# Main Controlling Factors and Formation Mode of Geothermal Anomaly in Eastern Chenghe Mining Area of Weibei Coalfield

Authors: Tao Peng; Kaixiang Liu; Yue Chen; Feng Wang; Bing Li
Journal: Geofluids
 (2022)

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## Abstract

Geothermal anomaly has gradually become a prominent issue affecting the efficient mining of coal with the depth of coal mining increasing in eastern Chenghe mining area of Weibei coalfield. Here, we comprehensively investigated the distribution characteristics of the present geothermal field, analyzed the main controlling factors, and constructed forming mode of geothermal anomaly by the temperature measurement in surface borehole and underground water, combined with coal-rock thermal conductivity test. The results show that the geothermal gradient ranged from 25.7°C/km to 54.3°C/km. The areas with geothermal gradient greater than 30°C/km accounted for 88.31%, and there was the highest gradient value in the southeast F1 fault zone. The heat flow was between 66.81 mW/m2 to 128.49 mW/m2, which belonged to the obvious high heat flow area. Under the action of the main controlling factors such as fault, fold, coal-rock thermal conductivity, and groundwater activity in the region, the geothermal gradient and geothermal heat flow values showed an increasing trend from northwest to southeast. According to the distribution characteristics of heat flow and the action mechanism of main controlling factors, the geothermal anomaly in the study area was finally divided into two forming modes, i.e., fault-deep circulating hot water uplifting type and coal seam heat resistance-fold type. The research provides guidance for the geothermal hazard prevention of coal mine and the rational development and utilization of geothermal resources.


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## Body

## 1. Introduction

Geothermal energy has been gaining more and more attention in the past decade due to the huge amounts of geothermal resources and their wide distribution, as well as its environmentally friendly advantages [1–3]. However, it must be noted that high geothermal hazard in coal mine has become a new problem following roof collapse, gas explosion, fire hazard, water inrush, and coal dust with the increase of mining depth [4–11]. Hence, the distribution characteristics and controlling factors of geothermal anomaly are important research contents of mine geothermal hazard prevention and comprehensive development of geothermal resources [12–17].In general, the geothermal gradient is higher in the eastern basins and lower in the western basins in China. The geothermal resource abundance is also higher in eastern and the Beibuwan basin in China, and the geothermal source forming condition is better, followed by the Ordos, Qaidam, and Sichuan basins [18]. Many scholars have conducted studies on the geothermal field and thermal evolution history of the Ordos Basin and made a series of important progress in the current geothermal field, geothermal evolution history, thermal events, and other aspects of the basin [19–25]. Geothermal anomalies are the result of a combination of geological factors, groundwater, tectonics, faults, and thermal evolution all affect geothermal distribution and heat flow [26–32]. Geothermal sources, channels, reservoirs, and caps constitute the entire geothermal system. Jolie et al. [33] considered that the main driver of geothermal activity is elevated crustal heat flow, which is focused in regions of active magmatism and/or crustal thinning. Permeable structures such as faults exercise a primary control on local fluid flow patterns, with most upflow zones residing in complex fault interaction zones. Uzelli et al. [34] found that Varto and the surrounding region have important geothermal fields, developing in strike-slip tectonic setting in East Anatolia, which resulted from the collision of the Arabian and Eurasian plates. The planes of strike-slip faults in transtensional areas are more favorable for secondary permeability and enhances the geothermal fluid circulation. Guo et al. [35] show that thermal conductivity differences among Cenozoic caprock, Proterozoic carbonate reservoirs, and basement rock mainly affect the geothermal distribution in the Xiong’an New Area. Convective type is a common geothermal system [36, 37]. Groundwater rises to geothermal reservoirs through heat-controlling faults, causing convective heat transfer and increasing the geothermal reservoir temperature. Therefore, high-temperature groundwater accumulates in the shallow uplift areas.Weibei Carboniferous-Permian coalfield located in the southeastern margin of Ordos Basin is an important coal mining base in China. According to the results of geothermal survey in coal mining areas, the potential risk of thermal hazard is relatively small in northwest coal mining areas [38–40]. But Xizhuo and Bailiang mine, located in Chenghe mining area of Weibei coalfield, have appeared relatively rare phenomenon of mine thermal hazard. When the mining depth is more than 400 m, the air temperature of underground working face is more than 32°C, and the water inrush temperature of mine floor is more than 40°C. Geothermal hazard in mines seriously restrict the safe and efficient mining of coal resources in this area. At present, no special research work on mine geothermal has been carried out in Weibei coalfield, and there is a lack of comparative study on various geological conditions affecting mine geothermal field. Therefore, the distribution characteristics of geothermal and the causes of geothermal anomalies need to be further studied.In this context, the current geothermal field distribution characteristics and geothermal differences were carried out a detailed analysis based on the borehole temperature logging data and the test of coal-rock thermal conductivity. Second, the spatial variation of geothermal transfer conditions was further studied, and main controlling factors of geothermal field were revealed. Finally, according to the distribution characteristics of heat flow and the action mechanism of main controlling factors, and the formation mode of geothermal anomaly was constructed. It provided scientific basis for geothermal hazard prevention and geothermal resources.


## 2. Geological Settings

### 2.1. Geological Structure

The study area is Xizhuo mine and Bailiang mine in the east of Chenghe mining area in Weibei coalfield. Due to the influence of regional tectonic setting and large fault at the edge of the mining area, the strata in the eastern part of Chenghe mining area are monocline strike NEE and dip NNW, accompanied by a series of secondary folds and secondary fault, with a relatively gentle dip angle, generally less than 15°.The southern boundary of the study area is the regional F1 Hancheng large fault belt, which strikes NE and dip SE with a dip angle of 70° and a maximum drop of 550 m. The northern boundary is the regional F10 fault belt, which strikes NEE, dip WNN, and dip angle 70°. The drop of F10 fault belt from west to east gradually increases to 420 m, and it intersects with F1 fault in the northeast of the study area. According to the actual exposure of the two mines, there are also a number of normal faults of small size in the area, with a drop of less than 15 m in general (Figure 1).Figure 1
Structure outline and temperature measurement point distribution of the study area ((a) structural outline of Ordos Basin; (b) study area structure outline).
(a)(b)


### 2.2. Strata and Coal Seam

The strata in the area are, respectively, from old to new: Middle Ordovician Fengfeng group (O2f), Upper Carboniferous Taiyuan group (C3t), Lower Permian Shanxi group (P1s), Lower Permian Xiashihezi group (P1sh), Upper Permian Shangshihezi group (P2sh), Upper Permian Sunjiagou group (P2s), Lower Triassic Liujiagou group (T1l), Lower Triassic Heshanggou group (T1h), Triassic Zhifang group (T2z), and Quaternary (Q). The main mining is no. 5 coal seam of Permian Shanxi Formation. The thickness of the coal seam is between 2.9 m and 9.9 m, with an average thickness of 5.5 m.


### 2.3. Thermal Hazard Phenomenon

In recent years, the temperature of air flow in underground working face of Bailiang coal mine is above 31°C. The water temperature of the Ordovician limestone aquifer in the floor of the roadway in the north, 508 working face and 502 working face of the mine, reaches 40°C. Xizhuo mine also has a relatively obvious high temperature abnormal phenomenon. The water temperature at the mine water inrush point is also over 37°C in roadway tunneling, and a relatively significant high temperature abnormal phenomenon occurs in both mines.


