Characteristics of karst geothermal system in intermountain fault basin: a case study of Taiyuan basin in north China

Introduction: The Taiyuan Basin of North China, as a typical intermountain fault basin, is located in the middle part of the Fen-Wei graben system-an extensional faulting belt. It is one of the areas where high-quality karst thermal storage is developed to match urban heating demand. The formation of its karst geothermal system and the study of its thermal storage characteristics are of great significance for analyzing the distribution pattern of this type of geothermal resource and its overall development.

Methods: Based on the synthesis of previous research results and the latest geothermal well data, this article analyzes the heat source, karst geothermal reservoir distribution, and hydrothermal dynamic characteristics of the karst geothermal system in the Taiyuan Basin, and evaluates the geothermal resources in 8 units.

Results: The results show as flow: (1) The karst geothermal reservoir strata in the Taiyuan Basin are mainly composed of the Lower Paleozoic Ordovician, widely distributed in the North China Plate. The evolution of karst geothermal systems has gone through five stages from the Early Paleozoic to the Quaternary. (2) The heat source comes from the high-altitude heat flow (>71 mW/m²) in the asymmetric fault basin of the Cenozoic era, and the heat transfer mode can be divided into the basin edge strong convolution type and the basin internal heat conduction type. (3) The cumulative thickness of the geothermal effective reservoir section is 160–180 m, including the main water bearing sections of 3-4 layers, which are prone to overflow during migration. (4) Controlled by the structure and topography of mountain fault basins, the geothermal water supply and transport mode of karst geothermal systems has the characteristics of bidirectional, near source, and rapid.

Discussion: According to the evaluation method of karst geothermal reservoir volume, the total geothermal resources of the karst geothermal system in Taiyuan Basin are estimated to be 83.03 × 10⁸ GJ, equivalent to 2.83 × 10⁸t standard coal, and the annual mining output can meet the heating area of 15 million square meters.

1 Introduction

The concept of geothermal system includes two connotations. One is the effectiveness of the geothermal system (Helgeson, 1968), that is, geothermal system is a relatively independent geological unit in heat and fluid circulation, with sufficient geothermal enrichment to constitute energy resources (Rybach et al., 1981; Wang, 2015); the other is the specific research content of the geothermal system, that is, the study of a geothermal system requires that while dissecting the four geological genetic factors of “source (including heat source and water source), reservoir (collective), channel (channel), cap (layer)”, more attention should be paid to the analysis of the geological process of “heat transmission, storage, preservation and loss” (Arnórsson and Andrésdóttir, 1995; Faulds et al., 2010; Deon et al., 2012; Moeck, 2014; He et al., 2017; Zhang et al., 2017). Previous studies of geothermal system mainly focus on the high-temperature convective geothermal system at the active edge of the plate, mainly used for geothermal power generation (Arnorsson, 1995). Those researches, however, contain little discussion of the low-and medium-temperature conductive geothermal system mainly used for urban heating in the sedimentary basin, especially the genetic model of the karst geothermal system in the intermountain fault basin.

The research on karst geothermal reservoir in Taiyuan Basin before 2014 was relatively low. Based on the data of more than ten wells constructed in the basin only, the hydrochemical types, geothermal geological conditions and karst water recharge are discussed (Ha et al., 1989; Han et al., 1993; Han et al., 2006). While the genetic mechanism of the karst geothermal system, the heterogeneity of the karst geothermal reservoir and the evaluation of the overall resources in the whole basin are seldom involved. Since 2014, Sinopec has exploited 64 geothermal wells (by the end of 2024) of lower-Paleozoic karst reserves in Xiwenzhuang uplift (also known as Xiwenzhuang geothermal field), Taiyuan Basin. Most of the wells have a water temperature of 60–75 °C, and single well water volume of 79–150 m³/h. The realization of heating capacity of 3.5 million square meters shows good development prospects. Based on the latest geothermal drilling data and test materials, combined with previous exploration and research results of karst geothermal reservoir in Taiyuan Basin (Ma et al., 2005; Ma, 2007; Han et al., 2009; Ma et al., 2009), this paper has established the genetic model of the karst geothermal system, compared spatial distribution differences of karst geothermal reservoirs in different secondary tectonic units, evaluated the amount of geothermal resources, and defined the exploration and development prospects of the karst geothermal reservoir in Taiyuan Basin.

2 Evolution of karst geothermal system

The Taiyuan Basin is a Cenozoic intermountain fault basin in the northern extension of the Jinzhong Rift, located in the middle part of the Fen-Wei Graben system-an extensional faulting belt with a length of 1,000 km in North China Plate. The basin contains Cenozoic sedimentary areas from north of Qingxu County to south of Yangqu County, Taiyuan City. The width of the basin is about 20 km in EW direction, and the length is about 50 km in EW direction, with a total area of about 1,000 km² (Figure 1). The urban area of Taiyuan City is basically the same as the outline of Taiyuan Basin, with a permanent population of 4.42 million and a potential geothermal heating demand of 30 million m².

Figure 1

Figure 1. Tectonic zoning and karst water system in Taiyuan Basin. 1-Ordovician; 2-Carboniferous-Permian; 3-Triassic; 4-Cenozoic and thickness; 5-section location; 6-normal fault; 7-reverse fault; 8-hidden fault; 9-anticline; 10-depression anticline; 11-syncline; 12-karst water system boundary; 13-tectonic unit boundary; 14-river; 15-hot spring point; 16-geothermal well. F1-Tianzhuang Fault Zone, F2-Jinci Fault, F3-Nanyan Fault, F4-Fenhe Fault, F5-Mingqian Fault, F6-Dongbianshan Fault, F7-Daliu Fault, F8-Xincheng Fault, F9-Guankou Fault, F10-Shanglan-Donglingjing Fault; I.1-Nitun Fault Terrace, I.2-Chezishan Horst, I.3-Mapotou Horst, I.4-Yangqu Depression, I.5-Xizhang Faulted Terrace, I.6-Xincheng Depression, I.7-Nanshe Fault Terrace, I.8-Sangi Horst; II.1-Ximing Fault Terrace, II.2-Urban Depression, II 3-Chengdong Fault Terrace, II 4-Yanxian Horst, II 5-Xibianshan Fault Terrace, II 6-Chengnan Uplift, II 7-Jinyuan Depression, II 8-Xiwenzhuang Uplift, II 9-Mingli Depression, III 1-Qingjiao Depression, III 2-Archaean Fault Depression.

