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Article

Deep Geothermal Resources with Respect to Power Generation Potential of the Sinian–Cambrian Formation in Western Chongqing City, Eastern Sichuan Basin, China

by
Xiaochuan Wu
1,2,
Wei Wang
3,*,
Lin Zhang
4,
Jinxi Wang
1,2,
Yuelei Zhang
3 and
Ye Zhang
1,2
1
National and Local Joint Engineering Research Center for Shale Gas Exploration and Development, Chongqing Institute of Geology and Mineral Resources, Chongqing 401120, China
2
Key Laboratory of Shale Gas Exploration, Ministry of Natural Resources, Chongqing Institute of Geology and Mineral Resources, Chongqing 401120, China
3
Chongqing Huadi Resources and Environment Technology Co., Ltd., Chongqing 401120, China
4
China National Petroleum Corporation Changqing Oilfield Branch Fifth Gas Production Plant, Xi’an 710016, China
*
Author to whom correspondence should be addressed.
Energies 2024, 17(16), 4045; https://doi.org/10.3390/en17164045
Submission received: 19 July 2024 / Revised: 9 August 2024 / Accepted: 13 August 2024 / Published: 15 August 2024
(This article belongs to the Special Issue Geophysical Geothermal Reservoir Exploration, Monitoring, and Development – Volume II)
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Versions Notes

Abstract

:
The Rongchang–Dazu region in western Chongqing (eastern Sichuan Basin, China), known for its seismic activity, is a promising area for deep geothermal resource development; however, practical development is limited. Key geological understandings, such as heat flux, geothermal gradients, the nature of heat sources, thermal reservoir rock characteristics, and the classification of geothermal resources, remain in need of further study. In this work, the targeted area is surrounded by Sinian–Cambrian carbonate gas fields. An analysis of the deep geothermal prospects was conducted using exploration and development data from the Gaoshiti–Moxi gas fields within the Longwangmiao and Dengying Formations. The results indicate that the Rongchang–Dazu area has relatively high heat flow values and geothermal gradients within the Sichuan Basin, correlating with fault structure and seismic activity. Gas test data confirm that the Longwangmiao Formation in the study area reaches depths of 4000 to 4500 metres and exhibits anomalous pressures and temperatures exceeding 140 °C. Meanwhile, the Dengying Formation of the Sinian system lies at depths of 5000 to 5500 metres, with normal pressure, minimal water production, and temperatures exceeding 150 °C, characterising it as a dry-hot rock resource. Adjacent to western Chongqing, the Gaoshiti area within the Longwangmiao Formation, with an estimated flow rate of 100 kg/s, shows that the dynamic investment payback period is significantly shorter than the estimated 30-year life of a geothermal power plant, indicating strong economic viability. Deep geothermal resource development aids in conserving gas resources and enhancing the energy mix in western Chongqing. Future research should prioritise understanding the links between basement faults, seismic activity, and heat flow dynamics.

