Borehole temperature measurement is the most effective and direct means of obtaining the present-day temperature field. Systematic steady-state temperature measurement refers to temperature measurement performed after the temperature in the borehole has reached the in-situ temperature of the formation. The drilling process severely affects the borehole temperature, mainly in the circulation of drilling fluid and frictional heat generation. Therefore, a considerable period is needed to recover the formation temperature from the end of drilling to the time of temperature measurement. Yu (1991) estimated the time required for recovery of drilling disturbance temperature: 0.5–1.5 times and 10–20 times of drilling time is required to recover 90 and 99% of formation temperature, respectively.

Although temperature measurements are required in most completion reports of a borehole, the vast majority of tests are conducted within 2 days of completion and cannot meet the requirements for the regional thermal regime studies. The temperature measurement curves of Borehole 016 (1500 m; drilling time, 37 days; mud circulation time, 2 days) in the NJB at 35 days post-stoppage, 23 days post-completion, and 45 days post-completion shows that the temperature at the neutral point gradually decreases at the shallow end and increases at the deep end as the time of static well progresses (Figure 2A). The ground temperature gradient at 45 days post-completion is about twice the initial measured ground temperature gradient and combined with the drilling time, the temperature data is usually considered as steady-state temperature measurement data. More recently, the steady-state temperature measurement conditions can be achieved after tens of days of static wells. As shown in Figure 2B, the deep temperature measurement curves of Borehole 004 (200 m; drilling time, 7 days; no mud circulation) do not differ much between 15 days and 9 months after completion, as such, the temperature measurement after 15 days of completion has reached the steady-state temperature measurement condition. In summary, drilling depth, drilling time, mud circulation time, and completion process closely affect the judgment of the steady-state condition of drilling temperature.

Geothermal work in the NJB began in the 1970s when oil companies conducted systematic temperature measurements on several exploration wells located in the southern NJB. Wang (1989) and Wang et al. (1995) collected steady-state temperature data from 21 wells and studied the geothermal field in the NJB in conjunction with the hydrostatic reservoir temperatures of 161 wells. The distribution of heat flow measurement points in the NJB and adjacent areas is generally uneven and very sparse (Figure 1).

From 2018 to 2020, temperature logs of 104 boreholes in the NJB and its adjacent areas were acquired, and the representative temperature profiles are shown in Figures 3A–G. Additionally, in this study, we collected 385 drill stem tests (DST) data from 58 boreholes in the NJB (Figure 3H). Temperature records below the water level of the borehole better reflect the true temperature of the formation, which exclude the effect of the thermal conductivity difference between air and water. We determine the water level of the borehole which is usually less than 40 m in the NJB (Figure 3) on the basis of the vertical variation of temperature, When the depth is greater than 50 m, the temperature increases linearly or nearly linearly, which is a conduction-type temperature measurement curve. Based on the temperature data, the temperature gradient of each log was calculated using the least-squares method, ranging from 22 to 59°C/km, with an average temperature of 32 ± 6°C/km for the NJB (weighted average based on the area of each tectonic unit).

The vast majority of the DST data are from the Dongtai Depression. A large number of test oil temperatures show that the temperature gradually increases with increasing depth. Moreover, the DST data are divided into three geothermal gradient sections (Figure 3H): 0–1500 m, 1500–3000 m, and >3000 m. The gradient of 0–1500 m and 1500–3000 m are about 24 and 28°C/km, respectively. As for the section deeper than 3000 m, the temperature gradient is up to 38°C/km. The distribution of the temperature gradient and depth (Figure 3I) reveals that the gradient gradually increases with depth in the shallow part (below 1000 m), and the average value of the gradient increases from 23°C/km to about 30°C/km. From 1000 m to 1800 m, the change of the gradient approximately repeats the above trend, increasing from 23°C/km to 31°C/km. The geothermal gradient stabilizes from a depth of 1800 m to about 3500 m, with the average value fluctuating around 30°C/km. The gradient tends to increase gradually at a depth of 3500 m, and the average value of the temperature gradient reaches 40°C/km at a depth of 4100 m.

