High-Quality Heat Flow Determination from Coastal Areas of Fujian Province, China

Abstract

The coastal region of Fujian is part of the southeastern hilly and coastal geothermal anomaly zone and is a significant high-heat-flow area in mainland China. However, existing heat flow data suffer from low quality due to insufficient calculation depth and inadequate corrections; this hinders theoretical research and geothermal resource exploration in the region. This study obtained high-quality geothermal heat flow data through steady-state temperature measurements in six boreholes and thermal conductivity tests on 79 core samples collected in situ. The results indicate that the average heat flow values from four newly analyzed representative boreholes (HDR1, LJSZ, JM-1, DS) are 65.2 mW/m², exceeding the mainland Chinese average of 60.5 mW/m², highlighting the region’s high heat flow characteristics. This finding corroborates previous high-heat-flow measurements in the coastal area of Fujian and aligns with earlier studies. The research fills a gap in providing high-quality heat flow data for the region, enhances our understanding of its thermal state, and is significant for studying the distribution and development of geothermal resources in the area.

1. Introduction

The coastal region of Fujian is situated within the Southeast Hills–Coastal Geothermal Anomaly Zone, representing a significant area of geothermal activity and constituting one of the high terrestrial heat-flow regions along the continental margin of South China [1]. As early as the 1980s and 1990s, extensive geological and geothermal investigations were conducted by researchers in the Zhangzhou Basin of this region. Subsequently, with advances in geothermal research, it has been determined that the Southeast Hills–Coastal Geothermal Anomaly Zone extends in a northeast–southwest direction, with heat flow values exceeding 70 mW/m², forming distinct high-heat-flow closures such as the Wuchuan–Sihui anomaly and the Southeast Coastal anomaly. Within these geothermal anomaly zones, peak terrestrial heat flow values have been recorded up to 220 mW/m² [2,3,4,5]. The geothermal anomalies of the Southeast Hills–Coastal region exhibit a strong correlation with crustal radiogenic heat production [6]. Previous studies on radiogenic heat production in various tectonic units across South China have identified characteristic values of approximately 2.8 μW/m³ in the southern domain, 2.1–2.8 μW/m³ in the eastern domain, and 2.8 μW/m³ in the northern domain [7,8,9]. Additionally, regional geothermal anomalies are significantly influenced by tectonic activity associated with fault structures [7]. Multiple elevated heat-flow anomalies (>80 mW/m²) occur in a bead-like distribution pattern aligned along the Wuchuan–Sihui deep fault zone. Furthermore, the region is characterized by a high density of hot spring occurrences, indicating abundant geothermal resources.

Compared to China’s inland regions, the southeastern coastal area has a relatively larger number and wider spatial distribution of heat-flow measurement sites, making it one of the regions in mainland China with the most extensively studied terrestrial heat flow. According to the most recent fourth edition of compiled heat-flow data, more than ninety heat-flow measurement points have been documented along the southeastern coastal region of China, which, tectonically, belongs to the Cathaysia orogenic belt (also referred to as the Wuyi–Yunkai orogenic belt). Nevertheless, these existing heat-flow measurements exhibit certain limitations; most of the data were obtained from mining or hydrological boreholes, typically at shallow depths ranging from several meters to tens of meters. Although groundwater corrections were applied to earlier data, the quality of these heat-flow data remains low. For instance, a reported heat-flow value of 220 mW/m² falls into category D data, indicating low reliability and accuracy. Recently, deep drilling activities have been initiated in this region, with an emphasis on exploring deep geothermal resources and hot dry rock potential, thereby enabling more accurate deep borehole heat-flow measurements [10,11]. Specifically, for the coastal region of Fujian, preliminary analyses based on existing data suggest relatively high terrestrial heat-flow values. However, fundamental geothermal parameters remain incomplete and geothermal datasets are not fully comprehensive in certain areas. Therefore, acquiring high-quality heat-flow data for this region is essential, as these data will provide crucial guidance for geothermal resource exploration, evaluation, and sustainable development.

In this study, steady-state temperature measurements were conducted in six boreholes, and thermal conductivity analyses were performed on 79 newly collected core samples from three of these boreholes. By integrating these measurements with previously collected thermal conductivity data from borehole cores within the region, we calculated the corresponding terrestrial heat flow values. These newly obtained heat flow results were subsequently compared with earlier published data from the same area. Through the acquisition of newly measured high-quality terrestrial heat flow data, this study fills critical gaps in the Fujian coastal region and enhances the geothermal database. It also provides robust data support for the exploration and evaluation of medium-to-high temperature geothermal resources in the southeastern coastal region of China. Additionally, the research attempts to analyze the causes of thermal anomalies in high-heat-flow areas, thereby promoting the development and utilization of geothermal resources in southeastern China.

