Investigating the Geothermal Potentiality of Hail Granites, Northern KSA: The Preliminary Results

Abstract

The work aims to give a preliminary investigation of the geothermal potentiality of the hot dry granitic rocks in the Hail area, Northern KSA. The Hail area is characterized by a massive exposed belt of radioactive granitic rocks in the southern part, while the northern part is covered by a sedimentary section. A comprehensive methodology utilizing different categories of mineralogical petrographic, geochemical, geophysical well logging and, radiometry datasets, was used to assess the radiogenic heat production capacity of this granite. The measured data are integrated and interpreted to quantify the potential geothermal capacity of the granite and estimate its possible power production. The radioactivity and radiogenic heat production (RHP) of the Hail granites are among the highest recorded values in Saudi Arabia. Land measurements indicate uranium, thorium, potassium, and RHP values of 17.80 ppm, 90.0 ppm, 5.20%, and 11.93 µW/m³, respectively. The results indicated the presence of a reasonable subsurface geothermal reservoir condition with heat flow up to 99.87 mW/M² and a reservoir temperature of 200 °C (5 km depth). Scenarios for energy production through injecting water and high-pressure CO₂ in the naturally/induced fractured rock are demonstrated. Reserve estimate revealed that at a 2% heat recovery level, the Hail granites could generate about 3.15 × 10¹⁶ MWe, contributing to an average figure of 3.43 × 10¹² kWh/y, for annual energy per capita Saudi share. The results of this study emphasized the potential contribution of the Hail granite in the future of the energy mix of KSA, as a new renewable and sustainable resource. It is recommended to enhance the surface geophysical survey in conjunction with a detailed thermo-mechanical laboratory investigation to delineate the subsurface orientation and geometry of the granite and understand its behavior under different temperature and pressure conditions.

1. Introduction

Saudi Arabia’s Vision 2030 has a clear task toward a sustainable future, aiming for reducing CO₂ emissions and net zero emissions by 2060. The vision aims to produce 50% of energy through other renewable resources to achieve the energy mix promised plan by 2030 [1]. For a long time, energy conservation in Saudi Arabia has relied mainly on conventional fuel resources, namely oil and natural gas. By the end of 2023, an estimated 453 terawatt hours (TWh) of electricity was generated, with 62% from natural gas, 38% from oil, and less than 1% from renewables. To increase the share of renewable energy, Saudi Arabia’s National Renewable Energy Program (NREP) initiated a 5 GW renewable energy project in 2020, primarily focused on solar and wind power. By the end of 2023, five of these projects, totaling 2.8 GW, represented Saudi Arabia’s installed renewable energy capacity (

Table 1. Renewable energy capacity in Saudi Arabia, 2023.

Project	Type of Project	Capacity (Megawatts)	Date of Operation (Year)
Sudair	Solar	1500	2023
Dumat Al-Jandal	Wind energy	400	2022
Sakakah	Solar	300	2020
South Jeddah	Solar	300	2023
Rabigh-1	Solar	300	2023
Total	2800

) [2].

Currently, Saudi Arabia aims to expand its electricity generation capacity from natural gas and renewable energy sources as part of its Vision 2030 initiative. The country seeks to increase the share of renewable energy to 50% of total electricity generation capacity by 2030, up from 3% at the end of 2023. As of mid-2024, more than 21 GW of renewable energy projects are planned, with 9.7 GW scheduled for completion by 2026 [3].

Geothermal resources in Saudi Arabia are a renewable energy source that could contribute to achieving the country’s 2030 vision and mitigating climate change. Despite some exploration activities during the last 10 years, these resources are yet to be explored and/or exploited in detail. Most geothermal research conducted over the past two decades has been primarily academic, carried out by researchers from Saudi universities, research centers, and the Saudi Geological Survey (SGS) [4,5,6,7,8,9,10,11,12,13,14,15,16]. Nowadays, several geothermal activities, both deep and shallow, are being carried out by various governmental and private sectors. i.e., Saudi Geological Survey [17] NEOM, TAQA, and Aramco. Saudi Arabia possesses a diverse and well-documented range of geothermal resources, including numerous hydrothermal systems and volcanic activity, particularly in the western parts of the Arabian Shield. Most of these geothermal features result from the tectonic activity associated with the Red Sea and the Gulf of Aqaba [18,19,20]. A large volume of granitic rocks in the Saudi Arabian Shield contains anomalously high concentrations of radioactive elements, such as uranium, thorium, and potassium. These rocks have a high heat capacity (hot dry rocks), forming what is known as an Enhanced Geothermal System (EGS), which is considered an excellent source of energy production [6,8,16,21]. EGS technology uses rock fracture, pumped water, and vertical and horizontal drilling to draw steam from underground heat and use it to power aboveground turbines. This method differs from conventional geothermal development, which pulls heat and steam from naturally occurring permeable rock.

Potential EGS can generate electricity without relying on the natural convective systems typically found in hydrothermal resources. Locations with deep-seated, highly radioactive granitic rocks that store significant heat, overlain by thick sedimentary layers (3–5 km), are favorable for such projects. The thick sedimentary cover acts as an insulator, preventing or slowing heat loss from the buried granite. The permeability and the fracture system (both natural and induced) of the radiogenic granitic rocks are key factors in facilitating thermal water flow. The fracture network can be enhanced through hydro-shearing by injecting high-pressure fluid (water and/or CO₂) into naturally fractured rock via injection wells [22,23,24]. The injected fluids will pass through the fractures, absorbing heat and acting as a heat transfer medium. At the surface, the heat component will be separated from the thermal fluids and converted into electricity using either a steam turbine or a binary power plant system. The cooled fluid can then be reinjected and cycled back into the subsurface to repeat the process.

