture zone that may serve as a potential migration pathway for deep thermal fluids and subsurface heat, and the excellent sealing of surrounding reservoirs. The abnormal gradient indicates high geothermal enrichment in the downthrown block of fault F1, the delineated core target area with superimposed Guantao and Dongying Formations, moderate burial and simple tectonics. It also promotes the enrichment of beneficial trace elements (Sr up to 64.9 mg/L, H₂SiO₃ up to 51.1 mg/L) in geothermal fluids, endowing high medical development potential. Isotopic analysis reveals regional geothermal fluids are dominated by paleowater with weak modern recharge, requiring geothermal reinjection for sustainable development. This research provides critical geological foundations for precise exploration, efficient development and sustainable utilization of geothermal resources in the study area.

1 Introduction

In recent years, as global attention has focused on climate change and the reduction of greenhouse gas (GHG) emissions from fossil fuels, interest in renewable energy alternatives has been steadily growing. Geothermal energy has the potential to provide long-term baseload power and heat, while reducing GHG emissions and serving as a sustainable and reliable energy source, crucial for the transition to a zero-carbon future (Fàbrega Ferrer, 2022; Rahman et al., 2022; Nguyen et al., 2024). It generates baseload electricity, achieving an average capacity factor of 85% (Zarrouk and Moon, 2014), only surpassed by nuclear power, while producing minimal emissions and avoiding dependence on earth (IRENA Coalition for Action, 2024). Heat is extracted from thermal-fluid-related reservoirs or enhanced geothermal systems, such as those developed in the FORGE project, which enables global deployment, with 16, 355 MW currently installed in 30 countries (IEA, 2025). The main producers are the USA (3,900 MW), Indonesia (2,418 MW), and Turkey (1,691 MW) (Villalba Ulberich et al., 2026). China is one of the countries with large reserves of geothermal resources in the world, especially medium and low-temperature geothermal resources, which have enormous potential for development and utilization (Lin et al., 2013).

Geothermal gradient is one of key parameters that describe the geothermal regime of the continental lithosphere (Ranalli, 1997; Rudnick et al., 1998; Artemieva and Mooney, 2001; Furlong and Chapman, 2013; Qiu et al., 2014; He, 2015). The geothermal gradient varies with different lithologies and tectonic conditions, so it can be used to reflect the subsurface geological conditions and is of great significance for geological exploration. Meanwhile, the geothermal gradient can be applied to design geothermal energy systems for the development and utilization of geothermal energy, and also to understand the physical and chemical processes in the Earth’s interior. Thus, it is also critical for investigating the evolutionary history of the Earth and other relevant disciplines (Ranalli, 1997; Ranalli and Rybach, 2005; Bürgmann and Dresen, 2008; Liu et al., 2008; Burov, 2011).

Under the influence of the extensional thinning tectonic mechanism of the Bohai Bay Basin, the Jiyang Depression is characterized by a prominent deep thermal background. Featuring a complex structural framework, this depression exhibits an alternating distribution of uplifts and sags, which results in substantial spatial variations in the geothermal field both vertically and horizontally (Wang et al., 2004; Qiu et al., 2011; Yang et al., 2024). However, current studies have not conducted systematic research on the geothermal gradient in this region, nor have provided a quantitative analysis of the geological-thermophysical coupling mechanism.

This study targets the northern Jiyang geothermal field as the research object. Based on borehole temperature and well log data, it analyzes the characteristics of the geothermal gradient in the study area and confirms the changes in the deep and shallow geothermal fields. The paper took the typical geothermal geologic profile in the northern Jiyang geothermal field as an example to analyze the formation mechanism and controlling factors of abnormal geothermal gradient.