2. Geological Setting

Fujian Province lies at the tectonic boundary between the southeastern Eurasian Plate and the Philippine Sea Plate. Its coastal region, situated along the active continental margin of the western Pacific Ocean, is renowned for intense tectonic and magmatic activity. In the Middle Jurassic, oblique northward subduction of the Pacific Plate resulted in the uplift of the Fujian coastal region from shallow marine conditions to terrestrial environments. Subsequently, during the early Late Jurassic, the subduction direction of the Pacific Plate rotated, shifting to a westward and frontal subduction mode, which triggered extensive magmatic intrusion and localized volcanic eruptions along coastal Fujian. By the mid-Cretaceous, subduction of the paleo-Pacific Plate had gradually weakened, leading to a cessation of magmatic activity and transitioning the region into an intracontinental extensional environment characteristic of a passive continental margin. Since the Cenozoic, renewed subduction processes have commenced with the oceanic crust east of Taiwan. The Philippine Sea Plate, subducting northwestward beneath the Ryukyu Arc at an angle of approximately 45°, has produced a nearly vertical plate boundary beneath the Taitung Longitudinal Valley [12]. Consequently, the coastal region of Fujian Province has experienced significant compressional stress from the southeast. The Coastal Range in eastern Taiwan represents the geological expression of this northwest-directed subduction and westward compressional interaction of the Philippine Sea Plate. After experiencing intense Mesozoic magmatic and tectonic deformation, the coastal area of Fujian has generally remained within a passive continental margin characterized by intracontinental extension. Since the Cenozoic, the intersection of intracontinental extension derived from the passive margin of South China and compressional stresses emanating from the Philippine Sea Plate beneath Taiwan and the Taiwan Strait has resulted in a northwest-trending, obliquely upward-thrusting, shovel-shaped thrust fault system [13,14].

Based on sedimentary structures, orogenic belts, intricate NE- and NW-trending fault systems, and widespread magmatic and metamorphic processes, the tectonic framework of Fujian Province can be delineated into two principal tectonic units separated by the Lishui–Zhenghe–Dapu Fault Zone: the Cathaysia Block and the Southeast Coastal Volcanic–Magmatic Belt (Figure 1). In fact, Fujian Province is characterized by an intricate fault network consisting of fifteen intersecting NE- and NW-trending faults. Four major NE-trending deep-seated faults controlled multiple episodes of magmatic intrusion and volcanic eruption. The continental margin region between the Pacific Plate subduction zone and the Cathaysia Block has experienced significant Cenozoic tectonic activity, with recent geological manifestations including frequent microseismic events and occasional strong earthquakes, accompanied by abundant geothermal indications and crustal uplift [15,16,17].

3. Heat Flow Determination

3.1. Borehole Temperature

High-quality heat flow values are indispensable for investigating the thermal regime of a region, including its lithospheric characteristics [19]. Continuous and precise temperature measurements obtained from boreholes, combined with laboratory-determined thermal conductivity values from drill cores, represent two ideal prerequisites for accurately determining terrestrial heat flow density [20,21]. Additionally, sufficient time must elapse between the cessation of drilling operations and subsequent temperature measurements to ensure that borehole temperatures have adequately recovered from disturbances induced by drilling [22]. Only under these conditions can the measured data be considered suitable and reliable for heat flow calculations.

When obtaining steady-state temperature data, conventional borehole temperature measurement instruments include maximum (mercury) thermometers and thermistor-based sensors. The former, known for their portability and stability, record the maximum temperature encountered but require pressure corrections. The latter category encompasses copper resistance, platinum resistance, semiconductor thermistors, quartz crystal, and thermocouple sensors, each with distinct advantages and limitations. Modern borehole temperature measurements emphasize continuous deep-well recording and extended temperature ranges, with platinum resistance thermometers being preferred due to their stability, despite challenges in frequent resistance–temperature calibration.

This study employs a borehole temperature logging system developed by Pioneer Petrotech Services Inc., Calgary, AB, Canada (Figure 2). The system features a PPS71 probe containing a platinum resistance circuit, with a response time of 0.1 s and the ability to operate continuously for four hours at extreme temperatures up to 350 °C (662 °F). The performance and operational principles of this device are the same as those described by Wang et al. (2020) [23]. The resolution and accuracy of the temperature measurements are ±0.01 °C and ±0.1 °C, respectively [23]. The system is equipped with a PPS36 portable depth recorder, allowing for the real-time monitoring of depth, speed, and tension during wireline operations. The temperature probe, combined with a 5000 m wireline winch, meets the requirements for temperature measurements across all stratigraphic units in the study area. The system’s maximum descent speed exceeds 20 m/s, with operational speeds adjusted to 6–7 m/s during this study to ensure data quality, while the retrieval speeds ranged from 15–20 m/s.