## 2.1. Geological Structure

The study area is Xizhuo mine and Bailiang mine in the east of Chenghe mining area in Weibei coalfield. Due to the influence of regional tectonic setting and large fault at the edge of the mining area, the strata in the eastern part of Chenghe mining area are monocline strike NEE and dip NNW, accompanied by a series of secondary folds and secondary fault, with a relatively gentle dip angle, generally less than 15°.The southern boundary of the study area is the regional F1 Hancheng large fault belt, which strikes NE and dip SE with a dip angle of 70° and a maximum drop of 550 m. The northern boundary is the regional F10 fault belt, which strikes NEE, dip WNN, and dip angle 70°. The drop of F10 fault belt from west to east gradually increases to 420 m, and it intersects with F1 fault in the northeast of the study area. According to the actual exposure of the two mines, there are also a number of normal faults of small size in the area, with a drop of less than 15 m in general (Figure 1).Figure 1
Structure outline and temperature measurement point distribution of the study area ((a) structural outline of Ordos Basin; (b) study area structure outline).
(a)(b)


## 2.2. Strata and Coal Seam

The strata in the area are, respectively, from old to new: Middle Ordovician Fengfeng group (O2f), Upper Carboniferous Taiyuan group (C3t), Lower Permian Shanxi group (P1s), Lower Permian Xiashihezi group (P1sh), Upper Permian Shangshihezi group (P2sh), Upper Permian Sunjiagou group (P2s), Lower Triassic Liujiagou group (T1l), Lower Triassic Heshanggou group (T1h), Triassic Zhifang group (T2z), and Quaternary (Q). The main mining is no. 5 coal seam of Permian Shanxi Formation. The thickness of the coal seam is between 2.9 m and 9.9 m, with an average thickness of 5.5 m.


## 2.3. Thermal Hazard Phenomenon

In recent years, the temperature of air flow in underground working face of Bailiang coal mine is above 31°C. The water temperature of the Ordovician limestone aquifer in the floor of the roadway in the north, 508 working face and 502 working face of the mine, reaches 40°C. Xizhuo mine also has a relatively obvious high temperature abnormal phenomenon. The water temperature at the mine water inrush point is also over 37°C in roadway tunneling, and a relatively significant high temperature abnormal phenomenon occurs in both mines.


## 3. Experiments and Methods

### 3.1. Temperature Logging Data

The main method to obtain the temperature of underground strata in coalfield exploration is the continuous temperature measurement of borehole fluid system, called temperature logging [41, 42]. Temperature logging data mainly includes two types: approximate steady-state temperature and simple temperature measurement. The approximate steady-state temperature data was generally measured three days after completion, and the drilling fluid and rock temperature have basically reached equilibrium. The measured data can more objectively reflect the real temperature of the stratum. The simple temperature data was usually measured within 1 day after completion, so the temperature data must be corrected before it can be used.The geothermal data used in this study mainly come from the temperature measurement data of geological exploration holes in the study area. There are 51 boreholes for temperature measurement, including 4 tests of approximate steady-state temperature and 47 tests of simple temperature measurement. On the plane, all temperature measuring holes are evenly distributed in Xizhuo and Bailiang mine, which is well representative (Figure1).


### 3.2. Underground Temperature Measurement

Surface drilling is affected by the mining activities and the changes of surface and underground hydrogeological conditions. In order to precisely understand the variation law of ground temperature, this study also compared the previous ground temperature data and measured the temperature of the surrounding rock of coal seam and the outlet point of coal seam floor in Bailiang coal mine.Underground temperature measurement adopted shallow hole temperature measurement method. This method used the pneumatic drill bolt construction technology to drill into the surrounding rock at the target point. Two drilling holes with an included angle ≥45° and a depth ≥2 m at each position were grouped together, and the temperature was measured at the same time (Figure2). After the drilling was completed, the thermometer was sent to the bottom of the hole through the casing, and the hole top was blocked. The temperature measuring was read every 10 minutes. Stopping the temperature measurement after the thermometer data was stable. The temperature measurement time was generally between 90 minutes and 120 minutes. The temperature measuring instrument used in underground work was composed of a temperature sensor probe and a temperature display, which can be applied to conditions above 100°C. The display device was an AD display device, which adopt COMS micropower inheritance circuit and wide-temperature large-field wide-screen LCD display. The characteristics of AD display device was waterproof, explosion-proof, and seismic (Figure 2).Figure 2
Schematic diagram of temperature measurement arrangement.


### 3.3. Rock Thermal Conductivity Test

Rock thermal conductivity refers to the characteristics of heat transfer of rock, which is the main thermal parameter of rock and the basic data to obtain terrestrial heat flow [43]. The test samples in this study were collected from two surface supplement exploration holes XC1 and XC2 in Xizhuo mine. Twenty-five representative samples were tested, and the depth range of the samples ranged from 120 m to 600 m. The strata from old to new were involved, basically representing the main rock types of coal measure strata in Weibei coalfield.


### 3.4. Calculation of Geothermal Gradient

Because the approximate steady-state temperature measurement results are high accuracy, the ground temperature gradient can be calculated and obtained directly by the following formula.(1)T=GH−H0+T0,whereG is the average ground temperature gradient of temperature measuring borehole, °C/km; T is the bottom hole temperature, °C; T0 is the temperature of constant temperature zone, °C; H is the bottom hole depth, m; H0 is the depth of constant temperature zone, m. H0 in the area is 30 m, and T0 is 16.8°C.When using the simple temperature measurement data, the borehole bottom hole temperature (BHT, the same below) must be corrected. In this paper, the Waples and Ramly [44, 45] method was used to correct the bottom hole temperature. This method holds that for BHT data with depth less than 3000 m, the following correction formula was used for prediction:
(2)Tc=Ts+fTm−Ts,whereTC is the correction temperature within a certain depth at the bottom of the well, °C; TS is the ground temperature corresponding to the correction point, °C; f is the correction factor; Tm is the BHT data obtained during temperature measurement, °C.f is given by the following formula:
(3)f=−0.1462lnTSC+1.6990.572Z0.075,wheref is the correction factor; TSC is the drilling stop time, h; Z is the drilling depth of the corresponding test point, m.


### 3.5. Calculation of Terrestrial Heat Flow

Terrestrial heat flow refers to the heat transmitted from the earth’s interior to the surface per unit area of the surface or near surface shallow layer. Numerically, it is equal to the product of rock thermal conductivity and vertical geothermal gradient [46].
(4)Q=−k•ΔTΔZ,whereQ is the earth heat flow, mW/m2, k is the thermal conductivity, W/m·K, ΔT/ΔZ is the ground temperature gradient, °C/km.In order to make the calculated heat flow value more reliable, this paper selected a representative approximate steady-state temperature measurement borehole and used the least square method to fit the linear relationship between ground temperature and depth, so as to obtain the ground temperature gradient value. The calculation range of ground temperature gradient at the calculation point of heat flow value is below the bedrock surface. According to the requirements of obtaining heat flow value, well temperature logging and rock thermal conductivity test should be the same borehole, but it is difficult to do in practical work. Considering that the sampling depth of rock samples of Permian coal measure strata is evenly distributed and has sufficient representativeness, the difference of rock thermal conductivity of the same horizon in the study area is small. This paper calculated the average thermal conductivity of each temperature measuring hole by using the “weighted average method” of various lithologic thicknesses of boreholes [47, 48].