Worldwide, the first EGS project was carried out at Fenton Hill (USA) in the early 1970s, where a small reservoir was created by hydro-fracturing granite at a depth of 3 km at a temperature of 185 °C. The Soultz-sous-Forêts project, which began in 1984, successfully achieved completion with a pilot plant generating 1.0 MWe of power in 2012. The Soultz project renamed the earlier HDR (Hot Dry Rock) concept to EGS after the drill hole encountered large volumes of hot brines in the basement granite of the Rhine Graben. Since then, HDR has been referred to as EGS [25]. The first demonstration of EGS geothermal electricity generation in Australia was at Cooper Basin in 2013. As a first phase, a 1 MWe pilot plant was operated at Habanero for 160 days at a flow rate of 19 kg/s and 215 °C production wellhead temperature. The project did not proceed to the next stages given there would not be a potential end user co-located [26].

Since the technology involves the creation of a heat exchanger in granites, in which cold water is injected and circulated in the fracture system of the granites where it warms up and is then pumped to the surface, it was named the EGS.

Recently, EGS technology has been implemented and utilized in many projects around the world. However, it is still under development and growing rapidly [27]. A total of 18 EGS sites around the world (e.g., the United States, Australia, the EU, South Korea, and Japan) are either operating or under development. An EGS plant is expected to have an economic lifetime of 20–30 years using current technology [28]. It is clear that granites with significant heat generation will be a major energy source for the world in the future [29]. CO₂ storage and heat recovery from hot granites via CO₂ circulation are two key benefits of the EGS system [8,30,31].

In China, a promised enhanced geothermal project was established at the Gonghe Basin, northwestern China, which manifests a massive potential source of geothermal energy within the hot dry rock (HDR). Numerical simulation was used to investigate the feasibility of power generation from a fractured granite reservoir. A feasibility study for power generation at depths between 2900 m and 3300 m was forecasted over 30 years with a production rate of 40 kg/s. The results demonstrated a promising production performance with electric power in the range of 2.86–3.23 MW over the 30 years [32]. According to the Energy Information Administration (EIA) data, the geothermal energy of the USA is contributing to 1.6% of the country’s main energy consumption in 2022. This measure encompasses transportation, industrial, residential, and commercial energy usage. By the end of 2023 Fervo Energy, in collaboration with Google, announced that it has started up a new 3.5-MW EGS geothermal plant in Nevada to supply electricity to data centers in Las Vegas and other parts of the state [33]. Northern India is enriched by a variety of older granites that have intruded into basement rocks. These granites occupy an area of about 150,000 km², extend to great depths, and sometimes form large batholiths several kilometers thick. An estimate of a small volume of these granites indicates that there is potential to generate a minimum of 61,160 × 1012 kWh [34].

In 2021, Japan considered the supercritical geothermal power generation from the superhot rock energy, as a key technology for the government’s Green Innovation Strategy, a strategy to spur economic growth, while reaching carbon neutrality by 2050. The Kakkonda geothermal field is one of the promised geothermal projects that is currently under research considering the EGS of the Quaternary granite intrusion in the Tohoku Volcanic Arc. Some wells were drilled into this granite to determine the thermal structure and deep fracture systems. The hottest acquired temperature was 580 °C. The Kakkonda geothermal power plant is a 80 MW operating geothermal power plant that could produce 666 GWhe annually, assuming 95% capacity factor [35,36,37].

The Arabian Shield of Saudi Arabia is rich in granitic and volcanic rocks, with granitic rocks covering an area of 161,467 km² and Harrats occupying approximately 90,000 km² [6,8,38,39,40]. The Midyan and Hail granites are among the promising locations of alkaline and peralkaline granites with anomalous radioactivity, which could be suitable for potential EGS projects [5,6,7,8,16,21]. This study primarily aims to provide a preliminary investigation of the geothermal potential of the granitic rocks in the Hail area, in northern Saudi Arabia. A comprehensive methodology involving the use of various types of mineralogical, petrographic, geochemical, geophysical well-logging, and radiometric datasets is applied. A detailed mineralogical study including thin section analysis, XRF, and XRD investigations was conducted on selected granitic samples. Geochemical analyses of water samples collected from available wells along with well-logging data were interpreted to support a geothermometric study and to determine the properties of the geothermal reservoir. Radiogenic heat production was estimated using available gamma spectrometry logs and granite samples. These datasets were integrated and interpreted together to assess the heat generation capacity of the granites and to evaluate their potential for geothermal energy and possible power production.

2. Geological Setting

The Hail region is located in the northern part of the Arabian Peninsula. It represents one of the major provinces of Saudi Arabia covering an area of 118,322 km² and represents 6% of the total area of the country (Figure 1). The northern part of the Hail area is covered by a thick sequence of sedimentary layers ranging in age from the early Paleozoic Cambrian (Saq Formation) to the recent deposits mainly represented by widespread sheets of Quaternary sediments, as well as, some gravel and aeolian sandstone. The Hail granite consists mainly of alkaline and calc-alkaline granites and granodiorites. As a part of the Arabian Shield, it is exposed in the central and southern parts of the Hail province (See red-colored rock units, Figure 2).