When selecting boreholes, we coordinate with relevant units or construction parties to gather basic information such as well coordinates, drilling depth, drilling schedule, rest time, and stratigraphic lithology. For steady-state temperature field measurements, the logging vehicle and equipment are first positioned near the borehole. The process begins with data recording as soon as the temperature probe enters the wellhead. The probe records temperature and pressure data, which are transmitted in real time to the display, as well as data about the cable tension and descent speed. To ensure data quality, the probe is lowered at an average speed of 6–7 m per minute, approximately 400 m per hour. During the measurement, the researchers remain by the logging equipment to operate the winch, control the probe’s descent speed, and monitor temperature and pressure data via the display to ensure that the probe descends smoothly without obstruction. If complex conditions arise, such as a borehole collapse, the descent may need to be adjusted or halted. Upon reaching the bottom, the probe is retrieved with careful attention to the “tension” data to avoid sudden increases indicating potential obstructions. After completing the temperature measurements, we collect core and outcrop samples for thermal property testing in the laboratory.

3.1.1. Temperature Logging

The thermal stabilization periods during actual temperature measurements in this study are detailed in

Table 1. Borehole specifications.

Well	Longitude (°N)	Latitude (°E)	Depth (m)	Test Depth (m)	Water Surface Depth (m)	Stabilization Period (Months)
ZK02	118.0313759	24.39676251	858.00	620.00	10.50	36
HDR1	117.7381504	24.3719023	2980.00	2975.00	65.00	57
LJSZ	118.0487962	24.23562462	1870.00	1830.00	22.00	86
JM-1	118.0783775	24.60604684	2000.00	2000.00	10.00	30
DS	117.4820691	23.73851644	800.00	710.00	1.00	51
ZA	117.1395296	23.7589869	1080.00	1020.00	0.80	34

, with these durations substantially exceeding the time required for drilling operations. Given the relatively small diameters of the boreholes investigated and the high thermal conductivity of the bedrock, the temperature logs obtained from the boreholes provide an accurate representation of equilibrium thermal conditions.

Borehole ZK02 is situated within the Zhangzhou Merchants Port area, which is predominantly characterized by biotite monzogranite in its lower strata, exhibiting grain sizes ranging from fine to medium-coarse textures. The lithological sequence is interspersed with quartz porphyry and diabase units. The lower formations are marked by well-developed fracture zones, which serve as favorable reservoirs and migration pathways for geothermal fluids, facilitating the accumulation of subsurface thermal energy.

Borehole HDR1 is located in an intermontane basin, overlain by Quaternary deposits and volcanic cover sequences and underlain by Yanshanian granitic formations. Prominent surface thermal anomalies are observed northwest of the borehole, approximately 17 km from the Zhangzhou geothermal field, with the Longhai Dongsongling hot springs located about 9 km to the southeast.

Borehole LJSZ is positioned southwest of Longhai Longjiao Township, where northwest-trending fault systems are prominently developed. These structural features are manifested as dense fracture networks at the surface, accompanied by granitic porphyry and quartz vein intrusions, forming distinct northwest-oriented topographic depressions.

Borehole JM-1 is located in Jimei District, Xiamen City, where the stratigraphy is relatively simple, comprising mainly Jurassic Lishan and Nanyuan formations, overlain by Quaternary deposits. The area is influenced by regional tectonics, exhibiting NE-trending, NW-trending, and approximately E–W-trending fault systems.

Borehole DS is situated on Dongshan Island, at the intersection of the Pingtan–Dongshan NE-trending deep fault zone and the Shanghang–Yunxiao major fault zone. The surrounding terrestrial area exhibits significant geothermal activity. A series of NW-trending reef formations are distributed along the tidal flats, with corresponding NW-trending faults dipping NE. The Chen Dai Yuanqian hot springs are exposed north of these faults, while the DS borehole is located in the low-lying area of Zhangtang Village along the fault’s extension.

Borehole ZA is positioned in the southern part of Xitan Township, Zhao’an County, within a granite terrain lacking significant cover layers. The area shows numerous surface geothermal manifestations, with abundant hot spring occurrences.

The measured temperature curve is shown in Figure 3.

