Discussion on Deep Geothermal Characteristics and Exploration Prospects in the Northern Jiangsu Basin

Abstract

The Northern Jiangsu Basin (NJB), located at the northeast edge of the Yangtze block, is not only rich in oil and gas resources but also contains abundant geothermal resources. Nevertheless, the distribution of geothermal resources at medium depth in the NJB is still unclear due to its complex geological structure and tectonic–thermal evolution process, which restricts its exploitation and utilization. The characteristics of the geothermal field and distribution of geothermal reservoirs within the NJB are preliminarily analyzed based on available temperature measurements and geothermal exploration data. The prospective areas for the exploration of deep geothermal resources are discussed. The analysis results show that (1) Mesozoic–Paleozoic marine carbonate rocks are appropriate for use as principal geothermal reservoirs for the deep geothermal exploration and development within the NJB; (2) the geothermal field is evidently affected by the base fluctuation, and the high-temperature area is mainly concentrated at the junction of the Jianhu uplift and Dongtai depression; (3) the southeast margin of Jinhu sag, Lianbei sag, the east and west slope zone of Gaoyou sag, the low subuplifts within the depression such as Lingtangqiao–Liubao–Zheduo subuplifts, Xiaohai–Yuhua subuplifts and the west of Wubao low subuplift, have good prospects for deep geothermal exploration.

1. Introduction

With the growing global population and economic development, energy demand is rapidly increasing. The increasing focus on environmental quality and the continuous advancements in scientific and technological capabilities have led to a growing awareness of the significance of renewable, clean energy [1]. In the future, with the support of policies and the maturity of technology, renewable clean energy is bound to become one of the major energy sources in the world. Due to the advantages of being clean, stable, and flexible, geothermal resources are considered as a kind of energy with significant potential development in the future [2]. However, at present, the utilization of geothermal resources is mostly concentrated in the middle shallow layers [3]. Making full development and utilization of geothermal resources’ potential means drilling deeper into unexplored high-temperature resources. The Enhanced or Engineering Geothermal System (EGS) is a significant method for exploiting and utilizing deep, high-temperature geothermal resources, particularly hot-dry rock resources [4].

The NJB is an important oil and gas production area in eastern China. In addition, the NJB also boasts abundant geothermal resources, not only medium–low-temperature hydrothermal geothermal resources in the shallow layer but also high-temperature geothermal resources in the deep layer [5]. The drilling data reveal that the temperature can reach 155 °C at a depth of 4700 m [6,7]. The available subsurface geology and geophysics data indicate the potential presence of anomalously high temperatures within the NJB [7]. The NJB, however, has undergone repeated tectonic superimposition of extrusion and extension, resulting in dramatic horizontal changes in the distribution of thermal storage. The geothermal conditions exhibit noticeable variations across different tectonic units, posing substantial constraints on exploring and utilizing deep geothermal resources.

Based on the regional geological and geophysical exploration data, a large number of drilling lithology and temperature measurement data in the NJB, this paper comprehensively analyzes geothermal geological conditions, reveals the distribution of deep geothermal reservoirs and the characteristics of the geothermal field, discusses the genetic model of geothermal resources in the basin, and optimizes the geothermal exploration prospective area by means of the spatial superposition of the buried depth of favorable reservoirs and temperature, providing a useful reference for the subsequent exploration and development of the NJB.

2. Geothermal Geological Conditions

2.1. Geological Background

Located on the northeast edge of the Yangtze plate, the NJB is the onshore part of the northern Jiangsu-South Yellow Sea Basin. This region is bounded by the Lusu uplift to the north, the Tongyang uplift to the south, the Pacific subduction zone to the east, and the Tanlu fault to the west (Figure 1a). The NJB is a rift basin situated above the Paleozoic Lower Yangtze depression area, encompassing an approximate area of 34,000 km². The NJB can be tectonically divided into one uplift and two depressions: Yanfu depression, Jianhu uplift, and Dongtai depression from north to east. Each depression comprises several northeast-trending subuplifts and sags (Figure 1c and Figure 2).

