Sådhanå (2020) 45:47 Ó Indian Academy of Sciences

https://doi.org/10.1007/s12046-020-1278-7Sadhana(0123456789().,-volV)FT3](0123456789().,-volV)

In-situ stress partition and its implication on coalbed methane occurrence in the basin–mountain transition zone: a case study of the Pingdingshan coalfield, China

JIANGWEI YAN1,2,3, TIANRANG JIA1,2,3,*, GUOYING WEI1,2,4,*, MINGJIE ZHANG1,2 and YIWEN JU5

1 State Key Laboratory Cultivation Base for Gas Geology and Gas Control, Henan Polytechnic University, Jiaozuo 454003, China 2 College of Safety Science and Engineering, Henan Polytechnic University, Jiaozuo 454003, China 3 Collaborative Innovation Center of Coalbed Methane and Shale Gas for Central Plains Economic Region, Jiaozuo 454003, Henan Province, China 4 Collaborative Innovation Center of Coal Safety Production of Henan Province, Jiaozuo 454003, China 5 Key Laboratory of Computational Geodynamics, College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China e-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]

MS received 25 June 2019; revised 2 December 2019; accepted 5 December 2019

Abstract. The basin–mountain transition zone presents complex geologic structures and non-uniformly dis- tributed in-situ stress. Studying the spatial distribution laws of in-situ stress and their influences on coalbed methane (CBM) occurrence in coal seams plays a significant role in CBM extraction and prevention of coalmine disasters. Based on the actual measured in-situ stress data, CBM content and gas pressure data in the Pingdingshan coalfield, located in the basin–mountain transition zone in the south of the late Palaeozoic basins in the North China block, this research investigated the distribution characteristics of geologic structures and partition of in-situ stress as well as the effects of in-situ stresses on CBM occurrence in the research area using evolution theories of geologic structure and a statistical analysis method. The research results show that geologic structure and in-situ stress distribution in the research area have obvious partition characteristics. The research area is divided into three tectonic zonations. In-situ stress distribution is controlled by tectonic types and tectonic stress field evolution of different tectonic zonations, which are divided into high tectonic stress zonation, tectonic stress zonation and vertical stress zonation from east to west. Also, the research results reveal the characteristics of each stress zonation and the relationship between CBM occurrence and in-situ stress in this research area.

Keywords. Partition of in-situ stress; control of geological structure; coalbed methane occurrence; basin– mountain transition zone; geologic structure evolution; coalfield.