The Taiyuan Basin shows an obvious structural pattern of the north-south segmentation and the east-west zonation in the plane (Guan and Li, 2001). Due to the differential tectonic subsidence of the extensional fault blocks in the Cenozoic, the basin was divided into three parts in the north-south direction by the nearly EW-oriented Sanji horst and the NE-oriented Tianzhuang fault. These three parts are the northern part, the central part and the southern part with the Cenozoic thickness of less than 500 m, 300–1000 m, and 1,000–3500 m, respectively. In the east-west direction, the basin was divided into 19 second-grade tectonic units by 3–4 nearly SN- oriented faults (Figure 1).

The geologic configuration of the Taiyuan Basin consists of 5 structural layers vertically (Figure 2): the Precambrian metamorphic basement; the Lower Paleozoic dominated by the platform-type marine carbonate buildups; the Upper Paleozoic dominated by the coal-bearing strata developed in transitional facies; the Mesozoic dominated by terrestrial clastic rocks; and the Neogenes dominated by poorly cemented or semi-diagenetic sediments. The maximum thickness of the sedimentary cover bed above the basement is over 6000 m. In the basin, karst geothermal reservoir developed in the Lower Paleozoic, overlying cover beds of the reservoirs developed in the Upper Paleozoic, and the regional cover bed developed in the Neogenes.

Figure 2

Figure 2. The structure-strata framework in Taiyuan basin and its adjacent area.

According to the controlling effect of regional tectonic evolution on the formation of genetic elements of Karst geothermal system, the evolution process of karst geothermal system in Taiyuan Basin is divided into the following five stages (Figure 3).

1. Marine carbonate deposition and the epigenic karstification of the stable uplift during early Paleozoic (Figure 3a). In the southwestern part of North China Plate, the platform-type sedimentation from Middle to Neoproterozoic was gradually overlying from south to north along the Jin-Henan fracture trough and reached the study area in the late Middle Cambrian. Therefore, above the basement, Taiyuan Basin only developed the Middle Cambrian-Ordovician widespread carbonates with a total thickness of 1200 m. During this period, two tectonic movements, the Huaiyuan Movement and the Caledonian Movement, dominated by overall lifting happened. The Huaiyuan Movement, dominated by episodic lifting, happened in late Cambrian- Early Ordovician (Song, 2001), and the Caledonian Movement, dominated by regional uplift, happened in the Late Ordovician with a depositional hiatus of about 120 Ma. As a result, three disconformity surfaces and three dolomite-limestone cycles of sedimentation could be identified in the Ordovician (Figure 2). Dolomite is favorable for karst geothermal reservoir, and argillaceous limestone and argillaceous dolomite is favorable for high-quality interlayer. At the same time, the depositional hiatus and flattened karst paleogeomorphology, formed by multipahse tectonic movements, resulted in the interlayer karst, forming karst caves and dissolutional pores, which distributed along the layer surface (Luo et al., 2008).

2. The direct caprock deposition in the late Paleozoic (Figure 3b). The Silurian-Lower Carboniferous didn’t develop in the study area. Until the Late Carboniferous, the overall settlement of North China Plate resulted in the deposition of the transitional facies coal-bearing strata with a thickness of about 800–1200 m, forming the overlying caprock of the Ordovician karst geothermal reservoir.

3. The uplift type karstification and the initial formation of karst geothermal system during Mesozoic (Figure 3c). The tectonic movements of the Mesozoic Indosinian and Yanshanian Movement caused the regional strata to be strongly compressed and uplifted, and formed the modern NE-oriented structural pattern. The study area was located in the southern flank of the Wutaishan anticlinorium formed during the period. The regional karst topographic feature showed the buried hill-type karst, the Wutaishan anticlinorium core developed the karst highland facies with an outcrop of the Archaeozoic crystalline basement, the Qinshui Basin developed the karst depression facies (the Triassic residual thickness here is about 1000 m), and the study area developed the karst slope facies. The strata of the study area was gently dipping, and from north to south the Middle Ordovician, the Carboniferous-Permian and the Lower Triassic were outcropped respectively. A relatively uniform karst geothermal system was formed under the continuous leaching of atmospheric precipitation in carbonate exposed areas.

4. Extensional faulting-block type karstification and karst system transformation in Himalayan period. The large scale NE-oriented rift basin formed under the control of the NS-oriented stress field during the Himalayan (Zhang, 1990). Due to the differential tectonic subsidence of the extensional fault blocks, the early relatively uniform karst geothermal system was damaged. The Tianzhuang fault zone were contemporaneous faults, the karst geothermal reservoir south of the fault zone gradually subsided, while in the north of the fault zone, sediments overlapped gradually to the north, and the reservoirs developed concealed-exposed epigenetic karstification (Figure 3d).

5. The regional caprock deposition and the setting of karst geothermal system during Quaternary. Due to the Quaternary depression of the Taiyuan Basin, the sediments overlapped from south to north on the Nitun-Yangqu area north of the Sanji horst, and as a result, the regional cover bed formed. At the same time, the mountains on both sides of the east and west were tilted, and the basin was severely stretched and subsided. This tectonic pattern made the atmospheric precipitation flow from mountains to the foreland and the basin, and focus and accumulate in the carbonate reservoirs in the basin, forming the current karst geothermal system (Figure 3e).

Figure 3

Figure 3. The evolution of karst geothermal system in Taiyuan Basin. (a) Early Paleozoic marine carbonate deposition stage and the epigenic karstification stage. (b) Late Paleozoic overlying caprock development stage. (c) The Mesozoic uplift-type karstification and karst geothermal system development stage. (d) The Himalayan karstification and karst system reconstruction stage. (e) The Quaternary regional cover bed deposition and the karst geothermal system formation stage (AA’ in Figure 1).