## 3.1. Temperature Logging Data

The main method to obtain the temperature of underground strata in coalfield exploration is the continuous temperature measurement of borehole fluid system, called temperature logging [41, 42]. Temperature logging data mainly includes two types: approximate steady-state temperature and simple temperature measurement. The approximate steady-state temperature data was generally measured three days after completion, and the drilling fluid and rock temperature have basically reached equilibrium. The measured data can more objectively reflect the real temperature of the stratum. The simple temperature data was usually measured within 1 day after completion, so the temperature data must be corrected before it can be used.The geothermal data used in this study mainly come from the temperature measurement data of geological exploration holes in the study area. There are 51 boreholes for temperature measurement, including 4 tests of approximate steady-state temperature and 47 tests of simple temperature measurement. On the plane, all temperature measuring holes are evenly distributed in Xizhuo and Bailiang mine, which is well representative (Figure1).


## 3.2. Underground Temperature Measurement

Surface drilling is affected by the mining activities and the changes of surface and underground hydrogeological conditions. In order to precisely understand the variation law of ground temperature, this study also compared the previous ground temperature data and measured the temperature of the surrounding rock of coal seam and the outlet point of coal seam floor in Bailiang coal mine.Underground temperature measurement adopted shallow hole temperature measurement method. This method used the pneumatic drill bolt construction technology to drill into the surrounding rock at the target point. Two drilling holes with an included angle ≥45° and a depth ≥2 m at each position were grouped together, and the temperature was measured at the same time (Figure2). After the drilling was completed, the thermometer was sent to the bottom of the hole through the casing, and the hole top was blocked. The temperature measuring was read every 10 minutes. Stopping the temperature measurement after the thermometer data was stable. The temperature measurement time was generally between 90 minutes and 120 minutes. The temperature measuring instrument used in underground work was composed of a temperature sensor probe and a temperature display, which can be applied to conditions above 100°C. The display device was an AD display device, which adopt COMS micropower inheritance circuit and wide-temperature large-field wide-screen LCD display. The characteristics of AD display device was waterproof, explosion-proof, and seismic (Figure 2).Figure 2
Schematic diagram of temperature measurement arrangement.


## 3.3. Rock Thermal Conductivity Test

Rock thermal conductivity refers to the characteristics of heat transfer of rock, which is the main thermal parameter of rock and the basic data to obtain terrestrial heat flow [43]. The test samples in this study were collected from two surface supplement exploration holes XC1 and XC2 in Xizhuo mine. Twenty-five representative samples were tested, and the depth range of the samples ranged from 120 m to 600 m. The strata from old to new were involved, basically representing the main rock types of coal measure strata in Weibei coalfield.


## 3.4. Calculation of Geothermal Gradient

Because the approximate steady-state temperature measurement results are high accuracy, the ground temperature gradient can be calculated and obtained directly by the following formula.(1)T=GH−H0+T0,whereG is the average ground temperature gradient of temperature measuring borehole, °C/km; T is the bottom hole temperature, °C; T0 is the temperature of constant temperature zone, °C; H is the bottom hole depth, m; H0 is the depth of constant temperature zone, m. H0 in the area is 30 m, and T0 is 16.8°C.When using the simple temperature measurement data, the borehole bottom hole temperature (BHT, the same below) must be corrected. In this paper, the Waples and Ramly [44, 45] method was used to correct the bottom hole temperature. This method holds that for BHT data with depth less than 3000 m, the following correction formula was used for prediction:
(2)Tc=Ts+fTm−Ts,whereTC is the correction temperature within a certain depth at the bottom of the well, °C; TS is the ground temperature corresponding to the correction point, °C; f is the correction factor; Tm is the BHT data obtained during temperature measurement, °C.f is given by the following formula:
(3)f=−0.1462lnTSC+1.6990.572Z0.075,wheref is the correction factor; TSC is the drilling stop time, h; Z is the drilling depth of the corresponding test point, m.


## 3.5. Calculation of Terrestrial Heat Flow

Terrestrial heat flow refers to the heat transmitted from the earth’s interior to the surface per unit area of the surface or near surface shallow layer. Numerically, it is equal to the product of rock thermal conductivity and vertical geothermal gradient [46].
(4)Q=−k•ΔTΔZ,whereQ is the earth heat flow, mW/m2, k is the thermal conductivity, W/m·K, ΔT/ΔZ is the ground temperature gradient, °C/km.In order to make the calculated heat flow value more reliable, this paper selected a representative approximate steady-state temperature measurement borehole and used the least square method to fit the linear relationship between ground temperature and depth, so as to obtain the ground temperature gradient value. The calculation range of ground temperature gradient at the calculation point of heat flow value is below the bedrock surface. According to the requirements of obtaining heat flow value, well temperature logging and rock thermal conductivity test should be the same borehole, but it is difficult to do in practical work. Considering that the sampling depth of rock samples of Permian coal measure strata is evenly distributed and has sufficient representativeness, the difference of rock thermal conductivity of the same horizon in the study area is small. This paper calculated the average thermal conductivity of each temperature measuring hole by using the “weighted average method” of various lithologic thicknesses of boreholes [47, 48].


## 4. Results and Discussion

### 4.1. Current Geothermal Field Distribution Characteristics

According to the test results, the characteristics of the geothermal field in this area were analyzed from the vertical and plane aspects, respectively.


#### 4.1.1. Vertical Distribution

The last test data in the approximate steady-state temperature measurement can basically represent the distribution of the original vertical ground temperature [49]. Therefore, according to the approximate steady-state hole temperature measurement curve in the study area (Figure 3), the vertical ground temperature in this area increases with the increase of depth and shows a good linear relationship.Figure 3
Typical well temperature logging curve.The results of the rock thermal conductivity in the study area range from 1.95 W/m·°C to 5.07 W/m·°C, with an average value of 3.32 W/m·°C, and the mosts are distributed between 2.5 W/m·°C and 3 W/m·°C (Table1).Table 1
Statistics of thermal conductivity of rock samples.
Hole numberDepth/mSerial numberLithologyThermal conductivity W/m·°CXC1136-138XC1-1Fine sandstone2.41178-181XC1-2Medium sandstone2.72338-340XC1-3Sandy mudstone2.89345-347XC1-4Coarse sandstone3.60493-496XC1-5Sandy mudstone4.14509-511XC1-6Siltstone4.59524-525XC1-7Fine sandstone3.43558-570XC1-8Mudstone2.63571-579XC1-9Fine sandstone4.12587XC1-10Mudstone3.06604XC1-11Sandy mudstone5.07XC2127-130XC2-1Sandstone4.12145-148XC2-2Sandy mudstone2.24154-157XC2-3Fine sandstone3.98162-163XC2-4Mudstone2.79280-290XC2-5Sandy mudstone3.75334-340XC2-6Medium sandstone3.99406-409XC2-7Fine sandstone2.24415-418XC2-8Medium sandstone3.98421-424XC2-9Mudstone2.79436-439XC2-10Fine sandstone3.29442-444XC2-11Mudstone2.56465-470XC2-12Mudstone1.95455-456XC2-13Mudstone3.71473-475XC2-14Mudstone2.99