1. Introduction [1, 2]. Further, stress measurement results have indicated that tectonic stress generally exists in the rock mass of the In-situ stress in the earth’s crust is formed under the effects earth’s crust, and its value is generally much larger than the of earth gravity, crustal movement and artificial distur- gravity stress of the rock mass and the lateral stress caused bance. In-situ stress can be divided into gravity stress, by gravity. The direction of tectonic stress is generally tectonic stress and mining-induced stress. Before mining, horizontal or near horizontal [3]. The horizontal stress is mainly gravity stress and tectonic stress exist. Tectonic generated by the tectonic stress [1, 4]. The value of maxi- stress, as in-situ stress generated during the movement of mum horizontal stress is generally 1.0–2.0 times that of the crust and existing in the crust (paleo-tectonic stress and gravity stress within the current depth of coal mining present tectonic stress included), primarily comes from the (\1200 m). Under the squeezing, expansion and shear collision of plates and deep material activities in plates effects due to multi-stage paleo-tectonic stress, coal-bear- ing strata are deformed and displaced, thus changing the formation, migration and preservation conditions of gas in *For correspondence 47 Page 2 of 17 Sådhanå (2020) 45:47 coal seams and damaging the structure of the coal body field in the Mediterranean region. Martin [48] measured a [1, 2, 4–6]. As a result, various types of deformed coals and significant increase in stress magnitude beneath the sub- diverse structures are formed, including open and closed horizontal fracture zone. Jia et al [7] considered that faults faults and folds. This also causes the relative lift and set- significantly affect in-situ stress distribution and that the tlement of coal-bearing strata and therefore controls the relationship between the fault trend and action direction of production, migration and storage of gas in coal seams. The principal stresses is an important factor influencing gas current structural feature and framework, gas content in outburst. Homand et al [49] identified two distinct stress coal seams and the development and distribution laws of areas in an arc syncline in the Provence coal basin. Han deformed coal are closely related to the effects of paleo- et al [34] reported that the orientation and magnitude of tectonic stress. As the main body of the current in-situ principal stress in the Kaiping syncline diverge widely from stress, the present tectonic stress exerts squeezing and the regional stress field and the stress regime, with 77% tension effects on existing geologic structures, provides strike-slip faulting and the remainder thrust faulting. Jia conditions in favour of the storage and release of gas in coal et al [21] studied the stress distribution laws of folded seams and is also the major cause of coal and gas outbursts structures under the modern stress field and their influences [2, 7–9]. Thus, it has a direct part in the gas outbursts. Due on CBM occurrence. Moreover, they found that the shear to the existence of geologic structures enhancing the stress concentration in a certain range of both sides of the heterogeneity of coal seams and the inhomogeneity of anticline is the primary cause of the strip distribution of tectonic stress distribution, in-situ stress is different in coal and gas outburst. different types of geologic structures and different locations Located in the southern margin of the North China plate of geologic structures; therefore, stress-concentrated zones and to the north of the Qinling orogenic belt [50], the and stress-increased zones are likely to be formed [1, 10]. Pingdingshan coalfield has complex geologic structures High tectonic stress determines the existence of high gas [51, 52], unevenly distributed in-situ stress [53–55], high pressure and content. The gravity stress primarily depends gas content and gas pressure, and severe risk of coal and on the thickness, lithology and other characteristics of the gas outburst [51, 52]. Penetrating studies on CBM occur- overlying strata. The larger the gravity stress, the lower the rence laws in the Pingdingshan coalfield have shown that possibility of gas to migrate and dissipate from coal seams in-situ stress significantly influences CBM occurrence and to the surface and it is easier for the gases to be stored. that in-situ stress distribution is controlled by geologic Otherwise, the gases are more likely to be dissipated. structures [53–55]. However, some issues yet will not be In-situ stress is an important parameter for academic clarified, such as distribution of in-situ stress, the control research and engineering applications in earth sciences, relation between geologic structures and in-situ stress, and energy science and safety science. For example, in-situ the influences of in-situ stress on gas occurrence. At