As illustrated in Figure 3a, the temperature profile of Borehole ZK02 can be segmented into three distinct intervals. The first interval, extending from the water table at 10.5 m to 280 m, exhibits a temperature increase from 22.99 °C (with a local minimum of 28.2 °C at 25.5 m due to atmospheric influence) to 26.57 °C, followed by an accelerated thermal gradient between 280 m and 300 m. The second interval, spanning 300 m to 506 m, shows a gradual temperature rise from 27.89 °C to 28.68 °C, with another thermal acceleration between 506 m and 508 m. The third interval, from 508 m to 620 m, demonstrates a temperature increase from 29.02 °C to 29.71 °C. The observed thermal accelerations are attributed to well-developed fracture zones, facilitating the upwelling of deeper geothermal fluids.

Borehole HDR1, with a water table at 65 m and a total depth of 2975 m, exhibits a clay overburden extending to 36 m, underlain by biotite granodiorite. The temperature profile (Figure 3b) displays excellent linearity, increasing from 22.89 °C (minimum at 80.6 m) to 83.49 °C.

Borehole LJSZ, featuring a 22 m water table and 1830 m depth, contains basaltic residual clay and semi-consolidated weathered boulder–cobble layers interbedded with sand down to 21 m, underlain by coarse-grained biotite granite. The thermal profile (Figure 3c) shows consistent linearity, with temperatures rising from 24.09 °C (minimum at 31.5 m) to 60.96 °C.

Borehole JM-1, with a 22 m water table and 2000 m depth, demonstrates a well-defined thermal gradient (Figure 3d), increasing from 22.81 °C (minimum at 39.5 m) to 65.25 °C.

Borehole DS, characterized by a 5 m water table and 710 m depth, exhibits a stable thermal profile (Figure 3e), with temperatures rising from 23.03 °C (minimum at 19.0 m) to 36.05 °C.

Borehole ZA, with a shallow water table at 0.8 m and 1020 m depth, shows a convex thermal profile (Figure 3f) within granitic formations, increasing from 23.30 °C (minimum at 13.5 m) to 52.56 °C. The convex curvature indicates convective heat transfer, suggesting the fracture-controlled upwelling of high-temperature fluids, consistent with surrounding thermal manifestations.

According to the temperature measurement results, the depth of the four wells in this steady-state temperature measurement exceeded 1000 m. The shallow thermal regime is significantly influenced by surface temperatures, with the measured temperatures initially decreasing with depth due to elevated surface conditions during measurement, reaching approximately 23 °C before transitioning to a linear conductive gradient. The newly acquired temperature profiles generally exhibit an excellent linear correlation with depth, indicative of conductive heat transfer. However, Borehole ZA demonstrates a convective profile, which is significantly influenced by groundwater circulation patterns.

3.1.2. Temperature Gradient

The geothermal gradient represents the rate of temperature change with depth below the Earth’s subsurface neutral zone, typically expressed as the temperature increase per 0.1 km or 1 km of depth. Under conductive heat transfer conditions, the geothermal gradient remains relatively stable. However, on larger spatial scales or at greater depths, such as within the solid lithosphere, the geothermal gradient is not constant and tends to decrease with increasing depth, although this trend is influenced by the structural and compositional characteristics of the lithosphere or crust. Geothermal gradients are valuable for studying geological structural features and play a significant role in understanding the formation and distribution of mineral resources, including oil, gas, and geothermal energy.

Using the linear least squares regression method [24,25], the temperature gradients for six boreholes were calculated with a depth interval of 20 m. The selected calculation intervals and corresponding temperature gradient variations are shown in Figure 3 and

Table 2. Calculation range and geothermal gradient.

Well	Longitude (°N)	Latitude (°E)	Calculation Range (m)	Geothermal Gradient (°C/km)
ZK02	118.0313759	24.39676251	25.50–620.00	11.32
HDR1	117.7381504	24.3719023	80.00–2975.00	20.92
LJSZ	118.0487962	24.23562462	100.00–1830.00	20.33
JM-1	118.0783775	24.60604684	100.00–2000.00	21.87
DS	117.4820691	23.73851644	120.00–710.00	20.13
ZA	117.1395296	23.7589869	13.50–1020.00	29.20

.

Borehole ZK02 exhibits a temperature gradient range of 0.83–64.47 °C/km. The temperature profile reveals abrupt increases at 280 m and 506 m, which can be attributed to hydrothermal fluid influx through fractures. Excluding these anomalies, the temperature profile approximates a uniform gradient. The selected calculation interval spans 25.5 m to 620 m, yielding an estimated average geothermal gradient of 11.32 °C/km.

Borehole HDR1 demonstrates a temperature gradient range of 13.32–29.05 °C/km. The gradient stabilizes below approximately 80 m, with the calculation interval set at 80 m to 2975 m. The gradient slightly increases with depth, resulting in an estimated average of 20.92 °C/km.