3 Genesis elements of karst geothermal system

3.1 Geothermal source analysis

3.1.1 Dynamic mechanism of geothermal anomalies in the deep basin

The high geothermal flow generated by the Cenozoic rifting provides a good geothermal source for the Taiyuan Basin, which belongs to the anomaly area of high thermal flow (Deng et al., 1999; Luo et al., 1988). Its geothermal flow value exceeds 71 mW/m² (Figure 4a), which is much higher than the global average geothermal flow value of 61.5 mW/m². It can be seen from the Curie isothermal surface depth map of the five major fault basin groups in Shanxi Province (Figure 4b) that there is a NE-trending belt distribution convex belt on the Curie isothermal surface of the Jinzhong Rift, which is about 60 km long, 20 km wide, less than 20 km deep and much less than 32 km in the surrounding mountainous areas. The Curie isothermal surface located in the upper crust has characteristics of low velocity, high conductivity and high temperature (600 °C). It is a physical surface to indicate and judge the thermal state of the crust. The thermal flow distribution in the Jinzhong Rift is exactly the same as the distribution of the Curie isothermal surface, indicating that the thermal anomalies in shallow crust are mainly controlled by deep structures.

Figure 4

Figure 4. Distribution map of terrestrial heat flow (a) and Curie surface depth (b) in the fault basins of Shanxi province (according to cited from Li et al., 1994 slightly changed in).

The sedimentary range of the Cenozoic faulted basin group in Shanxi is basically the same as the Curie isothermal surface uplift zone (or high thermal flow anomaly zone), but the distribution characteristics are obviously different (Figure 4). In Xinding Basin and Datong Basin in the north of Shanxi, the anomaly zones of high thermal flow are mainly located in the middle of the basins, and they have symmetrical distribution characteristics. In Yingcheng Basin, Linfen Basin and Jinzhong Rift in the south of Shanxi, the anomaly zones of high thermal flow are located at the western boundary of the basins and have asymmetrical distribution characteristics. The origin of symmetrical and asymmetrical distribution is related to the different dynamic mechanism of fault basin formation.

The dynamic mechanism of the formation of faulted (or rift) basins generally includes pure shear model and simple shear model (Fossen, 2010), in which pure shear model forms a symmetrical rift with hot spots in the central part of the rift basin, while simple shear model is generally dominated by a low-angle shear zone and produces an asymmetric rift, with hot spots located at the edge (or even the periphery) of the rift basin. It can be inferred that the dynamic mechanism of Xinding basin and Datong basin in northern Shanxi is a pure shear model, while that of Yingcheng basin, Linfen basin and Jinzhong rift in southern Shanxi is a simple shear model (Zhang and Deng, 1992) (Figure 5a).

Figure 5

Figure 5. Comparison of simple shear model of crustal extension (a) and crustal section model of Jinzhong Rift (b). (a-according to b-according to cited from Li et al., 1994 slightly changed).

To sum up, the heat source of Taiyuan basin comes from the high earth heat flow (>71 mW/m²) of Cenozoic asymmetric fault basin, and its dynamic genetic mechanism is a simple shear model under the background of crustal extension. This can also be evidenced by the tectonic form of the Cenozoic sedimentary pattern in Taiyuan Basin, which is faulted in the west but overlapped in the east, and the largest subsidence center located on the west side of the basin (Figure 1).

3.1.2 Crustal structure model of geothermal anomalies in the basin

According to the interpretation results of magnetotelluric sounding profiles in Shanxi, the former researchers divided the crust-upper mantle geological structure into five electrical layers, and the high-conductivity layer exists in the middle crust and upper mantle. The high-conductivity layer is thick and shallow in the basin. The burial depth of high-conductivity layers in crust and upper mantle are mirrored with Cenozoic thickness of the fault basin. On the basis of summarizing these research results, this paper has compiled a schematic diagram of the crustal section of Jinzhong Rift. Among them, the deep crust thickness of the basin is about 43 km and the depth of Curie is about 20 km. The high-conductivity layer in the middle crust plays a decisive role in the thermal anomaly distribution in the shallow crust of the basin. The buried depth is about 14 km, the resistivity is 2–5 Ω m and the thickness is 4 km. The buried depth becomes larger, the thickness gradually becomes thinner and even disappears towards the mountain areas on both sides, and steep in the west and gentle in the east. The abnormal high point is located on the western boundary of the basin. These determine the asymmetric distribution pattern of fault basins in the shallow crust under the action of mechanical mechanism of simple shear model (Figure 5b).

3.1.3 Thermal transfer modes in the shallow level of the basin

As a Cenozoic faulted basin with three sides surrounded by mountains and relatively narrow area, the shallow geothermal field of Taiyuan Basin is inevitably affected by deep faults in the basin margin and groundwater activities, resulting in a variety of thermal transfer modes in different secondary tectonic units in the basin. Based on the relationship between the geothermal gradient of karst geothermal reservoir and caprock revealed by the stratigraphic temperature-depth curves of geothermal wells in different tectonic units of Taiyuan Basin (Figure 6), the thermal transfer modes in Taiyuan Basin are divided into two distinct types: convection type and conduction type. Among them, the convection type can be sud-divided into two types: the recharge water convection type and the deep thermal flow convection type.

Figure 6

Figure 6. (a,b) show the wellhead temperature is from 15-17°C. (c,d) show the temperature trend of deep thermal flow convection typed. (e,f) show the temperature trend of normal conduction type.