1. Introduction

Geothermal resources, as a clean and renewable energy source, have a significant potential for advancing the goal of both carbon neutrality and reduced carbon emissions, and they aid in the adjustment of the energy structure [1,2]. The increasing exploration of geothermal resources has led to greater scholarly attention to oilfield geothermal opportunities [3,4]. The geothermal potential of hydrothermal energy and hydrocarbons within oil and gas fields are often interconnected [4,5]. Oil and gas fields may be considered geothermal reservoirs since their hydrocarbon reservoirs overlap with thermal ones, and the thermal fluids extracted during oil and gas production qualify as geothermal resources [6,7]. Utilising geothermal water from oil fields can overcome the challenge of separating oil, gas, and water while also providing opportunities for sustainable applications like green power generation, district heating, industrial processes, agriculture, hydrotherapy, and recreational hot water bathing, further enhancing the recovery rate of heavy oil [8,9,10,11,12,13,14].
The oilfield geothermal resources in the basin are differentiated into hydrothermal and dry-hot rock types. The hydrothermal type is further classified into co-production, high-pressure, and conventional types [15,16,17]. Geothermal resources associated with hydrocarbons typically manifest as medium to low-temperature hydrothermal reservoirs [18]. Their evaluation is influenced by a variety of factors, including the availability of regional data, the depth of exploration, and the precision of evaluation methods [19]. These factors contribute to varying assessments of the depth of hydrothermal geothermal resources in oil-producing regions.
A depth of 10 km is widely accepted as the fundamental threshold for assessing geothermal resource potentials. Currently, the typical depth for evaluating geothermal potential in oil fields does not exceed 5000 m [20]. However, due to economic constraints and technological limitations, drilling geothermal wells beyond 5000 m rarely occurs. As a result, geothermal reserves below this 5000 m mark are often classified as prospective or speculative resources, indicating their potential for future development if drilling technology improves and economic assessments become favourable [20]. In the context of China’s energy exploration, it is noteworthy that the depths of some oil wells have already exceeded 5000 m. The data obtained from these oil and gas wells are crucial for exploring and potentially tapping into deep geothermal resources. For example, in the northern part of the Songliao Basin, the Daqing Oilfield’s Yingshen No. 2 well reached a depth of 5520 m with a recorded temperature of 183 °C at the bottom within the Yingcheng Formation [21,22]. The geophysical logging data from such wells provide essential baseline conditions for simulating the hydraulic properties of dry-hot rock formations at similar depths. Information from oil and gas field explorations is invaluable in creating lithological, thermal, and porosity profiles that are fundamental to deep geothermal studies [23,24].
Existing productive or abandoned wells could serve as potential sites for geothermal energy extraction in oil-rich regions [25]. Similarly, ultra-deep dry wells with temperatures above 150 °C and no hydrocarbon presence offer opportunities for conversion into Enhanced Geothermal Systems [26]. Moreover, integrating geothermal energy for power generation in oil fields has significant advantages. Oil fields come with critical infrastructure that can support geothermal power plants, such as existing drilling, gathering stations, pipelines, and transportation infrastructure [4,12]. This reduces the economic burden associated with the high costs of developing geothermal energy. Nevertheless, the primary focus in oil and gas fields remains on hydrocarbon exploration and production, with geothermal energy research lagging comparatively behind.
Chongqing, as a municipality directly under the administration of China’s central government (Figure 1), where wind, solar, and hydropower have all been suitably developed, is in urgent need of high-temperature geothermal resources to alleviate its power supply shortages. However, research on these resources is significantly limited. There has been no comprehensive analysis of deep geothermal conditions, leaving their potential for installation uncertain. Fortunately, Chongqing’s area, which is identified as having promising deep geothermal resources, is within the Sichuan Basin’s marine carbonate gas fields’ operational area. The intensive exploration and development of Sinian–Cambrian carbonate rocks have accumulated extensive foundational data, creating a conducive condition for analysing deep geothermal resources in western Chongqing.
This paper focuses on western Chongqing, harnessing this extensive oil and gas data from the Cambrian Longwangmiao Formation and the Sinian Dengying Formation. This study analyses pivotal factors such as heat flow, geothermal gradient, thermal conductivity, and heat sources. It also categorises deep geothermal resources in alignment with the Cambrian and Sinian systems and assesses their economic feasibility and potential for power generation. Finally, the article concludes with suggestions for future research avenues.

2. Methodology

This study collected high-precision seismic profile data, geochemical data of oil and gas well water samples, regional seismic activity data, geothermal heat flow, and geothermal gradient data from references. The interpretation of deep seismic profiles clarified the crustal structure and crustal-scale faults in the study area. Meanwhile, 3D seismic reflection profiles were used to determine the fault patterns and stratigraphic sequences. Based on the heat flow values and regional tectonic activity characteristics, the main heat sources were analysed. The hydrochemical composition of the studied strata and the type of geothermal resources were determined using drilling test results. A comprehensive analysis of the heat flow, deep structures, and seismic activity established the development model of deep geothermal resources in the area. The volume method was applied to determine the heat content of the Cambrian strata water encountered during drilling. There is a certain positive correlation between temperature and pressure in their planar distribution. A decrease in temperature will lead to a decrease in the pressure coefficient. The relationship between the stratigraphic pressure and temperature of the Longwangmiao Formation is as follows:
P = 0.0981 × T + 62.459
where P represents the Stratigraphic pressure, Ma; T indicates the Stratigraphic temperature, °C.
Furthermore, the economic evaluation model established by predecessors was used to preliminarily assess the economic viability of geothermal power generation.

