A Method for Determining Target Areas of Hot Dry Rock Resources: A Case Study in Continental China

Abstract

Geothermal resources have been recognized as important sources of clean renewable energy. The exploration, development, and utilization of geothermal resources, especially hot dry rock (HDR) resources, are highly important for achieving peak carbon and carbon neutrality. However, there is no comprehensive evaluation method for determining HDR target areas, and the evaluation scale and application disciplines are relatively simplistic. In this paper, we sought to optimize the identification of HDR target areas through a multiscale and multidisciplinary method and formed a set of generalized and demonstrative processes to guide the exploration of HDR resources. Through practical application to the Gonghe Basin and the Zhangzhou Basin, it was found that the comprehensive geothermal conditions of the Gonghe Basin are superior to those of the Zhangzhou Basin, and the geothermal reservoir depth, geothermal reservoir temperature, geothermal gradient, and heat flow are the four most important factors affecting hot dry rock geothermal resources. Using this method, the prioritization of target areas changes from a qualitative study to a quantitative and semiquantitative study, increasing the specificity and reliability of the decision-making process.

1. Introduction

In recent years, there has been a crisis in energy sources such as oil and natural gas, with reserves continually diminishing. Moreover, the carbon dioxide gas produced from the use of fossil fuels is emitted into the atmosphere as a greenhouse gas, artificially causing global warming. Additionally, for China, it is necessary to import large amounts of oil and natural gas annually to meet social development needs [1,2]. “Hot dry rock geothermal resources”, as a new form of energy, are stable, reliable, green, low-carbon, clean, and renewable domestic resources. They are vast in scale and are not affected by periodic factors such as seasons, climate, or differences in sunlight. Therefore, the exploration and development of hot dry rock geothermal resources are becoming increasingly important.

The term “hot dry rock” was first coined by scientists at the Los Alamos National Laboratory in the USA studying the Fenton Hill hot dry rock mass and was originally defined as a rock mass below a depth of 2 to 3 km, without fractures, lacking fluids and with an appropriate temperature (~200 °C) [3]. Many scholars believe that as long as the temperature is at least 200 °C, the depth of the rock body is reasonable, and there are no or few cracks, then the rock mass can be called hot dry rock [4,5]. Currently, China generally refers to rocks with high temperatures, low porosities, low permeabilities, and temperatures higher than 180 °C as hot dry rocks. The term “hot dry rock resources” refers to the thermal energy stored in hot dry rock bodies that can be exploited by humans under existing economic and technological conditions. The extraction of thermal energy stored in hot dry rocks generally requires the creation of artificial reservoirs, i.e., enhanced geothermal systems (Figure 1).

Generally, under normal circumstances, the average geothermal gradient in the shallow part of the Earth’s crust is 30 °C/km. Based on this gradient, the temperature reaches 300 °C at a depth of 10 km below the surface [7]. However, existing studies have shown that hot dry rock resources appear in areas with high temperature gradients and high temperature anomalies. Examples include the Fenton Hill hot dry rock experimental site (the first dry heat rock experimental site in the world) in the United States with a temperature of 330 °C at a depth of 4500 m [8] (Figure 2); the Geysers experimental site in the United States with thermal reservoir temperatures of 280 to 400 °C at a depth of ~3 km [9,10]; the Habanero experimental site in the Cooper Basin, Central Australia, with a 250 °C hot dry rock body at 4300 m (drilling temperature data show that the geothermal temperature gradient in the upper sedimentary layer is 60 °C/km and that in the lower granite is 35–60 °C/km [11,12], which is approximately twice the global average geothermal temperature gradient); the Soultz site in France, with a geothermal gradient in the 1.5 km thick sedimentary section of up to 100 °C/km [13,14], which is approximately three times the average geothermal gradient; and the Gonghe hot dry rock experimental site in China with a temperature of 180 °C at a depth of 2886 m [15,16,17].

Based on data from the above hot dry rock geothermal resource development test sites, hot dry rock resources are very abundant in areas with high temperature gradients and high temperature anomalies and have significant practical application benefits and development potential. Currently, hot dry rock resources are extremely unevenly distributed around the world, and their discovery is often accidental. In response to this situation, scholars have conducted relevant research on the exploration and development of hot dry rock geothermal resources. For example, European scholars have developed a scale-related workflow describing the step-by-step process of how to locate reservoirs using different technologies; Chinese scholars have performed much work on hot dry rock site selection, summarized geothermal geological indicators, and examined heat source mechanisms in different regions [19,20]. However, there is no systematic and universal research method selecting target areas for hot dry rock geothermal resources. Therefore, we must summarize a set of process-based methods to guide the exploration of hot dry rock resources and improve the selection of hot dry rock geothermal resource target areas in China.