3.1.3.1 Recharge water convection type

It mainly distributes in the northern part of the basin, north to Sanji Horst. Because of the short distance from the recharge source area, the fast runoff speed, the thin geothermal caprock (<400 m) and the poor geothermal sealing performance, the strong geothermal convection reduces the temperature of the upper caprock, which results in that the karst water in the area basically maintains the temperature of the surface source area, or the temperature increase is very small, the water temperature increases by 13.0–17.0 °C, and the caprock geothermal gradient increases by 0.5 °C/100 m. For example, well RN-31 (Figure 6a) in Nitun Fault Terrace is 460 m deep, the wellhead temperature is only 16.0 °C. Well TS-2 on Sanji Horst is 823.33 m deep, the wellhead temperature is only 17.1 °C (Figure 6b). The karst geothermal system in the northern part of Taiyuan Basin is ineffective, because the geothermal water temperature is less than 25 °C, which can not meet the basic conditions for the utilization of geothermal resources.

3.1.3.2 Deep thermal flow convection type

It mainly occurs in Xibianshan Fault Terrace in the western part of the basin. Because the thermal anomaly zone in the deep level of the basin is located in the secondary tectonic zone, and the Jinci Major Fault connects the deep geothermal source, it leads to strong geothermal convection heating. For example, well S2 located in Xibianshan Fault Terrace has a well depth of 801.18 m and karst geothermal reservoir temperature of 45 °C. The geothermal convection in the karst geothermal reservoir accelerates the thermal transfer rate and reduces the geothermal gradient by about 2.17 °C/100 m (Figure 6c). Correspondingly, the thermal aggregation of the upper caprock makes the average geothermal gradient reach to 4.69 °C/100 m (Figure 6c). In addition, similar karst reserves convective thermal transfer and shallow caprock thermal aggregation phenomenon exist in well TD-1 near Tianzhuang Fault Zone. Due to the upward vertical movement of karst water in the deep fault, the geothermal gradient of the Ordovician karst geothermal reservoir and Carboniferous-Permian direct caprock has been homogenized, with the geothermal gradient difference is less than 1.40 °C/100 m. The high geothermal gradient section with rapid temperature increase mainly occurs in Cenozoic regional caprock, and the geothermal gradient is greater than 7.0 °C/100 m (Figure 6d).

3.1.3.3 Normal conduction type

This type of thermal transfer mainly occurs in the middle part of the basin south to Sanji Horst and north to the Tianzhuang Fault. Its characteristics are that the temperature of the caprock increases linearly with normal (or slightly lower) geothermal gradient as the depth increases. For example, well SSSS-1 and well SLYF-1 in the Xiwenzhuang Uplift have an average gradient of 2.0–2.5 °C/100 m (Figures 6e,f).

3.2 Characteristics of karst geothermal reservoir

3.2.1 Residual thickness and top burial depth of karst geothermal reservoir

The residual thickness of karst geothermal reservoirs is the basis for the development of hot water reservoirs as well as an important parameter for resources evaluation. Because the original sedimentary environment of Ordovician karst geothermal reservoir is a stable shallow sea platform, the initial thickness of the reservoir varies little, ranging from 600 m to 800 m. Therefore, the thickness difference of the remaining Ordovician is mainly related to the denudation degree caused by the later tectonic movement. The foregoing study shows that the structural position of Taiyuan Basin during Mesozoic denudation period is on the wing of wide and gentle anticline, and the overall denudation degree is not strong (Figure 3c), which results in little difference in the remaining thickness of karst geothermal reservoir. In the basin area north of Sanji horst, most of the area is exposed to the Upper Ordovician, and the thickness of karst geothermal reservoir is about 550–600 m. In the basin area south of Sanji horst, the Ordovician is not exposed, and the initial thickness of the karst geothermal reservoir is basically preserved, ranging from 600 m to 800 m (Figure 7a).

Figure 7

Figure 7. Description of Ordovician karst reservoir in the middle part of Taiyuan Basin. (a) Reservoir residual thickness; (b) Buried depth of reservoir; (c) Reservoir temperature; (d) Geothermal gradient of caprock.

The top burial depth of karst geothermal reservoir is the final manifestation of denudation (e.g., Mesozoic) and sedimentation (e.g., Cenozoic) caused by tectonic movement. It is also one of the main indicators for distinguishing favorable areas of geothermal resources. Based on the previous achievements and the latest data of geothermal wells, the topographic map of Ordovician karst geothermal reservoirs in Taiyuan Basin is compiled in this paper (Figure 7b). The map reveals the difference of top burial depth and thermal sealing performance between the northern, middle and southern segments of the Ordovician karst geothermal reservoir in Taiyuan Basin. The main performances are as follows: (1) In the area north of Sanji horst, the thickness of the cover is less than 500 m, and the thermal sealing performance is poor. (2) In the middle segment which located from the south of Sanji horst to the north of Tianzhuang fault zone, the burial of the karst geothermal reservoir roof is controlled by the basin structure. Overall, it shows a gradual deepening rule from south to north, from the basin margins on both sides of the east and west to the sedimentary center-Jinyuan sag with the buried depth of the top surface of the karst geothermal reservoir is 800–2000 m. This area is a favorable area for the development and utilization of karst geothermal reservoir because of its moderate burial depth and good sealing performance. (3) In the area south of Sanji horst, the burial depth of karst geothermal reservoir is more than 3000 m, and the economic risk of geothermal development and utilization is relatively high.

3.2.2 Temperature of karst geothermal reservoir and geothermal gradient of caprock

3.2.2.1 Temperature of karst geothermal reservoir

This paper refers to the measurement temperature of geothermal fluid at the wellhead, which is actually the mixing temperature of geothermal water in different depths of reservoirs (neglecting the heat loss during pumping), and can be regarded as the average temperature of karst geothermal reservoirs. It is one of the main parameters for evaluating the quality of geothermal resources. According to the geothermal fluid temperature (Ma et al., 2011; Ma et al., 2012) of existing geothermal wells and the latest 54 geothermal wells in Taiyuan Basin, the temperature distribution map of karst geothermal reservoirs in Taiyuan Basin is compiled (Figure 6b). This map shows that the temperature of karst geothermal reservoir is obviously controlled by its top burial depth and geological structure. It is mainly manifested in three aspects: (1) Because of the thin thermal cover, the temperature of karst water is less than 25 °C, the basic condition of geothermal resources utilization is not reached in Sanji horst and the basin area north of it. (2) In the middle segment which located from the south of Sanji horst to the north of Tianzhuang fault zone, the karst geothermal reservoir temperature increases gradually from south to north, from the edge of mountainous areas on the east and west sides to the center of the basin with the increase of the thickness of the caprock, ranging from 30 °C to 75 °C. (3) The high temperature distribution area of karst geothermal reservoir is located in the southwest of Xiwenzhuang uplift to the east of Jinyuan depression. The temperature range from 65 °C to 75 °C. It is a high-quality geothermal resource development area for geothermal heating utilization (Figure 7c).