3. Geological Setting

Integrated Mohorovičić discontinuity (Moho) depth data from the CRUST2.0 global crustal model and seismic stations in Sichuan Province and Chongqing City have been used to create a Moho depth map for the Chongqing region [27,28,29]. This map reveals two areas with crustal depths under 40 km in the northeastern and western parts of Chongqing (Figure 2a). Notably, the study area in western Chongqing has the thinnest crustal thickness in the region, not exceeding 39 km. The deep reflection profile in the study area shows clear layering in the basin caprock reflections, while the basement reflections of the basin are less distinct [30]. The profile identifies three clear reflection interfaces, Rc1, Rc2, and Rc3 (as shown in Figure 2b), which correspond to the lower boundaries of the upper, middle, and lower crust, respectively. Rc1 and Rc2 reflection interfaces exhibit arc-shaped composite phase characteristics, with intermittent continuity and apparent vertical displacement in some areas. This displacement reduces lower in the crust and stops before reaching Rc3. The Rc3 reflection is remarkably continuous and slopes gently upward from southeast to northwest, which the Moho depth map confirms. At the northwest end of the profile, the Moho is approximately 39.27 km deep, consistent with the Moho depth illustrated at the survey line in Figure 2b.
The study region, located in the central Sichuan Basin (Figure 1a), comprises four distinct evolutionary stages: the formation of the Middle–Late Proterozoic basement, the Sinian-Middle Triassic marine platform development, the emergence of the Late Triassic-Jurassic terrestrial basin, and the Cretaceous–Quaternary folded uplift [31,32]. The region features several karst heat reservoirs, including the Sinian Dengying Formation and the Lower Cambrian Longwangmiao Formation, which are important both as natural gas strata and karst formations [33] (Figure 1b). The grain-beach deposits of the Longwangmiao Formation are widespread, with holes ranging from 2 to 10 mm in diameter, which have a low degree of fill and strong connectivity [34,35]. The Dengying Formation reservoirs can be classified as the pore type, fracture-cavity type, and pore-cave type. Pore-type reservoirs have porosities between 2% and 3% and limited connectivity with permeabilities generally below 0.01 mD, resulting in restricted storage and permeability capacities [36,37]. Conversely, fracture-cavity and pore-cave reservoirs typically have porosities above 3% and permeabilities over 0.1 mD [38].

4. Results

4.1. Terrestrial Heat Flow and Geothermal Gradient

The Sichuan Basin is characterised by an average crustal heat flow of 28.8 mW/m2 and an average mantle heat flow of 24.4 mW/m2, indicating that it has a typically cold crust and mantle [39,40,41]; however, the geothermal gradient in the study area can exceed 30 °C/km. According to the latest heat flow map [42], the study area’s geothermal activity ranges from 64 to 68 mW/m2 [43], significantly higher than the Sichuan Basin’s overall average of 50.93 to 53.6 mW/m2. This discrepancy could be due to variations in thermal conductivity or localised heating within the study area.

4.2. Thermal Conductivity and Heating Conditions

Investigators studying the Middle and Lower Triassic karst heat reservoirs near the surface of high-rise anticlines in Chongqing have suggested that the absence of magma intrusion rules out a magmatic heat source [44]. They also argue that the contribution of heat from mantle-derived materials is negligible and that radioactive elements within sedimentary rocks offer a low heat generation potential [45,46]. Hence, forming a regional deep heat source seems improbable, with the subsurface temperature increase largely dependent on the burial depth. In contrast, our present study proposes that the relatively high heat flow in the study area can be ascribed to several factors.
First, it is notable that the Mohorovičić discontinuity (Moho) is relatively shallow in the study area, at approximately 37.5 km deep, representing the thinnest crust in Chongqing (refer to Figure 2a). This undeniably provides a regional advantage for accessing heat sources within the study area. Second, petroleum resource assessments have revealed that zones of advanced source rock maturation are often located near the extension of basement faults, and the development of these highly mature zones to localised thermal anomalies from basement fault activities [47]. The study area is marked by prominent large-scale crustal faults and multiple strike-slip faults (Figure 1c and Figure 3). The frequent seismic events in this area suggest that these basement faults are highly permeable (Figure 1c). Lastly, the seismic activity in the study area is mainly due to stress accumulation, as indicated by the high ratios of longitudinal to transverse wave velocities in the lithosphere’s velocity structure. The concentration of geostress around these faults is likely to generate additional heat.

