Genesis Mechanism of Geothermal Water in Binhai County, Jiangsu Province, China

Abstract

Taking the coastal area of Binhai County, Jiangsu Province, as an example, this study first investigated the basic natural geography and the regional geological and hydrogeological conditions of the study area, and then carried out in-depth geophysical prospecting, hydrogeological tests, geothermal temperature monitoring, hydrochemistry and isotope analyses, and other studies based on the results to comprehensively and systematically reveal the genesis mechanism of the geothermal water resources of this coastal area from multiple perspectives. The results showed the following: the geothermal water in this area mainly comes from atmospheric precipitation; the deep east–northwest interlaced fracture is the recharge and transportation channel; the Cambrian–Ordovician carbonate rock layer, enriched by the development of cavernous fissures, forms the thermal storage layer; the underground heat mainly comes from the upward heat flow along the deep fracture and the natural warming of the strata; and the thermal reservoir cover comprises Paleozoic and Mesozoic clastic rocks that have a high mud content and form a thick layer. The genesis mode of this area is as follows: the atmospheric precipitation infiltrates and is recharged through the exposed alpine carbonate fissures in the Lianyungang area, and then it is transported to the south along the large deep fracture under the action of a high hydraulic pressure head; meanwhile, it is heated by the heat flow in the deep part of the fracture and water–rock interactions with the strata occur. Geothermal water with a calculated thermal storage temperature of 83.6 °C is formed at a depth of 2.9 km, which is blocked by the intersection of the northeast and northwest fractures to form a stagnant zone in the coastal area.

1. Introduction

Geothermal energy resources are mineral resources that integrate the resources of “heat, minerals and water” and are also a source of clean and renewable energy, with the advantages of a widespread distribution, being clean and renewable, and the ability to be directly utilized. However, there are still some limitations in the current development and utilization of geothermal energy resources. The limitations of this technology include the heat and cold accumulation that can occur during the process of underground thermal energy extraction [1], ground subsidence caused by over-exploitation, the unclear calculation of medium- and high-temperature resources, and irrational development and utilization [2]. Further limitations include the clogging [3], scaling, and corrosion [4,5,6] of heat pumps due to the over-exploitation of changes in water temperature and the poor quality of the primary groundwater [7]. Constrained by the utilization technology of medium- and high-temperature geothermal resources, the average annual growth rate of geothermal power generation was about 4% from 2010 to 2020 [8].

Research on the genesis of geothermal water resources was first conducted overseas [9,10,11]. After the 1970s, domestic research on the genesis mechanism of geothermal energy resources entered a stage of rapid development [12,13,14], and since the beginning of the 21st century, research on the genesis of geothermal water resources has undergone further development [15,16,17,18]. Domestic and foreign research on geothermal water recharge and discharge conditions, heat source analysis, geological structure, conceptual model construction, etc., has been widely conducted, and various methods, such as geogravitational physical exploration, drilling, geological scanning, hydrochemical isotope analysis, etc. [19,20,21,22,23], have been proposed, but insufficiently detailed geological models and the lack of systematic and comprehensive analyses of the mechanisms restrict the long-term sustainable exploitation of geothermal water resources [24,25,26,27].

Binhai is a key potential geothermal area in Jiangsu’s North Jiangsu Basin (located in eastern China and northern Jiangsu Province), with its unique geologic features and promising prospects for geothermal resource development [28]. The input of sediments from multiple sources (Yangtze River, Yellow River, and Paleo-Huaihe River) and the strong tidal dynamics along the eastern coast of China have resulted in the formation of a very thick (200–300 m) Quaternary loose sedimentary layer in the Yangtze River Delta (e.g., in Shanghai, southern Jiangsu, and northern Zhejiang), which is characterized by a highly compressive, silty, soft clay and a sequence of land–sea interbedded sediments [29]. This sedimentary system contrasts with the sand-dominated sediments of the Pearl River Delta (Guangdong Province) and the clay–chalk–sand interbedded structure of the Bohai Bay (e.g., coastal areas of Tianjin, Hebei, and Shandong) [30,31].

In the groundwater system, the interactions between the dynamic changes in the salt- and freshwater interface and human activities lead to regional ground subsidence, the mechanism of which is different from that of the deep freshwater overexploitation-dominated subsidence pattern in the North China coastal area. The evolution of the coastal geomorphology shows extreme duality: the Yellow River estuary in the north continues to erode and recede due to sediment supply cutoffs, while in the south, the Yangtze River sediment replenishment forms rapidly silting tidal flats and has developed a rare group of radial sand ridges in the South Yellow Sea (the tidal channel–sand ridge system) [32,33,34], which is significantly different from the estuarine bay geomorphology dominated by human poldering in South China.

Since the founding of the People’s Republic of China, different scholars and scientific research groups have investigated and evaluated this area. From the 1950s to the 1980s, 1/200,000 regional gravity mapping and physical and chemical exploration were carried out and a 1/200,000 regional hydrogeological census report of Lianyungang (I-50-18) and Batan (I-51-13) was completed, which outlines the distribution of aquifers, the abundance of water resources, the hydrochemistry characteristics, the conditions of the groundwater replenishment, drainage, etc. Since entering the 21st century, 1/250,000 regional geological surveys of Yancheng City (I51C003001) and Binhuai Farm (I51C002001), 1/50,000 regional geological surveys of Dingsansanxu (I50E009023) and Kaisando (I50E009024), and nine other maps have been completed, which further identified the Quaternary System, the bedrock structure, and other geological characteristics of the region. However, the overall degree of work is relatively low, and investigations and evaluations of medium and deep geothermal energy resources are not often conducted, while no detailed analysis or research has been carried out on the genesis mechanism of geothermal water in the study area of Binhai County, Jiangsu Province. In particular, the genesis mechanism of middle- and high-temperature geothermal energy resources in this coastal area needs to be investigated and evaluated in depth in order to support the demand for clean energy in the long term.