The NJB was a foreland basin before the late Jurassic. The basin has been predominantly influenced by the tectonic activity of the Western Pacific Plate, marking the onset of the late tectonic evolution stage since the Mesozoic and Cenozoic eras. Since the late Indosinian period, the NJB mainly experienced tectonic events, including the late Cretaceous Yizheng movement (83 Ma), Eocene Wubao movement (54.9 Ma), Eocene Zhenwu movement (50.5 Ma) and Oocene Sanduo movement (38~24.6 Ma) [9]. After undergoing multiple periods of extensional faulting, compressive uplift, and other tectonic evolution activities, as well as sedimentation [10], the present characteristic of “one uplift and two depressions” was ultimately formed.

The NJB has developed a complex fault system due to multi-stage tectonic shifts, including tension faults, strike-slip faults, and associated faults influenced by the Tanlu strike-slip fault. The basement faults generally exhibit a NE-NEE direction [11], which indicates a large-scale occurrence and is characteristic of long-term activity. These faults are closely associated with the origin and distribution of geothermal resources. In addition, the NE-NEE faults play a crucial role as the primary conduits for magma, exerting control over large-scale magmatic activity in the basin.

2.2. Distribution Status of Geothermal Resources

The NJB is an important region of high heat flow anomalies in eastern China (Figure 1b), characterized by abundant geothermal resources [12]. The presence of hot springs from the emergence of thermal groundwater is limited within the basin. Up to now, numerous geothermal wells have been developed, which have primarily focused on meeting the demands of hot spring tourism. The wells are mainly distributed in the Yancheng area of the Jianhu uplift, the Xuyi area of Laozishan, Baoying, and Yangzhou areas of the Dongtai depression, as well as Taizhou and other regions. (Figure 1c). The outlet water temperature of geothermal wells typically falls within the range of 43–63 °C (Figure 3). The maximum water output can reach 3936 m³ a day. The geothermal well with the highest outlet temperature is located in Qili Village, reaching 93 °C, followed by Xiaoyangkou, Laozi Mountain, Baima Lake, etc.

3. Characteristics of Deep Geothermal Heat Resource

3.1. Heat Resource Types

The overlying sedimentary covers of the NJB mainly consist of Cretaceous-Quaternary continental clastic rock deposits, which can be categorized from bottom to top as follows: the Taizhou Formation (K₂t), Funing Formation (E₁f), Dainan Formation (E₂d), Sanduo Formation (E₂s), Yancheng Formation (N₂y), and Dongtai Formation (Qd), with the absence of Oligocene strata [10]. The basement is primarily composed of three types, namely, the Middle Proterozoic metamorphic rock basement, Mesozoic–Paleozoic marine basement, and middle and upper Triassic-lower Cretaceous continental–marine transition facies, which are continental clastic rocks with an active continental margin type, and an intermediate acidic volcanic rock basement [13]. Most existing middle shallow geothermal reservoirs within the NJB are Paleogene–Neogene sandstone and part of shallow buried Sinian–Ordovician carbonate rocks. Deep high-temperature geothermal resources primarily occur in the underlying basement rock layers.

Geophysical exploration results indicate that the depth of basement burial in the Yancheng–Baoying area, located in the central part of the NJB, is relatively shallow (<2000 m). In contrast, it is slightly deeper (2000 m~4000 m) in the Binhai–Funing area and generally deeper (4000 m~7600 m) in the Jinhu–Gaoyou–Haian area situated to the south of the basin. The areas with greater depths are primarily concentrated within the central depression and along the downslopes of major fractures, where depths exceed 5000 m (Figure 2).

3.2. Invasive Rock Mass

Magmatic rocks, predominantly consisting of igneous rocks, are widely distributed in the NJB. Intrusive rocks were mainly formed during the Yanshanian and Himalayan periods, mainly as dikes and sills controlled by basement fault zones [14]. The thickness typically ranges from several to dozens of meters, with the maximum thickness exceeding 100 m.

The invasive rocks formed during the Himalayan period are predominantly composed of mafic rock types, such as diabase, with an invasion depth ranging from 500 to 3200 m and covering an area exceeding 1000 km² [14]. There are two peak periods that occur at 58.0 Ma and 42.5 Ma, respectively [15]. During the Yanshanian period, medium acidic rocks, such as granite and diorite, predominantly occurred as veins, exhibiting sporadic distribution within the Xuyi tectonic belt and its surrounding areas. The intrusive rock is most extensively distributed in the Gaoyou sag, followed by the Jinhu and Qintong sags, while being scattered in the Hai’an sag. On the whole, although the intrusive rocks in the NJB are widely distributed, they primarily consist of dike and dyke intrusions with shallow intrusion depths and small thicknesses. On the other hand, the intrusive rocks are dominated by basic rocks with low heat conductivity and high hardness. Therefore, the intrusive rocks are not conducive to the ideal geothermal reservoir for deep geothermal development.