4.3. Types of Deep Geothermal Resources

In the study area, the Longwangmiao Formation commonly displays overpressure (Figure 4a), with pressure coefficients varying from 1.51 to 1.70 [43,48]. Conversely, the fourth member of the Dengying Formation typically has normal pressure (Figure 4b), with coefficients between 1.06 and 1.14 [43]. The upper section of the Dengying Formation’s second member exhibits normal pressure, whereas the lower section presents anomalously low pressure, where coefficients span from 0.41 to 1.10 [49]. These divergent formation pressure coefficients imply that the Longwangmiao and Dengying Formations are representative of two separate geothermal systems within the area under examination.
Gas test results for the Longwangmiao Formation near the study location have detected condensate water and formation water within this stratigraphic layer. The presence of condensate water is inferred from its relatively low mineralisation, generally below 20 g/L, contrasting with the formation water’s higher mineralisation level, which often surpasses 100 g/L [50]. The water-to-gas ratio in the condensate water is approximately 1 to 105 [51], showing scarce and stable water production rates. The detection of condensate water suggests the existence of a minimal amount of saturated vapour in the karst thermal reservoir zones of the Longwangmiao Formation.

5. Discussion

5.1. Significant Emphases in the Exploration of Hydrothermal Geothermal Systems within Cambrian Carbonate Formations

Geothermal water generation is linked to the infiltration of surface water via natural pathways and the absorption of profound heat from deep circulation, particularly in regions with developed faults, which are pivotal for the formation of geothermal waters [52]. The Triassic karst heat reservoirs in Chongqing’s main urban area boast conducive conditions for recharge, with atmospheric precipitation permeating down from various aquifers near the surface and through fault zones [44,53]; therefore, the surface water is predominantly of the CaSO4 type. In western Chongqing, the deep Cambrian karst thermal reservoirs also contain significant geothermal water reserves, characterised mainly by the CaCl2 type with a high total salinity and notable sealing properties, which limit underground fluid recharge [48].
Despite overpressure in the Cambrian karst heat reservoir, the Longwangmiao Formation’s geothermal water at considerable depth maintains low salinity and is primarily of the Na2SO4 type [48]. It may originate from a combination of surface and shallow waters. The 1:50,000 geological survey report of western Chongqing identifies many faults and fractures in the study area’s anticlinal core, acting as potential pathways for water migration.
Recent advanced 3D seismic explorations have detected multiple active strike-slip faults that extend from the Sinian and Cambrian up to the Permian [54]. Interpreted as extensional and torsional faults from the Himalayan orogeny, they overlap with or superpose onto the upper Mesozoic faults (Figure 3). These intersecting fault structures at different depths exhibit notable permeability and serve as conduits for geothermal water flow [55], causing the emergence of varied water types in the Cambrian Longwangmiao Formation, such as NaHCO3, MgCl2, and Na2SO4. The water mineralisation levels can be as low as 0.573 g/L [48]. Additionally, the Caledonian orogeny led to the exposure of the Longwangmiao Formation along the study area’s northwest margin, enabling groundwater to infiltrate downward along fault structures, which predominantly delineate the permeable zones [56]. Water production analysis from wells in the Longwangmiao Formation demonstrates a correlation between the proximity to faults and increased water yield, with wells closer to faults producing more water in comparison to those further away [57].
Consequently, the accurate mapping and characterisation of the distribution of hydro-conductive and thermo-conductive faults are crucial for exploring the Cambrian geopressure karst thermal reservoir. Overcoming these challenges holds the promise of identifying prime groundwater conduits and pinpointing areas with potential hot water accumulations at elevated temperatures.