This study aims to strengthen the research on the mechanism of geothermal water genesis in the coastal study area of Binhai County, Jiangsu Province. The main tasks are as follows: (1) to fully analyze the existing data in the study area, carry out hydrogeological and geothermal monitoring tests, conduct geophysical drilling and geophysical exploration works, conduct hydrochemical and isotopic analyses of groundwater, identify the tectonic and hydrogeological conditions of the study area, calculate the hydrogeological parameters, and determine the thermal parameters by combining the findings with past data; and (2) to combine the results of physical exploration and drilling, investigate the distribution and characteristics of tectonic structures, and analyze the role of tectonic structures in the water conductivity and heat control of underground hot water resources. Based on the hydrogeological and thermophysical parameters obtained from the test, we will identify the stratigraphic characteristics and determine the source and age of groundwater based on isotopes. Ultimately, based on the above test results, we will deduce the conditions of the heat source, water source, tectonic connection, thermal reservoir, and cover in the Binhai Research Area and systematically and comprehensively reveal the mechanism of geothermal water genesis in the Binhai Research Area.

2. Geological and Geothermal Geological Conditions

2.1. Physical and Geographic Profile

The coastal study area is located in the northeastern part of Jiangsu Province, in the central part of the North Jiangsu Basin (Figure 1), with a study area of 3200 km². The local annual mean temperature is 15.4 °C, the multi-year annual mean precipitation is 970 mm, and the multi-year annual mean water surface evaporation is 1196.6 mm. Summer temperatures are high, and evaporation is high, while winter temperatures are low, and evaporation is low. The continuous maximum evaporation usually occurs from May to August, with a value of about 620 mm, which is about 52% of the annual evaporation.

The study area of Binhai County is located in the shallow-lying plain area of the Lixia River and the coastal plain area of North Jiangsu Province, with ground elevation ranging from 3.6 m to 5.5 m. The regional water system mainly consists of the North Jiangsu Irrigation Main Canal, the Sheyang River, the waterway into the sea, the Zhangbao Main Canal, the Waste Yellow River, and the Tumbledown River. The surface water system is more developed, while the Yellow Sea lies to the east (Figure 2).

2.2. Geothermal Conditions

The stratigraphy of the coastal area belongs to the Lower Yangzi stratigraphic subdivision of the Yangzi stratigraphic zone. There are no bedrock outcrops exposed on the surface of the area, and all of them are deeply covered by the Quaternary stratum, with the depth of the Quaternary System ranging from 200 to 490 m [35].

The study area is located in the northeastern part of the North Jiangsu Basin to the south of the Huaiyin–Yangshui rupture and has formed a tectonic pattern in which faults and traps are nested with each other under the folding effect of the Indo-Chinese period and the block faulting effect of the Yanshan period since the Mesozoic Era. The fold structure is more developed for the Indo-Chinese–Yanshan early products at a large scale, with an axial extension of several kilometers to dozens of kilometers, and the fold axis is roughly parallel, being angled more to the northeast, with a south–west tilt. Regarding fracture development, the fracture originated in the late Yanshan to the early Himalayan, while the fracture directions are near east–west, north–northwest, and north–east. Among these fractures, the north–east fracture controlled the distribution of folds and stratigraphy (Figure 3). A series of north–northwesterly advective fractures cut the early north–easterly fractures.

Relying on regional geologic data and physical exploration drilling tests, a set of Paleozoic and Mesozoic clastic rocks with a high level of thickness were identified to have developed in the upper part of the carbonate rocks in the coastal study area, including loose sedimentary layers composed of Quaternary clay and sandy soil, Neoproterozoic and Paleoproterozoic mudstones and sandy mudstones, Jurassic–Cretaceous siltstones and mudstones, Carboniferous–Diapirs mudstones and siltstones, Silurian mudstones, etc. The upper covering layer generally becomes thicker from north to south. In the northern backslope development area, the depth of the base plate of the cover is about 1000~1500 m. According to the boreholes, the overlying layer of Silurian gray-white and gray-green sandstone, feldspathic quartz fine sandstone, and siltstone, interspersed with mottled-color and purple mudstone and locally containing siliceous strips, has a thickness of about 800 m. The thickness of the cover to the south gradually increases, containing Ordovician mudstone, siltstone, and other mudstone. The layers overlying the Ordovician carbonates include the Pukou Formation of the Cretaceous System, the Paleocene System, the Neocene System, and the Quaternary System, and the depth of the cover layer in the southwestern areas is about 2500~3000 m.

2.3. Basic Hydrogeological Conditions

The coastal study area is located in the subtropical monsoon zone, with abundant precipitation and no large river systems; atmospheric precipitation is the main source of groundwater recharge. The lateral recharge of surface water, lateral runoff recharge, irrigation infiltration recharge, and recharge from low hills are supplementary modes of recharge in the area. The groundwater in the area mainly consists of pore water from loose rocks, fissure water from clastic rocks and bedrock, and fissure water from carbonate rocks. The loose rock pore water is roughly divided into six pressurized aquifers, namely submerged aquifers, I, II, III, IV, and V, according to the formation age, cause, burial depth, and hydraulic connection of the water-bearing loose sand layer (Figure 4). The submerged aquifer is composed of Holocene sub-sand soil and silt in the fourth system, with a thickness of 5–20 m, a submerged depth of 0.3–3.5 m, an annual variation in water level of about 2 m, and poor water richness. The I pressurized aquifer is composed of sub-clay and silt from the Upper Pleistocene, with a depth of 40–60 m. The II pressurized aquifer is composed of silt and fine sand from the Middle Pleistocene (Q2), with a thickness of 15–85 m and a depth of 80–120 m. The III pressurized aquifer is composed of river and lake-phase powder, fine sand, and gravelly sand from the Lower Pleistocene (Q1), with a thickness of 20–40 m and a depth of 160–220 m. This aquifer has poor water richness and is the main mining aquifer in the coastal area. The IV pressurized aquifer is composed of coarse sand and fine sand in the river and lake phase sedimentary stratum from the Pliocene Series, with the depth of the bottom plate mostly being under 400 m; the thickness of the aquifer ranges from 20 to 60 m, and the aquifer is relatively water-rich. The V pressurized aquifer is composed of powdery clay, clay sand, medium sand, and medium coarse sand from the Miocene Series river and lake phase, and the depth of the top plate is generally deeper than 450 m.

Carbonate rock-like water aquifers are the main focus of geothermal water exploration and development in the coastal area at present. According to the borehole analysis, the carbonate fissure water in the coastal study area is mainly present in the hidden Ordovician (O) and Cretaceous Pukou Formation (K2p) strata below the loose sediments. Under the influence of long-term tectonic movement in the study area, the northeast-trending and northwest-trending faults intersect and displace each other, and the widely developed chert, dolomite caves, and fissures and fracture zones provide good storage space for groundwater. The aquifer is highly water-rich, being the main layer of geothermal water extraction, and the water influx of a single well is between approximately 1500 and 3000 m³/d, exhibiting a high potential for extraction.