In the extreme south-western part of the Hail province, a thick sheet of basaltic lava intrusions is present. The alkaline and calc-alkaline granites and granodiorites of the Hail area are enriched in radioactive minerals (Th, K and U) and can be considered as a promising geothermal source of potential energy production. The stratigraphic column of the Hail area is shown in Figure 2.

Radioactivity and Geothermal Signatures

The Saq Formation is exposed in various locations and is considered the main aquifer in the area. Elevated levels of radioactivity have been recorded in the water samples collected from the Saq aquifer. This radioactivity is primarily attributed to the leached radioactive elements from the erosion of radiogenic granitic rocks [41,42].

Evidence of ancient volcanic activity is well-documented in the Hail province (Figure 3). These features are mainly associated with multiple episodes of eruptions that occurred across the Arabian Shield forming extensive fields of volcanic basalts known as ‘Harrats’. The presence of a thick sheet of basaltic lava intrusions in the south-western part of the Hail area is among these volcanic activities. Another notable geological feature is the Hatima Crater, considered one of the most prominent volcanic craters in the highlands of the Hail region. Hatima Crater is an impact crater with a diameter of approximately 40 m and a depth of about 10 m. The crater is believed to have been formed by a meteorite impact and serves as a reminder of the dynamic geological processes that have shaped our planet’s surface over millions of years (Figure 3).

3. Available Data and Methodology

The data used in this work are represented mainly by the chemical analyses of the 29 water wells scattered in the study area and the geophysical well logs of shallow wells penetrating through the exposed granites, as well as, the thick sedimentary layers (See Figure 2 for locations). Some rock samples from the exposed granitic rocks in the Hail area were selected and analyzed for petrographic characteristics and radiogenic heat production potential. For data calibration and validation, the common standards are followed throughout the whole workflow for the analytical technique, each alone.

Microscopic thin section analysis was conducted alongside detailed mineralogical investigations, including XRD, XRF, and ICP-MS. A petrographic comparison with some selected granite samples from different parts of the Arabian Shield (i.e., Midyan and Quawayh) was conducted. The major elements were determined using X-ray fluorescence spectrometry. Trace elements, including REE, were determined using inductively coupled plasma mass spectrometry (ICP-MS).

The geochemical analyses (major, minor, and trace) of water samples were used to enhance a geothermometric study. The liquid chemistry plotting spreadsheet v. 3 provided by Powell Geoscience [43] was mainly used in the analyses. A selected number of geothermometers (Quartz, Chalcedony, Na, K, Ca geothermometers and combinations (i.e., Na–K and Na–K–Ca) were applied to estimate subsurface formation temperature, discharge enthalpy, and heat flow [44,45,46]. These geothermometers have an intermediate temperature range of 25–250 °C, which is expected to align with the thermal properties of the Hail granites and the overlying sediments.

Well, logging datasets (Gamma ray, spectrometry gamma ray ‘U, Th and K’, resistivity, density, neutron, etc.) from three selected wells penetrating the granitic rocks and the sedimentary cover were analyzed to determine the heat generation potential of these rocks. The academic license of the Interactive Petrophysics (IP) software (v. 6.2) (Schlumberger donation) is used in the well-logging analysis. The IP software includes all the modules necessary for performing a full petrophysical characterization of the reservoir (lithology, porosity, fluid content, etc.), in addition to generating the necessary analogs and cross-plots. These plots are supported by some important statistical parameters (Mean, median stranded error, P10–P90, etc.), for data validation. The radioactivity measurements from geophysical well logs and surface measurements were used to quantify heat production in granitic rocks. The empirical relationship proposed by [47,48] was applied to calculate radiogenic heat production.

The radiogenic heat production rate (A) is a scalar and isotropic petrophysical property independent of the in situ temperature and pressure. It can be calculated using the concentration of uranium (U), thorium (Th), and potassium (K) elements and the density (ρ) of the rock according to the following equation [48]:

A = 10⁻⁵ ρ (9.52 cU + 2.56 cTh + 3.48 cK)

where cU is the concentration of uranium in ppm, cTh is the concentration of thorium in ppm, cK is the concentration of potassium in %, and ρ is the rock density in kg/ m³.

Furthermore, the radiogenic heat generation was estimated from the surface measurements of the available samples of the Hail granites, as well as, samples from Midyan and Quwaiyah granites for matching and correlation. RS-230 device was used for measuring the radioactivity of granitic samples. The measurements were made for collected maples both in the field and at the laboratory. For data calibration, the background radioactivity is measured and taken into account, while measuring the radioactivity of rocks. Furthermore, for lab-based radioactive measurements, the data were calibrated with field measurements for some referenced samples. The measured data were integrated and interpreted to quantify the potential geothermal capacity of the granite and estimate its possible power production. A geothermal reserve estimation was conducted based on the analyzed data. The following mathematical forms were used [29,49,50]:

Average temperature of resource (°C) = (Temperature at 5 km depth (°C) + cut-off temperature)/2

Peta Joule PJ (per km³) = 2.2 × (Average temperature − cut-off temperature)

Volume of resource (Above 5 km) = Granite area × Thickness (Above 5 km and >cut-off temperature)

Geothermal Resource (PJ) = PJ (per km³) × Resource volume (Above 5 km)

Geothermal Resource (kWh) = Resource (PJ) × 278 × 10⁹

Geothermal Resource (MWe) = Geothermal Resource (kWh)/(7.88 × 10⁹)

Scenarios for energy production using CO₂ as an energy transfer medium were discussed. Two wells (one production and one exploration) were proposed for potential energy generation.