2 Geological setting

The northern Jiyang geothermal field is located in the south of the Jiyang Depression, which is located on the southeastern margin of the Bohai Bay Basin and is a Meso-Cenozoic superimposed rift basin developed on the North China Craton basement (Figure 1a). It belongs to the Huiming Sub-Sag, and it is the separated from the Luxi Uplift by the Qihe-Guangrao Fault in the south (Figure 1b). The tectonic evolution of the northern Jiyang geothermal field is basically synchronous with that of the Bohai Bay Basin and is deeply influenced by the dextral strike-slip activity of the Tan-Lu Fault Zone (Ren et al., 2009). Previous studies indicate that the study area has experienced multi-stage tectonic movements. During these processes, this area experienced alternating uplifts and sags (Zhang et al., 2006; Liu, 2018). The basement of the northern Jiyang geothermal field comprises of Precambrian crystalline rocks, which is mainly consists of migmatite granite (monzonitic and gneissic), with a small amounts of amphibole granulite, plagioclase amphibolite, and other metamorphic rock inclusions (Mu et al., 2025). The strata in this area consist of the Ordovician, Permian, Triassic, Jurassic, Cretaceous, Neogene and Quaternary Systems. Among them, the Ordovician System is mainly composed of limestone; the Permian System is dominated by interbedded sandstone and limestone; the Triassic System consists of interbedded sandstone and mudstone; the Jurassic System is made up of sandstone, conglomerate and tuff; the Cretaceous System comprises sandstone, basalt and andesite; the Paleogene System is composed of mudstone, siltstone, fine-grained sandstone, oil shale and carbonate rocks; the Neogene System is dominated by clastic sediments of variegated mudstone, intercalated with sandstone and minor pebbly or conglomeratic sandstone. The Paleogene System in the study area is dominated by the Dongying Formation, whose lithology mainly consists of grayish-green and gray mudstones, with purplish-red mudstones interbedded with sandstones and conglomeratic sandstones of varying thicknesses. The Neogene System mainly comprises the Guantao Formation and the Minghuazhen Formation. The Guantao Formation represents a suite of interbedded deposits of grayish-white conglomeratic sandstones, fine sandstones, and brownish-red mudstones. The lithology of the Minghuazhen Formation is dominated by earthy-yellow and brownish-red mudstones, sandy mudstones, and grayish-white sandstones. The strata of each period are controlled by multiple tectonic movements. During the Paleogene rifting stage, a half-graben structure characterized by northward faulting and southward onlap developed, and during the Neogene overall depression period, drape-style sedimentary cover was formed (Yu et al., 2026).

FIGURE 1

3 Methodology

The northern Jiyang geothermal field was studied for geological, stratigraphic and structural, coupled with an analysis of the geothermal gradient to understand the geothermal potential of deep confined reservoirs. Geological studies based on surface and subsurface information were used for determining the stratigraphic limits and depth of potential reservoirs, for evaluating the properties of the different lithological units that may contain or seal geothermal fluids and performing preliminary structural analyses of the reservoirs associated with the thermal fluids.

Field construction method for Wide-field Electromagnetic Sounding (WFEM): In this geophysical prospecting work, there are survey lines with three different azimuths, and to meet the construction requirements of WFEM. Three groups of separate transmitting sources were deployed; each survey line was equipped with an independent transmitting source system; the signal transmitter of the WFEM instrument consists of three parts, namely, a high-power diesel AC generator, a rectifier-inverter, and a pseudorandom signal transmission controller, with its configuration parameters being three-phase 380 V AC, 180 kW/50 Hz, and the WFEM exploration receiving system is mainly composed of a WFEM receiver, receiving electrodes and a computer workstation, and the transmitted signal covers a frequency range of 0.01171875 Hz–8192 Hz with a total of 80 frequencies.

Magnetotelluric Sounding (MT) survey: A cross-shaped array was adopted in the survey area. Two pairs of horizontal measuring electrodes were laid perpendicular to two magnetic sensor rods, with the azimuth deviation of each electrode pair controlled within 1°. The horizontal magnetic rods were buried at positions 8–10 m away from the center at their top ends. The V8 Electromagnetic Workstation was used for this survey, which is equipped with 3 magnetic channels and 9 electrical channels. The magnetic channels can be connected to either standard magnetic rods or 1 to 3 axial TDEM probes. The V8 can also operate as a standalone unit (typically for AMT and MT surveys). In addition, it can serve as a local network hub for auxiliary data acquisition with 2 electrical channels, supporting both wireless and wired communication modes. All recording units are synchronized via GPS time (±0.2 microseconds), and the transmitters are also synchronized through GPS time.