The lithology of clastic fissure water aquifers in the study area is clastic rocks, such as gravel, sandstone, and quartz sandstone, of the Pukou Formation from the Cretaceous to the Mesozoic era, and the water richness is affected by the tectonic structure and the degree of fissure development. The aquifers are mainly distributed in the nucleus or the flexural concave part of the monoclinic rock layer of the Kangzhuang–Xintan salt field backslope (A1), the Binhuai inverted backslope (A2), and the Batananan–Dasiluotian backslope (A3), and result from the tectonic cleavage of tensile and torsional fracture zones and sandstone conglomerates and sandstone with a brittle lithology and a very thick single layer. Mud shale is mostly used as a water barrier in these aquifers. The water richness is poor, and the TDS content is approximately between 0.3 and 3 g/L, mostly consisting of HCO₃-, Cl-Na-, Ca-, SO₄-, and HCO₃-type water.

3. Materials and Methods

Hydrogeological Tests

In order to identify the hydrogeological characteristics of the study area, the hydrogeological parameters of the study area were obtained by using the steady flow pumping test method based on the existing borehole BH01 in the study area. The depth of the pumping well is 2919 m, the water outlet section is 1350~2310 m deep, and the thermal reservoir mainly consists of Ordovician tuff. The pumping equipment has a pumping volume of 1200 m³/d, which is controlled by frequency conversion and valves; an electronic water level meter with an effective range of 0–60 m is used for water level observation; and a turbine flow meter with an effective volume of 120–1200 m³/d is used for water volume observation.

The pumping test was conducted between 6 September 2018 and 11 September 2018 at three different depths of depressions and water level recovery observations were made after the completion of the last small depressions. The initial static degree burial depth was 9.47 m. The maximum depth reduction pumping time was from 8:00 on 6 September to 8:00 on 8 September 2018, with a depth reduction of S₁ = 50.28 m, a stabilized dynamic water level burial depth of 59.75 m, a real-time pumping volume of 2150.8 m³/d, a continuous pumping time of 48 h, and a stabilization time of 26.5 h. The pumping time of medium depth reduction was from 8:00 on 8 September to 12:00 on 9 September 2018, the depth reduction was S₂ = 26.40 m, the stabilized dynamic water level was 35.87 m, the real-time pumping volume was 1441.92 m³/d, the continuous pumping time was 28 h, and the stabilization time was 17.5 h. The low depth reduction pumping time was from 12:00 on 9 September to 12:00 on 10 September 2018, the depth reduction was S₃ = 11.86 m, the stabilized dynamic water level burial depth was 21.33 m, the real-time pumping volume was 745.68 m³/d, the continuous pumping time was 24 h, and the stabilization time was 13.5 h. After observation, the water level of the aquifer after stopping pumping was restored to the initial water level from 12:00 on 10 September 2018 to 6:00 on 11 September 2018. The pumping test results are shown in

Table 1. Well BH01’s pumping test data sheet.

Project		Results
Pumping test section (m)		1199.48–2919
Cumulative thickness of hot storage aquifers (M, m)		274.30
Depth of static water level (m)		9.47
Pumping down order	S1	S2	S3
Abatement (S, m)	50.28	26.40	11.86
Pumping flow rate (Q, m3/d)	2150.8	1441.92	745.68
Unit inflow of water (L/s·m)	0.50	0.63	0.73
Runtime (h)	48	28	24
Stabilization time (h)	26.5	17.5	13.5

and Figure 5.

As shown in Figure 5, the water level in the pumping test at different depths enters into a stable state faster, indicating that the water output capacity of the aquifer is more stable. Recovery occurs faster according to the three different downward depth pumping test results of the Q-S and q-s curve (Figure 6 and Figure 7) based on the parameter fitting comparison, which we used to determine the pumping test equation for the exponential equation, R2 = 99.48%, lgQ = 0.7472 lgS + 2.0693 (Figure 6).

Accurately predicting the water output of the thermal storage aquifer at different depths of descent can serve the development and utilization plan under different working conditions. Therefore, according to the above equation, the water output at different depths of precipitation can be predicted, as shown in

Table 2. Predicted water influx versus depth of descent for Marina BH01 wells.

Drawdown (s, m)	10	11.86	20	26.4	30	40	50	50.28
Predicted pumping capacity of a single well (Q, m3/d)	655	744	1100	1353	1489	1846	2181	2190
Measured pumping capacity of a single well (q, m3/d)	/	745.68	/	1441.92	/	/	/	2150.8

. When the depth of precipitation is 11.86 m, 26.4 m, and 50.28 m, the pumping capacity of a single well calculated by empirical equations is 744 m³/d, 1353 m³/d, and 2190 m³/d, respectively, and the pumping capacity of a single well is 745.68 m³/d, 1441.92 m³/d, and 2150.8 m³/d, with a small error, which indicates that the empirical formula can predict the pumping capacity under different depths of precipitation more accurately and provide the basis for the formulation of different mining programs.

Based on the observation data from the pumping test, combined with the theoretical formula of hydrogeological parameters, the main hydrogeological parameters of the Ordovician Middle and Lower Unified Thermal Reservoir Aquifer were finally obtained (

Table 3. Summary of hydrogeological parameters for well pumping tests of BH01.

Number of Depth Drops	Drawdown (m)	Pumping Rate (Q, m3/d)	Hydraulic Conductivity (K, m/d)		Coefficient of Transmissivity (T, m2/d)		Radius of Influence (R, m)
			Calculation Result	Average Value	Calculation Result	Average Value	Stabilized Flow Calculation Results
S1	50.28	2150.8	0.20	0.22	54.31	61.26	223.72
S2	26.40	1441.92	0.23		63.91		127.31
S3	11.86	745.68	0.24		65.56		57.99

). From the permeability coefficients obtained from the pumping tests at different depths, it can be seen that the distribution of fissures in this area is relatively uniform, and the capacity of the aquifer to produce water is stable.

4. Geophysical Exploration

For the exploration of large-scale deep stratigraphy and geological formations, traditional drilling methods are often too costly or the exploration results cannot accurately reflect the macroscopic features. In recent years, the controlled-source audio geomagnetic method (CSAMT), the wide-field electromagnetic method (WFEM), and high-precision gravity profiling have become more reliable in the exploration of geothermal resources [36,37,38,39,40], providing the most direct and effective means of exploration for identifying fracture tectonics related to geothermal water, determining the distribution range of hot water reservoirs, and determining the thickness of the cover layer.