4. Data Analysis and Results

4.1. Lithological Characteristics of the Hail Granite

The lithological constituents of Hail granite were primarily determined through XRF, XRD, and petrographic thin section analyses.

Table 2. Geochemical analyses of trace elements, oxides, and rare earth elements (REE) of HS1 sample, Hail granite [21].

Trace Elements (ppm)
Ba	Rb	Sr	Y	Zr	Nb	Th	Pb	Ga	Zn	Cu
47.00	226.10	10.00	110.30	282.40	111.74	203.79	54.40	29.96	1331.10	2265.60
Ni	V	Cr	Hf	Cs	Sc	Ta	Co	Li	Be	U
11.40	8.00	317.00	11.01	4.30	1.00	7.70	1.10	12.80	5.00	15.40
Oxides (%)
SiO2	TiO2	Al2O3	Fe2O3	MnO	MgO	CaO	Na2O	K2O	P2O5	Cr2O3
76.52	0.13	11.56	2.60	0.06	0.05	0.48	3.88	4.56	<0.01	0.07
Rare Earth Elements (REE, ppp)
Mo	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Er
89.83	95.30	188.99	21.60	76.60	17.20	0.30	18.20	3.20	20.90	12.60

presents the geochemical data for trace elements, rare earth elements (REEs), and oxides from one representative rock sample of the Hail granites. The high SiO₂ content (>70%) indicates a highly evolved felsic rock, primarily composed of alkali feldspar. The table shows very high concentrations of trace elements, such as Rb (226 ppm), Y (110.3 ppm), Zr (282.4 ppm), Hf (11.01 ppm), Nb (111.74 ppm), U (15.4 ppm), Th (54.4o) and Ta (7.7 ppm). Furthermore, the high concentration of the light REE (Mo 89.83 ppp, Ce 188.99 ppp, Nd 76.6 ppp) and heavy REE (Dy 20.9 ppp, Er 12.6 ppp), together with the high radioactive elements suggests a potential heat production source [21].

Figure 4A,B presents the thin sections and XRD analysis of the Hail granite, revealing quartz, potash feldspar, biotite, and albite as the main mineralogical constituents.

4.2. Water Type and Geothermometric Analysis

The water type characteristics and geothermometric analyses are based on the chemical analyses of the 29 water samples in the Hail province (

Table 3. Shows the chemical analyses of the available water samples.

Sample No.	TDS	EC	pH	T	SiO2	Ca2+	Mg2+	Na+	K+	Cl−	F−
	ppm	mS/cm		°C	[mg/L]	[mg/L]	[mg/L]	[mg/L]	[mg/L]	[mg/L]	[mg/L]
1	729	1461	7.93	25	13.2	112	19	141	8	328	0.9
2	2090	4040	8.00	26.4	13.1	191	96	606	29	792	1.3
3	332	681	7.82	28.8	12.1	51	9	62	5	114	0.8
4	789	1589	7.99	27.9	16.7	112	16	218	5	371	0.8
5	303	623	7.28	26.6	14.4	44	9	44	5	70	0.1
6	647	1302	7.5	29.6	15.4	55	22	129	21	256	0.4
7	368	747	7.2	24.6	13.0	61	10	49	4	117	0.7
8	501	1017	6.77	23.7	11.6	64	20	75	6	150	1.3
9	502	1014	6.87	37.3	20.3	62	20	63	26	104	0.2
10	1718	3340	6.41	25.5	11.4	103	93	384	13	776	1.1
11	625	1260	7.74	26.4	14.4	34	28	157	3	182	2.6
12	322	660	7.02	26.8	30.6	49	8	46	3	43	2.5
13	370	757	7.64	25.8	11.5	55	12	53	6	109	0.5
14	478	971	7.69	27.7	13.2	50	40	50	12	110	0.4
15	969	1924	7.6	27.6	14.0	58	27	246	10	479	0.8
16	437	890	7.64	28.5	13.4	50	13	74	6	171	0.8
17	399	814	7.47	25.8	11.0	65	10	53	5	144	1.0
18	477	969	7.62	24.1	13.5	46	12	93	8	169	0.2
19	284	585	7.54	22.4	21.6	47	5	39	2	53	1.8
20	470	955	7.54	22.3	12.6	82	11	58	5	204	0.9
21	621	1252	7.67	22.7	17.3	83	11	113	3	256	1.4
22	473	961	7.77	22.9	27.8	101	10	44	4	141	0.9
23	689	1385	7.74	22.1	11.6	103	14	108	5	329	1.1
24	685	1377	6.83	21.9	12.6	56	31	145	15	199	0.4
25	230	476	7.83	22.6	19.4	44	4	25	2	35	2.1
26	196.8	408	7.87	22.6	19.9	45	5	17	3	14	2.2
27	1319	2590	7.37	37.0	17.8	205	90	205	27	468	0.8
28	466	947	7.03	44.2	23.6	46	15	100	25	156	0.3
29	810	1620	8.51	28.5	13.9	137	18	213	6	428	1.0

). This table presents the concentrations of major chemical constituents along with key physico-chemical parameters such as total dissolved solids (TDS), electrical conductivity (EC), and pH.