#### 4.1.2. Plane Distribution

(1) Ground Temperature Distribution of No. 5 Coal Seam. The no. 5 coal seam is the main coal seam at present. The thickness of the coal seam is 2.2 m ~7.7 m, with an average of 4.6 m. The underground temperature measurement of Bailiang coal mine is shown in Table 2, and the temperature degree is between 32.4°C and 43.3°C. Combined with borehole temperature data, the coal seam temperature of the whole study area is in the range of 28.5°C ~39.2°C, with an average of 33.13°C, and the most of them are distributed in the range of 32~35°C.Table 2
Temperature measurement results in Bailiang coal mine.
Measuring pointPositionTemperature of measuring point/°C1516 working face left side of roadway, no. 5 coal seam34.82516 working face right side of roadway, no. 5 coal seam36.23Outlet point of 505 working face39.24The rock of 505 working face33.7550 m total ventilation roadway, no. 5 coal seam34.36100 m total ventilation roadway, no. 5 coal seam43.37300 m total ventilation roadway, no. 5 coal seam32.48Wellbore inspection hole, gushing water409Bu 3-2 poor sealing borehole, gushing water4110518 working face, water gushing point4211518 working face, roadway rock40.112Ordovician limestone hydrological observation hole, water temperature40It can be seen from Figure4 that the ground temperature generally shows the trend of gradual increase from SW to EN. The results indicate that compared with the overlying noncoal stratum, the no. 5 coal seam has a large range of relatively high temperature area. Almost the whole area is a first-class heat hazard area (>31°C), and the northeast is a second-class heat hazard area (>37°C).Figure 4
Ground temperature contour map of no. 5 coal seam.(2) Geothermal Gradient Distribution. The geothermal gradient in this area is between 25.7°C/km and 54.3°C/km, with an average of 36.6°C/km, and the most of them are 30°C/km~40°C/km. The area with geothermal gradient greater than 30°C/km accounts for 88.31% of the total study area, and the high geothermal is prominent. The regular distribution of geothermal gradient is obvious. The overall performance is the trend of gradually increasing from northwest to southeast, in which the gradient value of F1 fault zone in the southeast is the highest. And the northwest flank of Xizhuo syncline in the middle and north is the lowest (Figure 5).Figure 5
Contour map of geothermal gradient.


#### 4.1.3. Regional Heat Flow Distribution

The heat flow of study area ranges from 66.81 mW/m2 to 128.49 mW/m2, with an average of 90.96 mW/m2 (Table 3 and Figure 6). This is 17.21 mW/m2 higher than the average heat flow of Hancheng mining area [50], which belongs to an obvious high heat flow area. The distribution law of heat flow value in this area is also obvious, which gradually increases from northwest to southeast. The distribution characteristics is relatively low in northwest, medium in central and high in southeast, and showing an obvious high anomaly area near the proved large fault.Table 3
Statistics of regional heat flow value.
Drill hole numberCalculation section range (m)Geothermal gradient (°C/km)Lithology proportion/%Weighted average thermal conductivity (W/(m·°C))Terrestrial heat flow (mW/m2)Mudstone and sandstoneFine sandstone40-1136.7-510.16290.650.352.65877.08740-2118-532.44330.610.392.66988.08641-2108-492.7350.390.612.73595.72441-3130.42-423.2245.80.710.292.641120.97642-1134.43-531.90300.710.292.64179.23042-298.13-439.1300.900.102.58677.57542-3104.28-385.7948.50.680.322.649128.48943-1107.16-490.10300.710.292.63979.17343-270.65-424.38280.620.382.66674.65943-3109.75-385.70370.810.192.61396.688CH20977.0-429.5033.40.001.002.86095.524CH21084-508.8300.070.932.83885.128CH211119.5-464.827.10.520.482.69673.050J1-1★123.1-541.12270.560.442.68372.441J1-2128.65-510.03290.410.592.72879.102J2-1★143.41-556.80300.590.412.67480.214J2-2118-538.22320.530.472.69486.203J2-3129.5-513.38350.560.442.68393.900J3-1130.5-552.22290.720.282.63776.468J3-2113.45-535.46300.660.342.65479.608J3-3132.74-519.28270.660.342.65571.692J3-4116.49-491.1729.50.710.292.64077.868J3-5136.5-444.89360.820.182.60893.890J4-1104.8-529.87270.700.302.64271.347J4-2113.64-491.92140.710.292.64069.952J4-3119.2-478.15330.510.492.69989.060J4-4124-446380.890.112.58998.366J4-5122.12-409.67390.790.212.619102.129J5-1★109-501.01350.520.482.69494.307J5-2116.4-492.79250.600.402.67266.806J5-3107.55-443.95330.520.482.69588.941J5-4113-403.55420.950.052.574108.089J5-5106-371.04380.800.202.61599.368J6-199.5-482.75370.710.292.64197.718J6-2115.9-460.72340.610.392.67090.771J6-3108.4-410.85410.690.312.646108.472J6-4124-388.77350.620.382.66693.326J7-1110.95-455.35400.590.412.674106.960J7-2101.22-435.2440.690.312.646116.444J7-3113.5-390.52340.800.202.61588.915J8-191-529.94370.580.422.67999.115J8-299-326.26470.600.402.671125.550J9-1★84-502.64350.610.392.67093.462XC1120.3-541.732.90.580.422.67788.090XC2120.08-417.5842.80.700.302.644113.175Note: ★ is stable temperature hole.Figure 6
Distribution map of terrestrial heat flow.


### 4.2. Main Control Factors of Geothermal Field

#### 4.2.1. Fault Structure

The F1 fault on the southeast boundary and F10 fault on the northern boundary are the main coal controlling structures in the study area. It can be clearly seen from Figures 4 and 6 that there are obvious high-temperature anomalies at the intersection of F1, F10 fault zone, and two major fault zones. The two major faults are tensile normal faults with large drop, long extension, and high angle. This type of fault zone is the main channel for conducting deep underground hot water, resulting in the continuous transmission of deep heat flow to the upper part, changing the original heat flow state [51], and forming the current high heat flow area. The high temperature Ordovician limestone water emitted from the underground coal seam floor near the F1 fault in Bailiang coal mine also provides evidence for the geothermal control function of the water flowing fault.


#### 4.2.2. Fold Structure

In the A1 anticline and Xizhuo anticline, the geothermal gradient and terrestrial heat flow showed a decreasing trend from the axis of the anticline to the two flank. At the intersection of A1 anticline and Xizhuo anticline, the geothermal gradient is above 45°C/km, and the heat flow value is also as high as 120 mW/m2. On the contrary, the geothermal gradient near the axis of Xizhuo syncline is much lower than that in the anticline area. This feature is mainly due to the lateral difference of rock thermal conductivity, resulting in the enrichment of deep heat flow in the basement uplift area, forming a heat accumulation effect, and the overlying sedimentary cover with low thermal conductivity plays a good role in preserving the heat flow [52]. Hence, the control of basement fluctuation and fold structure on heat flow transmission is also prominent in the study area.