5.2. Advantages of Exploring the Anhydrous/Oligohydrous Carbonate Geothermal System in the Sinian System

The sealing capacity of the formation water in the Sinian Dengying Formation has been observed to be notably superior to that of the overlying Cambrian Longwangmiao Formation. Gas testing has revealed that the formation contains a limited amount of fluid, with temperatures consistently exceeding 150 °C [43]. Globally, common lithologies found in dry-hot rock reservoirs include granite, carbonate rock, sandstone, metamorphic rock, and rhyolite [58]. In China, the predominant dry and hot rock encountered during domestic drilling operations is granite [59]. Exploiting hot and dry rock resources should follow a methodical progression from simple to complex, starting with more accessible tasks and gradually taking on more challenging ones. Reservoirs with high temperatures, well-developed porosity and fracture systems, favourable permeability, and strong technical and economic potential should be prioritised. Currently, the development of hot and dry rock resources at depths of 4 to 7 km, where temperatures range from 150 to 250 °C, is considered feasible [58]
In terms of porosity and permeability, the Sinian carbonate rocks in the study area have shown higher porosity than those composed of granite and metamorphic granite. The Deng4 Member reservoir is notable for its burial depth of 5000–5500 m, high formation temperatures between 148.7 °C and 158.9 °C [43], and low porosity and permeability, with averages of 3.87% and 0.51 mD [37,48,60], respectively. The mineral composition of the Sinian Dengying Formation is primarily dolomite, which is characterised by its brittle nature. This formation features well-developed structural attributes such as solution pores, karst caves, micro-cracks, and sutures. It also contains paleo-karst collapse bodies that contribute to higher overall porosity and permeability due to additional pores, structural fractures, and collapse fractures [60]. Granite primarily consists of quartz, feldspar, and mica and is known for its dense structure and hard texture. The GR1 well in the Gonghe Basin of the Qinghai Province has penetrated high-quality dry-hot rock formations; however, their low permeability poses a challenge for exploitation [61]. Additionally, the average porosity and permeability of the thermal reservoir zone within the ZR1 well in the Zhachanggou geothermal field in Qinghai Province are 1.85% and 0.0195 mD [62], respectively. As a result, extracting geothermal energy solely through natural fractures or the inherent permeability of the reservoir rock is not viable, and artificial fracturing may be required to enhance the reservoir’s properties [62,63]. Dry-hot rock formations are characterised by high temperatures, considerable hardness, significant abrasiveness, and high compressive strength, which can impede efficient rock fragmentation. Contrarily, the average temperature in the carbonate formations of the Dengying Formation in central Sichuan reaches about 153.2 °C, and the fracture pressure ranges between 140 and 175 MPa at a burial depth of 5000 m, making them suitable for conventional acidising fracturing to create fractures [64].
Despite the challenging burial depths of geothermal resources within the Sinian system, the research area benefits from substantial exploration and development data acquired from oil and gas wells, such as the Anyue, Moxi, and Weiyuan Gas Fields. Coupled with advanced drilling, completion technologies, and reservoir fracturing techniques, this provides a significant advantage for exploring and developing deep, anhydrous or low water content and tight carbonate geothermal resources. Future efforts to explore and develop geothermal resources within the Sinian system should, therefore, integrate oil and gas data, with the expectation of making notable progress in uncovering tight thermal rock formations.

5.3. The Heat Accumulation Model of Deep Geothermal

After a comprehensive analysis of the crustal structure, thermal reservoirs, heat flow, heat sources, occurrences of geothermal resources, as well as the focal points and advantages of thermal reservoir exploration, a deep geothermal heat accumulation model has been established (Figure 5). The research area is located in a region where the Moho surface is uplifted, marked by frequent seismic activity and the accumulation of geostress, indicating favourable heat source conditions [65]. The characteristics of thermal reservoirs within the area have been significantly influenced by various karst processes, resulting in the formation of high-quality thermal reservoirs within the Longwangmiao and Dengying Formations [35]. Regarding the caprocks, the overlying Dengying Formation above the Qiongzhusi Formation consists of a sedimentary sequence primarily composed of mudstone with a thickness of approximately 200 m. In a similar vein, the layer overlying the Longwangmiao Formation is made up of muddy dolomite, about 150 m thick, providing favourable conditions for an effective cap rock. In terms of pathways, the area under investigation features an intricate network of basement faults, such as the pre-Sinian faults and strike-slip faults in the basement identified by prior research. These faults may serve as channels to aid in thermal conductivity. Additionally, various surface faults present in the area, together with permeable cracks or multi-stage faults, both surface and subsurface, act as crucial conduits for groundwater movement. These geological structures support the supply of geothermal water to the geothermal resources in the Cambrian Longwangmiao Formation. The permeable catchment area at lower structural levels has proven to be an advantageous site for harnessing geothermal resources in the Longwangmiao Formation. Meanwhile, the underlying Sinian Dengying Formation has developed into a system with minimal or no free water, characterised as an anhydrous or oligohydrous carbonate geothermal system.