The controlled-source audio geomagnetic method (CSAMT) can realize exploration at different depths and with different accuracies through variable-rate frequency change, which has the advantages of a great exploration depth, a high lateral resolution of interpreted profiles, and not being subject to high-resistance shielding, etc. [41,42,43,44]. Gravity surveys are an effective complement to other methods by analyzing gravity anomalies in the study area to find hidden rock bodies and hidden structures [45,46].

The wide-area electromagnetic method is an exploratory method that sends pseudo-random signals through an artificial field source (horizontal electric dipole or vertical magnetic dipole) and then observes and calculates the wide-area apparent resistivity so as to detect the geologic target bodies at different burial depths. The observation speed and observation quality are greatly improved compared to other methods [47,48].

Based on the above principles, the survey lines BHC1, BHC2, BHC3, BHG1, BHG2, BHW1, and BHW1 were selected for geophysical exploration to further analyze the geotectonic conditions in the area (Figure 8).

4.1. Constructive Inference

According to the comprehensive interpretation of the controlled-source audio geomagnetic method (CSAMT) and the wide-area electromagnetic method resistivity inversion cross-sectional map (Figure 9, Figure 10, Figure 11 and Figure 12), at 2350 m, 3900 m, and 6500 m along the BHC1 and BHW1 lines; 2000 m, 4300 m, and 5500 m along the BHC2 line; and 2600 m, 3650 m, and 5900 m along the BHC3 line, most of the resistivities show inclined funnel-shaped low-resistance anomalies, and the field records show that there are no evident disturbances in the vicinity of these low-resistance anomalies. Therefore, it is inferred that these low-resistance anomalies may be caused by the fracture structure that breaks up the rock layer and the mud-conducting water, which indicates the development of faults.

Combined with the regional geological data, it is inferred that there are three faults passing through the survey line, named BH-F1, BH-F2, and BH-F3 (Figure 12). Among them, BH-F1 strikes north–west, trending toward the southwest, with a steeper dip angle and a fault distance of 130–280 m, and it is a positive fault. BH-F2 strikes north–east, trending toward the southeast, with a steeper dip angle and a fault distance of 130–600 m, and it is a positive fault. BH-F3 strikes northeast–east, trending toward the south–southeast, with a steeper dip angle and a fault distance of 260–480 m, and it is a positive fault.

Further based on the high-precision gravity profile (Figure 13 and Figure 14), and with reference to the results of the CSAMT projection near the survey line, it is finally inferred that there may be three fractures within the control range of the BHG1 line, which are BH-F2, BH-F1, and BH-F4. The BH-F2 fracture is located at a horizontal distance of about 5900 m from the profile; the BH-F1 fracture is located at a horizontal distance of about 8800 m from the profile; and the BH-F4 fracture is located at a horizontal distance of about 10,400 m from the profile. For the control area of the BHG2 line, there may be four fractures, namely BH-F1, BH-F2, BH-F3, BH-F4, among which the BH-F1 fracture is located at a horizontal distance from the profile of about 3900 m; the BH-F2 fracture is located at a horizontal distance from the profile of about 7450 m; the BH-F3 fracture is located at a horizontal distance from the profile of about 9300 m; the BH-F4 fracture is located at a horizontal distance from the profile of about 5900 m; the BH-F1 fracture is located at a horizontal distance from the profile of about 8800 m; and the BH-F4 fracture is located at a horizontal distance from the profile of about 10,400 m (Figure 8 and Figure 14).

Four major faults were eventually identified in the area, BH-F1, BH-F2, BH-F3, and BH-F4, and the basic characteristics of the faults were identified (

Table 4. Summary of geophysical inference faults.

Serial Number	Fault Number	Inclination	Orientation	Tilt (Inclination of the Ship from Vertical)	Segment Distance	Upper Breakpoint Buried Depth	Characteristic
1	BH-F1	North–West	South–West	steeper	130–280 m	800–1170 m	Normal fault
2	BH-F2	North–East	South–East	steeper	130–600 m	1000–1420 m	Normal fault
3	BH-F3	Northeast–East	South–Southeast	steeper	260–480 m	480–500 m	Normal fault
4	BH-F4	North–East	South–East	steeper	100–120 m	500–840 m	Normal fault

). The overall characteristics of the faults are basically consistent with the overall distribution trend of the faults shown in Figure 3, which further suggests that the faults in the study area are predominantly northeasterly and northwesterly trending.

4.2. Stratigraphic Inferences

According to the field physical exploration test, the inverse resistivity cross-sections of three CSAMT survey lines and the inverse cross-sections of the wide-area electromagnetic method on the BHC1 line were obtained in the Marina North Collapsed Work Area (Figure 10, Figure 11, Figure 12 and Figure 13). Analyzing the inversion results of the three CSAMT lines and the wide-area electromagnetic method in the vertical direction, it is found that the three CSAMT line inversion resistivity cross-sections exhibit somewhat similar regularity in space. Firstly, in the shallow interval of −500 m, there is a clear resistivity gradient zone vertically, and the resistivity is generally in the range of 10 Ω.m. Combined with the drilling data, it is inferred that this is a reflection of the Quaternary System and the Neoproterozoic System. Secondly, in the BHC1 line inversion profile, when the horizontal distance is 1000 m~2400 m, the elevation −1700 m~−500 m; when the horizontal distance is 2400 m~3900 m, the elevation −1200 m~−500 m; when the horizontal distance is 3900 m~6500 m, the elevation is −1000 m~−400 m. In the BHC2 line inversion profile, when the horizontal distance is 500 m~2000 m, the elevation is −1700 m~−500 m, and when the horizontal distance is 2000 m~5500 m, the elevation is −1200 m~−400 m. In the BHC3 line inversion profile, when the horizontal distance is 1000 m~2500 m, the elevation is −1700 m~−500 m, and when the horizontal distance is 2500 m~5900 m, the elevation ranges from about −1300 m to shallow surface depths. Within the range of the above three lines, the resistivity of the stratum is medium–low. Combined with the regional re-magnetization data, drilling data, and analysis of electrical parameters, it is inferred that this layer is a Paleocene stratum. Similarly, the horizontal distance of the BHC1 line inversion profile is 1000 m~7000 m, and the elevation is −2500 m~−400 m, gradually becoming shallower at higher points. The horizontal distance of the BHC2 line inversion profile is 500 m~5500 m, and the elevation is −2500 m~−800 m, gradually becoming shallower at higher points. The BHC3 line inversion profile has a horizontal distance of 1000 m to 6100 m and an elevation of −3500 m to −500 m, and the thickness of the layer reduces at higher points. The resistivity of the stratum within the vertical level of the above line profiles shows that it is a moderately to highly resistive layer, and combined with the regional re-magnetization data, drilling data, and electrical parameter analysis, we deduce that this stratum is a stratum of the Cretaceous Pukou Formation. Secondly, at greater depths, a high-resistance layer can be seen under the Cretaceous Pukou Formation, with a depth of about −2000 m to −3000 m. Combined with the regional re-magnetization data, drilling data, and electrical parameter analysis, it is inferred that this layer comprises the Ordovician and Cambrian strata. The stratum inferred by the wide-area electromagnetic method is basically the same as that presented by the CSAMT at the shallow end of −3000 m, which further confirms the existence of Ordovician and Cambrian strata at a depth of about −2000 m to −4000 m and a Sinian System at the deepest levels of −4000 m to −5000 m.