#### 4.2.3. Thermal Conductivity of Coal-Rocks

The thermal conductivity of various rocks in the study area is a wide range. The sandstone is the highest thermal conductivity, followed by mudstone, and coal is the lowest thermal conductivity values. The overall thermal conductivity of all rocks increases with the increase of burial depth and stratigraphic. The thermal conductivity has a certain influence on the distribution of the geothermal field in the vertical direction, and the lower the thermal conductivity of the rocks, the larger the geothermal gradient. The no. 5 coal seam has the smallest thermal conductivity and the largest geothermal gradient. The geothermal gradient of the coal seam is also larger than that of the noncoal seam (Table4), which also reflects the influence of thermal conductivity on the geothermal gradient.Table 4
Statistics of geothermal gradient of stratum.
StratumNoncoal strata (°C/km)Coal-bearing strata (°C/km)No. 5 coal seam/km (°C/km)Ordovician strata (°C/km)Gradient ranges14~480514~82.521~10115~88Average31.14254.239.7


#### 4.2.4. Influence of Groundwater Activities

The main source of groundwater recharge in the study area is underground runoff outside the area, which flows from NW to SE in the direction and is discharged outside the area through runoff. Due to the influence of geological structure, there are drainage springs distributed in the SE boundary of the study area and F1 intersection zone. According to the water quality analysis of Xizhuo mine, it is known that the groundwater is highly mineralized and strongly alkaline. The hydrogeochemical environment is a reducing environment, which indicates that the runoff conditions are poor. From the well temperature curve, it is found that most of the northern and northwestern areas of the study area exhibit a linear type, showing the characteristics of good runoff conditions, while the central-eastern area exhibits an upward convex type (Figure 7), especially in the boundary zone along the direction of the F1 fault is obvious, indicating the influence of lateral high temperature hot water runoff. It is highly consistent with the pattern of the geothermal gradient, i.e., groundwater activities also have an important influence on the distribution of the geothermal field in the study area.Figure 7
Temperature profile of CH176 borehole.


### 4.3. Forming Mode of Geothermal Field

According to the above analysis, the main controlling factors of the geothermal field in the study area are fault, folding, rock thermal conductivity, and groundwater. The geothermal anomaly temperature control mode in the study area is divided into two categories, which are fault-deep circulation hot water conduction type temperature control mode and coal seam heat resistance-folding type temperature control mode (Figure8).
(1)
Fault-Deep Circulation Hot Water Conduction Type Temperature Control Model. After the formation of the coal basin in Chenghe mine area, along with the transformation of relatively strong tectonic movement, two large fault F1 and F10 form horst structural style under the action of stress. First of all, when the fault zone channel is less filled with material or the fracture is more developed, the fault zone itself can form a stable and penetrating heat-conducting channel, gathering a large amount of heat flow upward along the channel. When the temperature is higher than the surrounding environment, the heat will be transferred to the horizontal direction, resulting in the increase of heat flow density in a certain range around the channel. The temperature of the surrounding rock is raised, resulting in the formation of local geothermal anomalies. Second, the existence of water-conducting faults will form a good discharge channel for deep groundwater, and the deep circulating hot water from the deep basement will be discharged to the surface along the discharge channel. The local water circulation will also be generated in the fault zone, which will promote the expansion of heat exchange range and form the geothermal anomaly zone(2)
Coal Seam Heat Resistance-Fold Type Temperature Control Mode. The heat flow is easily concentrated in the basement uplift area or the back-slope tectonic axis, with high geothermal temperature, geothermal gradient, and heat flow value. This feature is mainly due to the lateral difference of rock thermal conductivity, which causes the deep heat flow to be enriched in the basement uplift area and forms the heat gathering effect. In addition, influenced by the low thermal conductivity of coal seam, the heat blocking effect is obvious, and the spatial spreading pattern of coal seam is influenced by the folded structure, most of the heat flow occurs horizontally along the layer, which plays the role of heat blocking and flow guiding, resulting in the ground temperature of coal seam and its lower area is obviously higher than that of upper strataFigure 8
Schematic diagram of the temperature control mode in the study area (1-1′ profile line in Figure 1). The arrow in the figure indicates the direction of heat flow; A is the fault-deep circulation hot water conduction type temperature control mode; B is the heat resistance—folding temperature control mode.


## 4.1. Current Geothermal Field Distribution Characteristics

According to the test results, the characteristics of the geothermal field in this area were analyzed from the vertical and plane aspects, respectively.


### 4.1.1. Vertical Distribution

The last test data in the approximate steady-state temperature measurement can basically represent the distribution of the original vertical ground temperature [49]. Therefore, according to the approximate steady-state hole temperature measurement curve in the study area (Figure 3), the vertical ground temperature in this area increases with the increase of depth and shows a good linear relationship.Figure 3
Typical well temperature logging curve.The results of the rock thermal conductivity in the study area range from 1.95 W/m·°C to 5.07 W/m·°C, with an average value of 3.32 W/m·°C, and the mosts are distributed between 2.5 W/m·°C and 3 W/m·°C (Table1).Table 1
Statistics of thermal conductivity of rock samples.
Hole numberDepth/mSerial numberLithologyThermal conductivity W/m·°CXC1136-138XC1-1Fine sandstone2.41178-181XC1-2Medium sandstone2.72338-340XC1-3Sandy mudstone2.89345-347XC1-4Coarse sandstone3.60493-496XC1-5Sandy mudstone4.14509-511XC1-6Siltstone4.59524-525XC1-7Fine sandstone3.43558-570XC1-8Mudstone2.63571-579XC1-9Fine sandstone4.12587XC1-10Mudstone3.06604XC1-11Sandy mudstone5.07XC2127-130XC2-1Sandstone4.12145-148XC2-2Sandy mudstone2.24154-157XC2-3Fine sandstone3.98162-163XC2-4Mudstone2.79280-290XC2-5Sandy mudstone3.75334-340XC2-6Medium sandstone3.99406-409XC2-7Fine sandstone2.24415-418XC2-8Medium sandstone3.98421-424XC2-9Mudstone2.79436-439XC2-10Fine sandstone3.29442-444XC2-11Mudstone2.56465-470XC2-12Mudstone1.95455-456XC2-13Mudstone3.71473-475XC2-14Mudstone2.99


### 4.1.2. Plane Distribution

(1) Ground Temperature Distribution of No. 5 Coal Seam. The no. 5 coal seam is the main coal seam at present. The thickness of the coal seam is 2.2 m ~7.7 m, with an average of 4.6 m. The underground temperature measurement of Bailiang coal mine is shown in Table 2, and the temperature degree is between 32.4°C and 43.3°C. Combined with borehole temperature data, the coal seam temperature of the whole study area is in the range of 28.5°C ~39.2°C, with an average of 33.13°C, and the most of them are distributed in the range of 32~35°C.Table 2
Temperature measurement results in Bailiang coal mine.
Measuring pointPositionTemperature of measuring point/°C1516 working face left side of roadway, no. 5 coal seam34.82516 working face right side of roadway, no. 5 coal seam36.23Outlet point of 505 working face39.24The rock of 505 working face33.7550 m total ventilation roadway, no. 5 coal seam34.36100 m total ventilation roadway, no. 5 coal seam43.37300 m total ventilation roadway, no. 5 coal seam32.48Wellbore inspection hole, gushing water409Bu 3-2 poor sealing borehole, gushing water4110518 working face, water gushing point4211518 working face, roadway rock40.112Ordovician limestone hydrological observation hole, water temperature40It can be seen from Figure4 that the ground temperature generally shows the trend of gradual increase from SW to EN. The results indicate that compared with the overlying noncoal stratum, the no. 5 coal seam has a large range of relatively high temperature area. Almost the whole area is a first-class heat hazard area (>31°C), and the northeast is a second-class heat hazard area (>37°C).Figure 4
Ground temperature contour map of no. 5 coal seam.(2) Geothermal Gradient Distribution. The geothermal gradient in this area is between 25.7°C/km and 54.3°C/km, with an average of 36.6°C/km, and the most of them are 30°C/km~40°C/km. The area with geothermal gradient greater than 30°C/km accounts for 88.31% of the total study area, and the high geothermal is prominent. The regular distribution of geothermal gradient is obvious. The overall performance is the trend of gradually increasing from northwest to southeast, in which the gradient value of F1 fault zone in the southeast is the highest. And the northwest flank of Xizhuo syncline in the middle and north is the lowest (Figure 5).Figure 5
Contour map of geothermal gradient.