This paper proposes a set of universal multiscale and multidisciplinary comprehensive processes for selecting hot dry rock geothermal target areas, emphasizing a focus on exploration in different research stages and the main research methods, with special emphasis on geothermal measurements. As the most direct and effective method for resource exploration, geothermal measurements play an important role in the entire hot dry rock exploration process. In addition, we also emphasize that geophysical methods are used as identification methods, and the large risks involved in the solution process should be treated with caution.

2. Multiscale Multidisciplinary Approach to Target Area Selection for Geothermal Resources in HDRs

2.1. Geothermal Survey Stage

The first stage is the resource survey stage, where the scale of the investigation is the plate scale, the size of the study area is approximately 103 km², and the objective of the task is to carry out the preliminary selection of a target area. The focus of the survey is to clarify plate boundaries and the relative movement types of the plates, which are usually classified as convergent or discrete, by studying plate tectonics and the geological background. The thermal structure of the lithosphere and asthenosphere can be analyzed by studying geological data on volcanoes, magmatic activity, rift valleys, and mantle upwelling. Additionally, the regional geological section and crustal structure are analyzed by seismic stratigraphic imaging.

2.2. Geothermal Exploration Stage

The second stage is the detailed resource exploration stage, where the study scale is at the regional and hot field scales, with study area sizes of approximately 102 km² and 101 km², respectively. The objective is target area selection and drilling.

2.2.1. Regional Scale

The focus of exploration on a regional scale is to obtain the steady-state ground temperature of drilled wells, the thermal physical parameters of cores or outcrops through geothermal measurements, calculations of heat flow and thermal structure, and observations of hot springs to obtain the regional thermal background and identify thermal anomalies.

It should be emphasized that geothermal measurements are a fundamental part of the geothermal survey and exploration phase and are also the most direct method of geothermal exploration. High-quality temperature data are essential for basic geothermal research and for evaluating the potential of geothermal resources. Steady-state temperature measurements are the most effective way to obtain high-quality temperature data. Steady-state temperature measurement refers to the measurement after the borehole temperature has reached equilibrium with the surrounding rocks. Drilling causes large temperature perturbations. The main drivers of these perturbations are the circulation of the drilling fluid and the frictional heat generated by the drill bit. After drilling, the temperature gradually approaches the true formation temperature over time. It takes time for the amplitude of the drilling temperature disturbance to recover to 90%. This length of time is approximately 0.5 to 1.5 times the drilling time [21]. Therefore, to obtain the true formation temperature, measurements must be taken after a sufficient period following the drilling of the well.

The regional geothermal geological data, hydrological data, structural and other regional thermal background, and thermal anomaly data should be fully utilized and combined for target selection. For example, after comprehensive analysis, favorable areas for medium- and high-temperature geothermal resources in mainland China include the southwestern region of the Tibetan Plateau, the eastern region of the Xuefeng Mountains–Taihang Mountains, and the central region of the Cenozoic volcanic rift. The overall manifestations are positive anomalies in geothermal heat flow, widespread distribution of hot springs, frequent Cenozoic volcanoes, and widespread distribution of molten material in the crust [22].

Heat source analysis is carried out using geophysical prospecting methods. Geophysical prospecting technology is an effective means of identifying the depth and location of geothermal resources in hot dry rocks. However, a single geophysical prospecting technique cannot be used to effectively determine the conditions under which geothermal resources occur in hot dry rocks. Therefore, it is necessary to explore a variety of combined geophysical prospecting methods. This technique provides reliable and accurate data for the selection of favorable hot dry rock target areas and the selection of exploration well locations [23,24]. According to the different physical characteristics of the exploration target, different geophysical prospecting methods need to be selected. Based on the characteristics of geothermal and hot dry rocks, the current commonly used geophysical technical methods include the following: the remote sensing infrared method, the gravity exploration method, magnetic exploration, electrical exploration, and seismic detection [25].