Ternary, Piper, and Giggenbach diagrams were generated to determine the water type and degree of maturity (Figure 5, Figure 6 and Figure 7). The Cl- SO₄-HCO₃ ternary diagram commonly used to classify thermal fluids, helps differentiate between various types of thermal waters. Chloride-rich waters typically represent high-temperature geothermal fluids and plot within the ‘mature water’ field, indicating deep circulation of hot fluids. In contrast, sulfate-rich waters fall within the field associated with volcanic and/or steam-heated waters [51,52].

Regarding Figure 5, most of the samples plot near the mature water area but below the 75% weighted Cl⁻ line, indicating chloride concentrations lower than those typically associated with deep, hot geothermal fluids. This suggests weak subsurface water circulation, which may correspond to relatively low predicted reservoir temperatures. A considerable number of data points are shifted towards the peripheral water field, while two samples (Nos. 2 and 7) fall within the volcanic water domain. Data points that are located within the peripheral water region indicate high bicarbonate concentration and possible mixing with cold groundwater. These samples are mostly derived from shallow depths or sedimentary layers overlying or adjacent to granitic bedrock. Notably, the samples located near the volcanic area exhibit significantly higher TDS and Cl⁻ concentrations (2090 and 1391 ppm for TDS; 792 and 468 ppm for Cl⁻, respectively) compared to other samples, indicating greater mineralization and potential geothermal influence.

Figure 6 shows the Piper plot, which indicates the characteristics and type of water samples. The figure shows data clustering in regions 2 and 3 indicating mixed Ca-Mg-Cl and Na-Cl water types. The associated SO₄-HCO₃-Cl and Mg-Ca-Na+K ternary diagrams assign Cl and Na-K water-dominated types (regions F and C).

The geothermometric analysis is based mainly on the Quartz, Chalcedony, Na, K, and Ca geothermometers, as well as combinations of these. The analysis indicated that the Na-K-Ca geothermometer gave comparatively higher reservoir temperature estimates ranging from 160 to 350 °C, while the chalcedony geothermometer gave the lowest values (<50 °C). As a result, the interpretation avoids using these geothermometers since they are deemed to be unrepresentative of the reservoir temperature. The saturated phase was represented by the Quartz geothermometers [44,45], which provided significantly better-matched reservoir temperatures in the range of 42–80 °C and 41–81 °C, respectively. Given the geothermal gradient of a sedimentary section of the Hail area that is 2000 m thick (as shown by the available drilled wells), these estimates appear to be plausible.

Table 4. Shows the reservoir temperatures from different geothermometers and heat flow.

Sample No.	Quartz Geothermometer		Na-K Geothermometer	Heat Flow	Heat Flow
	Temp. [44]	Temp. [45]	Temp.	HF-Qz [44]	HF—Na-K
1	48.52	47.86	143.53	52.62	194.19
2	48.26	47.58	130.21	52.23	174.34
3	45.56	44.65	174.03	48.21	239.63
4	56.80	56.70	81.27	64.96	101.42
5	51.53	51.11	208.40	57.11	290.85
6	53.90	53.63	250.60	60.64	353.73
7	47.99	47.30	175.16	51.84	241.32
8	44.14	43.09	173.28	46.10	238.52
9	64.00	64.23	404.23	75.70	582.63
10	43.56	42.46	105.60	45.24	137.67
11	51.53	51.11	70.90	57.11	85.98
12	80.23	80.74	155.10	99.87	211.43
13	43.85	42.78	207.99	45.67	290.24
14	48.52	47.86	305.35	52.62	435.31
15	50.55	50.06	118.22	55.65	156.48
16	49.03	48.42	174.53	53.39	240.38
17	42.38	41.15	189.12	43.47	262.12
18	49.29	48.70	180.12	53.77	248.71
19	66.36	66.66	135.52	79.21	182.25
20	46.93	46.14	180.32	50.25	249.01
21	58.08	58.05	90.03	66.87	114.48
22	76.29	76.78	185.46	94.01	256.67
23	44.14	43.09	127.72	46.10	170.63
24	46.93	46.14	198.47	50.25	276.06
25	62.30	62.46	173.28	73.16	238.52
26	63.25	63.45	261.11	74.58	369.39
27	59.12	59.14	224.88	68.42	315.40
28	69.78	70.16	311.78	84.31	444.89
29	50.30	49.79	93.71	55.28	119.95

presents the selected reservoir temperatures derived from both the quartz and Na–K geothermometers. The Na–K geothermometers are also presented since the water type in the the Hail area is mainly dominated by Cl and Na-K (regions F and C, Figure 6). Corresponding heat flow (HF) estimates based on the quartz geothermometer range from 50 to 85 mW/M², which aligns with an intermediate heat flow regime, whereas the HF values from the Na–K method appear anomalously high and less dependable.

The Giggenbach diagram shows that all water samples are located in the immature area, very close to the Mg corner (Figure 7). This indicates immature water with limited or weak interaction of surface water with the deep-seated rocks (EGS granitic rocks).

4.3. Radiogenic Heat Production

The radiogenic heat production of Hail granites was estimated using available well-logging datasets (Spectral Gamma Ray, SGR) and land measurements of the granitic rock samples. In addition, the available well-logging data of three selected wells (H-1, H-3, and H-6) were used to assess the radiogenic heat production (RHP).