5.4. Power Generation Potential of Deep Geothermal Resources

Geothermal power generation is predominantly positioned in high-temperature geothermal zones located at the edges of tectonic plates [65]. This includes the use of dry steam power generation, flash steam power generation, combined cycle geothermal power generation, and hybrid power generation technologies [66]. Notably, dry steam power generation is a rare phenomenon, occurring exclusively in geothermal fields associated with the San Andreas Fault Zone, Sunda Arc, and the Apennine Belt [55]. In carbonate geothermal fields, numerous instances of power generation are reported, with temperatures ranging from 63 to 123 °C and installed capacities between 2.3 and 40 MW [67]. These figures surpass those of geothermal power stations located at tectonic plate boundaries and within China operating at similar temperatures (refer to Figure 6a) [68]. Consequently, the deep carbonate rock thermal reservoir geothermal field in the study area is expected to provide distinct advantages in power generation.
It is important to note that the temperature and thermal content of geothermal fluids are closely correlated with the dynamic payback period of combined cycle geothermal power plants [68,69,70]. Prior research has shown that as the temperature of geothermal fluids rises, there is a notable decrease in the dynamic payback period. Given a geothermal water flow rate of 10 kg/s, an increase in temperature from 110 °C to 150 °C can reduce the dynamic investment recovery time of the organic Rankine cycle power station system from 62.3 years to 10.8 years [68] (Figure 6b). However, when the temperature of the heat source falls below 110 °C, the revenue generated by the organic Rankine cycle system becomes insufficient to cover the annual operating and maintenance costs, making the project unprofitable and economically unsustainable [68]. The operational lifespan of geothermal power plants is often extensive, generally spanning 20 to 30 years. A shorter dynamic investment recovery period, typically under 20 years, indicates better economic viability and investment potential for geothermal power projects.
In the vicinity of the research area, the Longwangmiao Formation is characterised by a high water content. Currently, the confirmed extent of the Moxi high-permeability zone covers 240 km2 (Figure 1c), while the Gaoshiti high-permeability zone covers 380 km2 (Figure 1c). Production tests have confirmed that the water layer has a porosity of 4% and a vertical thickness of approximately 30 m. Based on these parameters, the estimated water storage capacity in the Moxi area is 1.9 × 1011 kg, and that in the Gaoshiti area is 3.0 × 1011 kg. The water bodies around the research area, covering an area of 170 km2, have drilling data substantiating temperatures reaching up to 135 °C. At an extraction rate of 100 kg/s, these water bodies can be utilised for a period of 35 years, which would result in a dynamic investment recovery period of less than 20 years. This suggests a matching mining lifespan and economic value.
Furthermore, the temperature of geothermal resources is a crucial factor for evaluating power generation potential. For a geothermal resource to be economically viable from a power generation perspective, the fluid temperature must exceed 90 °C [71]. In estimating geothermal power generation capacity, the volumetric method is initially employed to calculate the inherent thermal energy in rocks and fluids. Additionally, it is essential to determine the average recovery factor for extracting geothermal energy from thermal reservoirs, as well as the average conversion efficiency of converting geothermal energy into electricity. Assuming a projected lifespan of 30 years for a geothermal power plant utilising a carbonate rock thermal reservoir in the study area, the formula for estimating power generation capacity should be structured accordingly [71].
Q = A · d · ρ s · C s · 1 − φ + ρ w · C w · φ · t r − t o
Q e = R e · Q
P e = η · Q e / 9.46 × 10 14
where Q represents the heat energy stored in the thermal reservoir, J ; A represents the distribution area of the hot reservoir, m 2 ;   d represents the thickness of the thermal reservoir, m; ρ s denotes the rock skeleton density of the hot reservoir in kilograms per cubic meter, k g / m 3 ;   C s signifies the specific heat capacity of the rock skeleton, J / k g · ° C ;   φ represents the porosity of the thermal reservoir without dimension; ρ w stands for the geothermal fluid density, k g / m 3 ;   C w denotes the specific heat capacity of the geothermal fluid, J / k g · ° C ;   t r represents the average temperature of the geothermal reservoir, ° C ;   t o is the lowest temperature limit for utilising geothermal reservoirs, ° C ; Q e is the heat energy recovered from the thermal reservoir. R e denotes the recovery rate of geothermal energy, set at 15% [71]; P e represents the power generation capacity, M W ;   η signifies the conversion coefficient of geothermal energy into electric energy, which is 7% for karst reservoirs [72]. The conversion constant of heat energy to electricity over 30 years is 9.46 × 1014.
The analysis of formation water samples from the Longwangmiao Formations in the western region of Chongqing has revealed a density range from 1.08 × 103 kg/m3 to 1.12 × 103 kg/m3. For the purposes of this study, a value of 1.1 × 103 kg/m3 has been used. Additionally, the porosity of the thermal reservoir in the Longwangmiao Formation is estimated at approximately 4%. The density of the rock matrix has been considered as 2.7 × 103 kg/m3, the specific heat capacity of the rock matrix as 920 J/(kg·°C), and the specific heat capacity of the geothermal fluid as 419 J/(kg·°C). Furthermore, the minimum temperature requirement for power generation in karst reservoirs has been established at 90 °C. The high-temperature zone above 135 °C within the Gaoshiti–Longwangmiao Formation, located near the study area, covers an area of 1.70 × 108 m2 (refer to Figure 1c) with an average thickness of 30 m. After an in-depth analysis and calculation, the estimated installed capacity of this geothermal resource is 6.78 megawatts (Mw).
The potential for implementing deep geothermal development could contribute significantly to reshaping the power infrastructure in the western Chongqing research area. This region is predominantly dependent on electricity imported from outside, and there is a notable lack of readily available renewable energy resources, especially in terms of hydropower, wind power, and photovoltaics. This deficit limits the opportunities for the continuous improvement of the energy consumption structure. Hence, an in-depth analysis of the potential of geothermal resources is crucial in steering the restructuring of the local energy landscape.