Based on the inferred results of the CSAMT and the wide-area electromagnetic method, in order to further identify the stratigraphic characteristics, the high-precision gravity profile method was adopted (Figure 13 and Figure 14). It was found that the overall thickness of the Quaternary and Neoproterozoic Systems, inferred by the inversion of the BHG1 line, gradually thins from south to north, with a thickness of about 700 m to 280 m, and the underlying strata are the Paleoproterozoic System, Cretaceous Pukou Formation, Ordovician System, and Cambrian System, in that order. The thickness of the Paleocene system south of the center of the survey line is about 1500 m on average, and it gradually becomes thinner to the north, with the thickness being about 100 m near the end of the survey line. The Cretaceous Pukou Formation is gradually uplifted from the middle to the north of the survey line, and the depth of the top is raised from 2100 m to about 400 m. The thickness of the Quaternary System and Neoproterozoic System, inferred by the inversion of the BHG2 line, gradually increases from the south to the north, ranging from about 700 m to 300 m, and the subordinate strata are, in order, the Paleocene System, the Cretaceous Pukou Formation, the Ordovician System, and the Cambrian System. The thickness of the Paleocene system south of the central part of the survey line is about 1500 m on average, and it gradually becomes thinner at higher points, with the thickness being about 100 m near the end of the survey line. The depth of the top of the Cretaceous Pukou Formation from the center to the north of the survey line gradually changes from about 1200 m to about 300 m.

Finally, based on the survey results of the CSAMT, the wide-area electromagnetic method, and the high-precision gravity profile method, combined with the geological data of this area and the data of the neighboring holes 332, 333, and 370, it is inferred that within the shallow range of 3000 m below the survey line, the area can be roughly classified into five large electrical layers: Quaternary and Neoproterozoic, Paleocene, Cretaceous, Ordovician, and Cambrian (Figure 15). The faults are prominent, causing the entire Cambrian to Paleocene strata to have a greater uplift.

4.3. Synthesis and Analyze

Comprehensive analysis shows that the Buge gravity anomalies in the survey area are spreading in a northeast direction, with a trend of high in the northwest and low in the southeast, corresponding to the coastal uplift and the Yafu depression, respectively. Magnetic anomalies are mainly positive anomalies, with small magnetic field variations, smooth and monotonous anomalies, many local anomalies and irregularities, and anomalous axes mainly in the northeast direction. The stratigraphy has a thick Ordovician carbonate stratum, which provides good development conditions for the formation of geothermal water. The faults are mainly oriented in the northeast direction, and most of them are deep and large positive faults with a large dipping angle, which are interrupted by the northwest faults, forming interlaced groundwater transportation and storage channels, and they all show good hydraulic conductivity and a water-rich nature.

5. Analysis of Hot Water Recharge Sources

5.1. Groundwater Hydrochemical Characteristics

Based on two geothermal wells in the region (Figure 16), hydrochemical analysis of the mid-depth geothermal water was carried out and conventional hydrochemical indices of each water sample were obtained (

Table 5. Statistical analysis of water chemistry data.

Pound Sign	BH01	Milligram Equivalents in %	BH02	Milligram Equivalents in %
Na+ (mg/L)	933	38.28	330	39.70
K+ (mg/L)	19.4	0.47	4.32	0.31
Ca2+ (mg/L)	147	6.94	27.2	3.76
Mg (mg/L)	49.5	3.59	11.8	2.51
HCO3− (mg/L)	290	4.49	477	11.20
Cl− (mg/L)	1266	33.66	247	37.18
SO42− (mg/L)	646	12.57	93.5	5.33
Dissolved solids (mg/L)	3243	/	1178	/
PH	7.5	/	7.7	/
H2SiO3	36.5	/	46.63	/
Total hardness	571	/	117	/
Mineralization	3388	/	1219	/
Type of water chemistry	Cl-Na type	/	Cl-Na type	/

). The cations in the geothermal water were mainly Na⁺, K⁺, Ca²⁺, and Mg²⁺. The anions, on the other hand, were dominated by Cl⁻, SO₄²⁻, HCO³⁻, and F⁻. The pH value of the groundwater is 7.5~7.7. TDS content ranges from approximately 1178 to 3243 mg/L, with an average TDS content of 2210.5 mg/L. The total hardness of the groundwater ranges from approximately 117 to 571 mg/L, with an average content of 344 mg/L.

The hydrogeochemical geological radar maps of the two water samples (Figure 17 and Figure 18) show that the major cation in the two water samples is Na⁺ and the major anion is Cl⁻, but the only difference is that the secondary dominant anion in well BH01 is SO₄², whereas the secondary dominant anion in well BH02 is HCO³⁻, which is probably due to the slight difference in the major dissolved minerals. The hydrochemical type is Cl-Na according to Piper’s trilinear map (Figure 19).

