A Study on the Application of a Deep Thermal Reservoir by Using a Magnetotelluric Sounding Method: Taking an Example of Geothermal Resources’ Exploration in the Western Taikang Uplift of the Southern North China Basin

Abstract

Geothermal resources are abundant in the Southern North China Basin, which is one of the prospective areas hosting low–medium-temperature geothermal resources in sedimentary basins in China. The purpose of this work is to reveal the formation and storage conditions of the geothermal resources in the western margin of the Taikang Uplift and delineate the range of potential geothermal reservoirs. This paper uses five magnetotelluric sounding profiles for data processing and analysis, including the calculation of 2D skewness and electric strike. Data processing, analysis, and NLCG 2D inversion were performed on MT data, which consisted of 111 measurement points, and reliable two-dimensional resistivity models and resistivity planes were obtained. In combination with drilling verification and the analysis of geophysical logging data, the stratigraphic lithology and the range of potential geothermal reservoirs were largely clarified. The results show that using the magnetotelluric sounding method can well delineate the range of deep geothermal reservoirs in sedimentary basins and that the MT method is suitable for exploring buried geothermal resources in deep plains. The analytical results showed that the XZR-1 well yielded 1480 cubic meters of water per day, with the water temperature of the wellhead being approximately 78 °C, and combined with the results of this electromagnetic and drilling exploration, a geothermal geological model and genesis process of the west of the Taikang Uplift area was constructed. The water yield and temperature were higher than those of previous exploration results, which has important guiding significance for the future development and utilization of karst fissure heat reservoirs in the western Taikang Uplift.

1. Introduction

Geothermal resources, as a renewable and clean energy source, have the characteristics of a wide storage space, a large thickness, multiple types of thermal storage, and high resource availability. With the proposal of the “dual carbon target”, China will vigorously develop and utilize clean geothermal energy in the future [1,2,3].

The Taikang Uplift in the Southern North China Basin has abundant geothermal resources and belongs to the category of medium–low-temperature sedimentary basin geothermal resources [4]. In recent years, in order to accurately identify regional thermal storage characteristics and geothermal resource conditions, the Taikang Uplift has successively implemented the exploration of multiple geothermal resource survey wells, including the Weire 1 well (WR-1), Shangqiu Re 1 well (SR-1), Tongxu Re 1 well (DZK1), Airport Re 1 well (HR01), Airport Re 2 well (HR02), etc. [5,6,7,8].

The exploration of geothermal resources is the foundation for the development and utilization of geothermal resources, and it is also an important application field of geophysical methods. Currently, a relatively complete detection technology theory and data processing and interpretation methods have been formed [9,10,11,12,13]. Therefore, applying geophysical exploration methods to search for deep layered thermal reservoirs has become the most effective technical method in geothermal resource exploration. Among various methods, magnetotelluric sounding is currently the most widely used technique in geothermal resource exploration [14,15,16,17]. Magnetotellurics (MT) is a natural-frequency-domain electromagnetic sounding method that has the advantages of simple construction, a large exploration depth, and the strong resolution of low-resistance layers. MT has advantages that other geophysical methods do not have in distinguishing lateral heterogeneous bodies such as faults and basement undulations [18,19,20]. In the view of this, in the exploration of geothermal resources in the Taikang Uplift, magnetotelluric sounding provides an effective means and research example for searching for deep thermal reservoirs. The drilling results confirm that the XZR-1 well yielded 1480 cubic meters of water per day, with the water temperature of wellhead being approximately 78 °C. The water yield and temperature were higher than those in previous exploration results, which has important guiding significance for the future development and utilization of karst fissure heat reservoirs in the west of the Taikang Uplift.