3.3. Mesozoic–Paleozoic Marine Carbonate Rocks

The lower Yangtze block has witnessed the deposition of marine carbonate rocks and clastic rocks, with a thickness ranging up to 10,000 m, from the Upper Sinian period to the Middle Triassic period [16]. As a significant component of the lower Yangtze block, the marine carbonate rocks in the pre-Mesozoic period exhibit extensive distribution and remarkable thickness within the NJB, which has been substantiated by deep well findings (Figure 4). The carbonate rocks are generally characterized by low porosity and low permeability [17]. The porosity ranges predominantly from 0.82% to 2.09%, and the permeability is below 0.1 mD [18]. Meanwhile, abundant secondary cavities, fissures, and fractures are present within the carbonate formations adjacent to the unconformity surface [19,20]. The characteristics of marine carbonate rock within the NJB make it an ideal geothermal reservoir for geothermal development. The carbonate formations can be divided into three sets based on their age, from old to new, namely the Sinian–Ordovician carbonate construction (Z+∈+O), the Permian–Carboniferous carbonate construction (C-P), and the Triassic carbonate construction (T) (Figure 5).

(1)

Sinian–Ordovician carbonate construction (Z+∈+O)

The Sinian carbonate is mainly composed of dolomite and dolomitic limestone, while Cambrian carbonate is predominantly composed of dolomite, calcareous dolomite, dolomitic limestone, and limestone, which is interbedded with shale and carbonaceous. The middle-lower Ordovician carbonate mainly comprises dolomite, dolomitic limestone, and bioclastic limestone. In contrast, the upper Ordovician carbonate is mainly composed of mudstone and calcareous mudstone, interbedded with shale and silty-fine sandstone. According to the seismic interpretation and limited drilling exposure (with some areas unexplored), the apparent thickness of Dengying in the Sinian period ranges from 0 to over 1500 m, the apparent thickness of the Cambrian period ranges from 0 to over 1750 m, and the Ordovician period ranges from 0 to over 1250 m. The structural units control the buried depth of early Paleozoic and Sinian carbonate rocks, exhibiting an increasing trend from uplifted areas to subuplifts and, subsequently, sags. The Sinian formation is primarily distributed in the southwestern region of Xuyi County (mostly exposed), as well as in the Guannan–Huaiyin area (buried at depths ranging from 1500 to 4000 m) and the eastern part of Hai’an (buried at depths ranging from 1500 to 3000 m). Cambrian rock is primarily distributed in Yangzhou–Xinghua (buried at depths ranging from 2500 to 7000 m), Taizhou–Anfeng (buried at depths ranging from 2000 to 4000 m), and Huai’an subuplifts (buried below 4000 m). The Ordovician is mainly distributed in Lianshui–Binhai (buried at depths ranging from 1000 to 4000 m) and north of Baoying (buried below 1000 m).

(2)

Permian–Carboniferous carbonate construction (C-P)

The Permian-Carboniferous strata consist primarily of dolomitic limestone, bioclastic limestone, and limestone interbedded with mudstone and shale. These formations are predominantly found in the Jinhu–Liubao region (at depths ranging from 3000 to 6000 m), Yancheng–Dafeng region (at depths ranging from 1000 to 4500 m), and Qintong–Dongtai region (at depths ranging from 3000 to 8000 m).

(3)

Triassic carbonate construction (T)

Triassic carbonate is mainly composed of limestone, dolomite, shale, and mudstone interbedded with gypsum, and the strata are mainly distributed in Jiangdu–Jiangyan–Hai’an–Xiaohai (buried at depths from 2000 to 7000 m) and Funing–Sheyang (buried at depths from 2000 to 6000 m).