5.2. Groundwater Isotope Characterization

5.2.1. Hydrogen and Oxygen Isotope Characterization

In order to analyze the source of groundwater in the study area, 10 samples of atmospheric precipitation, river water, underground cold water, and geothermal water were obtained for groundwater isotope analysis. Surface water was mainly taken from the North Jiangsu Irrigation Canal, the Waste Yellow River, and the Sheyang River in the study area, while the geothermal water was taken from geothermal wells and the underground cold water was taken from the second and fifth pressurized aquifers. The samples were submitted to the Chinese Academy of Sciences (CAS) in Beijing for testing after sampling was completed. δ²H and δ¹⁸O isotopes were determined using an MAT253 Gas Isotope Mass Spectrometer (the O isotope error was better than 0.1‰; the H isotope error was better than 1‰), and the δ²H and δ¹⁸O isotope ratios were determined using the SMOW standard [49]. The δ²H and δ¹⁸O isotopic signatures of groundwater were finally obtained (

Table 6. Statistical table of δ²H and δ¹⁸O data of water samples in the study area.

Original Number	Typology	δ2H	Average Value	δ18O	Average Value
BHDO01	Geothermal water	−57.9	−57.1	−8.30	−7.8
BHDO02	Geothermal water	−56.3		−7.37
BHDO03	V pressurized water (cold water)	−49.9	−49.5	−7.21	−7.3
BHDO04	II pressurized water (cold water)	−49.2		−7.33
BHDO05	Rainwater	−49.0	−49.0	−7.61	−7.6
BHDO06	Creek	−40.971	−36.71	−6.35	−5.65
BHDO07	Creek	−36.909		−5.64
BHDO08	Creek	−35.909		−5.52
BHDO09	Creek	−33.032		−5.106

).

The hydroxide isotope test data were cast onto the δ²H-δ¹⁸O relationship map (Figure 20). Based on the monthly precipitation isotope data from the Global Network for Isotope Monitoring of Precipitation (GNIP), Nanjing Observatory (IAEA/WMO, 2003), the LMWL of atmospheric precipitation in the study area was obtained by means of regression as δ²H = 8.4δ¹⁸O + 17.4‰, which was used as the reference line; the global reference line of atmospheric precipitation, GMWL, was δ²H = 8δ¹⁸O + 10‰.

As can be seen from Figure 20, both the geothermal water and the cold underground water originate from atmospheric precipitation, but there is a large difference in the enrichment of stable δ²H and δ¹⁸O isotopes between the cold underground water and the geothermal water. δ²H and δ¹⁸O isotopes in the cold groundwater are more enriched than those in the geothermal water, which suggests that there is no significant connection between the upper pressurized water and the lower geothermal water. In addition, it can be seen clearly in Figure 20 that the surface water is more enriched than the cold groundwater and the geothermal water, indicating that the surface water in the upper part of the aquifer is affected by strong evaporation. The cold groundwater is more enriched than the geothermal water, while the hydrogen and oxygen isotopes of the geothermal water are the most depleted, which indicates that the hydraulic relationship between the cold groundwater and the deep geothermal water in the upper second and fourth pressurized aquifers is relatively weak. The sample point of deep geothermal water is shown in the lower left of Figure 20, and there is some oxygen drift, which is inferred to be influenced by the interaction between the geothermal water and the surrounding rock under the high-temperature and high-pressure conditions in the deep region.

5.2.2. Hot Water Recharge Elevations

Based on the above differences, based on the δ²H and δ¹⁸O isotope height effects of atmospheric precipitation, the δ²H value decreases with increasing groundwater recharge elevation, and the principle of isotope height effect can be calculated to obtain the groundwater recharge elevation (1)

$$\mathrm{H}=\mathrm{Hr}+\mathrm{gadD}$$

(1)

where

H: groundwater recharge elevation (m);

Hr: elevation of the reference point (m);

D: δ²H value of groundwater, ‰ (SMOW);

Dr: δ²H value of atmospheric precipitation at the reference point, ‰ (SMOW);

gradD: gradient of δ²H decreasing with elevation, ‰ (SMOW)/100 m.

According to (1), the calculated groundwater recharge elevation ranges from 383.5 to 463.5 m, which is significantly different from the cold groundwater recharge area (

Table 7. Groundwater recharge elevation in the study area.

Original Number	Typology	Underground Hot Water δ2H	Elevation of Reference Point (m)	Reference Point Atmospheric Precipitation δ2H	GradD	Recharge Elevation (m)	Average Value
BHDO01	Geothermal water	−57.9	26	−49.15	−2	463.5	365.1667
BHDO02	Geothermal water	−56.3	26	−49.15	−2	383.5
BHDO03	V pressurized water (cold water)	−49.9	26	−49.15	−2	63.5	46
BHDO04	II pressurized water (cold water)	−49.2	26	−49.15	−2	28.5

).

According to the basic geological background data, the study area is characterized by the development of deep and large north–west- and north–east-oriented fractures. Combined with the hydrochemical and isotopic characteristics, it is hypothesized that the atmospheric precipitation comes from the Lianyungang peaks, such as Huaguo Mountain and Jinping Mountain, which are outcrops of northwesterly carbonate rocks in the study area, with elevations ranging from 385 m to 624 m. The atmospheric precipitation enters the deeper part from the outcrops of carbonate rocks and then is transported southward through the northwesterly deep faults to the lower coastal area.

5.2.3. ⁸⁷Sr/⁸⁶Sr Ratio

Sr isotopes (⁸⁷Sr/⁸⁶Sr) are chemically stable, and groundwater contains Sr, mainly originating from the dissolution of minerals or through ion exchange with clays and other minerals in the recharge area in the runoff process. In dolomite and limestone, the ⁸⁷Sr/⁸⁶Sr ratio is relatively low, generally 0.708~0.710; in loose layers and sand shale with more clay minerals, the ⁸⁷Sr/⁸⁶Sr ratio is generally high, at 0.716~0.720 or higher. Therefore, in order to identify the connection between the geothermal water and the rest of the groundwater in the study area, further studies using Sr isotopes were needed [50,51].

A total of four water samples were collected in the study area for Sr isotope determination, including two geothermal water samples and two pressurized water samples (

Table 8. Statistical analysis of Sr isotopes in groundwater.

Borehole Number	Typology	Well Depth	Sampling Time	87Sr/86Sr	Average Value	Content/(ug/L)
BHS01	II Pressurized water (cold water)	120	23 March 2023	0.711661	0.7114815	467
BHS02	V Pressurized water (cold water)	568	23 March 2023	0.711302		625
BHS03	geothermal water	2000.06	23 March 2023	0.709507	0.709556	2515
BHS04	geothermal water	2919	23 March 2023	0.709605		5130

).