3.2.2.2 Geothermal gradient of caprock

The geothermal gradient of the caprock in the basin is a comprehensive reflection of the temperature of the karst geothermal reservoir, the thickness of the overburden and the mode of heat transfer. It is also one of the important parameters for optimizing the exploration target area. In the basis of the temperature of karst karst geothermal reservoir and the burial depth of the top surface, the geothermal gradient distribution map of the caprock (Figure 7d) shows that: (1) The geothermal gradient of the cover layer is less than 0.5 °C/100 m in the basin area north of Sanji horst, which is the result of intense convection of the recharge water. (2) In the middle segment of the basin to the south of Sanji horst and north of Tianzhuang fault zone, the geothermal gradient value gradually increases from the negative anomaly area in the basin margin (<2.0 C/100 m) to the normal area uplift area in the basin (∼3.0 C/100 m), and then to the positive anomaly area on the basement uplift zone in the basin (>4.0 C/100 m). (3) The positive anomaly of the geothermal gradient in the northern part of the western mountain fault terrace (>5.0 C/100 m) is related to the local thermal convection caused by deep faults.

Based on the analysis of karst geothermal reservoir characteristics, the Chengnan uplift zone and the Xiwenzhuang uplift zone, that be seated in the middle part of the basin are the most favorable exploration targets for karst geothermal reservoirs because of their relatively shallow burial depth (800–1700 m) and high karst geothermal reservoir temperature (50–75 °C).

3.2.3 Reservoir heterogeneity

Reservoir heterogeneity refers to the uneven changes in spatial distribution and internal attributes of reservoirs due to the influence of sedimentation, diagenesis and tectonics during the formation process. The reservoirs in the study area are karst geothermal reservoirs in the lower Paleozoic, which go through longer in geological period and have a long evolutionary history. Influenced by tectonic and weathering effects in the later stage, the reservoirs have many types and strong heterogeneity. It is rare that the researches on the heterogeneity of carbonate reservoirs in this area. Based on the productivity test and well interpretation results of the geothermal wells in Xiwenzhuang uplift, Taiyuan Basin, this paper intends to analyze the differences between the horizontal distribution and vertical stratification of reservoirs.

3.2.3.1 Horizontal zoning of reservoirs

The inhomogeneity of karst geothermal reservoirs in the study area is mainly manifested in the following aspect: in the same geothermal field within a geothermal system, the water volume of single well varies greatly. Taking the water test results of 43 geothermal wells in Xiwenzhuang geothermal field of Taiyuan Basin as an example (Figure 8). The depth and the water-intaking sections of these 43 wells is basically similar, which is about 2,500 m deep and 600–800 m long. The water-intaking horizons include the Ordovician and the Fengshan formation in the upper Cambrian. Although most of the effluent temperatures are concentrated in the range of 60–70 °C, the water quantity of a single well varies greatly. According to statistics, there are 20 wells whose water volume is more than 110 m³/h, accounting for 46.51%; 11 wells whose water volume is 50 m³/h to 110 m³/h, accounting for 25.58%; 12 wells whose flow rate is less than 50 m³/h.

Figure 8

Figure 8. Distribution of water yield of single well in Xiwenzhuang geothermal field in Taiyuan Basin.

It can be seen from the contour map of total water volume in Xiwenzhuang geothermal field (Figure 8): (1) In the southeastern area of Xiwenzhuang uplift (FHJD—JLQC—the east of GXWL—NMC), there exists a high water volume zone with NE direction. The water volume of a single well is more than 100 m³/h, and the highest is 178 m³/h (JLQC-2). (2) The water production in the north (SSSS) and south (TD) of Xiwenzhuang uplift is the second, about 60–100 m³/h. (3) The water production in the central (SLYF) and the southwest part (west of GXWL) is relatively low, less than 60 m³/h. The lowest well is THY-1, which is only 8.7 m³/h.

Combined with regional tectonic analysis, the pre-Cenozoic bedrock of Xiwenzhuang uplift is a broad anticline structure, which is the plunging (extension) part of the NE-trending Dongshan anticline in the basin (Figure 1). The strata involved in anticline are Cambrian-Triassic. The ridge line in the core of the anticline is exactly in the high-yielding water zone along the NE-trending strip. It can be inferred that the hidden NE-trending ridge line belt in the Dongshan anticline is the most favorable channel for karst water migration and the best water-rich reservoir in the basin. On the flanks of the core zone of the anticline, the water-enrichment of thermal reservior decreases; at the turning end of the disappearance of the concealed anticline, the water-enrichment of reservoir is poor.