pre- stress is a key factor for permeability prediction and fluid sent, mining in the Pingdingshan coalfield has entered deep flow in coalbed methane (CBM) reservoirs and develop- areas (overburden depth of about 1,200 m), and a large ments [11–20], and coal and gas outburst prediction amount of in-situ stress data have been measured, which [4, 7, 21–23]; it also facilitates roadway support and rock provide a basis for studying in-situ stress distribution in the burst control in underground coal mines [24–27]. In addi- Pingdingshan coalfield. Using evolution theories of geo- tion, vertical stress is significant in the numerical simula- logic structure and statistical analysis method, this study tion of faulted zones, rock stiffness and rock falls [28–30]. investigated in-situ stress distribution laws and its control In a word, estimation of in-situ stress for coal-bearing strata factors in the Pingdingshan coalfield and analysed the has been applied widely in underground mines. influences of in-situ stresses on CBM occurrence. The Along with study of in-situ stress, numerous macro- and research results offer significant guidance for the develop- micro-tectonic studies have been undertaken to understand ment of CBM resources, prediction of gas outburst and local and far-field stress activities [31], and increasing roadway supports in the Pingdingshan coalfield. numbers of researchers have realized that in-situ stress is deeply affected by tectonic and gravitational forces and is particularly associated with horizontal tectonic movements 2. Geological setting [32, 33]. Many other factors are related to in-situ stress, such as geological structures [34–37], layering [38, 39], 2.1 Structural characteristics of the Pingdingshan rock heterogeneities [40–42] and so on. coalfield The magnitude and orientation of in-situ stress are sig- nificantly affected by geological structures [43–45]. Carls- The collision and connection of the North China plate and son and Christiansson [46] found that large faults the South China plate formed the Qinling–Dabie orogenic influenced the orientation of in-situ stress, and small faults belt between them. As a composite orogenic belt formed influenced not only the orientation but also the magnitude. due to collision, the orogenic belt comprises the southern Rebaı¨ et al [47] found evidence for variation in stress continental margin of the North China block, the northern directions at different scales in a modern tectonic stress continental margin of the Yangtze block and the Qinling Sådhanå (2020) 45:47 Page 3 of 17 47 ocean (suture zone) between them [50] (figure 1). The NW–SE direction, and the two sides are basically Pingdingshan coalfield is located on the southern margin of symmetric. The relatively wide and gentle area is located in the North China block and the northern margin of the the northwest of the Likou syncline. While it uplifts and Qinling orogenic belt. Therefore, it is in a typical basin– converges in a SE direction, the syncline is relatively closed mountain transition zone in the south of the late Palaeozoic in the southeast. The dip angles of stratum in the two wings basins in the North China block (figure 1a). are gently inclined, generally ranging from 5° to 15°, and The Pingdingshan coalfield has been controlled and secondary folds are developed in the two sides. Particularly, reformed by the Qinling orogenic belt for a long time [50]. Xiangjia anticline, Lingwushan syncline, Baishishan anti- Therefore, the coalfield and its periphery show a series of cline, Guozhuang anticline and Niuzhuang syncline are complex WNW- and NW-trending parallel folded struc- distributed in the eastern mines in the coalfield from the tures and fault structures, accompanied by NEN- and NE- north to the south [52]. trending faults (figure 1b, c). Due to boundary faults – such The main sedimentary Carboniferous and Permian coal- as the Jiaxian fault, Xiangjia fault, Luogang fault and Luye bearing strata from top to bottom in the coalfield are the fault – with a large angle and a height difference of about coal seams of A–G formations. The thickness of minable 1,000 m distributed across, the Pingdingshan coalfield has coal seams is 15–18 m, and the main minable coal seams been lifted and has become a separate horst structure unit. are D–G coal formations (figure 1d). In Pingdingshan The main structure of the coalfield is the Likou syncline, a coalfield, the CBM (gas) content and gas pressure are very wide and gentle complex syncline for the trend NW, con- high. Coal and gas outburst hazards are severe. This coal- trolling the tectonic form of the whole coalfield. The syn- field is considered high CBM (gas) and coal–gas outburst cline, showing an orientation of NW50° and a nearly dangerous area. However, CBM (gas) occurrences are vertical axial surface, is inclined and stretched towards the obviously different in the western, middle and eastern parts.