Based on the non-D quality data (heat flow can be classified into A,B,C and D categories according to the quality, where A data is the best and D data is for reference only) collected from the basin and neighboring areas (Jiang et al., 2019), we mapped the temperature gradient above the Yancheng Formation (N_1-2 yn, inclusive) in the NJB by Kriging interpolation method (Figure 4) (Stein, 1999). The east-central NJB and the eastern edge of Dongtai depression are high gradient areas on the plane, with an average geothermal gradient value higher than 35°C/km. The average temperature gradient of the NJB, the Dongtai depression, the Yanfu depression and Jianhu uplift are 31 ± 6°C/km, 39°C/km, 30°C/km and 39°C/km, respectively. Notably, it is crucial to compare and study the temperature gradient over the region with specific layers in mind.

Vertically, there is a general trend of gradual increase from top to bottom based on the statistical results of single wells. The geothermal gradient in the NJB increases with depth, and the gradient in Ny, E₂s-E₂d is generally lower than that in the E₁f-K₂t. The Neoproterozoic and Quaternary formations are characterized by a low temperature gradient of 15–30°C/km, probably as a result of the influence of lithology and groundwater activity in the shallow formations. The high sandy content of the Ny, E₂s, and E₂d formations leads to high stratigraphic thermal conductivity and low gradient, resulting in generally lower gradient in the shallow. The Paleocene and Upper Cretaceous rocks have relatively high mud content, with gradient values between 30 and 40°C/km.

Rock thermal physical parameters are the basis for conducting regional geothermal field studies. Since 1986–1988, outcrop and core samples have been collected three times in the NJB to test the thermal conductivity of some strata (Wang, 1989; Wang et al., 1995). During 2018–2020, the Institute of Geology and Geophysics, Chinese Academy of Sciences and Geological Survey of Jiangsu Province systematically collected a large number of rock samples in the geothermal study of the NJB and carried out a series of thermal conductivity tests, which greatly improved the regional thermal conductivity column. Because of the large stratigraphic span and uneven distribution, the thermal conductivity values compiled in this study were combined with previous research results (Wang, 1989; Wang et al., 1995) to obtain weighted average values of thermal conductivity for different strata based on the number of test blocks. The thermal conductivity test results are shown in Table 2 (the thermal conductivity values are those of the dry rock samples measured at room temperature).

The results show that the average rock thermal conductivity the strata in the region fluctuates widely, and the minimum value in the test results is the Quaternary clay and sand with thermal conductivity of 0.6 W/m/K; the maximum value is the Silurian S₂m¹ with thermal conductivity of 7.99 W/m/K (Figure 5A). In general, the thermal conductivity shows a gradual decreasing trend from old to new with the stratigraphy. The thermal conductivity of the Paleozoic strata is generally higher than 3.0 W/m/K; the Mesozoic strata have the next highest thermal conductivity, mostly 2.0–3.0 W/m/K; and the Cenozoic strata have the lowest thermal conductivity, between 1.5 and 2.5 W/m/K. The stratigraphic thermal conductivity of the NJB can be roughly divided into two different layers, namely, the shallow low thermal conductivity layer and the deep high thermal conductivity layer. The thermal conductivity of the shallow Cretaceous-Quaternary is low (<2.7 W/m/K), especially thermal conductivity of the Late Paleogene-Quaternary is mostly less than 2.0 W/m/K, which can provide good cover for heat preservation. The thermal conductivity of the deeper Ediacaran-Jurassic is relatively high, generally greater than 3.0 W/m/K, especially the thermal conductivity of the Silurian-Devonian and Ediacaran-Cambrian can exceed 4.0 W/m/K, and the thermal conductivity of the Late Ediacaran Dengying Group can exceed 6.0 W/m/K, all of which can be used as good thermal conductivity and thermal storage layers. The thermal conductivity of magmatic rocks gradually increase with the change of composition from basic to acidic, and the thermal conductivity of medium-acidic to basic magmatic rocks is low overall, especially the thermal conductivity of Cenozoic basic volcanic rocks is comparable to that of Cenozoic sedimentary cover.

The most important factor affecting large differences in thermal conductivity is the difference in lithology, while other influencing factors such as the degree of metamorphism and porosity (Figure 5B) also have a large effect on thermal conductivity, and the effects of temperature, pressure, and anisotropy on thermal conductivity should not be ignored (Wang, 2020; Wang et al., 2021b).