3.2.3.2 Vertical stratification of reservoirs

There are thirteen geothermal wells in total at the Xiwenzhuang geothermal field of Taiyuan Basin have been tested for fluid production profiles, which are distributed in the northern, central, southwestern and southern parts of Xiwenzhuang uplift. Comprehensive analysis of productivity profile shows that there are four main aquifers in Ordovician in the study area from top to bottom: the lower member of Fengfeng Formation to the upper member of Shangmajiagou Formation, the lower member of Shangmajiagou Formation, the upper member of Xiamajiagou Formation, the lower Liangjiashan Formation to the upper Yeli Formation, these constitute four sets of geothermal fluid reservoir-cap assemblages in Ordovician (Wang et al., 2018). However, the main aquifer of the four sets of reservoirs in the Ordovician vary in different structural positions, sometimes transiting to the Fengshan Formation on the top of the Cambrian. For instance, the No.33 institutes in the north of Xiwenzhuang uplift, the main aquifer is Fengshan Formation of Cambrian and the second is Fengfeng Formation. The hatching base block in the central part of Xiwenzhuang uplift regards the Shangmajiagou Formation and Liangjiashan Formation as the main aquifers and the lower member of Fengfeng Formation and Xiamajiagou Formation as the secondary aquifers. In the high-tech logistics area in the southwest of Xiwenzhuang uplift, the main aquifers are Xiamajiagou Formation and Yeli Formation, and the secondary aquifers are Fengfeng Formation and Liangjiashan Formation. Taiyuan University area lies in the south of Xiwenzhuang uplift, the main aquifer is Liangjiashan Formation, and the next is Xiamajiagou Formation. The whole Lower Paleozoic can be regarded as a large reservoir, it can be divided into three or four main water-bearing sections, where the main aquifer is prone to “overflowing” phenomenon in the process of migration.

Taking GXWL-2 well in the southwest of Xiwenzhuang uplift as an example to illustrate the vertical zoning features of reservoir (Figure 9).

Figure 9

Figure 9. Log curve interpretation and productivity testing of GXWL-2 well in Taiyuan Basin.

Productivity profile analysis of GXWL-2 well (Figure 8; Table 1): (1) Thirteen aquifers with different thickness can be identified in the reservoir section, and the total effective thickness is 28.5 m. According to the three dolomite-limestone sedimentary cycles, it can be classified into 4 sections. From new to old, the sequence is the Fengfeng Formation to the upper member of Shangmajiagou Formation, the lower member of Shangmajiagou Formation, the upper member of Xiamajiagou Formation and the Yeli Formation. The average porosity is 3.7%, 6.5%, 3.7% and 7.1%, respectively. (2) Yeli Formation is the main aquifer, with productivity accounting for 51.31% of the total. The other three layers are secondary production layers, accounting for 15.05%, 16.29% and 17.35% of the total respectively from new to old. (3) The effective water production thickness of Yeli Formation is only 5 m, but it contributes more than half of the production capacity, which may be related to its relatively high permeability (average 0.33 mD).

Table 1

Table 1. Corresponding tables for log interpretation of GXWL-2 well in Taiyuan Basin.

3.3 Analysis of water source and transport channel

3.3.1 Hydrogen and oxygen isotopes analysis

Hydrogen and oxygen isotopes are the most commonly used stable isotopes. The linear correlation between the hydrogen-oxygen isotope compositions in meteoric precipitation in a particular region is called the meteoric precipitation line. As the geothermal water enters the basin from the recharge mountainous area, it gradually deviates from the meteoric precipitation line as the water-rock interaction progresses. Because of the different effects of different regions, it is possible to distinguish between different fluid systems and to track the direction of fluid transport. Three points of understanding can be drawn from theδ2H-δ18O plot of different layers of the Taiyuan Basin (Figure 10): (1) The isotope composition of the karst water in the East and West Hills has a similar trend with the current Fen River, reflecting the overall composition of the isotope is affected by the current meteoric precipitation isotope and isotope exchange with the sedimentary layer. (2) The isotope composition of karst water in the East and West Hills does not have obvious distinguishable structure, indicating that the karst geothermal system in the central part of the Taiyuan Basin has been recharged from the karst water from the East and West Mountains. (3) The hydrogen and oxygen isotope composition of the Permian pore water has obvious isotope depletion phenomenon, and its change trend is obviously deviated from the isotope variation trend of the Fen River. This indicates that as a karst geothermal reservoir caprock, the formation pore water is not homologous to the underlying Ordovician karst water and may be older than deep karst water. At the same time, it also reflects the good sealing effect of the Carboniferous-Permian coal strata on the karst geothermal system.

Figure 10

Figure 10. Map of hydrogen and oxygen isotopic composition of geothermal water in Taiyuan Basin (revised from Zhang and Liu, 2009).

3.3.2 Water chemical analysis

The karst water temperature at edge of Taiyuan Basin is low (15–25 °C) and the mineralization is small (558–1199 mg/L). The composition of water chemical ions is relatively scattered (Ma et al., 2012; Li et al., 2006; Zhang, 1990). According to Shukalev classification method, the water chemistry can be classified into three types: HCO₃-Na type, SO₄-Ca·Mg type, and HCO₃-Ca·Mg type. It reflects the characteristics of the diverse sources of geothermal water at the edge of the basin (Figure 11a).

Figure 11

Figure 11. Comparison of geothermal water Piper map between the edge of Taiyuan Basin (a) and Xiwenzhuang geothermal field (b).

The water chemical data analysis of 34 geothermal wells in the Xiwenzhuang geothermal field in Taiyuan Basin indicates that: (1) the wellhead water temperature of karst geothermal reservior is higher (51–74 °C), the mineralization degree is higher (1,592–3279 mg/L), the pH values are weakly alkaline (6.34–8.31). (2) The water chemistry type is dominated by SO₄-Ca type, and the smallest part is SO₄-Ca•Mg type, Cl•SO₄-Ca•Mg type and SO₄-Ca•Na type (Figure 11b). (3) With the increase of TDS, the concentration of cation Ca²⁺ and the concentration of anion SO₄^2- increased significantly and showed a good linear relationship. It indicates that the water-rock interaction in the Ordovician limestone groundwater is dominated by the dissolution of minerals containing Ca²⁺ and SO₄^2- ions (Figure 12).

Figure 12

Figure 12. Relationship between ionic concentration and TDS in Ximenzhuang geothermal field, Taiyuan Basin.

The change of salinity and ion content of karst geothermal water provides an important basis for studying the flow direction of geothermal water and the process of water rock interaction (Zhang, 1990; Li et al., 2006). Comparing the mineralization degree and water chemistry type of the karst water in the East and West Mountain Faults on the edge of the basin and the Xiwenzhuang geothermal field in the central part of the basin, it can be inferred that the geothermal water transport process of the karst geothermal system in the Taiyuan Basin is: The karst water in the exposed areas of the east and west mountains of the basin is recharge along the deep fault and the karst unconformity, it gradually migrates and accumulates to the Xiwenzhuang uplift in the middle of the basin, the water and rock interaction in the transport process is mainly the dissolution of carbonate rock and gypsum minerals. It reflects the change law of HCO₃→HCO₃+SO₄→SO₄ type from the recharge area to the basin confined zone (Hou, 2002).