5.5. Prospects for Deep Geothermal Resources

The research area, situated in close proximity to the central Sichuan Basin, benefits from a wealth of oil and gas exploration data. Utilising this data for a comprehensive analysis has revealed favourable conditions for the potential exploitation of deep geothermal resources in the area [7,12]. The Longwangmiao Formation, which serves as a deep karst reservoir within the research area, is a critical stratum for the accumulation of deep geothermal resources. Moreover, it is also a significant site for the occurrence of overpressure geothermal resources.
The presence of significant water bodies within the Longwangmiao Formation, especially in the Gaoshiti–Moxi area, presents the potential risk of water breakthrough into gas wells during natural gas extraction, thus impacting the efficiency of development [51]. Consequently, the extraction of geothermal water from the Longwangmiao Formation has emerged as a viable strategy. This allows for the protection of natural gas development and the efficient utilisation of geothermal resources in gas-rich regions.
Future initiatives should emphasise the importance of collaborating closely with industry stakeholders who hold mineral rights in the area to collectively enhance knowledge of geothermal resource distribution and potential. This collaboration will be key to exploring and developing deep geothermal technologies and establishing a pathway for the efficient use of clean energy.
Furthermore, the increased seismic activity in the southwestern region of the study area offers a favourable opportunity to investigate the interconnected relationship between seismic events and geothermal resources. This includes research on the heating mechanisms associated with seismic activity within intraplate regions. Therefore, the research area bears significance not only for the exploitation and utilisation of geothermal resources within the gas fields but also as an ideal setting for addressing cutting-edge geothermal issues. Subsequent research should prioritise examining the interplay between basement faults, seismic activity, and geothermal energy to advance understanding in this field.

6. Conclusions

The study site is situated in a region distinguished by a thermal anomaly compared to the rest of the basin, as evidenced by terrestrial heat flow measurements and the average geothermal gradient. Heat flow values within the research area range from 64 to 66 mW/m2, surpassing the Sichuan Basin’s average of 53 mW/m2. Similarly, the average geothermal gradient in this area is higher than 30 °C/km but does not exceed 35 °C/km, which again exceeds the Sichuan Basin’s norm of 25 °C/km.
The Longwangmiao Formation at the study site has been confirmed to host a geothermal water reservoir extending over an area of at least 170 square kilometres, featuring temperatures that exceed 135 °C. With a fluid production rate assumed to be 100 kg/s for power generation purposes, the estimated dynamic payback period for the investment is expected to be less than 20 years. Furthermore, the potential installed capacity for a geothermal power facility in this context is estimated at approximately 6.7 megawatts (MW).
The Longwangmiao Formation is recognised as the principal reservoir for geothermal exploitation, defined by abnormal overpressure and vulnerability to water encroachment during gas extraction. Therefore, the extraction of geothermal fluids from these reservoirs can be an effective approach to safeguarding the integrity of gas formations. Moreover, the southern portion of the study area is positioned within an active seismic zone, underscoring the need for focused analysis on the interplay among basement faults, seismic events, and heat flow values in subsequent research phases.

Author Contributions

Conceptualization, X.W. and W.W.; methodology, X.W.; software, Y.Z. (Yuelei Zhang); validation, Y.Z. (Yuelei Zhang), L.Z. and Y.Z. (Ye Zhang); formal analysis, X.W.; investigation, J.W.; resources, X.W.; data curation, X.W.; writing—original draft preparation, X.W.; writing—review and editing, W.W.; visualization, W.W.; supervision, J.W.; project administration, X.W.; funding acquisition, X.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Chongqing Municipal Scientific Research Institutions Performance Incentive and Guidance Project grant number [cstc2022jxjl90001] and the National Natural Science Foundation of ChongQing grant number [CSTB2022NSCQ-MSX1406] And The APC was funded by [National and Local Joint Engineering Research Center for Shale Gas Exploration and Development, Chongqing Institute of Geology and Mineral Resources].