(a) Yuxian (d)

ˉ Stratigraphic Stratigraphic Coal Seam and North China Jia Hujianshan Kunlun Column ˉ Block xian Units Marker Bed Qinling Pingdingshan coalfield Fold System Sync Huixian Anticline Zhangde Anticline Faul Quaternary line Zhangde t System Yangtze Block a

hina Se Palaeogene- Fault uth C Fault Neogene o X S ia Qingcaoling Fault Likou ngjia Dafengkou Syncline Jiaxian Fault Jing jiaw sandstone a Shangsh Syncline Research Area u A3 Baofeng i Fault ˄(°1°˅ Likou 3 2 Guodishan Syn 1 (b) B2 Jiulishan cline Likou 5 4 Fault Tianjiagou Fault Pingdingshan g Permian sandstone Baiguis System Lushan han reservoir ˄(1˅ Luogan C2 Fault

Lu Luye 1 Xiangjia anticline ye Anticline Faul 2 Lingwushan syncline 0 5 10Km t 3 Baishishan anticline 4 Guozhuang anticline D5-6 5 Niuzhuang syncline

(c) E8 Luye fault N39°E Xiangjia fault A' S39°W Likou syncline N+E Q Baiguishan 20m 9-10 Jiulishan fault Guodishan fault Q E A reservoir 2 1 2 P2 --T1 P2 --T1 20m Q 2 P2 1 Q 1 C1 P2 -1000m Shaguoyao sandstone C3 P1 P2 2 1 2 C3 P1 N+E C2 P1 P1 1 Dazhan sandstone C3 Z P1 -1000m C1 C2 Z C3 C2 Ar Ar Ar C1 C3 -2000m -2000m Ar 0 24km F15

F16-17 Ar C P Z N Q T E Legend L8 limestone Reverse SynclineAnticlineCoalfield ArcheanCambrian Permian Sinian Neogene Quaternary TriassicPaleogene City Provincial Normal G20 Fault Fault Boundary Period Period capital Carboniferous System G21

Kun-Qinling Research Coalbed Overburden Fine Medium Coarse Mudstone Sandy Limestone Bauxite Cambrian G22 Fault Coal Seam Siltstone Bauxitic mudstone Fold System Area Outcrop Sandstone Sandstone Sandstone Mudstone System

Figure 1. Geological structure setting and research area with (a) the geotectonic location of Pingdingshan coalfield, (b) distribution of major geological structural of research area and its surrounding, the position of research area, (c) A–A0 geological section in (b) and (d) comprehensive histogram of coal strata in research area, in which A–G are the number of coal seams. 47 Page 4 of 17 Sådhanå (2020) 45:47

2.2 Tectonic zonation of the Pingdingshan NW-trending fold–fault belt. Therefore, this zonation coalfield belongs to the complex structural zone that has developed folded structures and reverse faults, and the structures are Due to the location of the Likou syncline and its tectonic closed. No. 10, No. 12 and No. 8 mines are mainly dis- distribution characteristics, the structure of the SW side of tributed in this zonation. the Likou syncline in the Pingdingshan coalfield was divi- Based on the tectonic distribution and tectonic charac- ded into three tectonic zones. Figure 2 shows the division teristics of tectonic zonations, the characteristics of struc- of tectonics, the position of measurement points and the tures in the eastern, central and western areas are different. orientation of maximum horizontal principal stress of each The tectonic structure is the most complex in the east as a measurement point in each tectonic zonation, which was series of WNW- and NW-trending closed folds are well- prepared according to the main control tectonic, table 1, developed. The WNW- and NW-trending fault structures and the coordinate of each in-situ stress measurement point. are dominant in the west and structures are complex, and Tectonic zonation I is located in the wide and gentle area the areas close to the Guodishan fault have more complex in the NW direction, and the controlling structures are the structures. No large-scale controlling structures have been Guodishan fault and the Jiaxian fault, belonging to a identified in the central area, and the tectonic structure is complex tectonic zonation. One part of this zonation, close relatively simple. to the Guodishan fault, has developed NW- and WNW- trending fault structures including the Guodishan fault and associated faults, revealing a more complex structure. The other part – the western area far from the Guodishan fault – 3. Measurement results and analysis of in-situ has a simple structure. The range of this zonation mainly stress includes No. 9 mine, No. 11 mine, No. 7, No. 3 mines and the middle and west of the No. 5 mine (No. 6 mine). 3.1 In-situ stress measurement Tectonic zonation II lies in the transition zone from the With the successive mining of the deep areas of each mine convergence area in the SE direction to the wide and gentle in the Pingdingshan coalfield, more and more attention is area in the NW direction on the SW side of the Likou being paid to in-situ stress because of coal and rock syncline, showing a simple structure without large control- dynamic disasters, especially rock burst in mines. Many ling structures. The tectonic zone is far from the Guodishan engineers involved in coal mine safety in China have tested fault and the NW-trending fold–fault belt (consisting of 44 groups of in-situ stress data in the Pingdingshan coal- Likou syncline, Niuzhuang syncline, Guozhuang anticline, field [53–55] using the hollow inclusion stress-relief Niuzhuang reverse fault, reverse fault of the original No. 11 method [34]. Of these, 41 groups are on the SW side of mine, Zhuyuan reverse fault, Zhangjia reverse fault and F2 Likou syncline (table 1, figure 2). reverse fault). It mainly comprises No. 4 mine, No. 1 (No. 2) mine and the eastern part of No. 5 mine (No. 6 mine). Tectonic zonation III is in the convergence area in the SE 3.2 In-situ stress distribution characteristics direction and the SW side of the Likou syncline. The controlling structures include a series of NW- and WNW- Based on table 1 and figure 2, measurement points of in- trending folded structures and reverse faults, such as the situ stresses are distributed from the western No. 11 mine to