In this study, the radioactive element contents of 47 samples were tested and compiled for U, Th, and K. The method proposed by Rybach (1976) was used to calculate the stratigraphic heat production. The content of U and Th was measured by ICP- MS, and the content of K was measured using atomic absorption spectroscopy. The test and summary results are shown in Table 2.

The results show that the average heat production of the rocks of all rock types in the varies little between 1.1 and 2.7 mW/m³, with the lowest heat production in the Middle Proterozoic Picheng Group and the highest heat production in the fourth section of the Paleogene Fu’ning Formation. The difference in heat production is closely related to lithological changes, and rocks with more silt and mud content tend to have larger heat production. Therefore, more work on heat production test work should be carried out to obtain precise constraints on the heat flow contribution and deep temperature variation in different depths of the strata.

The characteristics of the deep temperature field are an important basis for studying both the thermal structure of the lithosphere and the deep geothermal resources. The parameters that determine the temperature distribution at different depths z include surface temperature T₀, surface heat flow q₀, rock thermal conductivity K, and rock heat generation rate A. The surface temperature can be obtained from the multi-year surface average temperature; the surface heat flow comes from the drilling temperature and rock thermal conductivity measurements; and the thermal conductivity and heat production of the rocks come from the latest results of this update and compilation. Based on the previously published and newly added heat flow values of the NJB and its adjacent areas, the kriging interpolation method was used to interpolate the values in the whole area, with one point taken at 50 km, and a total of 480 interpolation points were taken in the study area.

Under the condition of one-dimensional steady-state heat conduction, for the distribution area of uniformly layered rocks, the thermal conductivity and heat production within a single layer can be approximated as constants (the measured averages are obtained according to different lithologies), and the corresponding deep temperature can be calculated by the following equation: T ( z ) = T 0 + ( q 0 · z ) / K - ( A · z 2 ) / 2 K (1)

Where T ₀ is the temperature of the constant temperature zone, °C; q ₀ denotes the surface heat flow, mW/m²; K represent the thermal conductivity of certain crustal layer, W/m/K; A is the heat production of each layer, mW/m³.

Based on Fourier’s law, the temperature distribution at different depths of the NJB was calculated using parameters such as surface temperature, heat flow, rock thermal conductivity, and heat production, as shown in Figure 7. The favorable distribution area of geothermal resources in the NJB is wide, and its high temperature areas are mainly distributed in the Jinhu sag and its northern margin, the east-central part of Jianhu uplift, and the eastern margin of Dongtai depression, which reach 80°C at 2 km and about 200°C at 5 km.

Since the “one uplift and two depressions” pattern in the NJB charactered the NEE-SWW spreading, we choose the profiles that are perpendicular to it when simulating the 2D temperature field. Accordingly, we choose three profiles, BB′, CC′, and DD’ in Figure 1, to simulate the two-dimensional temperature field. To reduce the influence of the shallow thermal refraction and the setting of the bottom boundary heat flow on the results, we performed the simulations at a depth of 20 km.

The bottom boundary heat flow was assigned in segments based on the combination of the regional average heat flow value and the measured heat production. For each different strata or rock body, the assignment of thermal physical parameters, including the density, specific heat capacity, thermal conductivity and heat production of rocks, is also required. The initial values of the temperature in the thermostatic zone of the profile model were set regarding the annual average temperature of the province (Liu et al., 2003), and adiabatic boundaries were set at on both sides of the profile. Therefore, the temperature profiles of different profiles have certain differences in the three profiles. In order to more clearly portray the variation of the regional temperature field, we use different scale bars for the 2D temperature field profiles.

In summary, we simulated the temperature distribution at different depths on the profiles using parameters such as surface temperature, rock thermal conductivity, heat generation rate, and so on, and continuously optimized the parameters to match the observed heat flow using an interactive method. Finally, we obtained three large-scale 2D temperature field simulations (Figure 8).