### 4.1.3. Regional Heat Flow Distribution

The heat flow of study area ranges from 66.81 mW/m2 to 128.49 mW/m2, with an average of 90.96 mW/m2 (Table 3 and Figure 6). This is 17.21 mW/m2 higher than the average heat flow of Hancheng mining area [50], which belongs to an obvious high heat flow area. The distribution law of heat flow value in this area is also obvious, which gradually increases from northwest to southeast. The distribution characteristics is relatively low in northwest, medium in central and high in southeast, and showing an obvious high anomaly area near the proved large fault.Table 3
Statistics of regional heat flow value.
Drill hole numberCalculation section range (m)Geothermal gradient (°C/km)Lithology proportion/%Weighted average thermal conductivity (W/(m·°C))Terrestrial heat flow (mW/m2)Mudstone and sandstoneFine sandstone40-1136.7-510.16290.650.352.65877.08740-2118-532.44330.610.392.66988.08641-2108-492.7350.390.612.73595.72441-3130.42-423.2245.80.710.292.641120.97642-1134.43-531.90300.710.292.64179.23042-298.13-439.1300.900.102.58677.57542-3104.28-385.7948.50.680.322.649128.48943-1107.16-490.10300.710.292.63979.17343-270.65-424.38280.620.382.66674.65943-3109.75-385.70370.810.192.61396.688CH20977.0-429.5033.40.001.002.86095.524CH21084-508.8300.070.932.83885.128CH211119.5-464.827.10.520.482.69673.050J1-1★123.1-541.12270.560.442.68372.441J1-2128.65-510.03290.410.592.72879.102J2-1★143.41-556.80300.590.412.67480.214J2-2118-538.22320.530.472.69486.203J2-3129.5-513.38350.560.442.68393.900J3-1130.5-552.22290.720.282.63776.468J3-2113.45-535.46300.660.342.65479.608J3-3132.74-519.28270.660.342.65571.692J3-4116.49-491.1729.50.710.292.64077.868J3-5136.5-444.89360.820.182.60893.890J4-1104.8-529.87270.700.302.64271.347J4-2113.64-491.92140.710.292.64069.952J4-3119.2-478.15330.510.492.69989.060J4-4124-446380.890.112.58998.366J4-5122.12-409.67390.790.212.619102.129J5-1★109-501.01350.520.482.69494.307J5-2116.4-492.79250.600.402.67266.806J5-3107.55-443.95330.520.482.69588.941J5-4113-403.55420.950.052.574108.089J5-5106-371.04380.800.202.61599.368J6-199.5-482.75370.710.292.64197.718J6-2115.9-460.72340.610.392.67090.771J6-3108.4-410.85410.690.312.646108.472J6-4124-388.77350.620.382.66693.326J7-1110.95-455.35400.590.412.674106.960J7-2101.22-435.2440.690.312.646116.444J7-3113.5-390.52340.800.202.61588.915J8-191-529.94370.580.422.67999.115J8-299-326.26470.600.402.671125.550J9-1★84-502.64350.610.392.67093.462XC1120.3-541.732.90.580.422.67788.090XC2120.08-417.5842.80.700.302.644113.175Note: ★ is stable temperature hole.Figure 6
Distribution map of terrestrial heat flow.


## 4.1.1. Vertical Distribution

The last test data in the approximate steady-state temperature measurement can basically represent the distribution of the original vertical ground temperature [49]. Therefore, according to the approximate steady-state hole temperature measurement curve in the study area (Figure 3), the vertical ground temperature in this area increases with the increase of depth and shows a good linear relationship.Figure 3
Typical well temperature logging curve.The results of the rock thermal conductivity in the study area range from 1.95 W/m·°C to 5.07 W/m·°C, with an average value of 3.32 W/m·°C, and the mosts are distributed between 2.5 W/m·°C and 3 W/m·°C (Table1).Table 1
Statistics of thermal conductivity of rock samples.
Hole numberDepth/mSerial numberLithologyThermal conductivity W/m·°CXC1136-138XC1-1Fine sandstone2.41178-181XC1-2Medium sandstone2.72338-340XC1-3Sandy mudstone2.89345-347XC1-4Coarse sandstone3.60493-496XC1-5Sandy mudstone4.14509-511XC1-6Siltstone4.59524-525XC1-7Fine sandstone3.43558-570XC1-8Mudstone2.63571-579XC1-9Fine sandstone4.12587XC1-10Mudstone3.06604XC1-11Sandy mudstone5.07XC2127-130XC2-1Sandstone4.12145-148XC2-2Sandy mudstone2.24154-157XC2-3Fine sandstone3.98162-163XC2-4Mudstone2.79280-290XC2-5Sandy mudstone3.75334-340XC2-6Medium sandstone3.99406-409XC2-7Fine sandstone2.24415-418XC2-8Medium sandstone3.98421-424XC2-9Mudstone2.79436-439XC2-10Fine sandstone3.29442-444XC2-11Mudstone2.56465-470XC2-12Mudstone1.95455-456XC2-13Mudstone3.71473-475XC2-14Mudstone2.99


## 4.1.2. Plane Distribution

(1) Ground Temperature Distribution of No. 5 Coal Seam. The no. 5 coal seam is the main coal seam at present. The thickness of the coal seam is 2.2 m ~7.7 m, with an average of 4.6 m. The underground temperature measurement of Bailiang coal mine is shown in Table 2, and the temperature degree is between 32.4°C and 43.3°C. Combined with borehole temperature data, the coal seam temperature of the whole study area is in the range of 28.5°C ~39.2°C, with an average of 33.13°C, and the most of them are distributed in the range of 32~35°C.Table 2
Temperature measurement results in Bailiang coal mine.
Measuring pointPositionTemperature of measuring point/°C1516 working face left side of roadway, no. 5 coal seam34.82516 working face right side of roadway, no. 5 coal seam36.23Outlet point of 505 working face39.24The rock of 505 working face33.7550 m total ventilation roadway, no. 5 coal seam34.36100 m total ventilation roadway, no. 5 coal seam43.37300 m total ventilation roadway, no. 5 coal seam32.48Wellbore inspection hole, gushing water409Bu 3-2 poor sealing borehole, gushing water4110518 working face, water gushing point4211518 working face, roadway rock40.112Ordovician limestone hydrological observation hole, water temperature40It can be seen from Figure4 that the ground temperature generally shows the trend of gradual increase from SW to EN. The results indicate that compared with the overlying noncoal stratum, the no. 5 coal seam has a large range of relatively high temperature area. Almost the whole area is a first-class heat hazard area (>31°C), and the northeast is a second-class heat hazard area (>37°C).Figure 4
Ground temperature contour map of no. 5 coal seam.(2) Geothermal Gradient Distribution. The geothermal gradient in this area is between 25.7°C/km and 54.3°C/km, with an average of 36.6°C/km, and the most of them are 30°C/km~40°C/km. The area with geothermal gradient greater than 30°C/km accounts for 88.31% of the total study area, and the high geothermal is prominent. The regular distribution of geothermal gradient is obvious. The overall performance is the trend of gradually increasing from northwest to southeast, in which the gradient value of F1 fault zone in the southeast is the highest. And the northwest flank of Xizhuo syncline in the middle and north is the lowest (Figure 5).Figure 5
Contour map of geothermal gradient.