2. Geological and Geophysical Background

2.1. Geologic Background

The Taikang Uplift is a secondary tectonic unit located in the northern part of the Nanhuabei Basin, bordered by the Jiyuan Kaifeng Depression to the north and the Zhoukou Depression to the south. The overall structural pattern is “two depressions sandwiching one uplift” (Figure 1) [21,22,23]. This area is located on the crystalline basement of the Precambrian metamorphic rock series and has deposited the Songshan Group of the Paleoproterozoic period, the limestone of the Cambrian–Ordovician marine platform facies, dolomite of the Lower Paleozoic period, the sandstone of the Carboniferous–Permian marine–land interaction facies, mudstone interbedded with thin coal seams of the Upper Paleozoic period, the sandstone of the Triassic river–lake facies, mudstone of the Mesozoic period, and the interbedded sandstone and mudstone of the Cenozoic river–alluvial plain facies [24,25].

According to the geological data of the five geothermal wells that were explored, the main type of thermal storage in the Taikang Uplift is layered thermal storage. The thermal reservoirs could be mainly divided into two categories: Cenozoic sandstone pore-type thermal storage and Paleozoic karst fissure-type thermal storage. The pore-type thermal reservoirs of the Cenozoic era are mainly the Minghuazhen Formation and Guantao Formation of the Neogene period. The thermal reservoirs are mainly composed of sandstone and conglomerate sandstone. The water inflow of a single geothermal well is generally 800–1200 m³/d, and the outlet water temperature is 35–55 °C. The Paleozoic karst fissure-type thermal reservoir is concealed by the Neogene or Carboniferous–Permian sedimentary layers, and the temperature of the thermal reservoir is relatively high. The water inflow of a single geothermal well is generally 1000–1500 m³/d, and the outlet water temperature at the wellhead is 51.5–75 °C.

2.2. Geophysical Characteristics

The electrical parameter data used in this study mainly included the resistivity inversion from MT depth measurements near the well and the logging resistivity of the surrounding wells, including Weichen 1, Tongxu 2, Mouye 1, and Weire 1 [4,25,26]. Among them, Weichen 1 and Tongxu 2 are oil exploration wells, Mouye 1 is shale gas exploration well, and Weire 1 is geothermal exploration well. The statistical results show that the resistivity of the Quaternary (Q) system was between 5 and 20 Ω·m; the resistivity of the Neogene (N) and Paleogene (E) systems ranged from 2 to 15 Ω·m; the resistivity of the Triassic system (T) was between 20 and 100 Ω·m; the resistivity of the Carboniferous–Permian (C-P) system was 30–200 Ω·m; the Cambrian–Ordovician (C-O) resistivity ranged from 200 to 500 Ω·m; and the resistivity of the Archean (Ar) and Paleoproterozoic (Pt₁) systems was generally greater than 500 Ω·m.

3. Data Collection, Processing, and Inversion

Five MT survey profiles were designed for this magnetotelluric exploration work, and a total of 111 MT depth measurement points were completed, as shown in Figure 2. The strike of the profiles MT01, MT02, and MT03 was basically perpendicular to the structural strike of the main fault zone in the region. The survey line ran at around NE60°, with a distance of 100 m between the survey points. Each survey line had 22 MT depth measurement points completed. The MT04 and MT05 profiles had a strike of around NW30°, with a distance of 100 m between the measurement points. A total of 22 and 23 MT depth measurement points were completed, respectively. The total length of the five sections was about 10.60 km.

3.1. Data Acquisition and Processing

The MT data collection work was completed between August and September 2022, using three sets of MTU-5A broadband magnetotelluric depth sounders produced by Phoenix Corporation in Canada. The field data collection adopted a tensor measurement method, setting true north as the positive x-axis direction and true east as the positive y-axis direction. Each measurement point recorded two mutually orthogonal horizontal electric field components (E_x, E_y) and magnetic field components (H_x, H_y). Due to the target detection depth of 4000 m, the single point data collection time was determined to be no less than 4 h after field observation experiments. The quality of the processed magnetotelluric data was significantly improved, and impedance tensors within the frequency range of 320–0.01 Hz were obtained for each measurement point.