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Figure 2. Tectonic zonations and distribution of in-situ stress measurement points in research area, which is the grey shaded area in figure 1b, and the number of in-situ stresses is consistent with the number in table 1. S å dhan

Table 1. Statistics of in-situ stress measurement results in the research area. å 22)4:7 ae5o 747 17 of 5 Page 45:47 (2020) Principal Principal Principal Principal Category stress Azimuth stress Azimuth stress Azimuth stress Azimuth of stress Number H (m) (MPa) (deg) Number H (m) (MPa) (deg) Number H (m) (MPa) (deg) Number H (m) (MPa) (deg) rH 1 495.4 28.10 119.23 12 793 36.2 60.3 23 633 22.06 120.7 34 718 13.13 126.3 rV 12.46 28.49 25.1 49.3 14.99 209.2 17.96 rh 5.97 209.50 19.1 - 30 17.63 31.4 6.90 rH 2 807.3 34.15 224.25 13 869 44.4 56 24 556 19.74 98.1 35 718 12.8 118.2 rV 22.38 - 16.13 17.2 21.5 16.75 39.7 17.96 rh 14.69 153.31 25.5 - 26.7 14.83 141.8 6.96 rH 3 602.9 29.06 251.71 14 869 44.3 61.5 25 555 28.13 149.7 36 844 13.1 109.4 rV 17.90 5.03 18.5 6.1 19.06 - 12.7 21.1 rh 10.22 159.98 26.1 - 29.7 15.26 239.9 8.16 rH 4 1090 41.34 255.04 15 514 31.4 53.2 26 900 31.7 32.4 37 645 16.41 107.03 rV 19.28 - 25.00 17.5 131.1 18.7 206.8 17.87 rh 17.32 165.72 15.4 146 15.27 rH 5 830 48.25 122.89 16 514 29.3 - 130.9 27 764 23.4 145.11 38 700 16.84 91.23 rV 20.98 44.34 17.1 160 19.50 33.68 18.54 rh 18.84 211.45 18.3 137.4 14.83 249.86 15.41 rH 6 620 33.46 110.24 17 914 40.2 43.1 28 740 22.99 241.21 39 690 13.61 61 rV 16.94 - 21.50 14.2 27.5 19.10 352.75 17.24 rh 11.81 200.80 28.3 132.2 12.81 139.29 8.25 rH 7 1100 43.56 246.44 18 914 43.4 - 130.8 29 520 14.02 13.58 40 743 13.28 123 rV 25.13 310.02 16.4 42 13.39 18.57 rh 20.81 165.24 23.1 133 10.09 8.14 rH 8 1123 65.5 60.1 19 440 19.03 111.2 30 657 28.6 268.0 41 763 13.98 39 rV 38.1 - 150.6 12.66 49.4 15.51 - 43.0 19.08 rh 31.3 149 11.40 196.4 10.14 180.0 8.84 rH 9 1061 43.1 - 131.9 20 490 18.64 180.8 31 547 25.74 240.0 rV 26.1 60.5 15.12 88.3 14.11 - 2.0 rh 22.4 138 14.39 91.7 10.05 150.0 rH 10 1061 44.1 60.4 21 652 22.39 170.2 32 960 29.21 94.92 rV 28.4 155.3 14.20 155.2 18.23 98.62 rh 24.2 149 17.65 79.3 19.03 6.17 rH 11 785 34.3 - 157.6 22 692 30.83 109.6 33 965 27.60 110.52 rV 22.2 - 141 14.68 35.1 26.23 30.26 rh 18.3 - 67 12.85 193.7 18.21 123.42