2.2.2. Geothermal Field Scale

The focus of investigation at the geothermal field scale is to determine the source and migration path of the fluid through chemical and geothermal temperature analysis of the geothermal fluid. The classification and evaluation of geothermal resource types are followed by target area selection and well siting. Then, geothermal drilling (exploration wells, production wells, and recharge wells) is carried out.

2.3. Reservoir Construction Stage

The reservoir construction stage is the resource development stage. The research scale at this stage is the reservoir scale. The research area is in the order of 100 km². The objective is to carry out fracturing support and pumping tests. The focus of exploration at the reservoir scale is the analysis of stress fields and structures (joints and fractures), the integration of physical and mineralogical rock properties, and the performance of fracturing experiments, including hydraulic fracturing, chemical fracturing, and blast fracturing.

The process of multiscale and multidisciplinary methods for identifying hot dry rock geothermal resource targets is as follows (Figure 3):

3. Decision Analysis Methods for the Optimization of Target Areas for Geothermal Resources in HDRs

3.1. Expert Scoring Method

Based on the hot dry rock geothermal resource target area selection process, an analysis method for management decision-making—the expert scoring method—is introduced. The expert scoring method is a method of quantitative description and qualitative description. It is suitable for the early stage of project decision-making. At this stage, there is often a lack of specific data. The conclusions drawn are mainly based on expert experience and the intentions of decision-makers. The approximate score can be used as a basis for further analysis. The process of decision analysis and evaluation involves first assigning different weights to the different data mentioned above and then evaluating each parameter to obtain the objective function F:

$$\mathrm{F}={\sum }_{\mathrm{k}=1}^{\mathrm{n}}{\mathrm{X}}_{\mathrm{k}}{\mathrm{W}}_{\mathrm{k}}$$

(1)

where F is the objective function, X_k refers to the score of the k-th indicator (four levels, excellent, good, medium, and poor, are based on 100, 75, 50, and 25 points, respectively), and W_k refers to the k-th indicator on the stem. The influence weight of the hot rock geothermal resource target area selection has score levels of 4, 3, 2, and 1 points, which are obtained by expert evaluation and vary among different areas.

3.2. Analysis of Factors Influencing the Selection of Target Areas for Hot Dry Rock Geothermal Resources

The construction of a reasonable influencing factor evaluation system based on the characteristics of hot dry rock geothermal resources is critical to the efficient and accurate selection of target areas for hot dry rock geothermal resources. Through a comprehensive analysis of the multiscale and multidisciplinary methods for identifying hot dry rock geothermal resource target areas, a decision-making analysis system for target area optimization was established, as shown in Figure 4.

3.2.1. Regional Scale

Geological resource conditions include geothermal reservoir depth, geothermal reservoir temperature, geothermal gradient, heat flow, cover thickness, and cover fracture development conditions and can be divided into two categories: geothermal reservoir conditions and caprock conditions.

The geothermal reservoir depth should neither be too shallow nor too deep. If it is too shallow (<1000 m), the rock temperature will not meet the requirements for hot dry rock resource development, or the resource preservation conditions will be poor; if it is too deep (>5000 m), the conditions will be unfavorable because excessive pressure will lead to poor reservoir permeability, increasing the difficulty of drilling and reservoir engineering and significantly increasing the cost. Moreover, the geothermal reservoir temperature is also a very important influencing factor. The geothermal reservoir temperature of most hot dry rock geothermal resource projects globally is greater than 200 °C, and only a few geothermal reservoir temperatures are between 150 and 200 °C [26].

The geothermal gradient is highly important for the development and operation of hot dry rock geothermal resources, and a large geothermal gradient is an important indicator for identifying hot dry rocks [27,28]. If an area has a high geothermal gradient (≥30 °C/km), the geothermal temperature will increase rapidly with depth, and higher-temperature rock bodies can be found at relatively shallow locations. A large geothermal heat flow value (≥70 mW/m²) indicates the possibility of high-temperature rock bodies deep in the Earth. The geothermal heat flow is a comprehensive thermal parameter of the geothermal field and can accurately reflect the characteristics of the geothermal field in the region.