For field data collection, the numerically controlled logging system SKD3000C, manufactured by the 22nd Research Institute of China Electronics Technology Group Corporation (CETC-22), was adopted. At the time of logging, the total well depth was 1812.0 m, the logging interval ranged from 6.0 m to 1812.0 m, and the logging series employed was SKD3000C. Following technical design specifications, nine logging datasets were collected from this well, including 2.5 m RA bottom gradient electrode logging, RA0.4 m potential electrode logging, dual laterolog, caliper logging, spontaneous potential logging, natural gamma logging, compensated acoustic logging, continuous temperature logging, and continuous deviation logging. The logging data processing and interpretation software adopted is the Complex Lithology Processing and Interpretation Module (CRA) of the Comprehensive Logging Application Platform (FORWARD), independently developed by China University of Petroleum.

The geothermal gradient of the northern Jiyang geothermal field was calculated based on steady-state temperature taken from exploration borehole. Borehole temperature measurements were conducted 2 months after borehole production stabilized to thermal equilibrium, which can accurately reflect the actual thermal regime of the basin. The equipment for the borehole temperature logs operation in this study has a measurement sensitivity of 0.01 °C and a temperature measurement range from 0 °C to 180 °C. The equipment was operated by the professional team from the Institute of Geology and Geophysics, Chinese Academy of Sciences. Temperature logging was conducted during the descent of the probe. Continuous measurements have been done provided that the probe is lowered at speed 3 m/min, and the movement is mechanically controlled to ensure precise recording of the depth. Calculation method of steady-state temperature and geothermal gradient is in the Supplementary Material.

For the ion analysis in this hydrochemical testing, major cations (K⁺, Na⁺, Ca²⁺, Mg²⁺) were determined by an Inductively Coupled Plasma Optical Emission Spectrometer (ICAP7200), with detection limits ranging from 0.001 mg/L (for partial trace metals) to 0.12 mg/L (for Na⁺). For anions, chloride (Cl⁻), sulfate (SO₄^2-), and bicarbonate (HCO₃⁻) were analyzed via ion chromatography using an Ion Chromatograph (ICS-900), where the detection limits for Cl⁻, SO₄²⁻, and HCO₃⁻ were 0.02 mg/L, 0.05 mg/L, and 0.10 mg/L, respectively. All instruments involved in the ion analysis were equipped with valid calibration certificates, ensuring the accuracy and reliability of the test results. All the hydrochemical analysis tests were conducted at the Laboratory of Shandong Provincial Institute of Geology and Mineral Engineering.

4 Results

4.1 Structure of the northern Jiyang geothermal field

Based on data from three geothermal wells (JYB01, DS1 and JR025), one petroleum borehole (JYR1), and one seismic well (Qu1), the new structural analysis performed in this present study shows that the stratigraphic distribution of the northern Jiyang geothermal field exhibits distinct vertical layering and lateral variations (Figure 1c). The uppermost layer is the Quaternary, which continuously covers the entire area. Below it lies the Neogene Minghuazhen formation, which extends over a large area. The Neogene Guantao Formation distributed in a banded pattern lies beneath the Minghuazhen Formation and pinches out gradually from the northwest to the southeast. The Paleogene (such as the Kongdian Formation and Dongying Formation), Cretaceous and Triassic strata are deeply buried. The Dongying Formation is only distributed in the northwest, the Kongdian Formation is distributed in the central part, and the Cretaceous and Triassic strata are only distributed in the southeast. In addition, the upper Quaternary and Minghuazhen Formation exhibit good continuity, while the distribution of the lower Guantao Formation, Paleogene, and older strata is clearly controlled by structures. In particular, the Dongying Formation is cut through by a deep, large-scale fault (F1, Figure 1c).

Compared with the other two geothermal wells, Well JYB01 not only has a geothermal temperature that is more than 10 °C higher (Table 1), but also shows significant differences in geothermal reservoirs. Therefore, Well JYB01 was selected for a more comprehensive study in this research.

4.2 Data wells, steady-state temperature and hydrochemistry

We have recognized 70 aquifers using boreholes records (JYB01) with depths ranging from 310.5 m to 1806.7 m (Table 2). Among them, 68 are classified as conventional water layers and 2 (Layers 50 and 51, 1362.8 m–1377.5 m depth) as low-yield water layers. The thickness of the aquifer layers varies from 1.5 m to 42.8 m, with an average thickness of 10.1 m. The thickest layer (Layer 63) is 42.8 m (1635.9 m–1678.7 m), while the thinnest (Layer 57) is 1.5 m (1550.0 m–1551.5 m), reflecting significant vertical heterogeneity in the aquifer system.