3.3.3 ¹⁴C age analysis

The ¹⁴C age of geothermal water reflects the time when geothermal water is infiltrated from the recharge area, through deep circulation, runoff in the reservoir, and transported to the sub-structural belts of the basin. This paper synthesizes the ¹⁴C dating results of previous predecessors (Ma, 2007; Li et al., 2006), and compiled a distribution map of the geothermal retention time in Taiyuan Basin (Figure 13). On the whole, the retention age of geothermal water in Taiyuan Basin is obviously controlled by the structure of the basin, and it has the characteristics of short transport distance, fast recharge speed and younger retention age. The specific performances are as follows: (1) From the runoff of the east and west mountain recharge areas to the edge of the basin, the retention age of geothermal water is less than 500a, which is very fast for the distance of the runoff distance of 40–50 km. For example, the ¹⁴C relative age of karst geothermal water in the Jinci fault zone is 318a. (2) In the east and west of the Bianshan fault zone, an obvious age gradient zone is formed, with an age range of 500-1000a, reflecting the vertical movement of karst geothermal water along the edge of the basin and towards the deep basin. For example, the geothermal age of the TS₁ well is 978a. (3) The age of the fluid in the basin is about 2000 a, and the seepage velocity is relatively fast. (4) In the core section of the Xiwenzhuang uplift in the main confined area, the age of geothermal water retention is relatively old, exceeding 10 ka. Such as the ¹⁴C age of Xiaodian area is 11,330 a.

Figure 13

Figure 13. Distribution map of geothermal water retention time in Taiyuan Basin.

4 Transport model of karst geothermal system

The karst geothermal reservoir temperature in the northern part of Taiyuan Basin is less than 25 °C, the resource conditions cannot reach the requirements for development and utilization. Therefore, the study object of the karst geothermal system refers only to the Lower Paleozoic between the Sanji horst-Tianzhuang faults zones in the middle section of Taiyuan Basin. Through the above-mentioned research on reservoir performance, water recharge source, transport channel and heat transfer mode of karst geothermal reservoir, the transport model of karst geothermal system in Taiyuan Basin can be summarized as: Under the background of high geothermal flow in asymmetric rift basin, the meteoric precipitation recharge into the karst water system from the exposed areas of the karst in the East and West Mountain, gradually transport into the basin area along the karst unconformity surface and fault as the transport channel, and drain to deep basin through the basin deep fault, thus entering the hidden reservoirs in the basin. During the flow process, the karst water continuously absorbs the heat of the surrounding rock mass and gradually increases the temperature. The average temperature of the karst geothermal reservoir in the core of the Xiwenzhuang uplift zone can reach 65–75 °C. At the same time, the Upper Paleozoic-Mesozoic sand, shale and Cenozoic clay rocks in the basin provide a good thermal insulation cap for thermal water, forming a medium-low temperature conductive karst geothermal system that can be used for heating (Figure 14). Part of the karst thermal water is blocked in front of the piedmont fault zone, moves upwards, and mixes with the shallow cold water to form a convective low temperature spring (such as Jinci Spring).

Figure 14

Figure 14. Geothermal water migration model map of karst geothermal system in Taiyuan Basin F₁- Tianzhuang fault zone, F₂-Jinci fault, F₃-Nanyan fault, F₄- Fenhe fault, F₅-Mingqian fault, F₆-Dongbianshan fault.

5 Evaluation of geothermal resources

The evaluation range of the geothermal resources of the karst geothermal system in Taiyuan Basin is Sangi Horst-Tianzhuang Fault Zone in the middle part of the basin. According to the division boundary of secondary structural units in the basin, it can be divided into eight units. From north to south, they are Urban Depression, Ximing Fault Terrace, Chengdong Fault Terrace, Yinxian Horst, Chengnan Uplift, Western Mountain Fault Terrace, Jinyuan Depression and Xiwenzhuang Uplift. The evaluated karst geothermal reservoir is Ordovician.

5.1 Computational formulas

The method of “thermal storage volume” is adopted in the evaluation. The calculation of “thermal storage volume method” includes two parts: the heat storage of rocks in karst geothermal reservoir and the carrying heat of geothermal water. Its variable parameters are the evaluation area, effective thickness, porosity and thermal storage temperature of Ordovician karst geothermal reservoir, which can be determined by the research results mentioned above. Specific formulas are as follows:

$Q=Ad\left[P_c{C}_c\left(1-\phi \right)+P_w{C}_w\phi \right]\left(t_r-t_j\right)$

Q-geothermal resources, J; A-evaluation area, m²; d-karst geothermal reservoir thickness, m; t_r-karst geothermal reservoir temperature, °C; φ-rock porosity, %; t_j-reference temperature, °C; P_c、P_w-densities of rock and water respectively, kg/m³; C_c、C_w-specific heat capacity of rock and water respectively, J/(kg °C).

5.2 Evaluation parameters of thermal storage

The accuracy of geothermal resource evaluation mainly depends on the reliability of the evaluation area, effective thickness, porosity and temperature of karst geothermal reservoir. Among the eight secondary tectonic units to be evaluated in this paper, more than 60 geothermal wells have been drilled in the Xiwenzhuang uplift, so the evaluation of its resources is more accurate. The parameters of the other seven tectonic units may be estimated by comparison with those of the Xiwenzhuang uplift (Table 2). The specific steps are as follows:

1. Evaluation area (A): The area of karst geothermal reservoir evaluation is the area of secondary tectonic units, that is, the area bounded by boundary faults of tectonic units. It can be calculated automatically by using the software of resource evaluation. For example, the area of Xiwenzhuang Uplift is 124.94 km².