## 4.1.3. Regional Heat Flow Distribution

The heat flow of study area ranges from 66.81 mW/m2 to 128.49 mW/m2, with an average of 90.96 mW/m2 (Table 3 and Figure 6). This is 17.21 mW/m2 higher than the average heat flow of Hancheng mining area [50], which belongs to an obvious high heat flow area. The distribution law of heat flow value in this area is also obvious, which gradually increases from northwest to southeast. The distribution characteristics is relatively low in northwest, medium in central and high in southeast, and showing an obvious high anomaly area near the proved large fault.Table 3
Statistics of regional heat flow value.
Drill hole numberCalculation section range (m)Geothermal gradient (°C/km)Lithology proportion/%Weighted average thermal conductivity (W/(m·°C))Terrestrial heat flow (mW/m2)Mudstone and sandstoneFine sandstone40-1136.7-510.16290.650.352.65877.08740-2118-532.44330.610.392.66988.08641-2108-492.7350.390.612.73595.72441-3130.42-423.2245.80.710.292.641120.97642-1134.43-531.90300.710.292.64179.23042-298.13-439.1300.900.102.58677.57542-3104.28-385.7948.50.680.322.649128.48943-1107.16-490.10300.710.292.63979.17343-270.65-424.38280.620.382.66674.65943-3109.75-385.70370.810.192.61396.688CH20977.0-429.5033.40.001.002.86095.524CH21084-508.8300.070.932.83885.128CH211119.5-464.827.10.520.482.69673.050J1-1★123.1-541.12270.560.442.68372.441J1-2128.65-510.03290.410.592.72879.102J2-1★143.41-556.80300.590.412.67480.214J2-2118-538.22320.530.472.69486.203J2-3129.5-513.38350.560.442.68393.900J3-1130.5-552.22290.720.282.63776.468J3-2113.45-535.46300.660.342.65479.608J3-3132.74-519.28270.660.342.65571.692J3-4116.49-491.1729.50.710.292.64077.868J3-5136.5-444.89360.820.182.60893.890J4-1104.8-529.87270.700.302.64271.347J4-2113.64-491.92140.710.292.64069.952J4-3119.2-478.15330.510.492.69989.060J4-4124-446380.890.112.58998.366J4-5122.12-409.67390.790.212.619102.129J5-1★109-501.01350.520.482.69494.307J5-2116.4-492.79250.600.402.67266.806J5-3107.55-443.95330.520.482.69588.941J5-4113-403.55420.950.052.574108.089J5-5106-371.04380.800.202.61599.368J6-199.5-482.75370.710.292.64197.718J6-2115.9-460.72340.610.392.67090.771J6-3108.4-410.85410.690.312.646108.472J6-4124-388.77350.620.382.66693.326J7-1110.95-455.35400.590.412.674106.960J7-2101.22-435.2440.690.312.646116.444J7-3113.5-390.52340.800.202.61588.915J8-191-529.94370.580.422.67999.115J8-299-326.26470.600.402.671125.550J9-1★84-502.64350.610.392.67093.462XC1120.3-541.732.90.580.422.67788.090XC2120.08-417.5842.80.700.302.644113.175Note: ★ is stable temperature hole.Figure 6
Distribution map of terrestrial heat flow.


## 4.2. Main Control Factors of Geothermal Field

### 4.2.1. Fault Structure

The F1 fault on the southeast boundary and F10 fault on the northern boundary are the main coal controlling structures in the study area. It can be clearly seen from Figures 4 and 6 that there are obvious high-temperature anomalies at the intersection of F1, F10 fault zone, and two major fault zones. The two major faults are tensile normal faults with large drop, long extension, and high angle. This type of fault zone is the main channel for conducting deep underground hot water, resulting in the continuous transmission of deep heat flow to the upper part, changing the original heat flow state [51], and forming the current high heat flow area. The high temperature Ordovician limestone water emitted from the underground coal seam floor near the F1 fault in Bailiang coal mine also provides evidence for the geothermal control function of the water flowing fault.


### 4.2.2. Fold Structure

In the A1 anticline and Xizhuo anticline, the geothermal gradient and terrestrial heat flow showed a decreasing trend from the axis of the anticline to the two flank. At the intersection of A1 anticline and Xizhuo anticline, the geothermal gradient is above 45°C/km, and the heat flow value is also as high as 120 mW/m2. On the contrary, the geothermal gradient near the axis of Xizhuo syncline is much lower than that in the anticline area. This feature is mainly due to the lateral difference of rock thermal conductivity, resulting in the enrichment of deep heat flow in the basement uplift area, forming a heat accumulation effect, and the overlying sedimentary cover with low thermal conductivity plays a good role in preserving the heat flow [52]. Hence, the control of basement fluctuation and fold structure on heat flow transmission is also prominent in the study area.


### 4.2.3. Thermal Conductivity of Coal-Rocks

The thermal conductivity of various rocks in the study area is a wide range. The sandstone is the highest thermal conductivity, followed by mudstone, and coal is the lowest thermal conductivity values. The overall thermal conductivity of all rocks increases with the increase of burial depth and stratigraphic. The thermal conductivity has a certain influence on the distribution of the geothermal field in the vertical direction, and the lower the thermal conductivity of the rocks, the larger the geothermal gradient. The no. 5 coal seam has the smallest thermal conductivity and the largest geothermal gradient. The geothermal gradient of the coal seam is also larger than that of the noncoal seam (Table4), which also reflects the influence of thermal conductivity on the geothermal gradient.Table 4
Statistics of geothermal gradient of stratum.
StratumNoncoal strata (°C/km)Coal-bearing strata (°C/km)No. 5 coal seam/km (°C/km)Ordovician strata (°C/km)Gradient ranges14~480514~82.521~10115~88Average31.14254.239.7


### 4.2.4. Influence of Groundwater Activities

The main source of groundwater recharge in the study area is underground runoff outside the area, which flows from NW to SE in the direction and is discharged outside the area through runoff. Due to the influence of geological structure, there are drainage springs distributed in the SE boundary of the study area and F1 intersection zone. According to the water quality analysis of Xizhuo mine, it is known that the groundwater is highly mineralized and strongly alkaline. The hydrogeochemical environment is a reducing environment, which indicates that the runoff conditions are poor. From the well temperature curve, it is found that most of the northern and northwestern areas of the study area exhibit a linear type, showing the characteristics of good runoff conditions, while the central-eastern area exhibits an upward convex type (Figure 7), especially in the boundary zone along the direction of the F1 fault is obvious, indicating the influence of lateral high temperature hot water runoff. It is highly consistent with the pattern of the geothermal gradient, i.e., groundwater activities also have an important influence on the distribution of the geothermal field in the study area.Figure 7
Temperature profile of CH176 borehole.