The Mesozoic–Paleozoic marine carbonate formations, which serve as excellent geothermal reservoirs, are extensively developed in the NJB. These reservoirs typically have a burial depth of over 3000 m in low subuplifts, while in sags, the burial depth of their top surface is significantly greater, often exceeding 4500 m [17]. The Zhen1 well in the Gaoyou sag and the Sure1 HDR exploration well in Xinghua utilize this stratum as a heat reservoir. However, noticeable local variations are observed due to the deep burial depth of the Sinian and Lower Paleozoic strata and the impact of multi-stage tectonic superposition, such as the late thrusting and extension. Bounded by the Huaiyin–Dongtai fault, the denudation and deformation strength of the marine carbonate rocks exhibit significant spatial variations, with strong values in the west and north and weak values in the east and south [21]. The complex fault system and tectonic evolution processes make the pre-Mesozoic marine carbonate thermal reservoir deep in NJB and show the complex distribution characteristics of being “small, broken and scattered” [22]. The distribution of the reservoir remains uncertain due to these factors.

4. Geothermal Characteristics

4.1. Heat Flow Characteristics

The NJB is an area of high geothermal activity in eastern China, with a heat flow value exceeding 65.0 mW·m⁻² and an average value of 67.1 mW·m⁻² [6]. The statistical analysis of heat flow values within the basin reveals that the central and eastern sections of Jianhu uplift, the northern section of Jinhu sag, and the eastern edge of Dongtai depression exhibit relatively high heat flow rates, with an average heat flow exceeding 70 mW·m⁻². The Huai’an subuplift exhibits the highest average heat flow, reaching 73.1 mW·m⁻², followed by the Jianhu uplift, the Sujiazui subuplift, and the Yuhua subuplift [6]. The heat flow in the uplifted area is higher than in the depressed area, indicating the significant control effect of basement fluctuation on heat flow within the basin. Additionally, some of the subuplifts and low subuplifts within the depression exhibit elevated heat flow rates, and some even exceed those observed in the Jianhu uplift. This suggests that these subuplifts within the depression may also represent favorable areas for deep geothermal resources.

4.2. Distribution of the Geothermal Field

According to the analysis of the collected drilling temperature measurement data, the overall geothermal gradient ranges from 2.8 to 3.1 °C/100 m, with a sediment cover geothermal gradient ranging from 2.2 to 3.6℃/100 m, and the carbonate heat resource is about 2.1 °C/100 m (Figure 6). The shallow temperature field within the basin (<3500 m) exhibits a characteristic of “low in sag and high in subuplift”, with higher temperatures observed in the uplift and subuplift areas compared to the slope and depression areas at the same depth [23]. The statistics show that the temperature of the basin at depths of 1000 m, 2000 m, and 3000 m is about 36–57 °C, 59–88 °C, and 85~116 °C, respectively. The high temperature in the deep is mainly concentrated within the Jinhu sag and its northern edge, the junction between the middle-eastern Jianhu uplift and Dongtai depression, and the eastern margin of the Dongtai depression. The average geothermal gradient value can reach up to 35 °C/km. It is estimated that the maximum temperature at the depth of 5 km is up to 200 °C [6].

Additionally, the local area within the NJB is clearly governed by the north–east fault structure. As a result of convection’s influence, regions with abnormally high temperatures are formed in shallow depths, such as Yancheng and Baoying.

5. Discussion

5.1. Thermogenic Background

The subduction and retreat of the Pacific plate during the Paleocene–Oligocene period resulted in the stretching and thinning of the basin and mantle upwelling, which constituted the primary dynamic background for deep, high-temperature geothermal activity in the sedimentary basin of eastern China (Figure 7). The Moho is deeply buried in the central uplift area of the NJB, while it is shallowly buried in the depression. The burial depth ranges from 27 to 35 km, predominantly below 31 km, exhibiting a predominant east–west orientation that reflects the concave and convex pattern of the basin. The Moho depth near Gaoyou–Liuhe–Tianchang and Huaian–Lianshui exhibits a shallow burial with a crustal thickness of less than 28 km. Conversely, the Moho in the central and eastern basin near Baoying–Yancheng, as well as Taizhou in the southern basin, displays burial depths exceeding 34 km.

The Curie surface serves not only as the lower boundary of demagnetized magnetic rocks but also as a significant isothermal interface (~560 °C) in the investigation of the lithospheric thermal structure. The buried depth can reflect the regional temperature to a certain extent. The NJB is a local anomaly area with shallow burial depth in eastern China [24]. Typically, the burial depths of the Curie surface range between 20 and 34 km. The presence of a shallowly buried Curie surface and the occurrence of commonly developed low-speed and high-conductivity layers suggest the existence of high-temperature geothermal resources in the NJB. It shows the characteristics trending in the northeast, with shallow characteristics common in the southwest and relatively deep areas in the north and east. The buried depth of the Curie point isotherm is relatively shallow in the Tianchang–Gaoyou area, with the shallowest buried depth being less than 16 km. The northern part of the basin, adjacent to the coastal uplift and the eastern coastal areas, exhibit relatively greater depths, typically exceeding 28 km. The high conductivity layers in Jinhu–Baoying–Huaian and Dafeng–Dongtai–Jiangyan exhibit a relatively shallow burial depth, ranging from 8 to 10 km. The buried depth of the high conductivity layers in Gaoyou, northeast of Funing, southeast of Sheyang, and east of Dafeng is slightly deeper, ranging from 10 to 12 km.