This article used Swift and Bahr decomposition methods to analyze the dimensionality of all data. As shown in Figure 3, the two-dimensional deviation values of most frequency points were less than 0.3, indicating that each profile had good two-dimensional properties and could be interpreted by two-dimensional inversion. After confirming that the construction satisfied the two-dimensional assumption, it was necessary to further determine the direction of the construction. This article used the Groom–Bailey impedance tensor decomposition method to perform an electrical spindle angle statistical rose plot (Figure 4) for all measurement points in the MT01~MT05 section in different frequency bands (320–10 Hz, 10–1 Hz, 1–0.1 Hz, 0.1–0.01 Hz), with long lobes indicating the dominant electrical spindle angle direction. According to the regional geological data, the overall structural orientation of the surveyed area was NW30°.

3.2. D Inversion

The nonlinear conjugate gradient inversion (NLCG) algorithm integrated with WinGLink 2.20.02 software was used for TE+TM-mode joint 2D inversion of the MT01~MT05 profiles. The parameter settings used in the inversion were as follows: the error base of the apparent resistivity and phase was set to 5%, the initial model was a 100 Ω·m uniform half-space, and the value of the regularization factor τ was compared and analyzed. It was found that τ = 7 balanced the roughness of the model and the data fitting. Therefore, τ = 7 was used as the regularization factor value for all MT profiles’ 2D inversions. During the inversion process, data within the frequency range of 320–0.01 Hz were used. After 200 iterations of calculation, the root mean square (RMS) errors were 2.25, 1.92, 2.30, 2.03, and 1.86, respectively. Figure 5 shows the comparison of the resistivity and phase before and after the two-dimensional inversion of the MT02 profile. The measured data and the simulated profile of the model response had a high degree of agreement, indicating good fitting.

4. Discussion and Interpretation

4.1. Comprehensive Interpretation Results of MT02 Profile

The MT02 profile had a total length of 2.2 km and an orientation of 70°, with the surface mostly covered by farmland. Figure 6a shows the two-dimensional geoelectric structure model obtained from the MT02 profile’s magnetotelluric inversion. The resistivity profile shows a variation of low resistance to medium–high resistance to high resistance from shallow to deep in the vertical direction. The shallow low-resistance layer is speculated to reflect the Quaternary and Neogene systems, with a uniform and continuous horizontal distribution of resistivity, with a value of about 5–30 Ω·m, and a bead-shaped, low-resistance block vertically sandwiched in the middle. The overall shape of the Neogene bottom boundary is relatively gentle. The burial depth of the Neogene bottom boundary is generally between 450 and 500 m, and the lithology of this formation is mainly sandy clay, sandy mudstone, and fine sandstone. The resistivity value of the middle resistive layer in the middle is 30–100 Ω·m, which is speculated to be a reflection of the Triassic system of the Mesozoic era, and this layer is in integrated contact with the underlying Permian system. The burial depth of the bottom boundary is generally between 1300 and 1400 m. The lithology of this formation is mainly mudstone and siltstone, and the lithology is relatively dense. The horizontal distribution of resistivity in the Carboniferous–Permian system is uniform and continuous, and the resistivity value gradually increases vertically, showing a medium to high resistivity, characteristic of about 100–300 Ω·m. The lithology of the strata is mainly mudstone and argillaceous sandstone, with a bottom boundary burial depth generally between 2400 and 2500 m. The Cambrian–Ordovician system has obvious high resistivity characteristics, with vertically stable resistivity values generally greater than 300 Ω·m. The formation lithology is mainly composed of limestone, dolomitic limestone, and dolomite.