Theoretically, the initial state of rock bodies deep in the Earth involves relatively high temperatures, and these high temperatures can be preserved in some places but not in others. This preservation is closely related to the caprock overlying the geothermal reservoir. The thickness of the caprock and the degree of fracture development play important roles in temperature preservation in geothermal reservoirs [29]. At present, hot dry rock target layers are generally located at depths of 3 to 6 km, so the cap layer, i.e., the rock shallower than 3 km, is equivalent to a protective layer overlying the high-temperature material. The geological characteristics and thermophysical properties of this cap layer determine whether the underlying solid rock is hot dry rock. If the hot dry rock is covered with a stable cover layer with good water insulation and good thermal insulation, the cap layer can effectively prevent heat from escaping to the surface. Therefore, to form high-temperature geothermal resources at depth, the cover layer typically has a low thermal conductivity (<2 W/mK), moderate thickness (>1 km) (with good thermal insulation), high geothermal gradient (≥30 °C/km), and high earth heat flow value (≥70 mW/m²) (indicating the existence of high-temperature characteristics at depth).

3.2.2. Neotectonism and Surficial Evidence

Neotectonism is an important indicator that is often overlooked when searching for hot dry rock. Neotectonic features include volcanoes, earthquakes, and active fault structures [30]. Earthquakes and volcanoes are extremely destructive natural hazards. The occurrence of both phenomena indicates that the thermal energy within the Earth is relatively high and that the Earth’s internal balance has been disrupted, thereby releasing heat in the form of earthquakes or volcanoes [31]. Potential indicators include earthquakes with large magnitudes (>3) and shallow hypocenter depths (10–15 km) and volcanic activity with high frequencies and young ages (e.g., active volcanoes, dormant volcanoes, and volcanoes that have gone extinct since the Miocene). These features are mostly indicative of the existence of an unstable and high heat state in the deep part of the Earth, indicating the possible formation of hot dry rocks. Large, deep, active faults can also generate heat and transfer heat from deep to shallow parts, especially active strike-slip stress fractures with ductile shear properties at depth. Such faults are a direct indication of the presence of high-temperature rock in the deep parts of faults.

The existence of high-temperature hot springs is usually closely related to deep heat accumulation. It is generally believed that groundwater penetrates downward along channels, contacts deep high-temperature materials, is heated, and then flows up to the surface along other channels to form hot springs. Therefore, there are high-temperature rocks (hot dry rocks) at depth. In areas with dense high-temperature hydrothermal fields, such as surface hot springs, hot dry rocks are expected to be found at depth. The use of hydrothermal resources is a conventional method of finding heat (hot dry rocks).

3.2.3. Engineering Conditions

Engineering conditions mainly include engineering construction and related support conditions. The development of hot dry rock resources requires a series of related support technologies, such as high-temperature drilling and well completion technology, testing technology, reservoir stimulation technology, and geophysical monitoring technology.

Drilling into geothermal reservoirs and forming stable wellbores are prerequisites for realizing deep geothermal resource exploitation [32]. Drilling is the main technology for developing hot dry rock resources (accounting for approximately 35% to 60% of the total investment). Establishing a reliable circulating injection and production wellbore channel in ultrahigh-temperature and extremely hard reservoirs is an important step in geothermal resource development and production improvement and is the most important engineering link for reducing costs [33]. Fracturing geothermal reservoirs is the core step in the development of hot dry rock geothermal resources and directly determines the success or failure of hot dry rock geothermal mining and the overall economic benefits. Reservoir creation in hot dry rock requires the formation of a large-scale interconnected complex three-dimensional fracture network. The requirements for fracture creation are high, modification is difficult, and communication between injection and production wells is difficult. Traditional hydraulic fracturing technology used in the oil and gas industry cannot be applied to deep geothermal environments.

3.3. Determining the Weight of Each Influencing Factor and Calculating the Composite Score

Numerous experts who are familiar with the selection of hot dry rock geothermal resource target areas can evaluate the impact weights of the various indicators above, independently express their opinions on the indicator weights through a specific method, and use statistical methods for appropriate processing. The specific method is as follows:

First, m experts are selected to evaluate the influence of each influencing factor on the preferential selection of hot dry rock geothermal resource target areas; from this, the weight coefficient of each influencing factor is calculated separately. The evaluation scale is divided into (4, 3, 2, 1) according to the evaluation value, and the greater the evaluation value is, the greater the influence. Then, the weight coefficients for each factor are as follows:

$${\mathrm{W}}_{\mathrm{k}}={\sum }_{\mathrm{j}=1}^{\mathrm{m}}{\mathrm{q}}_{\mathrm{j}\mathrm{k}}/{\sum }_{\mathrm{j}=1}^{\mathrm{m}}{\sum }_{\mathrm{k}=1}^{\mathrm{n}}{\mathrm{q}}_{\mathrm{j}\mathrm{k}}$$

(2)

where m is the number of experts, n is the number of influencing factors, and q_jk is the scoring value.