5.2. Heat Control Factors

The NJB is an asymmetric fault basin formed by the lower Yangtze block under the influence of a single shear tension background. The Sanduo event during the middle–late Paleozoic period was a regional thermal uplift event in the basin, which reached its maximum heat flow and was accompanied by minor extrusion along the NE-SW direction. At this point, the evolution of the basin basically ended. The basin subsequently transitioned into the Yancheng–Dongtai depositional stage, characterized by a period of subdued crustal extension activity and thermal decay [25]. The long-term thermal decay process gradually transformed the high-temperature geothermal distribution, with the structure being dominated into a “concave and convex” thermal physical property difference as the primary factor controlling heat (Figure 7). Therefore, the area with a shallower Moho may not necessarily correspond to the region with the most intense tension. The delineation of the abnormal thermal area, based solely on the buried depth of the Moho surface, may deviate from the actual conditions. According to the steady-state heat transfer theory, it can be inferred that the uplifted area is expected to exhibit higher temperatures and heat flow in the shallow layer (3–4 km), while the depressed area may have elevated temperatures in the deeper layers. This observation is further supported by the drilling temperature measurement curve [26]. The formation of local geothermal fields within the NJB is controlled by deep-seated faults and groundwater activities. The intersection of fracture systems is a favorable area for geothermal fields.

Therefore, on the basis of understanding the knowledge of predecessors, we supposed that the crustal extension and thinning, caused by the Cenozoic subduction and retreat of the western Pacific plate, provide the deep dynamic background for the formation of high-temperature geothermal energy in the NJB. The lateral variation in thermal properties caused by the concave and convex patterns of the shallow crust is the primary factor influencing the specific distribution of high-temperature geothermal resources (Figure 7).

5.3. Deep Geothermal Exploration Prospective Area

In light of the statements identified above, we propose that marine carbonate rocks should be prioritized as the primary target reservoirs for deep geothermal exploration and development in the NJB. According to the analysis of temperature and reservoir information revealed by geophysical research and drilling, the following can be concluded: Firstly, Dongtai depression is found to have a more suitable deep geothermal reservoir compared to Yanfu depression and Jianhu uplift; secondly, Jinhu sag and Gaoyou sag contain deeply buried geothermal reservoirs with poor exploration and development conditions for HDR due to their reservoir occurrence conditions; thirdly, Mesozoic and Paleozoic marine carbonate strata can be drilled at depths ranging from 4000 to 5000 m with predicted temperatures reaching 140–190 °C in the southeast edge of the Jinhu sag, the Lianbei sag, the east–west slope zones of the Gaoyou sag, as well as the low subuplifts within the depression, such as the Lingtangqiao–Liubao–Zheduo low subuplifts, west of the Wubao low subuplift and Xiaohai–Yuhua subuplifts. It can be drilled into Mesozoic and Paleozoic Marine carbonate strata at a depth of 4000–5000 m, with the predicted temperature reaching 140–190. The areas mentioned above exhibit promising potential for geothermal exploration and development and are the prospective areas for deep geothermal exploration and development.

6. Conclusions

(1)

Mesozoic–Paleozoic marine carbonate rocks should be prioritized as the primary target reservoirs for future deep geothermal exploration within the NJB;

(2)

The geothermal field of the NJB is evidently influenced by basement fluctuations, with a concentration of high temperatures observed at the junction of the Jianhu uplift and Dongtai depression;

(3)

Deep geothermal rock exhibits more pronounced characteristics in the southeast margin of Jinhu sag, the slope belt on the east and west sides of the Lianbei sag and Gaoyou sag, and the low protrusions of the depression, such as the Lingtangqiao–Liubao–Zheduo subuplifts, west of the Wubao subuplift and the Xiaohai-Yuhua subuplift.