Borehole LJSZ shows a temperature gradient range of 17.88–23.66 °C/km. The gradient stabilizes below approximately 100 m, with the calculation interval spanning 100 m to 1830 m. The gradient exhibits a slight increase with depth, yielding an estimated average of 20.33 °C/km.

Borehole JM-1 has a temperature gradient range of 18.55–23.66 °C/km. The gradient stabilizes below approximately 100 m, with the calculation interval set at 100 m to 2000 m. The gradient increases slightly with depth, resulting in an estimated average of 21.87 °C/km.

Borehole DS exhibits a temperature gradient range of 11.87–21.32 °C/km. The gradient stabilizes below approximately 120 m, with the calculation interval spanning 120 m to 710 m. The estimated average geothermal gradient is 20.13 °C/km.

Borehole ZA demonstrates a temperature gradient range of 11.55–50.42 °C/km. The convex temperature profile indicates convective heat transfer in this shallow borehole. The calculation interval spans 13.5 m to 1020 m, yielding an estimated average gradient of 29.2 °C/km.

3.2. Thermal Conductivity Measurement

Rock thermal conductivity represents the ability of rocks to conduct heat, defined as the amount of heat passing through a unit area per unit time under a temperature gradient of 1 °C per unit length along the direction of heat transfer, with units of W/(m·K).

The optical scanning method [26] was employed for the rock thermal conductivity measurements in this study. This method offers several advantages: high measurement speed (10–20 samples per hour); no need for sample cutting or polishing; flexible operations, as the instrument can be transported to core repositories for on-site testing; continuous testing capabilities, enabling the measurement of thermal conductivity for a large number of cores in a single session, thereby reducing systematic errors caused by different methods or time intervals; non-destructive testing (a layer of water-based black paint 1 cm wide and 20–45 μm thick is applied to the sample surface to eliminate laser reflection during scanning, which can be washed off with water after testing without damaging the sample); and the ability to quantitatively assess the anisotropy and heterogeneity of rock thermal conductivity.

The instrument used was a thermal conductivity scanning (TCS) device manufactured in Germany, with a measurement range of 0.2–25 W/(m·K) and an accuracy of ±3%. Samples were prepared in advance by cutting to ensure a flat surface, with surface irregularities of the core or outcrop cross-sections and longitudinal sections within 1 mm, and a length of approximately 5 cm. The actual testing process for thermal conductivity is shown in Figure 4. Once the optical heat source scans the sample, heat penetrates the sample perpendicular to the scan line. The measured thermal conductivity corresponds to the heat conduction direction perpendicular to the scan line, which, in this case, is aligned along the core axis. Scanning each flat surface provides the thermal conductivity distribution along the scan line and the average thermal conductivity in the perpendicular direction.

3.2.1. Measured Thermal Conductivity

A total of 79 samples, all of granitic lithology, were collected for this study. From Borehole ZK02, 37 samples were obtained from the surface to the bottom at intervals of 10–50 m. From Borehole DS, 27 samples were collected from the surface to the bottom at intervals of 20–30 m. From Borehole ZA, 15 samples were acquired from the surface to the bottom at intervals of 50–100 m.

The measurement results are presented in

Table 3. Thermal conductivity.