The heat flow map of the NJB (Figure 6) shows that the there is a strong convergence of heat flow at the uplifts or highs, i.e., a significant thermal refraction effect occurs, with higher heat flow values relative to the depressions and sags (Xiong and Zhang, 1988; Wang et al., 2015). Two-dimensional temperature field simulations of the BB′ profile (Figure 1) reveal that the area where the isothermal surface uplift occurs is concentrated in the southern depressions, which does not correspond significantly to the convergence of heat flow. Heat flow distribution and two-dimensional temperature field simulation of the profiles CC′ and DD’ (Figure 1) show that the uplifts and highs also have a strong heat flow convergence. However, the area where the isothermal surface is uplifted occurs in the Dongtai depression in the south and the Yanfu depression in the north, rather than concentrated in the Jianhu uplift. Besides, we plotted the temperature variation at 0.5 and 4.5 km depths using profile CC’ as an example, as shown in Figures 8G,H. The trends of the two in the lateral direction indicate a very clear mirror image phenomenon, manifesting the variability of the lateral heat transfer of thermal conductivity at different depths. This aspect will be discussed in detail in the genesis mechanism section.

To check the reliability of the simulation results, we selected some boreholes adjacent to the simulated section and carried out a comparative analysis between the simulated and measured values. The analysis shows that most of the temperature profiles obtained from the simulations have a good fit with the measured data (both in the temperature gradient and the bottom-hole temperature). For example, the overall deviation of borehole B018 (Figure 8J) is about 1%, while this deviation is approximately 6% in Borehole Milu (Figure 8K). In addition, the preliminary logging work (bottom-hole temperature) on the borehole SR1 (Figure 1) similarly confirmed the reliability of the simulation results.

The polarization of the present-day geothermal background of the East Asian sedimentary basins (Figure 1A) seems to imply that they experienced different tectono-thermal events. As one of the most critical “hot” (≥65 mW/m²) and oil-bearing basins in the Yangtze Craton, the thermal evolution and dynamical background of the NJB is a scientific issue of interest.

Since the Mesozoic, the subduction of the Izanagi plate led to dehydration/decarbonization of the overlying plate, and the strong unsteady flow of the big mantle wedge caused the craton destruction in East China, peaking at the end of the Early Cretaceous (Zhu et al., 2012; Zhu et al., 2017), accompanied by strong convective erosion and peridotite-melt interactions (He, 2014; Zhang, 2005), while the upper crust underwent large-scale extensional tectonic deformation (Lin and Li, 2021; Lin and Wei, 2018) (Figure 9C). Large-scale magmatism from 135–120 Ma enriched mantle sources existed in the Lower Yangzi Craton (Wu et al., 2005; Wu et al., 2019) (Figure 9B). Compared with the ancient craton, the lithosphere of the Lower Yangtze Craton is getting weaker and warmer. However, the dramatic increase in surface heat flow often lags behind significant erosion and magmatism (He, 2014). Thermal history studies of oil boreholes in Jurong and Taixing show that the surface heat flow reached a maximum of 85–93 mW/m² at about 128 Ma (Figure 9A). During the Late Cretaceous, trench retreat and slab reversal occurred, often accompanied by lithospheric extension and re-emergence of magmatism (Sun et al., 2008) (Figure 9B), as well as partial tectonic deformation (biotite ⁴⁰Ar-³⁹Ar age) (Lin and Li, 2021) (Figure 9C). Cenozoic basalts are identified as coming from a barren mantle, reflecting lithospheric thinning (Xu, 2001; Xu et al., 2009). During the Eocene and Oligocene, surface heat flow in the NJB reached 78–86 mW/m² (Figure 9A), as evidenced by apatite fission track and vitrinite reflectivity data (Ro) (Zeng, 2005). After 30–40 Ma, the thickness of the lithosphere increased to varying degrees, mainly as a result of thermal decay (Xu, 2001). Thereafter, the NJB gradually enters the thermal subsidence process in the late extensional stage, characterized by a gradual decrease in surface heat flow and lithospheric temperature. Numerical simulations by He (2015) show that the difference between the thickness of the thermal and seismic lithosphere (rheological boundary layer) gradually decreases from west to east, indicating that the viscosity coefficient of the large mantle wedge gradually decreases to the east, and suggesting that this is the result of subduction and dehydration of the western Pacific plate. Therefore, we considered the effect of this result in the schematic diagram of the evolution of the thermal structure of the NJB lithosphere (Figure 9E).