4.2. Comprehensive Interpretation Results of MT04 Profile

The MT04 profile had a total length of 2.5 km and an orientation of 5°. From the resistivity cross-section diagram (Figure 7), it can be seen that the shallow part exhibits low resistivity characteristics, and the resistivity gradually increases with increasing depth. The MT04 profile electrical structure model can be divided into four layers vertically from shallow to deep: The first electrical layer is a shallow, low-resistance layer with a resistivity value between 5 and 30 Ω·m, and the bottom boundary is buried at a depth of about 500–550 m, which is inferred to be of the Cenozoic period. The second electrical layer is the middle-resistance layer, with a resistivity value ranging from 30 to 100 Ω·m, and the bottom boundary is buried at a depth of about 1400 to 1500 m, which is inferred to reflect the Mesozoic era. The second electrical layer is a medium–high-resistance layer with a resistivity value of about 100–300 Ω·m, and the bottom boundary is buried at a depth of about 2400–2600 m, which is inferred to reflect the Carboniferous–Permian system. The fourth electrical layer shows the high-resistance characteristics of the substrate, with a resistivity value generally greater than 300 Ω·m, suggesting a reflection of the Cambrian–Ordovician system.

4.3. Drilling Verification

According to the analysis of regional geothermal geological data, the thermal reservoirs in the western part of the Taikang Uplift are mainly pressure–conduction-type, medium- and low-temperature layered thermal reservoirs. The upper part is a sandstone pore-type thermal reservoir of the Minghuazhen Formation and Guantao Formation of the Neogene period, while the lower part is a karst fissure-type thermal reservoir of the Cambrian and Ordovician periods. Previous studies have suggested that carbonate rock formations composed of Paleozoic limestone and dolomite, located at the edge of a basin and covered by sedimentary cover layers, are generally considered favorable areas for the development of karst fractures. Thermal reservoirs are mainly controlled by the structure of carbonate rock formations and the development of karst fractures. Groundwater undergoes a long cycle and a large cycle depth, resulting in high temperatures. At the same time, due to the accumulation of groundwater at the edge of the basin, the water volume is abundant.

Through the analysis of regional geological structures and the conditions for the formation of geothermal resources, it is believed that the study area is located on the western edge of the Taikang Uplift sedimentary basin. Structurally, it belongs to the transitional zone from the tectonic uplift zone to the sedimentary basin area. Its groundwater has a large circulation depth and easily communicates with deep heat sources. The research area has developed Quaternary, Neogene, Triassic, Carboniferous–Permian, and Cambrian–Ordovician strata from top to bottom. The lithology of the Cenozoic, Mesozoic, and Paleozoic strata is mainly composed of mudstone and fine sandstone, with good thermal insulation and water-resistance effects and good cap rock conditions. Cambrian–Ordovician carbonate rocks have conditions for the development of karst fractures and the potential to form karst fracture-type thermal reservoirs.

Based on the analysis of the regional geothermal geological conditions and the comprehensive interpretation results of this magnetotelluric sounding, drilling field verification was carried out to verify the reliability of the magnetotelluric data interpretation, with a focus on verifying the burial depth of deep Cambrian–Ordovician karst fracture-type thermal reservoirs and the occurrence of geothermal water in the study area. Based on this, a geothermal well exploration hole named XZR01 was set up in the middle of measurement points 7 and 8 of the MT02 profile in the research area, with a designed depth of 3000 m. The final depth of the XZR-1 geothermal well was 2708.80 m. The encountered strata from top to bottom were Quaternary (105 m thick), Neogene (410 m thick), Triassic (932 m thick), Carboniferous–Permian (1116 m thick), Ordovician (82 m thick), and Cambrian (168.80 m thick, not penetrated). The geological conditions encountered during drilling are shown in

Table 1. Stratigraphic characteristics revealed by geothermal well XZR-1.