Data Availability Statement

The primary data in this research were obtained through meticulous review and synthesis of existing literature. However, the well and seismic data available in the study area are insufficient, which limits our comprehensive understanding of the geothermal gradients and terrestrial heat flow in this specific region.

Acknowledgments

Thanks to the reviewers and editor for their suggestions.

Conflicts of Interest

Authors Wei Wang and Yuelei Zhang were employed by the company Chongqing Huadi Resources and Environment Technology Co., Ltd. Author Lin Zhang was employed by the company China National Petroleum Corporation Changqing Oilfield Branch Fifth Gas Production Plant. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. (a) Regional location map of the Sichuan Basin, with the study area delineated by the orange box. Figure 1a marked on the global elevation map with a red box. (b) General stratigraphic column and tectonic movements within the Sichuan Basin, with karst reservoirs highlighted in a shallow grey box. (c) Figure 1c displays the water body area surrounding the study site, along with the distribution of seismic events and basement faults.
Figure 1. (a) Regional location map of the Sichuan Basin, with the study area delineated by the orange box. Figure 1a marked on the global elevation map with a red box. (b) General stratigraphic column and tectonic movements within the Sichuan Basin, with karst reservoirs highlighted in a shallow grey box. (c) Figure 1c displays the water body area surrounding the study site, along with the distribution of seismic events and basement faults.
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Figure 2. (a) The depth map of the Moho Surface in the Chongqing region and the deep reflection profile of the study area (b).
Figure 2. (a) The depth map of the Moho Surface in the Chongqing region and the deep reflection profile of the study area (b).
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Figure 3. The faults present on the 3D seismic section. The coloured solid lines represent the bottom reflection interfaces of the layers.
Figure 3. The faults present on the 3D seismic section. The coloured solid lines represent the bottom reflection interfaces of the layers.
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Figure 4. The superimposed map of buried depth and pressure coefficient of the Longwangmiao Formation (a) and Dengying Formation (b).
Figure 4. The superimposed map of buried depth and pressure coefficient of the Longwangmiao Formation (a) and Dengying Formation (b).
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Figure 5. Heat accumulation model of deep karst reservoirs in the western of Chongqing.
Figure 5. Heat accumulation model of deep karst reservoirs in the western of Chongqing.
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Figure 6. (a) A scatter plot depicting the correlation between reservoir temperature and installed capacity. (b) A graph showing the relationship between the dynamic investment payback period and the temperature of geothermal fluid.
Figure 6. (a) A scatter plot depicting the correlation between reservoir temperature and installed capacity. (b) A graph showing the relationship between the dynamic investment payback period and the temperature of geothermal fluid.
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MDPI and ACS Style

Wu, X.; Wang, W.; Zhang, L.; Wang, J.; Zhang, Y.; Zhang, Y. Deep Geothermal Resources with Respect to Power Generation Potential of the Sinian–Cambrian Formation in Western Chongqing City, Eastern Sichuan Basin, China. Energies 2024, 17, 4045. https://doi.org/10.3390/en17164045

AMA Style

Wu X, Wang W, Zhang L, Wang J, Zhang Y, Zhang Y. Deep Geothermal Resources with Respect to Power Generation Potential of the Sinian–Cambrian Formation in Western Chongqing City, Eastern Sichuan Basin, China. Energies. 2024; 17(16):4045. https://doi.org/10.3390/en17164045

Chicago/Turabian Style

Wu, Xiaochuan, Wei Wang, Lin Zhang, Jinxi Wang, Yuelei Zhang, and Ye Zhang. 2024. "Deep Geothermal Resources with Respect to Power Generation Potential of the Sinian–Cambrian Formation in Western Chongqing City, Eastern Sichuan Basin, China" Energies 17, no. 16: 4045. https://doi.org/10.3390/en17164045

APA Style

Wu, X., Wang, W., Zhang, L., Wang, J., Zhang, Y., & Zhang, Y. (2024). Deep Geothermal Resources with Respect to Power Generation Potential of the Sinian–Cambrian Formation in Western Chongqing City, Eastern Sichuan Basin, China. Energies, 17(16), 4045. https://doi.org/10.3390/en17164045

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