Number	Well	Depth (m)	Measured Thermal Conductivity W/(m·K)	Corrected Thermal Conductivity W/(m·K)
01	ZK02	40	3.33	3.62
02	ZK02	50	3.152	3.41
03	ZK02	60	3.673	4.04
04	ZK02	73	3.273	3.56
05	ZK02	80	3.466	3.79
06	ZK02	90	3.229	3.51
07	ZK02	100	3.228	3.51
08	ZK02	110	3.496	3.83
09	ZK02	120	3.806	4.21
10	ZK02	130	3.524	3.87
11	ZK02	140	3.468	3.80
12	ZK02	150	3.784	4.19
13	ZK02	160	3.191	3.47
14	ZK02	170	3.656	4.03
15	ZK02	180	3.581	3.94
16	ZK02	190	3.429	3.76
17	ZK02	200	3.483	3.82
18	ZK02	210	3.332	3.64
19	ZK02	220	3.732	4.13
20	ZK02	230	3.504	3.85
21	ZK02	240	3.924	4.36
22	ZK02	250	3.922	4.36
23	ZK02	260	3.774	4.18
24	ZK02	283	3.956	4.40
25	ZK02	296	3.195	3.48
26	ZK02	350	3.331	3.65
27	ZK02	400	3.552	3.92
28	ZK02	450	3.61	3.99
29	ZK02	500	3.123	3.40
30	ZK02	550	3.004	3.26
31	ZK02	600	3.451	3.81
32	ZK02	650	3.291	3.60
33	ZK02	700	3.216	3.52
34	ZK02	750	3.457	3.79
35	ZK02	800	3.582	3.94
36	ZK02	850	3.096	3.38
37	ZK02	858.23	3.125	3.41
38	DS	15	3.233	3.51
39	DS	30	3.28	3.56
40	DS	50	2.972	3.19
41	DS	73	2.746	2.93
42	DS	99	3.413	3.73
43	DS	120	2.767	2.95
44	DS	145	3.322	3.62
45	DS	170	3.074	3.33
46	DS	200	3.206	3.49
47	DS	224	2.884	3.10
48	DS	255	2.994	3.24
49	DS	280	2.784	2.98
50	DS	315	2.792	2.99
51	DS	340	2.796	3.00
52	DS	375	2.478	2.61
53	DS	410	4.195	4.72
54	DS	436	3.68	4.09
55	DS	455	3.535	3.92
56	DS	480	2.567	2.73
57	DS	504	3.586	3.99
58	DS	530	2.648	2.83
59	DS	562	2.618	2.79
60	DS	595	3.172	3.48
61	DS	620	3.09	3.38
62	DS	650	2.671	2.87
63	DS	680	2.939	3.20
64	DS	710	2.544	2.71
65	ZA	350	3.501	3.92
66	ZA	550	2.976	3.28
67	ZA	600	2.988	3.30
68	ZA	650	2.937	3.24
69	ZA	700	3.026	3.36
70	ZA	750	3.501	3.98
71	ZA	795	2.586	2.79
72	ZA	850	2.978	3.31
73	ZA	900	2.683	2.93
74	ZA	950	2.617	2.84
75	ZA	1000	3.1	3.48
76	ZA	1050	3.326	3.65
77	ZA	1100	2.979	3.25
78	ZA	1150	3.484	3.83

. The thermal conductivity values range from 2.48 to 4.19 W/(m·K). Specifically, the thermal conductivity measurements for Borehole ZK02 range from 3.00 to 3.96 W/(m·K), with an average of 3.46 W/(m·K). For Borehole DS, the measurements range from 2.48 to 4.19 W/(m·K), with an average of 3.04 W/(m·K). For Borehole ZA, the measurements range from 2.59 to 3.61 W/(m·K), with an average of 3.09 W/(m·K).

3.2.2. Corrected Thermal Conductivity

Thermal conductivity is temperature dependent, and we use the following empirical formula for correction [27]:

$$k_{\left(0\right)}=k_{\left(25\right)}\{1.007+25\times [0.0037-0.0074/k_{\left(25\right)}]$$

(1)

$$k_{\left(T\right)}=k_{\left(0\right)}\{1.007+T\times [0.0037-0.0072/k_{\left(0\right)}]$$

(2)

where T represents the in situ formation temperature in °C, while k₍₀₎ and k₍₂₅₎ denote the thermal conductivity at 0 °C and 25 °C, respectively, in units of W/(m·K).

These two empirical relationships can be used to correct all room-temperature values for crystalline rocks up to 300 °C [28,29]. In this study, the temperature range falls between 0 °C and 100 °C, ensuring the validity of these equations without introducing significant errors. The measured and corrected values are presented in Table 3, with a maximum correction of approximately 11%. Statistical analysis of the thermal conductivity data was conducted, and the distribution of thermal conductivity is illustrated in Figure 5.

Rock thermal conductivity is also influenced by other factors, such as pressure, porosity, and fluid saturation. For the boreholes studied, our core samples consist of granitic rocks with relatively low measured porosity (<1%). Consequently, no significant difference is expected between the thermal conductivity values in dry and water-saturated states [30]. Additionally, based on our estimates using the Chapman and Furlong [27] equation, the pressure effect is less than 1%. Therefore, corrections for water saturation and pressure were omitted for these samples.

Thermal conductivity data for core samples from Boreholes HDR1, LJSZ, and JM-1 were obtained from the literature [31]. For Borehole HDR1, the average thermal conductivity of 25 samples was 3.00 W/(m·K). For Borehole LJSZ, the average thermal conductivity of three samples was 3.18 W/(m·K). For Borehole JM-1, the average thermal conductivity of the core samples was 3.13 W/(m·K).

3.3. Heat Flow Determination

Terrestrial heat flow refers to the amount of heat conducted to the Earth’s surface per unit time and per unit surface area, measured in mW·m⁻². As a comprehensive thermal parameter, heat flow values provide a more robust representation of a region’s thermal state compared to individual parameters such as rock thermal conductivity or geothermal gradient. High-quality heat flow data are essential for studying contemporary geothermal fields and lithospheric thermal structures [32,33].