After determining the weights of the above influencing factors, each influencing factor is then rated as 100, 75, 50, or 25, corresponding to excellent, good, medium, or poor, respectively. The factors are scored by the selected experts according to their influence on the geothermal resource target area, and the average of the scores for each factor is calculated as follows:

$${\mathrm{X}}_{\mathrm{k}}=\frac{1}{\mathrm{m}}{\sum }_{\mathrm{j}=1}^{\mathrm{m}}{\mathrm{Q}}_{\mathrm{j}\mathrm{k}}$$

(3)

(j = 1, 2, …, m; k = 1,2, …, n; Qjk = [100, 75, 50, 25]).

Finally, the comprehensive score of the hot dry rock geothermal resource target area is calculated:

$$\mathrm{F}={\sum }_{\mathrm{k}=1}^{\mathrm{n}}{\mathrm{X}}_{\mathrm{k}}{\mathrm{W}}_{\mathrm{k}}$$

(4)

4. Application Examples

The expert scoring method was used to optimize the target areas for hot dry rock geothermal resources in the Gonghe Basin and Zhangzhou Basin.

4.1. Gonghe Basin

Located on the northeastern margin of the Tibetan Plateau, bordering the Qinghai Lake Basin to the north and the Qaidam Basin to the west, the Gonghe Basin is a Meso-Cenozoic diamond-shaped faulted basin [34]. The basin is surrounded by a group of conjugate shear zones, the north and south of which are the Animaqing Mélange and the Zongwulong–Qinghai Nanshan Fault, respectively, and the east and west of which are the NNW-trending Duohemao Fault and the Wahongshan–Elashan Fault, respectively [35] (Figure 5). The basin has gone through two stages of evolution, the plate tectonic regime and the intracontinental tectonic regime, which eventually formed the present tectonic landscape, and the transition from the plate tectonic regime to the intracontinental tectonic regime occurred in the Upper Triassic [36]. Inside the basin, there is a thick Cenozoic sedimentary layer, the maximum thickness of which can reach more than 5000 m. At the edge of the basin, Triassic strata and Indosinian granite are dominant. According to published data, more than 84 hot water or geothermal anomalies with water temperatures above 15 °C have been found in the basin, with the highest temperature reaching 96.6 °C.

Eight experts were invited to rate target areas for dry heat rock resources in the Gonghe Basin, and the scores are shown in

Table 1. Weighting of factors influencing the prioritization of hot dry rock resource targets in the Gonghe Basin.

Expert	k1	k2	k3	k4	k5	k6	k7	k8	k9	k10	Remarks
E1	3	4	4	3	3	2	1	1	2	2	qjk = [1, 2, 3, 4]
E2	3	3	4	4	3	2	2	2	1	1
E3	3	2	3	4	2	3	1	2	2	1
E4	3	4	3	3	2	4	2	1	1	2
E5	4	3	4	3	3	2	3	1	1	3
E6	2	3	4	4	2	3	1	2	1	1
E7	4	3	3	4	3	2	2	1	2	2
E8	2	2	3	4	2	3	3	2	1	3

and

Table 2. Evaluation of factors influencing the prioritization of hot dry rock resource targets in the Gonghe Basin.

Expert	k1	k2	k3	k4	k5	k6	k7	k8	k9	k10	Remarks
E1	100	100	75	100	50	75	50	75	50	50	Qjk = [100, 75, 50, 25]
E2	75	75	100	75	75	50	25	25	75	75
E3	50	75	100	50	50	75	75	50	50	75
E4	100	100	75	100	50	75	50	50	50	25
E5	75	100	50	75	50	100	25	50	50	75
E6	75	100	75	75	75	50	75	75	75	25
E7	100	75	700	100	75	100	25	75	75	50
E8	100	75	50	100	50	50	75	75	50	50

, where K₁, K₂ …, K₁₀ represent the geothermal reservoir depth, geothermal reservoir temperature, geothermal gradient, heat flow, cover thickness, fracture development, seismic and volcanic features, high-temperature springs, drilling cost, and geothermal reservoir stimulation, respectively.