Based on the above analysis, we suggest that the thermal control mechanism of the Lower Yangtze Craton, including the NJB, at the lithospheric scale, can be summarized as “two stages” thermal concentration (Figure 10).

Temporally, during the Late Mesozoic (ca. 140 Ma) (Figure 10A), the Izanagi plate subducted beneath the East Asian big mantle wedge, the trench retreated and the slab rollback, causing strong unstable flow in the overlying mantle, resulting massive magmatic activity and extensional tectonic deformation in the shallow (Zhu et al., 2017), and the craton destruction in East China led to a huge thinning of the lithosphere thickness of the Lower Yangtze Craton.

Since about 60 Ma (Figure 10C), the Pacific plate gradually succeeded the Izanagi plate and influenced the tectonic evolution of East Asia (Zhu et al., 2017). The subduction of the Pacific plate in the NW or W direction led to the previous Izanagi plate to gradually subduct into the mantle. Due to the presence of the mid-ocean ridge connecting two oceanic plates (Izanagi plate and Pacific plate), which may have caused regional extension of the lithosphere above the plate window (Zhu et al., 2017), the tectono-thermal event (45–60 Ma) of this period was recorded in the Lower Yangtze Craton under the influence of the subduction of the mid-ocean ridge. As the age of the subducted oceanic crust became older, high-angle subduction led to trench retreat and slab rollback, and regional extensional tectonics became more intense during the Oligocene-Miocene, along with large number of basaltic rocks forming in the back-arc region (Xu, 2001; Niu, 2005). Besides, it has been suggested that the Cenozoic volcanism and high heat flow in the basins of East China are mainly due to the back-arc extension caused by the subduction of the western Pacific plate (He, 2015; Liu et al., 2016; Qiu et al., 2016) (Figure 10D).

Most studies support that during this period, the Lower Yangtze experienced an extensional tectonic setting. Even though there was no magmatism during part of the period (46–56 Ma) (Wu and Wu, 2019; Liu et al., 2020), the high heat flow corresponding to the extensional setting has been widely accepted (Currie, 2004). Unlike central China, such as the Shanxi Rift System, which may has also been affected by subduction of the mid-ocean ridge and the Pacific plate (Liu et al., 2017; Su et al., 2021), the lithospheric thickness of the Lower Yangtze Craton has been substantially thinned due to the effects of Late Mesozoic craton destruction. Overall, the high heat flow in East China may be controlled by the combination of lithospheric extension, magmatic activity, and so on (Lei, 2012; Lei et al., 2013).

The influence of stratigraphic relief on the crustal-scale temperature field is mainly reflected in the difference of rock thermal conductivity (similar to the fact that water always preferentially transfers to the direction of lower resistance), resulting in the redistribution of heat in the lateral direction or the thermal refraction effect.

In this study, the thermal conductivity of Paleozoic and Ediacaran rocks in the NJB is much greater than that of sedimentary layers since the Mesozoic, and even much greater than that of Neogene rocks (Table 2). This makes the geothermal field not all converge toward the shallow uplift area, but is related to the stratigraphic thickness and distribution scale of the Paleozoic and Ediacaran strata. The larger the scale of the Paleozoic and Ediacaran strata with high thermal conductivity layers, the greater this control factor. Therefore, from the upper crustal scale, the Dongtai depression on the south side of Jianhu uplift has the strongest controlling effect on the regional geothermal field.

On the other hand, the control aspect of the temperature field. As shown in Figure 8I, high thermal conductivity rocks have higher thermal conductivity compared to nearby low thermal conductivity mudstones and sandstones, and heat will be preferentially transferred in the high thermal conductivity strata or rocks during the upward heat transfer. In the shallow section (about <3 km), heat converges from the depression to the uplift; in the deep section (about 3 km deeper), the situation is reversed, heat converges from the uplift to the depression. In detail, it can be generally divided into three parts: the first, (<1 km) uplift area with high temperature; the second, (about 2–3 km) depression area with high temperature; and the third section, (about 3.5–5 km) depression area with significantly higher temperature. Based on the above measurements and simulations, it is not difficult to derive the “sweet spots” of geothermal resources at different scales in the NJB.