The ⁸⁷Sr/⁸⁶Sr ratios for the two groups of geothermal water (Figure 21) are relatively similar, with a small range of variation between 0.7095 and 0.7096, falling within the range of ⁸⁷Sr/⁸⁶Sr ratios of carbonate stratigraphic sources, which suggests that geothermal water in the study area mainly flows through saline stratigraphy. In contrast, the ⁸⁷Sr/⁸⁶Sr ratios of the two groups of subsurface cold water are higher than those of the geothermal water. The high Sr content and low ⁸⁷Sr/⁸⁶Sr ratios in the geothermal water distribution areas and the low Sr content and high ⁸⁷Sr/⁸⁶Sr ratios in the cold groundwater distribution areas indicate a very weak connection between the two.

From the vertical distribution characteristics, the ⁸⁷Sr/⁸⁶Sr values show a decreasing trend with depth. The mean ⁸⁷Sr/⁸⁶Sr value for shallow subsurface cold water is 0.711482, while the mean value of ⁸⁷Sr/⁸⁶Sr for geothermal water is as low as 0.709556 (Figure 22). Based on the regional hydrogeological conditions, geothermal water with low ⁸⁷Sr/⁸⁶Sr values was formed by atmospheric precipitation at depth and transported through the strata, where long-term water–rock interactions occurred. This is also consistent with the geothermal water’s isotopic signatures of δ²H and δ¹⁸O, which suggests that the geothermal water underwent cyclic transport over a long timescale.

6. Results and Discussion

A set of thick Paleozoic and Mesozoic clastic rocks were observed to have developed in the upper part of the carbonate rocks in the coastal study area, including loose sedimentary layers composed of Quaternary clay and sandy soil, Neoproterozoic and Paleoproterozoic mudstones and sandy mudstones, Jurassic–Cretaceous siltstones and mudstones, Carboniferous–Diapirs mudstones and siltstones, Silurian mudstones, etc. The thickness of the upper covering layer generally increases from north to south. In the northern backslope development area, the depth of the base plate of the cover is about 1000~1500 m. According to the boreholes, the overlying layer of Silurian gray-white and gray-green sandstone, feldspathic quartz fine sandstone, and siltstone, interspersed with mottled-color and purple mudstone and locally containing siliceous strips, has a thickness of about 800 m. The thickness of the cover to the south gradually increases, containing Ordovician mudstone, siltstone, and other mudstone, the overlying layers on the Ordovician carbonates include the Pukou Formation of the Cretaceous System, the Paleocene System, the Neocene System, and the Quaternary System in order, and the depth of the floor of the cover layer is about 2500~3000 m, reaching 3000 m in the southwestern area. In conclusion, it can be seen that the overlying strata have a high mud content, high thickness, and poor water permeability, which makes them an ideal cover layer for thermal storage.

In the coastal study area, the thermal reservoir is dominated by carbonate formations that have developed in the fracture zone. Deep geothermal water in the coastal area is transported to the northwest to the deep fracture and then flows through the northeasterly and northwesterly faults to the intersection of the fracture area, where the flow is blocked and transported to the upper part of the area. This has resulted in the area at the convergence of the drainage section becoming a carbonate rock fissure karst water drainage or stagnant flow area. In the overlying layer of the Paleozoic and Meso-Cenozoic clastic rocks, the formation of thermal storage area has occurred under the effect of barrier insulation.

From the perspective of thermal storage space, the coastal area of Jiangsu Province spans two major geological and tectonic units, namely the Sulu orogenic belt and the Yangzi plate, and in the long history of geological development, it has gone through three major stages of development, namely the formation of the basement, the formation of the folded cover, and the activities of the continental margin of the Pacific Ocean, and due to the differences in tectonic activities, different secondary tectonic units have been formed. After many phases of geological activities, 1000~2300 m thick carbonate rock strata were eventually formed in the sedimentation, including the Aurignacian carbonate rock thermal reserve, the Cambrian–Ordovician carbonate rock thermal reserve, the Carboniferous–Permian carbonate rock thermal reserve, and Triassic carbonate rock.

According to the results of physical exploration and interpretation, the north–west fault BH-F1 and the north–east faults BH-F2, BH-F3, and BH-F4 have intersected with each other after many phases of tectonic action, resulting in the development of extensive fissures in the strata. It can be seen that there are carbonate rocks hidden in the base of the basin. In particular, the carbonate rocks hidden at the junction of tectonic uplift and depression in the central part of the basin are characterized by a high thickness, stable stratigraphy, widespread distribution, and the development of karst fissures. The water connectivity in the carbonate formation is good, and the hidden carbonate rocks in the basin have a tectonic connection and a certain hydraulic connection with similar rock formations around the basin, forming a suitable space for water storage and transportation.

Having a high thermal storage temperature is also an important condition for a geothermal water thermal reservoir. Based on the relationship between the content of certain chemical components in geothermal water and the temperature, the equilibrium temperature of chemical reactions can be used to estimate the temperature of the geothermal reservoir. Some commonly used methods are cation geothermometers, silica geothermometers, and gas chemical geothermometers [52,53]. Using the Giggenbach Na-K-Mg saturation equilibrium diagram (Figure 23) to analyze whether the geothermal water in the study area has reached reaction equilibrium, it is found that the geothermal water in wells BH01 and BH02 is immature; i.e., water–rock interactions have not reached equilibrium. Therefore, it is not suitable to use the cation temperature scale estimation method to calculate the thermal storage temperature, and thus the SiO₂ geothermometer can be used for this purpose (

Table 9. Calculation results of the silicon dioxide geometer (unit: °C).

Borehole	Actual Temperature	Quartz Scale (No Vapor Loss)	Quartz Scale (Maximum Vapor Loss)	Chalcedony Warm Label	Amorphous Silica Temperature Scale
BH01	58.1	85.3	83.6	52.3	/
BH02	58	96	98	68	−15

).