Figure 8, Figure 9, Figure 10, Figure 11, Figure 12, Figure 13 and Figure 14, show the gamma ray, spectral gamma ray logs, the estimated RHP, and the constructed histograms. The H-1 well is drilled mainly in the granitic rocks to a total depth of 75 m. Figure 8 shows the gamma ray, spectral gamma ray, and RHP plot of this well. A high gamma ray range of up to 120 API was detected coupled with high content of Th (up to 40 ppm), considerable U content (up to 20 ppm), and low K content in the range of 5%. The estimated RHP for H-1 reaches a maximum of 8.0 μW/m³, with an average value of approximately 4.0 μW/m³ (Track 7, Figure 8).

Figure 9 and Figure 10 demonstrate the logging data of the H-3 well. The well was drilled to a depth of 40 m in the Arabian Shield granitic rocks. The gamma ray, temperature, caliber, conductivity, and spectral gamma ray logs are presented in Figure 9. Elevated thorium concentrations are observed in the upper 20 m of the well, corresponding to high gamma ray values reaching up to 150 API. The caliper log indicates significantly enlarged borehole diameters (>14 inches) in this section, suggesting a highly fractured zone. The SGR and RHP plot (Figure 10) shows nearly similar behavior like that in H-1. However, the RHP tends to decrease (<4 μW/m³) at the lower section of the well (depth between 20 and 40 m). The average RHP value of this well is 8.33 μW/m with an average value of 3.06 μW/m³ (Figure 11D). The constructed Th, U, and K histograms show maximum values of 39.74 ppm, 31.42 ppm, and 6.45% with average values of 16.13 ppm, 9.67 ppm, and 3.42% respectively (Figure 11A–C). The statistical analysis (Mean, median standard error, P10-P90, etc.) is given below each plot for data validation.

On the other hand, Figure 12 and Figure 13 exhibit the conventional well logs (Resistivity, density, neutron, etc.), the spectral gamma ray logs (TH, K, and U), and the estimated heat production of H-7. The well is drilled in a thick sedimentary layer of the Saq sandstone Formation to a depth of 350 m. The well shows a unique high gamma ray response exceeding 350 API at depths between 260 and 350 m. The contents of Th and U are extremely high reaching up to 149.66 and 47.43 ppm. The constructed histograms show average values of 36.76, 5.92, and 0.36 for Th, U, and K, respectively (Figure 14). The RHP records high value of 20 μW/m³ in front of this section. The RHP records maximum value up to 18.42 μW/m³ in front of this section with an average of 5.18 μW/m³, as shown in Figure 14D.

Table 5. The radioactivity and RHP of the Hail granites compared with other granites in Saudi Arabia.

Area	Sample Number	Dose Ratio	U ppm	Th ppm	K %	RHP μW/m3
Hail Granite	HS1	497	17.80	90.20	5.20	11.93
Midyan Granite	65	251	9.70	31.10	5.40	5.44
Midyan Granite	64	209	7.00	25.00	5.10	4.23
Quwaiyah Granite	2	85	6.04	8.10	1.25	2.35
Quwaiyah Granite	1	82	4.53	9.67	1.25	2.06
Jabal Abha [53]	184018	--	9.12	29.80	3.40	4.70
Diorite [54]	165546	--	13.80	40.60	40.6	7.60
J. Abu Sadi [55]	155619	--	5.72	10.50	4.11	2.60

shows a comparison of Hail granites with other granites in Saudi Arabia. The radiogenic measurements of some other granitic samples (i.e., Midyan, Quwaiyah, etc.) are also indicated. The table shows that Hail granite attains the highest rate of radioactivity among other granites in Midyan and the other locality in Saudi Arabia. The concentrations of Th, U, K, and RHP are found to be 17.80 ppm, 90.20 ppm, 5.2%, and 11.93 μW/m³, respectively.

5. Discussion

5.1. Subsurface Thermal Regimes at Hail Area

The subsurface thermal regime of the Hail area—including surface temperature, subsurface reservoir temperature, and heat flow parameters are presented in a number of lateral distribution maps (Figure 15, Figure 16 and Figure 17). The surface temperature distribution map is generated to check for the possible surface signatures if found. It depends on the temperature of the water samples collected from different wells in the Hail area. It shows a wide range of variations from 20 to 45 °C with an obvious increase towards the eastern flank of the area (occupied by water samples 9, 27, and 28). The maximum reached temperature is 44.2 °C for sample 28.

Figure 16 shows the subsurface reservoir temperature as inferred from the silica geothermometers. The map shows a remarkable increase in reservoir temperature towards the eastern part of the Hail area, beside a well-recognized trend of increase in the central and southern regions, associated mainly with the well-developed volume of granitic rocks. The maximum estimated subsurface reservoir temperature falls within the range of 80–90 °C. The temperature range is less than expected due to many reasons. Most of the sampled water wells are drilled to depths between 300 and 500 m within the Saq Sandstone Formation. Only a few water wells were drilled within the granitic rocks (See, Figure 2 for wells 12, 22, 25, and 26). The chance of water contact/circulation within the granitic rocks is only limited to the shallower section of granites, while significant heat is likely stored in the deeper granitic layers. Therefore, it is expected that reservoir temperatures would increase with depth, particularly where deep-seated hot granitic rocks are present.

The heat flow (HF) map shows a similar trend of increasing values towards the eastern and central-southern parts of the Hail area (Figure 17). These HF values range from medium to high with the maximum recorded value reaching 99.87 mW/M². This value is associated with sample 12, which is located within the crater of the Hatima volcanic cone.