Porosity and permeability show strong positive correlation across the layers. Layers with porosity exceeding 30% (e.g., Layers 11, 12, 31) generally have permeability greater than 1,000 × 10⁻³ μm², while layers with porosity below 10% (none in this dataset) would likely exhibit low permeability. The high average porosity (25.4%) and permeability (602.3 × 10⁻³ μm²) suggest that the aquifer system has excellent storage and seepage capacity for geothermal or groundwater resources.

This study utilizes temperature data from a single exploration borehole, covering a depth range of 100 m–1,800 m. The dataset includes two key temperature parameters: logging temperature (directly measured during well logging) and steady-state temperature (corrected equilibrium temperature of the formation) (Table 3). A total of 33 pairs of temperature data points were collected at 25 m intervals, providing a comprehensive basis for analyzing the thermal regime of the subsurface. The logging temperature serves as the primary observed data, while the steady-state temperature is derived through correction to eliminate thermal disturbances caused by drilling activities (e.g., mud circulation cooling effect), similar to the bottom-hole temperature (BHT) correction method described by Villalba Ulberich et al. (2026). This corrected temperature more accurately reflects the true thermal state of the formation. Both logging temperature and steady-state temperature exhibit a continuous upward trend with increasing depth, consistent with the conductive heat transfer mechanism in sedimentary depression basins. Logging temperature ranges from 33.4 °C (100 m) to 61.2 °C (1,800 m), with a total temperature increase of 27.8 °C over the 1,700 m depth interval (Table 3; Figure 2). Steady-state temperature varies from 19.3 °C (100 m) to 79.2 °C (1,800 m), showing a more significant temperature rise of 59.9 °C over the same depth range.

FIGURE 2

The chemical characteristics of the aquifers were investigated by analyzing two water samples in the Guantao Formation and the Dongying Formation from the JYB01 geothermal well, one water sample in the Guantao Formation from the DS1 geothermal well and one water sample in the Guantao Formation in the JR025 geothermal well. Additionally, hydrogen and oxygen stable isotopes and ¹⁴C radioactive isotope tests were conducted on the water samples from the JYB01 geothermal well. According to the hydrochemistry analysis results (Table 4), the geothermal water in the northern Jiyang geothermal field is dominated by Na⁺, accounting for 85%–88% of the total cations, with significant differences in content among different thermal reservoirs—the Na⁺ content is the highest in the Dongying Formation geothermal water (6,923 mg/L), followed by the Guantao Formation (5,355 mg/L). The Ca²⁺ content ranges from 465 mg/L to 573 mg/L, Mg²⁺ from 96.0 mg/L to 126 mg/L, and K⁺ from 40.8 mg/L to 54.2 mg/L. Anions are dominated by Cl⁻, accounting for 82%–89% of the total anions, and their content distribution is positively correlated with Na⁺. The HCO₃⁻ content is relatively stable, ranging from 185 mg/L to 202 mg/L, with no significant difference among different thermal reservoirs. The total dissolved solids (TDS) ranges from 15,998 mg/L to 20,554 mg/L. Based on the analysis of the Piper trilinear diagram (Figure 3) and combined with the characteristics of conventional ion composition, the hydrochemical type of the study area is SO₄·Cl-Na type. The δ²H values of the water samples range from −64‰ to −59‰, and the δ¹⁸O values range from −9.3‰ to −8.9‰ (Table 5). All geothermal water samples plot slightly below the local meteoric water line (LMWL) without the positive δ¹⁸O enrichment typical of geothermal systems (Figure 4). The ¹⁴C data indicate that the ¹⁴C age of the Guantao Formation is 30,110 BP, and that of the Dongying Formation is 41,300 BP (Table 5). The measured radiocarbon ages are uncorrected apparent ages and represent upper-bound estimates of groundwater residence times.