This study employs the segmented method to calculate terrestrial heat flow values. This method involves calculating the geothermal gradient using temperature–depth data within the specified depth intervals of a borehole and then multiplying it by the representative rock thermal conductivity for the corresponding depth range. Under steady-state conductive heat transfer conditions, the terrestrial heat flow value is numerically equal to the product of the geothermal gradient and rock thermal conductivity, with the negative sign indicating the direction of heat conduction:

$$Q=-K{\displaystyle \frac{\mathrm{d}\mathrm{T}}{\mathrm{d}Z}}$$

(3)

where Q represents the terrestrial heat flow value in mW·m⁻², K denotes the rock thermal conductivity in W·m⁻¹·K⁻¹, and dT/dZ represents the geothermal gradient.

The heat flow was calculated by multiplying the least squares temperature gradient by the thermal conductivity. For depth intervals without samples, thermal conductivity values were estimated using the linear interpolation of adjacent samples with the same lithology. All thermal conductivity values for crystalline rocks were corrected for temperature. The calculated heat flow values for all boreholes are presented in

Table 4. Average temperature gradient (G), average thermal conductivity (K) and heat flow (Q).

Well	Calculation Range (m)	Lithology	Geothermal Gradient (°C/km)	Average Thermal Conductivity (W m−1 K−1)	Heat Flow (mW m−2)
ZK02	25.50–620.00	Granite	11.32	3.80	43.0
HDR1	80.00–2975.00	Granite	20.92	3.00	62.8
LJSZ	100.00–1830.00	Granite	20.33	3.18	64.6
JM-1	100.00–2000.00	Granite	21.87	3.13	68.5
DS	120.00–710.00	Granite	20.13	3.29	66.2
ZA	13.50–1020.00	Granite	29.2	3.41	99.6

. The data in the table indicate that Boreholes ZK02 and ZA are significantly influenced by groundwater activity, exhibiting anomalous geothermal gradients and thermal anomaly zones. The remaining boreholes yielded high-quality terrestrial heat flow values, with an average of 65.2 mW·m⁻².

4. Discussion

4.1. Determination of High-Quality Heat Flow Values

Previous studies have reported only 16 terrestrial heat flow values for the entire Fujian region. This study contributes new heat flow data, significantly enriching the geothermal dataset for this area. The four newly obtained representative heat flow values average 65.2 mW·m⁻², which is higher than the continental average of 60.5 mW·m⁻² for mainland China. This confirms the previously observed high heat flow values in the coastal region of Fujian and is consistent with earlier findings.

We also found that the newly acquired high-quality heat flow values are slightly lower than the previously reported average of 74.6 mW/m². The earlier data, consisting of 16 records, were mostly of B-class or lower quality. This underscores the need for deep borehole measurements to obtain high-quality heat flow data for more accurate geothermal theoretical research. According to the fourth and fifth editions of the Heat Flow Compilation for Mainland China, the classification criteria are as follows: A-class data: static well time greater than 0.5 times the drilling time; measurement depth ≥200 m; at least 10 temperature measurements; continuous steady-state temperature measurement method; calculation interval ≥50 m; thermal conductivity testing with ≥2 in-situ core samples or ≥10 samples from the same stratigraphic rocks; in-situ thermal conductivity correction. B-class data: static well time ≥48 h; measurement depth 100–200 m; 5–10 temperature measurements; single-point temperature measurement method; calculation interval 30–50 m; thermal conductivity testing with ≥1 in-situ core sample or ≥5 samples from the same stratigraphic rocks; partial thermal conductivity correction. C-class and D-class have lower requirements.

In 1993, researchers published 16 heat flow data points for the Fujian area, derived from mining and hydrogeological boreholes, which were mostly classified as B-class or lower. The specific reasons for this classification include: (1) shallow measurement depth, not exceeding 200 m; (2) a lack of continuous temperature measurement equipment at the time, resulting in single-point measurements; and (3) calculation intervals of less than 50 m, leading to shorter sections for geothermal gradient calculations. These factors collectively contributed to the classification of the data as B-class or lower-quality data.

In contrast, the new data points meet the following criteria for high-quality measurements: (1) thermal equilibration periods significantly exceed drilling durations; (2) continuous steady-state temperature measurements = conducted at depths ≥200 m, generally reaching approximately 1000 m, with two boreholes exceeding 2000 m; (3) calculation intervals that are much longer than 50 m; (4) thermal conductivity measurements that were performed on original location core samples and corrected for original location conditions; (5) the four representative boreholes are sufficiently deep to minimize the influence of shallow factors (Boreholes ZK02 and ZA meet other criteria but are more strongly affected by shallow factors).