The above scores were statistically analyzed, and the weighting coefficients for each factor are as follows: W₁ = 0.1206, W₂ = 0.1206, W₃ = 0.1407, W₄ = 0.1457, W₅ = 0.1005, W₆ = 0.1055, W₇ = 0.0754, W₈ = 0.0603, W₉ = 0.0553, and W₁₀ = 0.0754.

The average of the scores for each factor is as follows: X₁ = 84.375, X₂ = 87.5, X₃ = 153.425, X₄ = 84.375, X₅ = 59.375, X₆ = 71.875, X₇ = 50, X₈ = 59.375, X₉ = 59.375, and X₁₀ = 53.125.

The combined score of the Gonghe Basin hot dry rock resource target area is F = 82.76.

4.2. Zhangzhou Basin

The Zhangzhou Basin is located on the eastern margin of the Cathaysia Block of the South China Plate, where Upper Jurassic and Lower Cretaceous granitic rocks were widely developed as a result of large-scale magmatism during the Yanshan Period caused by the subduction of the Paleo-Pacific Plate (Figure 6). During the late Indosinian movement, the subduction of the Paleo-Pacific Plate caused compression and uplift, which led to the uplift of Fujian from a shallow sea to a land mass. In approximately the Upper Triassic, the leading edge of the plate reached the western lower part of the Fujian Block at a depth of more than 100 km, resulting in small-scale volcanic eruptions and intrusive activities that generated magma on the surface of the subducting plate [37,38]. As subduction continued, the dip angle became steeper, and the space above the subducted slab became a partially molten wedge zone due to the influx of asthenospheric material and the formation of eddies from the rising magma, which delivered a large amount of mantle heat to the overlying slab. At approximately 160 Ma, the plate subduction dip was close to 30°, and the rapid spreading of the Kula–Pacific double spreading ridge caused rapid plate subduction, strong erosion, landward migration of the paleotrench, and rapid generation of a large amount of magma, resulting in large-scale Upper Jurassic volcanic intrusion activity in eastern Fujian. At the same time, the mantle wedge zone vortex was greatly intensified, relatively large intrusions of mid- and upper-crustal remelted magma occurred in west-central Fujian in response to magma generation and strong mantle heat flow, and volcanic eruptions occurred locally. Subsequently, due to the formation of the late Yanshanian–Himalayan subduction zone in the oceanic crust east of Taiwan, subduction zone activity along the Fujian coast ceased, and the scale of magmatic activity significantly decreased [39,40].

Eight experts were invited to rate target areas for dry heat rock resources in the Zhangzhou Basin, and the scores are shown in

Table 3. Weighting of factors influencing the prioritization of hot dry rock resource targets in the Zhangzhou Basin.

Expert	k1	k2	k3	k4	k5	k6	k7	k8	k9	k10	Remarks
E1	3	3	2	2	1	4	1	1	4	3	qjk = [1, 2, 3, 4]
E2	4	4	3	3	2	2	1	3	1	2
E3	4	3	4	3	2	1	2	1	2	3
E4	4	4	3	3	3	2	2	2	1	1
E5	4	3	4	3	3	2	3	1	1	3
E6	2	3	4	4	2	3	1	2	1	1
E7	3	3	4	4	3	2	1	1	2	2
E8	2	2	3	3	3	2	1	1	4	4

and

Table 4. Evaluation of factors influencing the prioritization of hot dry rock resource targets in the Zhangzhou Basin.

Expert	k1	k2	k3	k4	k5	k6	k7	k8	k9	k10	Remarks
E1	25	100	100	100	75	50	50	50	75	50	Qjk = [100, 75, 50, 25]
E2	50	75	75	75	100	75	50	50	50	25
E3	50	100	50	75	100	75	50	50	50	75
E4	25	75	100	100	75	50	25	75	25	50
E5	75	100	75	100	50	100	50	25	75	25
E6	75	75	75	100	75	100	50	75	25	75
E7	50	75	100	75	700	50	75	50	50	25
E8	50	100	100	75	50	75	50	75	25	75

, where K₁, K₂ …, K₁₀ represent geothermal reservoir depth, geothermal reservoir temperature, geothermal gradient, heat flow, cover thickness, fracture development, seismic and volcanic features, high-temperature springs, drilling cost, and geothermal reservoir stimulation, respectively.