The Cl-Quartz discharge enthalpy plot shows that the subsurface discharge enthalpy of the reservoir is in the range of 200–450 kj/kg (Figure 18).

Based on the geothermometers and logging analyses, the subsurface temperature in the Hail area is estimated to range between 85 °C and 90 °C at an approximate depth of 2000 m (thickness of sedimentary cover from available drilled wells). While granitic rocks are extensively exposed in the central and southern parts of the Hail area, the northern region is characterized by a sedimentary cover with an average thickness of approximately 2.5 km overlying the basement granites. However, in the future, a more detailed geophysical survey should be executed to determine the exact thickness of the sedimentary cover overlying the basement granitic rocks. The estimated reservoir temperature of 85–90 °C at 2000 m implies a geothermal gradient of approximately 40–45 °C/km. Therefore, at a depth of 5 km, a temperature window of 200 °C could be reached.

5.2. EGS Geothermal Reserves

An estimate of the possible energy generation of the EGS of the Hail area is presented in

Table 6. Geothermal generation potential of the Hail EGS granite.

Parameter	Value
Estimated area of granite (km2)	52,072
Estimated temperature at 5 km depth (°C)	200
Cut-off temperature (°C) @ 3500 m	145
Average depth to granite and Thickness of the sedimentary cover (km)	2.50
Thickness (km) of granite above 5 km and Cut-off temperature of 145 °C	1.50
Estimated average temperature of resource (°C)	172.5
PJ (km3)	60.5
Estimated volume of resource in granite and above 5 km (km3) and Cut-off Temperature	70,108
Geothermal Resource in PJ (Cut-off temperature of 145 °C)	4.73 × 10 6
Geothermal Resource in kWh	1.3137 × 10 18
Geothermal Resource in Joule	4.72932 × 10 24
Geothermal Resource in MWe (Efficiency, 0.90)	1.67 × 10 8
Geothermal Resource in kWh (Heat recovery 2%)	2.63 × 10 16

. The assumption is based on a subsurface temperature of 200 °C at a depth of 5 km. A cut-off temperature limit of 145 °C is taken at a depth of 3500 m. Consequently, the thickness of granite above 5 km and above the cut-off temperature (145 °C) is 1.50 km. It is important to note that this estimate is restricted mainly to the northern parts of the Hail area where the sedimentary cover does occur (Figure 19). This thick sedimentary cover acts as an insulator that prevents heat loss from the deeply buried granite.

With an area of 52,072 km² and a reservoir thickness of 1.5 km, the estimated volume of granitic resource above a depth of 5 km and exceeding the 145 °C cut-off temperature is approximately 70,108 km³. Based on these parameters, the EGS of the Hail area can generate 1.313 × 10¹⁸ kWh of thermal energy. This is equivalent to 1.67 × 10⁸ MWe with efficiency of 0.90 (1 MW × 0.90 × 365 days × 24 h = 7884 MWh). As the nature and geometry of the subsurface fracture system of the Hail granite have not been investigated yet, a marginal low heat recovery of 2% is assumed. This value was utilized in similar geological conditions in the Cooper basin [56,57] and cross-ponding to the geothermal reservoir of low flow rate and non-uniform fracture system [58]. However, the actual heat recovery of the Hail granite may be higher than the assumed value. With a recovery factor of 2%, the Hail granites can generate about 2.63 × 10¹⁶ MWe. Considering that the average annual electric energy consumption per capita in Saudi Arabia has increased from 8840 kWh in 2021 to 9200 kWh in 2024 [59], the potential contribution of the Hail granite equates to approximately 2.86 × 10¹² kWh per capita per year. In comparison, previous studies [7,8] reported that the Midyan granites could generate around 160 × 10¹² kWh of electricity, based on subsurface temperatures exceeding 200 °C at a depth of 2.0 km and assuming a 2% heat recovery efficiency.

5.3. Scenarios for Energy Production and Limitations

Based on the results of this study and the analyzed data, a power plant for geothermal energy production from the EGS of the Hail area can be proposed and viable under certain circumstances. The characteristics of the Hail granite show similarities to those observed in the Soultz geothermal project in France [25]. The basement granitic rocks at Soultz consist of biotite-hornblende granite overlain by about 1 km thick sedimentary cover. A geothermal gradient of 45 °C/km was recorded, with a granite temperature reaching 200 °C at a depth of 4.5 km beneath the sedimentary layer [25].

5.3.1. Injection and Production Wells

Delineating the depth, extent, and nature of the fracture system of the granitic rocks are key parameters that could help in implementing the best locations for drilling the necessary wells, as well as detecting the type of circulating fluids. These fluids will be injected to induce/create a network of fracture within the granites.

The proposed EGS design includes two wells—one serving as the injection well and the other as the production well. Depending on the capacity of the circulating fluids in the fractured granite rocks, one production well (or more) can be used. The system operates as a closed loop, with circulating fluids typically being water or, more efficiently CO₂. A binary system can be connected at the surface to the loop to generate electricity.

5.3.2. Circulating Fluids

CO₂ is being used as a circulating fluid to carry the heat from the hot reservoir rocks. EGS concepts that use CO₂ (rather than water) as a working fluid have been proposed due to the superior thermophysical properties of CO₂, as demonstrated in preliminary modeling studies of CO₂ injection and production in fractured reservoirs [22,23,24]. Compared to water, CO₂ can be up to 50% more efficient for geothermal energy extraction. At pressures greater than 7.38 MPa and temperatures greater than 31.1 °C, CO₂ becomes supercritical. In its supercritical state, CO₂ has liquid-like densities and gas-like viscosities, allowing significant heat-carrying capacity whilst permitting ease of flow through fractured rock [22,23,24].