## 4.2.1. Fault Structure

The F1 fault on the southeast boundary and F10 fault on the northern boundary are the main coal controlling structures in the study area. It can be clearly seen from Figures 4 and 6 that there are obvious high-temperature anomalies at the intersection of F1, F10 fault zone, and two major fault zones. The two major faults are tensile normal faults with large drop, long extension, and high angle. This type of fault zone is the main channel for conducting deep underground hot water, resulting in the continuous transmission of deep heat flow to the upper part, changing the original heat flow state [51], and forming the current high heat flow area. The high temperature Ordovician limestone water emitted from the underground coal seam floor near the F1 fault in Bailiang coal mine also provides evidence for the geothermal control function of the water flowing fault.


## 4.2.2. Fold Structure

In the A1 anticline and Xizhuo anticline, the geothermal gradient and terrestrial heat flow showed a decreasing trend from the axis of the anticline to the two flank. At the intersection of A1 anticline and Xizhuo anticline, the geothermal gradient is above 45°C/km, and the heat flow value is also as high as 120 mW/m2. On the contrary, the geothermal gradient near the axis of Xizhuo syncline is much lower than that in the anticline area. This feature is mainly due to the lateral difference of rock thermal conductivity, resulting in the enrichment of deep heat flow in the basement uplift area, forming a heat accumulation effect, and the overlying sedimentary cover with low thermal conductivity plays a good role in preserving the heat flow [52]. Hence, the control of basement fluctuation and fold structure on heat flow transmission is also prominent in the study area.


## 4.2.3. Thermal Conductivity of Coal-Rocks

The thermal conductivity of various rocks in the study area is a wide range. The sandstone is the highest thermal conductivity, followed by mudstone, and coal is the lowest thermal conductivity values. The overall thermal conductivity of all rocks increases with the increase of burial depth and stratigraphic. The thermal conductivity has a certain influence on the distribution of the geothermal field in the vertical direction, and the lower the thermal conductivity of the rocks, the larger the geothermal gradient. The no. 5 coal seam has the smallest thermal conductivity and the largest geothermal gradient. The geothermal gradient of the coal seam is also larger than that of the noncoal seam (Table4), which also reflects the influence of thermal conductivity on the geothermal gradient.Table 4
Statistics of geothermal gradient of stratum.
StratumNoncoal strata (°C/km)Coal-bearing strata (°C/km)No. 5 coal seam/km (°C/km)Ordovician strata (°C/km)Gradient ranges14~480514~82.521~10115~88Average31.14254.239.7


## 4.2.4. Influence of Groundwater Activities

The main source of groundwater recharge in the study area is underground runoff outside the area, which flows from NW to SE in the direction and is discharged outside the area through runoff. Due to the influence of geological structure, there are drainage springs distributed in the SE boundary of the study area and F1 intersection zone. According to the water quality analysis of Xizhuo mine, it is known that the groundwater is highly mineralized and strongly alkaline. The hydrogeochemical environment is a reducing environment, which indicates that the runoff conditions are poor. From the well temperature curve, it is found that most of the northern and northwestern areas of the study area exhibit a linear type, showing the characteristics of good runoff conditions, while the central-eastern area exhibits an upward convex type (Figure 7), especially in the boundary zone along the direction of the F1 fault is obvious, indicating the influence of lateral high temperature hot water runoff. It is highly consistent with the pattern of the geothermal gradient, i.e., groundwater activities also have an important influence on the distribution of the geothermal field in the study area.Figure 7
Temperature profile of CH176 borehole.


## 4.3. Forming Mode of Geothermal Field

According to the above analysis, the main controlling factors of the geothermal field in the study area are fault, folding, rock thermal conductivity, and groundwater. The geothermal anomaly temperature control mode in the study area is divided into two categories, which are fault-deep circulation hot water conduction type temperature control mode and coal seam heat resistance-folding type temperature control mode (Figure8).
(1)
Fault-Deep Circulation Hot Water Conduction Type Temperature Control Model. After the formation of the coal basin in Chenghe mine area, along with the transformation of relatively strong tectonic movement, two large fault F1 and F10 form horst structural style under the action of stress. First of all, when the fault zone channel is less filled with material or the fracture is more developed, the fault zone itself can form a stable and penetrating heat-conducting channel, gathering a large amount of heat flow upward along the channel. When the temperature is higher than the surrounding environment, the heat will be transferred to the horizontal direction, resulting in the increase of heat flow density in a certain range around the channel. The temperature of the surrounding rock is raised, resulting in the formation of local geothermal anomalies. Second, the existence of water-conducting faults will form a good discharge channel for deep groundwater, and the deep circulating hot water from the deep basement will be discharged to the surface along the discharge channel. The local water circulation will also be generated in the fault zone, which will promote the expansion of heat exchange range and form the geothermal anomaly zone(2)
Coal Seam Heat Resistance-Fold Type Temperature Control Mode. The heat flow is easily concentrated in the basement uplift area or the back-slope tectonic axis, with high geothermal temperature, geothermal gradient, and heat flow value. This feature is mainly due to the lateral difference of rock thermal conductivity, which causes the deep heat flow to be enriched in the basement uplift area and forms the heat gathering effect. In addition, influenced by the low thermal conductivity of coal seam, the heat blocking effect is obvious, and the spatial spreading pattern of coal seam is influenced by the folded structure, most of the heat flow occurs horizontally along the layer, which plays the role of heat blocking and flow guiding, resulting in the ground temperature of coal seam and its lower area is obviously higher than that of upper strataFigure 8
Schematic diagram of the temperature control mode in the study area (1-1′ profile line in Figure 1). The arrow in the figure indicates the direction of heat flow; A is the fault-deep circulation hot water conduction type temperature control mode; B is the heat resistance—folding temperature control mode.


## 5. Conclusions

(1)
The geothermal hazard is obvious in Chenghe mining area, almost the whole area of the main mining no. 5 coal seam in the eastern part of the Chenghe mine area belongs to the first-class heat hazard zone (>31°C), and the northeastern part is the second-class heat hazard zone (>37°C)(2)
The present geothermal gradient ranges from 25.7°C/km to 54.3°C/km, and the geothermal heat flow value varies from 66.81 mW/m2 to 128.49 mW/m2. The distribution pattern of geothermal gradient and geothermal heat flow value shows a gradual increase from northwest to southeast, and it is especially prominent near the large fault zone(3)
The main controlling factors of geothermal anomaly are fault, fold, coal-rock thermal conductivity, and groundwater activity in the region. According to the distribution characteristics of heat flow and the action mechanism of main controlling factors, the geothermal anomaly in the study area is finally divided into two forming modes, i.e. fault-deep circulating hot water uplifting type and coal seam heat resistance-fold type



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*Source: 3209790-2022-09-07.xml*