Stratum	Bottom Depth/m	Layer Thickness/m	Main Lithology	Classification Basis
Quaternary	105	105	Scattered undiagenetic clay and sandy clay	The sediment structure is loose, the degree of cementation is poor, and the drilling rate is fast
Neogene	515	410	Brown–red and yellow–brown mudstone interbedded with brown and brown–yellow medium–fine sandstone, with brown sandstone and gravel at the bottom	Encountering the yellow–brown mudstone at the top of the Neogene Formation during drilling
Triassic	1342	932	Brown–red and purple–red mudstone, silty mudstone, fine sandstone	Encountering brownish–red mudstone at the top of the Triassic system during drilling
Carboniferous–Permian	2458	1116	Deep gray and bluish–gray mudstone interbedded with gray–white, gray–green, and bluish–gray argillaceous sandstone and medium–fine sandstone, interbedded with thin coal seams	Encountering dark gray mudstone at the top of the Permian system during drilling
Ordovician	2540	82	Dark gray limestone, mudstone, and dolomite	Encountering deep gray limestone at the top of the Ordovician system during drilling
Cambrian	2708.80	168.80 (No bottom seen)	Gray and gray–brown dolomite containing flint nodules and powder crystal dolomite	Encountering gray–brown dolomite with flint nodules at the top of the Cambrian system during drilling

. The drilling results show that the water intake section of the XZR-1 geothermal well was located at 2458.00–2708.80 m underground. The geophysical logging results show that the main aquifers were 2568.6~2586.4 m, 2594.5~2608.2 m, 2624.4~2632.6 m, 2645.3~2653.5 m, 2678.3~2682.8 m, and 2690.1~2694.5 m, with thermal reservoirs of Cambrian–Ordovician limestone and dolomite. The measured temperature at the bottom of the well was 81.3 °C, with a water output of 1480 m³/d and a wellhead water temperature of 78 °C (Figure 8). The interpretation results of the magnetotelluric data are in good agreement with the geological results encountered during the drilling, which verifies the feasibility of using magnetotelluric sounding methods to search for layered thermal reservoirs in sedimentary basins and the reliability of geological interpretation.

4.4. Analysis of Hydro-Geothermal System and Its Formation Mechanism

The North China Basin is a Mesozoic Cenozoic fault basin developed on the North China Platform, consisting of many secondary depressions and uplifts as the skeleton. The typical concave–convex structure of this basin basement forms a composite hydrothermal system consisting of sedimentary strata and ancient buried hills stacked together. The heat generation from the upper mantle and the radioactive decay of bedrock is mainly achieved through thermal conduction to heat the shallow strata. During geological history, this area belonged to the uplift zone of the upper mantle, and the heat source was relatively shallow, which was conducive to the formation of geothermal resources. Geothermal resources mainly accumulate heat through conduction [11,27]. The top sedimentary cover layer serves as a thermal reservoir for insulation, while the thermal convection of the deep bedrock reservoir serves as a beneficial supplement. Geothermal fluids are mainly transported through water and heat conduction channels through bedrock fractures and regional faults. The structural pattern of alternating concave and convex areas and multi-level faults in the research area constitute a good water and heat conduction channel for geothermal fluids. The main source of geothermal fluid supply is the vertical infiltration of fluids from the southwestern mountainous areas (Figure 9).

5. Conclusions

Through this magnetotelluric survey and actual drilling verification, it was proven that the western edge of the Taikang Uplift in the Southern China North Basin has the conditions to form Lower Paleozoic carbonate rock karst fracture-type thermal reservoirs. The study area belongs to the transitional zone from the tectonic uplift zone to the sedimentary basin area structurally, and the groundwater has a large circulation depth, making it easy for it to interact with deep heat sources. Atmospheric precipitation infiltrates along the karst fractures and thermal control faults and deeply circulates and heats up in the Lower Paleozoic carbonate rock karst fractures. Under the action of regional water head pressure, it rises to the shallow part, forming a layered thermal reservoir with strong water enrichment and a high water temperature.

Due to the small size of the research area, no major regional fault structures were found in the scope of this exploration work. It is recommended to carry out large-scale geothermal exploration work on the western edge of the Taikang Uplift to further verify the development characteristics and resource distribution conditions of the Lower Paleozoic karst fissure-type thermal storage, providing guidance for future geothermal exploration and the development of karst fissure-type thermal storage.