2. Karst geothermal reservoir effective thickness (d): In geothermal drilling and oil drilling logging data, the segments with porosity greater than 1.8% and permeability greater than 0.1mD are considered as effective aquifers. The sum of all aquifer sections is the effective thickness of karst geothermal reservoir revealed by the well. The average effective thickness of all geothermal wells can be a reliable evaluation parameter. The average effective thickness of 52 geothermal wells in Xiwenzhuang Uplift is 177.7 m. The average thickness of Ordovician strata is about 684 m, and the thickness ratio is 26.0%. As the initial sedimentary environment and karstification process of Ordovician in other tectonic units are basically similar, the average effective thickness of other evaluation units can be calculated according to the same thickness ratio. For example, the average thickness of residual strata in Jinyuan Depression is 673 m, and the effective thickness of aquifer is 175.0 m according to the same thickness ratio (Table 2).

3. The average temperature of karst geothermal reservoir (t_r): This is a parameter most affected by the cold water in the supply area of geothermal water. The parameters of the eight evaluation units can be calculated by weighted average method of area according to the above-mentioned temperature distribution map of karst geothermal water (Figure 5c). The average temperature of Xiwenzhuang reservoir with the most abundant data is 68.8 °C, other calculation results are shown in Table 2.

4. Porosity of karst geothermal reservoir: The average porosity of 52 geothermal wells in Xiwenzhuang uplift is 5.5%. Considering that the other seven tectonic units have the same tectonic evolution history and karstification process, the average porosity of other geothermal fields is 5.5%.

5. Other parameters: The average annual temperature is 12.5 °C, the density of geothermal water is 1,000 kg/m³, the density of rock is 2,700 kg/m³, the specific heat of water is 4180 J/(kg.°C), and the specific heat of rock is 920 J/(kg.°C).

Table 2

Table 2. Summary of evaluation parameters and calculation results of geothermal resources for Ordovician in Taiyuan Basin.

5.3 Analysis of evaluation results

The karst geothermal system in Taiyuan Basin is calculated by eight evaluation units as shown in Table 2. The total amount of geothermal resources is 83.03 × 10⁸GJ, which is equivalent to 2.83 × 10⁸t of standard coal (29.3 GJ of heat can be produced from 1t of standard coal). If the recovery rate of karst geothermal reservoir is 15% (according to the “Evaluation Method for Geothermal Resources” (DZ 40-85) and the “Geological Exploration Specification for Geothermal Resources” (GB/T 11,615-2010,the recovery rate of karst geothermal reservoir is set at 15%), the recoverable resources of the karst geothermal system in Taiyuan Basin are 12.45 × 10⁸GJ, equivalent to 4,251.0 × 10⁴t of standard coal. According to the calculation that the annual heat required for heating per square meter is equivalent to 0.0283 t standard coal, the heating area of the karst system can reach 15.02 million square meters. In view of the current built geothermal heating area of only 3 million square meters, there is a huge potential for resource development.

6 Conclusion

We have summarized the storage performance, supply water sources, migration channels, and heat transfer methods of karst thermal reservoirs in the Taiyuan Basin, and established a geothermal water migration model for karst geothermal systems to describe the genesis mechanism of geothermal systems in mountain fault basins. The results indicate that, the heat source of the karst geothermal system in Taiyuan Basin comes from the high terrestrial heat flow in asymmetric fault basin (>71 mW/m²). Influence by the genetic mechanism of the intermountain fault basin, the heat transfer mode can be divided into two categories: strong convolution type and heat conduction type, which can be further divided into three sub-categories: the strong convolution type of recharge water supply in the weak area of basin margin caprock, the deep thermal convolution type of basin margin deep fault zone and the heat conduction type of layered karst geothermal reservoir inside1016/j.rser.2014.05.0 the basin.

Controlled by the geological structure of the basin with the western faulting and the eastern Overlying, the burial depth of its top surface is larger in the west than that in the east, vary between 400 and 1900 m, and the temperature of the karst geothermal reservoir is 30–75 °C. The heterogeneity performance of karst geothermal reservoir is mainly controlled by the hidden structure in NE direction on the plane. In the longitudinal profile, the effective reservoir section of 15–20 layers with about 160 m cumulative thickness is identified, and it can be divided into 3–4 main water-bearing sections, where the main aquifer is prone to “overflowing” phenomenon in the process of migration. The recharge-migration mode of the karst geothermal system in Taiyuan Basin has the characteristics of two-way, near-source and rapid. The migration distance of the karst water from the exposed Ordovician area to the basin pressure area is about 30 km, the migration time is about 2000 a.

The total geothermal resources of the karst geothermal system in Taiyuan Basin are estimated to be 8.303 billion GJ, which is equivalent to million tons of standard coal. The annual exploitation of geothermal resources can meet the heating area of 1,502 million square meters with broad prospects for development.

Data availability statement

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding authors.

Author contributions

WT: Writing – original draft. WX: Conceptualization, Methodology, Supervision, Writing – original draft. ZX: Data curation, Visualization, Writing – review and editing. LL: Data curation, Methodology, Writing – review and editing. LH: Data curation, Resources, Writing – review and editing.

Funding

The author(s) declare that financial support was received for the research and/or publication of this article. This work was financially supported by the Deep Earth Probe and Mineral Resources Exploration-National Science and Technology Major Project (No.2024ZD1003600) and the Sinopec Science and Technology Project (Grant No. P25075).

Acknowledgments

We are grateful to the four reviewers for their invaluable comments which helped to improve the quality of this paper.

Conflict of interest

Authors WT, WX, ZX, LL, and LH were employed by Sinopec Star Petroleum Co., Ltd.

The authors declare that this study received funding from Sinopec. The funder had the following involvement in the study: all geothermal wells mentioned in this article are geothermal wells currently used by Sinopec. Sinopec provided data support, experimental instruments, experimental sites, and data analysis for the conclusions obtained in this article.

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The author(s) declare that no Generative AI was used in the creation of this manuscript.

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