FIGURE 3

FIGURE 4

4.3 Temperature gradient

Geothermal gradients were calculated from temperature logs via least-squares regression applied to linear segments of temperature–depth profiles. Firstly, temperature data in the shallow sections are excluded, because they are easily affected by groundwater flow, thermal conductivity heterogeneities, or surface temperature variations due to climate or land cover change (Huang et al., 2000; Kurylyk et al., 2014). Secondly, the gradient profile and lithological layer should be considered to evaluate the stable sections, which can be used to calculate the geothermal gradient compiled from the least-squares regression method.

Based on the steady-state temperature logging results of Well JYB01, the geothermal gradient is 4.66 °C/100 m for the Quaternary System, 4.66 °C/100 m for the Minghuazhen Formation, 5.31 °C/100 m for the Guantao Formation and 1.65 °C/100 m for the Dongying Formation; the average geothermal gradient of the thermal reservoir stands at 4.27 °C/100 m (Figure 2). For geothermal Well JR025, the corresponding geothermal gradients are 2.12 °C/100 m (Quaternary System), 3.91 °C/100 m (Neogene Minghuazhen Formation) and 1.91 °C/100 m (average for thermal reservoir). As for geothermal Well DS01, the geothermal gradient is 2.97 °C/100 m in the Quaternary System and 2.60 °C/100 m in the Neogene Minghuazhen Formation; the average geothermal gradient of the entire well is 2.48 °C/100 m (Figure 5).

FIGURE 5

An abrupt temperature rise from 54.92 °C at 1,150 m to 67.75 °C at 1,200 m over merely 50 m, yielding an anomalously high local geothermal gradient that deviates substantially from the regional thermal background. We first eliminated the possibility of correction-induced artifacts by re-examining Horner plot fitting outputs for depths ranging from 1,150 m to 1,200 m, which all exhibited fitting coefficients of determination (R² > 0.99) and extrapolation residuals lower than ±0.2 °C; the overall correction uncertainty was negligible compared with the 12.83 °C temperature jump, proving the temperature discontinuity was not caused by calculation errors. Further geological analysis verified that this drastic temperature shift represents an authentic subsurface thermal signal tied to the stratigraphic contact between the Neogene Guantao Formation and Paleogene Dongying Formation at 1,150–1,200 m in Well JYB01: the Guantao Formation above 1,180 m consists of porous and permeable sand-mud interbeds where shallow cold groundwater circulates vertically and dissipates heat, whereas the underlying Dongying Formation connects to the deep heat-transmitting fracture zone of Fault F1. Deep hot geothermal fluids ascend along this fault and accumulate at the Guantao–Dongying contact, developing a high-temperature fluid enrichment zone; vertical fluid convection across the stratigraphic boundary consequently triggers the prominent thermal jump, which is a natural geothermal anomaly governed by lithologic interfaces and fault-mediated fluid migration rather than faulty measurements.

4.4 Wide-field electromagnetic (WFEM) sounding and magnetotelluric (MT) sounding

In this study, two survey lines were designed for both WFEM and MT geophysical prospecting respectively. One line (A-A’/I-I’) is located in the northwest of the northern Jiyang geothermal Field, adjacent to Well JYB01, and the other (E-E’/V-V’) is in the central area.

The A-A’/I-I’ profile of the WFEM Sounding has a survey line length of 3 km, a station interval of 100 m and an azimuth of 120°. The station interval for MT Sounding is set at 50 m. The inverted cross-section of the A-A’/I-I’ line exhibits a clearly defined overall electrical structure with generally low resistivity values: the apparent resistivity varies in the range of 6 ∼ 27 Ω·m, while that of the MT ranges from 3.5 to 16.5 Ω·m. As observed from the resistivity contour cross-sections of both WFEM and MT, the resistivity curves are relatively continuous in the horizontal direction and show distinct layering in the vertical direction. The morphological variations of the resistivity curves well delineate the boundaries, geometries and intercontact relationships of each geological unit. In the vertical direction, the resistivity values exhibit a low resistance →high resistance →low resistance → high resistance trend from shallow to deep depths (Figure 6).