Based on the relationship between SiO₂ content and temperature (Figure 24), it can be seen that the geothermal water data point of well BH01 is located between the quartz and chalcedony control line. Although the water sampling point is closer to the chalcedony temperature scale, the thermal storage temperature calculated according to the chalcedony temperature scale is only 52.3 °C, which is not in line with the measured temperature of 58.1 °C. The thermal storage temperature should be higher than the measured temperature, which indicates that the equilibrium effect of SiO₂ in the geothermal water is mainly under the control of the quartz, which is realistic. The extension line upward along the water sample point first intersects with the quartz (no steam loss) dissolution line, and so it is more fitting to use the quartz (no steam loss) temperature scale to estimate the thermal storage temperature. Similarly, the chalcedony temperature scale can be applied in the geothermal water sample point of the shallower BH02 well to calculate the thermal storage temperature. The thermal storage temperatures of wells BH01 and BH02 are 83.6 °C and 68 °C, respectively, and thus they represent low- and medium-temperature geothermal resources. Since the thermal storage layer of this exploitation is the thermal storage aquifer exploited in well BH01, the thermal storage temperature of the target layer is 83.6 °C.

From a comprehensive point of view, the widely developed deep fractures in the area provide a good channel for the transportation and conduction of underground heat flow, and the upward conduction and transportation of heat flow causes the groundwater to increase in temperature and be stored in the Cambrian–Ordovician carbonate rock strata, which are richly developed in fissures. Eventually, the thermal reservoir in this area has a good degree of fissure development, high permeability, thicker aquifers, better water enrichment, and superior thermal storage conditions with higher geothermal fluid temperatures.

7. Geothermal Water Genesis Models

Geothermal water mineralization patterns in the coastal area are diverse, including convective geothermal systems and conductive geothermal systems. Convective geothermal systems usually form medium–high-temperature geothermal resources, while conductive geothermal systems often form medium–low-temperature geothermal resources [54]. Conduction-type geothermal systems are mainly distributed in shallow depressions, with loose porous sandstones of the Paleoproterozoic Sanlii Formation and Dainan Formation, the Neoproterozoic Yancheng Formation, and some shallow tuff fissures as the thermal reservoirs, which are generally distributed in layers spanning a large area. Convective geothermal systems are widely developed in the karst fissure zones of carbonate rocks in the Binhai Rise and Yafu Depression. Therefore, the simultaneous presence of both mineralization modes in the study area is inextricably linked.

The geothermal water system in the coastal study area is a typical deep-circulating carbonate karst rift convection–conduction geothermal system (Figure 25). In terms of heat source conditions, the heat source mainly comes from the deep crust and the upper mantle of the Earth’s heat flow through the thermal conductivity of the deep fractures and strata upward heat transfer, the northwest and northeast extension of the deep fractures constitute the storage and contact between the deep heat source channel, and the heat source conditions are good. In terms of water sources conditions, the recharge source is the atmospheric precipitation infiltrated through the fissures of carbonate rock strata, which is a sufficient source of recharge; the recharge channel is the deep and large north–west-oriented fracture, which presents good conditions for water recharge. The hot storage aquifer has good conditions for the water source and can be exploited in large quantities. From the condition of tectonic connection, the depth of fault cutting is deeper, and the interlocking fractures create good spatial conditions, which provide good channel conditions for the transportation of geothermal water and heat, and the geothermal water can be fully recharged by the heat source. From the cover conditions, the upper part of the carbonate rock has a set of Paleozoic and Mesozoic clastic rocks with a high level of thickness, a high mud content, and poor permeability, thus representing an ideal cover for thermal storage. From the thermal storage conditions, the widely developed deep and large fractures have created the Cambrian–Ordovician carbonate thermal storage layer in this area with good fissure development, high permeability, a good water-rich nature, and a high geothermal fluid temperature, with an estimated thermal storage temperature of 83.6 °C.

The genesis mechanism is shown in Figure 25: the geothermal water recharge source is mainly the atmospheric precipitation infiltration recharge of the exposed alpine carbonate rocks in the Lianyungang area, and water flows through the carbonate rock stratum and is then transported to the south along the deep large fracture under the action of high hydraulic pressure head. It is then heated by the deep heat source and forms the geothermal water resource with a thermal storage temperature of 83.6 °C at a depth of 2.9 km; meanwhile, water–rock interactions occur with the transport channel and the surrounding rocks of the storage layer, ultimately forming Cl-Na-type geothermal water. Under the condition of the intersection of the north–east-oriented and north–west-oriented fractures in the coastal area, this creates a large space for storing geothermal water. At the same time, it creates a certain blocking effect on the geothermal water and forms a stagnant area in the coastal area. Eventually, this leads to the formation of carbonate thermal storage geothermal water resources in the coastal study region.

8. Conclusions

(1) Taking the coastal area of Binhai County, Jiangsu Province, as an example, this study reveals the genesis mechanism of geothermal water resources in this coastal area from multiple angles by investigating the basic natural geography and regional geology and hydrogeology, and then carrying out complementary geophysical exploration, hydrogeological tests, hydrochemistry, isotope analyses, etc.

(2) The geothermal water in the coastal study area originates from atmospheric precipitation, and the hydrochemical type is mainly Cl-Na-type water. The recharge elevation analyzed according to δ²H and δ¹⁸O isotopes is 383.5~463.5 m, and the age according to ¹⁴C determination is 23,320–29,071 years. Hydrochemical characterization and Sr isotope (⁸⁷Sr/⁸⁶Sr) ratios show that there is no major hydraulic connection between the other aquifers and the thermal storage aquifer.

(3) Geothermal water genesis conditions have been identified. The underground heat source in the coastal research area mainly comes from the upward heat flow along the deep fractures and the natural warming of the strata; the water recharge conditions are good, and the aquifer thickness, water level, and water volume are stable; the heat storage layer is the Cambrian–Ordovician carbonate rock strata enriched by the development of cavernous fissures; and the deep fractures in the northeast and northwest directions interlacing with each other have created superior connecting conditions and connected the recharge area, the discharge area, and the hydraulic connection between different aquifers in different zones. The overlying Paleozoic and Mesozoic clastic rocks with high mud content and great thickness serve as a good thermal storage cover.

(4) The model of geothermal water genesis in the coastal study area has been analyzed. Atmospheric precipitation infiltration recharge occurs through the Lianyungang area of exposed alpine carbonate rock, and the water flows through the carbonate rock stratum and then in a higher-hydraulic-pressure head under the action of transport along the deep fracture to the south. At the same time, via heating from the deep heat source and water–rock interactions, Cl-Na-type geothermal water is formed with a thermal storage temperature of 83.6 °C at a depth of 2.9 km. A stagnation zone is formed in the coastal area under the intersection of the northeast-oriented fault and the northwest-oriented fault.

(5) Existing physical exploration and drilling data are concentrated in localized areas, but data coverage is insufficient, which may result in limitations of the study of genesis mechanisms in the area.