The above scores were statistically analyzed, and the weighting coefficients for each factor are as follows: W₁ = 0.1307, W₂ = 0.1256, W₃ = 0.1357, W₄ = 0.1256, W₅ = 0.0955, W₆ = 0.00905, W₇ = 0.0603, W₈ = 0.0603, W₉ = 0.0804, and W₁₀ = 0.0955.

The average of the scores for each factor are as follows: X₁ = 50, X₂ = 87.5, X₃ = 84.375, X₄ = 87.5, X₅ = 153.125, X₆ = 71.875, X₇ = 50, X₈ = 56.25, X₉ = 46.875, and X₁₀ = 50.

The combined score of the Zhangzhou Basin hot dry rock resource target area is F = 76.04.

4.3. Compared with Traditional Methods

By using the expert scoring method, we obtained a comprehensive score of 82.76 points for the hot dry rock geothermal resource target area in the Gonghe Basin and 76.04 points for the Zhangzhou Basin, indicating that the comprehensive geothermal conditions of the Gonghe Basin are superior to those of the Zhangzhou Basin.

If traditional methods are adopted, to evaluate the preferred target areas for hot dry rock geothermal resources in the above two regions from the perspective of a single variable or a few influencing factors, the following conclusions can be drawn: considering the geothermal reservoir depth, the temperatures at a depth of approximately 2700 m for the DR1, DR3, and DR4 in the Gonghe Basin all exceed 150 °C, while in the Zhangzhou Basin, geothermal drilling reaching 4000 m exceeds 110 °C; considering the geothermal gradient, the Gonghe Basin has a range of 39~45 °C/km, and the highest geothermal gradient in drill holes within the Zhangzhou Basin can exceed 37 °C/km; and considering heat flow, the Gonghe Basin ranges from 93.3 to 104.8 mW/m², whereas the Zhangzhou Basin is between 80 and 90 mW/m².

In summary, if traditional methods are used for evaluation, some data may show significant differences (such as geothermal reservoir depth and geothermal gradient), but there are also factors that vary only slightly (such as heat flow). Additionally, considering other conditions, such as the local level of economic development and drilling costs, it may not be easy to make accurate and clear decisions, and it is even possible to draw opposite conclusions from certain indicators (such as neotectonism and superficial). This introduces uncertainty into our decision-making process for the exploration and development of hot dry rock geothermal resources. The use of the expert scoring method elevates the prioritization of target areas from a qualitative study to a quantitative and semiquantitative study, making decisions more concrete and reliable.

Notably, this method has many advantages and value for promotion, but there are still aspects that could be improved, such as the independent score of the Zhangzhou Basin, which cannot directly prove that the basin lacks exploration value. We believe that as this approach is popularized and more scores from different regions are collected for comparison and validation, it might be possible to determine a score threshold that can directly indicate exploration prospects (for example, 70 or 75 points).

5. Conclusions

The following conclusions are drawn in this study:

• According to the geothermal geological indicators of hot dry rock resources, both the Gonghe Basin and the Zhangzhou Basin are favorable areas for the development of hot dry rock resources in mainland China, and both have good heat sources, good thermal conductivity channels, large heat-storing rock bodies, and good heat-preserving cover rocks.
• Based on the expert scoring method, although the weighting of the factors influencing the geothermal resources of hot dry rocks in different regions varies slightly among experts, the four influencing factors that have the greatest weights are the depth of burial of the thermal reservoir, the temperature of the thermal reservoir, the geothermal gradient, and the geothermal heat flow.
• Geothermal temperature measurements are the most direct and effective method for identifying hot dry rock geothermal resources. By analyzing the influencing factors, we find that the thermal storage depth, thermal reservoir temperature, geothermal temperature gradient, geothermal heat flow, and caprock thickness can all be obtained directly from geothermal temperature measurements. This approach occupies the most important position in the whole multiscale, multidisciplinary preference determination process.
• Through this decision analysis and evaluation tool, the target area conditions are divided and decomposed, the corresponding parameter evaluation index design is established, numerous experts in the relevant fields are invited to score the target area without consulting each other, and a comprehensive score is obtained through corresponding calculations. The advantage of this method is that it can quantify qualitative evaluations, and it is intuitive, simple, and easy to understand. Furthermore, this approach saves time and has strong applicability.