FIGURE 6

The E-E’/V-V’ profile has a survey line length of 3 km, an azimuth of 133° and a station interval of 100 m, with the station interval for MT Sounding set at 50 m. The inverted cross-section of the E-E’/V-V’ profile shows generally low resistivity values across the whole profile: the resistivity of WFEM Sounding varies from 7 to 25.5 Ω·m, while that of MT ranges from 3.5 to 16.5 Ω·m. It can be seen from the resistivity contour cross-sections of WFEM and MT that the resistivity curves are relatively continuous in the horizontal direction and exhibit distinct vertical layering. In the vertical direction, the resistivity values show a variation trend of low resistance →high resistance →low resistance →high resistance from shallow to deep depths (Figure 7).

FIGURE 7

5 Discusion

5.1 Spatial distribution characteristics and controlling factors of the geothermal reservoir

High-resolution inversion results of WFEM Sounding and MT Sounding, combined with data from geothermal wells, have clearly delineated the geological structure of the northern Jiyang geothermal field. From top to bottom, the Quaternary System, the Minghuazhen Formation and Guantao Formation of the Neogene System, and the Dongying Formation of the Paleogene System are identified in the study area (Figure 1c). The Minghuazhen Formation and Quaternary System are dominated by mudstone and silty clay, with a total thickness of 750–1,250 m (Figure 1c); the resistivity of WFEM in this interval is generally lower than 18.5 Ω·m, and that of MT ranges from 4.5 to 8 Ω·m (Figures 6, 7). Its dense lithology and low permeability render it an excellent aquiclude and thermal insulation cap rock. As the main geothermal reservoir distributed throughout the study area, the Guantao Formation has a buried depth of the top boundary ranging from 700 to 1,250 m (Figure 1c); well logging results show that its porosity is 18%–22%, permeability is 50–150 10^-3um², and lateral continuity exceeds 80% (Table 2), providing a fundamental basis for the extensive occurrence of geothermal fluids. In contrast, the Dongying Formation is only concentrated in the northern part of the study area, with a buried depth of the bottom boundary of 1,200–2,000 m (Figure 1c). It is mainly composed of interbedded sandstone and mudstone.

The northwest-dipping normal fault F1, accurately identified in the northwest of the study area, is the core structure governing energy conduction in the geothermal system. This fault has an extension depth of 10–12 km and exhibits a favorable vertical connection with the buried depth of the geothermal reservoir’s bottom boundary (1,200–2,000 m) (Figure 1c). In the vicinity of the fault, the resistivity of MT is significantly lower than that of the country rock; this low-resistivity anomaly originated from the high electrical conductivity of groundwater and increased rock porosity within the fault fracture zone. The corresponding high-value zone of geothermal gradient (4.27 °C/100 m) is in perfect coincidence with the low-resistivity anomaly zone, which confirms that this fault serves as a critical channel for the upward conduction of deep geothermal heat sources (Villalba Ulberich et al., 2026; Xu et al., 2021). The controlling effect of the fault on the geothermal reservoir is also reflected in its impact on reservoir sealability (Yu et al., 2026; Cai, 2008): the northern part is located in the downthrown block of the fault, characterized by large stratigraphic thickness and an intact cap rock (mudstone cap rock thickness > 1,000 m). Fault activity has not damaged the continuity of the cap rock, making geothermal fluids less prone to loss and forming a closed environment with high temperature and high pressure (Zhu et al., 2023). In the southern part, which is the upthrown block of the fault, stratigraphic uplift and denudation have occurred, resulting in a thinned cap rock (mudstone cap rock thickness <500 m). The fracture zone connects to shallow groundwater, weakening the sealing capacity of geothermal reservoirs and lowering the temperature and pressure of geothermal fluids. In addition, the measured water temperature of the Dongying Formation is 68 °C, 8.5 °C higher than that of the Guantao Formation (59.5 °C). This temperature difference stems not only from the disparity in burial depth but also from the thermal conduction effect of faults. Therefore, we consider that faults are an important factor controlling the geothermal reservoirs in this area.

5.2 Abnormal geothermal gradient and significance of well JYB01

Well JYB01 in the study area exhibits significant abnormal geothermal gradient characteristics and is a typical representative of regional geothermal wells. The abnormal geothermal gradient of Well JYB01 is mainly reflected in the obvious vertical temperature difference and the deviation from the regional average geothermal gradient. According to the measured data of Well JYB01, the geothermal gradient of the Guantao Formation of this well is significantly higher than the regional average level, showing obvious abnormal characteristics (Figure 5).