Based on these findings, we conclude that the average heat flow value for the coastal region of Fujian, represented by the new high-quality data, is 65.2 mW·m⁻². This indicates that the region remains an area of medium-to-high heat flow within China, characteristic of a “hot basin”. However, the average values are slightly lower than the previously reported 74.6 mW/m², which merits attention beyond data quality considerations. This discrepancy might be due to the location at the structural edge near the land–sea interface, where interactions between groundwater and seawater could transport heat away from the shallow subsurface. Unfortunately, the complex interactions between these factors and many unknown parameters make it difficult to quantify their contributions to the observed heat flow values. Further geological studies and thermal modeling are necessary to better understand these effects.

4.2. Analysis of the Causes of Geothermal Anomalies

The surface geothermal anomalies observed in the coastal region of Fujian Province result from the combined effects of deep and shallow heat sources. The deep thermal influence is primarily attributed to deep-seated heating from the southeast. In this study, Boreholes LJSZ, HDR1, JM-1, and DS, which penetrate the crystalline basement, all exhibit heat flow values higher than the continental average of 60.5 mW·m⁻². Influenced by the subduction of the Pacific Plate, the coastal region of Fujian and the Taiwan Strait represent one of the most complex tectonic units in eastern China. Heat from the deep high-temperature magmatic sources associated with the circum-Pacific tectonic system migrates upward along faults, contributing to the formation of geothermal anomaly zones [34,35]. The deep geodynamic setting of these anomalies aligns with the broader geodynamic framework of eastern China and the South China continental margin [36].

At shallow depths, the relatively high heat flow is partly attributed to the radioactive elements in granitic rocks. The coastal region of Fujian is a significant granitic distribution area in China, with granites rich in radioactive elements such as U, Th, and K. Previous studies indicate that the mantle heat flow in the Fujian–Guangdong coastal region is relatively low, while the crustal heat flow is high. The elevated crustal heat flow is primarily due to heat generation from shallow radioactive elements, accounting for over 40% of the total surface heat flow or more than 60% of the crustal heat flow [37,38].

Additionally, convective heat transfer caused by upwelling geothermal fluids contributes to the thermal anomalies. In this study, Boreholes ZK02 and ZA were significantly influenced by groundwater activity, primarily because the study area hosts numerous hot springs, most of which are medium- or low-temperature hydrothermal convection systems controlled by faults. The extensive fault networks and their intersections create structurally weak zones that facilitate the heating and upward migration of groundwater, forming conductive structures for heat transfer.

5. Conclusions

This study has made progress in understanding the thermal state of the coastal region of Fujian, part of the southeastern coastal geothermal anomaly zone in China. By conducting steady-state geothermal logging, thermal conductivity testing, and reliable geothermal heat flow calculations, we provide high-quality heat flow data for this tectonically active area. The main findings and their significance are summarized as follows:

1. By employing continuous steady-state temperature measurements and in situ rock thermal conductivity corrections, we determined high-quality geothermal heat flow values for the Fujian coastal region. The four representative values are 62.8 mW/m² (HDR1), 64.6 mW/m² (LJSZ), 68.5 mW/m² (JM-1), and 66.2 mW/m² (DS), with an average of 65.2 mW/m², exceeding the mainland Chinese average of 60.5 mW/m². This confirms previous characterizations of the area as a “thermal basin”. Notably, all newly determined heat flow data are A-class, enhancing the data quality for both the Fujian and southeastern coastal regions and providing a more precise understanding of the area’s thermal state.

2. The high surface heat flow in the Fujian coastal area results from the combined effects of deep and shallow thermal processes. Deep heating is primarily influenced by the subduction of the Philippine Sea Plate beneath the Eurasian Plate, facilitating upward heat transfer through an active fault system and introducing mantle heat into the crust. Shallow heat contributions mainly derive from the radioactive decay of uranium, thorium, and potassium in granites, accounting for over 40% of the surface heat flow. Hydrologically driven convection, evidenced by temperature profiles in the ZK02 and ZA boreholes, amplifies local heat conduction through fault-controlled hydrothermal systems.

3. This study fills a gap in the heat flow data for the Fujian coastal region, enriching the geothermal database and providing data support for geothermal resource evaluation and target selection. The area is characterized by extensive fault development and numerous hot spring manifestations, with high-temperature geothermal resources significantly influenced by surface structures and groundwater. Future research should focus on the spatial distribution and scale of thermal anomalies, with priority given to developing hydrothermal-type geothermal resources.