Moreover, the limited capacity for supercritical CO₂ to dissolve and transport mineral species in solution leads to less fouling and corrosion of circulation systems in the EGS [22,60]. Additionally, supercritical CO₂ displays a significant density fluctuation with variation in temperature and the buoyancy drive acquired from such density dependence creates a ‘thermosiphon effect’ that assists in the circulation of fluids from injection to production well and minimizes parasitic energy expenditure on pumps for fluid circulation [22,23,60].

5.3.3. Constraints and Future Work

However, some technical and environmental limitations/constraints are emerging that need to be fulfilled before proceeding forward regarding upscaling the results of this study in the Hail area. A comprehensive surface geophysical survey should be conducted to better delineate the geometry and the subsurface setting of the deep-seated granitic rocks. Drilling of deep drilling wells, coupled with a complete well-logging survey, including temperature logs, is very important to check for the actual subsurface regime of Hail granite, as well as, detecting petrophysical properties of the reservoir (Porosity, density, lithological components, etc.). Running detailed gamma ray spectrometry measurements is very important to detect the radioactivity of the granitic rocks in the new proposed wells, and to better calibrate the radioactive measurements from the surface rock samples. Interpreting more data from the old co-existing shallow wells and collecting more water samples will be needed to verify the geothermometric study.

Furthermore, detailed laboratory-based geo-mechanical measurements including uniaxial and triaxial tests on some selected granite samples are required to understand the mechanical behavior of these granites under a specific set of conditions of different temperatures and pressures. Numerical simulations will also help in deciphering the heat extraction potential of supercritical CO₂ in the Hail granite. It is also recommended to enhance an economic analysis including the Capital Expenditures (CAPEX), Operational Expenditures (OPEX), and Return on Investment (ROI), once the necessary numerical data/parameters become available. Such an analysis would provide a more comprehensive evaluation of the economic feasibility of geothermal energy production from Hail granites.

Considering the environmental and health implications of the radiogenic granite and water resources in the Hail area is very important, while planning any future related projects. It is reported that elevated levels of radioactivity (U, Th, and K) have been recorded in the collected water samples from the Saq aquifer, which is considered the main aquifer in the Hail area. Such high radioactivity is primarily attributed to the leached radioactive elements from the erosion of radiogenic granitic rocks [41,42]. Therefore, solutions for controlling the radioactivity distribution and waste management in the Hail area, and adopting measures to reduce radioactivity are required while planning for any geothermal activities. Fortunately, guidelines have been recently formulated by the Ministry of Environment, Water, and Agriculture (MEWA) in the context of environmental safety and groundwater quality. In order to improve institutional capacity and supply the community with safe water sources, water laws have been updated and reformulated by the KSA government [61,62].

6. Conclusions

This study aimed to investigate the potentiality of the highly radioactive granites in the Hail province-Northern Saudi Arabia as a geothermal resource through a comprehensive multi-disciplinary approach. Detailed mineralogical and petrographic analyses, geothermometric modeling, geophysical well logging, and radiometry measurements. These measurements were conducted to characterize and investigate the geothermal capabilities of these hot dry rocks. The acquired data were integrated and interpreted together to assess the potential energy-generating capacity and quantify the geothermal reserve of these granites. The main conclusions of this study can be summarized as

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The Hail granite forms part of the Arabian Shield and is exposed in the south-central part of the Hail area.

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It primarily consists of alkaline and peralkaline pink granite enriched in radioactive minerals, resulting in high radiogenic heat production.

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The geothermometric analysis relied mainly on the Quartz and Na-K geothermometers. Based on geothermometers and logging analyses, the subsurface temperature in the Hail area is estimated to range from 85 °C to 90 °C at an approximate depth of 2000 m (upscaled to 200 °C @ 5 km) and heat flow up to 99.87 mW/M².

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The ternary and Giggenbach plots indicated Na-Cl dominated water type, with water samples located in the immature area, indicating immature water with limited or weak interaction with the deep-seated rocks EGS granitic rocks.

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The natural gamma ray spectrometry showed elevated contents of Th and U up to 149.66 and 47.43 ppm, correlated with a high RHP record of 20 μW/m³.

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With an estimated volume of 70,108 km³, Hail granites have the potential to generate approximately 2.63 × 10¹⁶ MWe assuming a 2% heat recovery. Considering the Saudi average annual electric energy consumption per capita of 9200 kWh, the possible average contribution of the Hail granite will be 2.86 × 10¹² kWh per capita annually.

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Scenarios of energy production using two injection and production wells, with water and CO₂ as energy-transferring fluids, are demonstrated.

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Some technical limitations/constraints should be treated before upscaling the results of this study in the Hail area. Furthermore, considering the environmental and health hazards of the radiogenic granite and its impact on the groundwater resources in the Hail area are among the important parameters.

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The results of this study highlighted the role of the EGS in the Hail area as a new renewable and sustainable resource that could contribute to the future of the energy mix of KSA.

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A recommendation is to extend the geophysical field investigation and thermo-mechanical laboratory work to better image the subsurface setting of the Hail granite and understand its geo-mechanical behavior under different conditions of pressure and temperature.