The formation of the abnormal geothermal gradient of Well JYB01 is the result of the synergistic effect of multiple geological factors. Firstly, the average burial depth of the Guantao Formation drilled by Well JYB01 is 1,150–1,470 m in depth, and the temperature increases with the increase of burial depth, which is the basic condition for the formation of the abnormal geothermal gradient of this well. Secondly, advective heat transport via thermal fluid migration along the fracture zone of Fault F1 is a critical factor contributing to the localized abnormal geothermal gradient (Jaupart et al., 2015). Fault F1 near Well JYB01 formed an efficient thermal conduction channel, which allows deep mantle heat flow to be quickly conducted to the Guantao Formation reservoir drilled by Well JYB01, significantly increasing the geothermal water temperature and geothermal gradient of this formation. Thirdly, the sealing conditions of the geothermal reservoirs around Well JYB01 affect heat preservation and geothermal fluid evolution, thereby regulating the formation of the abnormal geothermal gradient. Hydrochemical data of Well JYB01 show that the salinity of geothermal water in the Guantao Formation and the Dongying Formation is 16,099 mg/L and 20,648 mg/L, respectively. Nearly three-fold salinity difference between JYB01 and the two peripheral wells (DS1 and JR025). JYB01 is located on the deep downthrown block of Fault F1, whereas DS1 and JR025 are situated on the upthrown side far from the fault fracture zone. The fault plane and associated damage zone form a low-permeability barrier that partitions the Guantao aquifer into two hydraulically isolated compartments. Sedimentary sand continuity exists, but cross-fault fluid exchange is severely restricted. At the same time, the δD value (−59‰ ∼ −64‰) and δ¹⁸O value (−8.9‰ ∼ −9.3‰) of the geothermal water in this well are close to the isotopic characteristics of deep fluids (Jia et al., 2024). As displayed in Figure 4, both geothermal water samples fall slightly below the LMWL with limited δ¹⁸O depletion, indicating that prominent oxygen isotope exchange is absent in the system. The potential mechanisms behind this phenomenon are extremely high TDS geothermal fluid dominated by NaCl-type brine. High dissolved Na⁺ and Cl⁻ ions reduce mineral surface activity and slow oxygen atom exchange kinetics between silicate/carbonate minerals and pore water, drastically delaying isotopic equilibrium. In addition, the ¹⁴C ages of geothermal water in the Guantao Formation and Dongying Formation are 30,110 years BP and 41,300 years BP, respectively. The measured radiocarbon ages represent upper-bound estimates of groundwater residence times. Therefore, these fluids are paleofluids formed since the Late Pleistocene, which indicates an extremely sluggish fluid circulation within the thermal reservoirs (Tamers, 1975). These integrated geochemical signatures demonstrate that geothermal fluids within the Guantao Formation around Well JYB01 are trapped in a closed geological system. In contrast, affected by the activity of the Qihe-Guangrao Fault, the caprock denudation thickness of the Guantao Formation in the southeastern part is relatively thin, the sealing is weakened, and the heat loss is fast, so the geothermal gradient is lower than that in the northwestern part.

Although continuous steady-state temperature profiles across both Guantao and Dongying Formations are only obtained at Well JYB01, the abnormal geothermal gradient of Well JYB01 has important geological significance and practical value. Firstly, the abnormal geothermal gradient of Well JYB01 records the evolution trajectory of geothermal fluid in a local closed environment (Xu et al., 2021). The high-temperature environment (60 °C ∼ 68 °C) of the Guantao Formation in Well JYB01 accelerates mineral dissolution and ion exchange reactions (Chen and Kanan, 2025), resulting in the relatively higher Ca²⁺/Mg²⁺ ratio and SiO₂ content of the geothermal water, which reflects the synergistic evolution characteristics of “temperature-mineral-ion” in the geothermal system around the well. In addition, Gibbs diagram analysis of the hydrochemical data of Well JYB01 shows that all data points fall in the evaporation-precipitation area (Figure 8), far from the atmospheric precipitation dilution area, indicating that fluid evolution is mainly controlled by evaporation-concentration and mineral dissolution, and the abnormal geothermal gradient provides temperature guarantee for this evolution process (Xu et al., 2021). For example, the Cl⁻ content of the Guantao Formation and Dongying Formation in Well JYB01 is as high as 12,710 mg/L, close to the halite saturation concentration (15,000 mg/L), indicating that halite dissolution is the main source of Cl⁻ (Cipriani et al., 2026), and the high temperature under the abnormal geothermal gradient promotes the dissolution of halite (Li et al., 2025). Secondly, the abnormal geothermal gradient of Well JYB01 provides an important target for regional geothermal resource exploration. The abnormal geothermal gradient of the well indicates that the surrounding area has good heat storage conditions and abundant geothermal resources. The hydrochemical indicators of the well (high salinity, high Cl⁻ and Na⁺ contents) further confirm the good sealing and heat storage capacity of the local geothermal reservoir, which can guide the layout of regional geothermal exploration wells and improve the efficiency and accuracy of resource exploration.

FIGURE 8

5.3 Geothermal potential analysis

Relying on integrated borehole, multi-geophysical, hydrochemical and isotopic datasets, this work systematically constructs a fine geothermal geological model specific to the northern Jiyang geothermal sub-area, and the stratigraphic absence of the Dongying Formation in the central and southern parts constitutes the most prominent regional geothermal geological feature. Combined with the NW-dipping normal fault F1 identified from geophysical prospecting results, the genesis of the heat source of the regional geothermal field has been defined—deep crustal heat is transported upward primarily via advection of geothermal fluids within the fracture network of Fault F1, which acts as a key potential migration pathway for heat-bearing fluids, providing a stable heat source supply for the geothermal field and ensuring the sustainability of geothermal resources (Muñoz Dada et al., 2024). The main geothermal reservoirs in this area are the Guantao Formation of the Neogene and the Dongying Formation of the Paleogene, both of which are porous-fractured sandstone geothermal reservoirs with a porosity of 25 ∼ 30%. The well-developed pores and fractures endow the reservoirs with sufficient heat and water storage capacity, making them the core carrier for the occurrence of regional geothermal resources.

The geothermal gradient exhibits an overall distribution characteristic of being lower in the southeast and higher in the northwest. The average geothermal gradient of the geothermal reservoirs reaches 4.27 °C/100 m, and even geothermal gradient of the Guantao Formation in Well JYB01 hits 5.31 °C/100 m, showing a remarkable geothermal temperature increasing characteristic and being much higher than the regional average geothermal gradient. Based on geothermal field data and interpretation results of geophysical prospecting strata, the northwest part has been accurately delineated as the core target area of geothermal anomaly. This area not only has the highest geothermal gradient in the whole field, but also features the superimposed development of the Guantao Formation and Dongying Formation geothermal reservoirs, moderate burial depth and simple tectonic conditions, resulting in a high enrichment degree of geothermal resources and making it the core area for subsequent geothermal resource development.

Hydrochemical characteristics of the regional geothermal fluid indicate that the geothermal fluid in this area is neutral salt water (pH 7.3 ∼ 7.7) with a total dissolved solids (TDS) content of 5.711 ∼ 20.55 g/L, and its hydrochemical type is SO₄·Cl-Na type, dominated by Na⁺ among cations and Cl⁻ among anions. The geothermal fluid in the Guantao Formation and Dongying Formation has relatively higher TDS content, higher concentrations of beneficial trace elements (Sr and H₂SiO₃) and higher water temperature. Meanwhile, the fluid is rich in beneficial trace elements such as Sr (up to 64.9 mg/L) and H₂SiO₃ (up to 39.3 mg/L), boasting high-value development potential for medical and healthcare, as well as bathing and recreation purposes (Zhu et al., 2020; Gong et al., 2021).

Isotopic dating results show that the geothermal fluid in this area is dominated by paleowater in aquifers, and the δD and δ¹⁸O isotopic characteristics reveal a weak modern recharge capacity. This genetic characteristic puts forward explicit requirements for the subsequent sustainable development of geothermal resources—it is necessary to attach importance to geothermal reinjection to achieve the extraction-reinjection balance of resources (Yang et al., 2025; Zhu et al., 2025).

6 Conclusion

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