No. 1–21 and No. 24 are located in tectonic zonation III, No. 22, No. 23 and No. 25–33 in tectonic zonation II and No. 34–41 in tectonic zonation I. 47 Page 6 of 17 Sådhanå (2020) 45:47 the eastern No. 8 mine and are basically in the existing tectonic zonations III and II, while vertical stresses play a mining range of the SW side of the Likou syncline. The major role in tectonic zonation I. measurement points range from 440 to 1,123 m under- The minimum horizontal principal stresses of tectonic ground, and the maximum horizontal principal stress rH zonations III and II increase with depth’s increasing, while ranges from 12.8 to 65.5 MPa (mean: 29.07). Moreover, the it decreases in tectonic zonation I, basically ranking tec- vertical principal stress rV and the minimum horizontal tonic zonations III, II and I from large to small (figure 3c). principal stress rh are in the ranges of 12.46–38.1 and The vertical principal stresses in the three tectonic zona- 5.97–31.3 MPa, with the average of 18.97 and 15.69 MPa, tions increase with depth, displaying a slight numerical respectively. In the measurement data, the maximum and difference, illustrating the basically consistent laws minimum values 65.5 and 12.8 MPa for maximum hori- (figure 3d). zontal principal stresses were measured in the eastern No. 10 mine and western No. 5 mine, respectively. Further- 3.2b Distributions of orientation of maximum horizontal more, the ratio of maximum to minimum horizontal prin- principal stresses: In accordance with the measurement cipal stress is 1.04–4.71, mainly varying from 1.24 to 2.84. data of in-situ stresses in table 1, figure 4 shows the ori- That is, there is a large difference between maximum and entation of maximum horizontal principal stresses using a minimum horizontal principal stresses, indicating strong Rose diagram. It can be seen from figure 4a and table 1 that directivity. In addition, the ratio of the maximum horizontal the orientation of maximum horizontal principal stresses is principal stresses to the vertical principal stresses varies in the ranges of 0.8–88° and 271.23–350.2°. Measurement from 0.62 to 2.83, mainly distributing in the range of data for two measurement points (#20 and #21), accounting 1.05–2.83. This implies that the in-situ stress field of the for 4.88% of the total points, indicate that the maximum Pingdingshan coalfield is mainly dominated by horizontal horizontal principal stress is in the SN orientation with tectonic stress fields. azimuth of 0–11.25° and 348.75–360°. Three measurement points (#11, #26 and #29, 7.32% of the total) demonstrate 3.2a Distribution of in-situ stresses: Being affected by the an orientation of NEN with an azimuth of 11.25–33.75°. Likou syncline, the maximum horizontal principal stresses Measurement data of eight points (#2, #9, #13, #15–18 and show obvious zonation (figure 3a). At the same depth, the #41) indicate that the maximum horizontal principal stress maximum horizontal principal stress in tectonic zonation I is in the NE direction with an azimuth ranging from 33.75° at a distance from the axis of the Likou syncline and to 56.25°. Ten measurement points (#3, #4, #7, #8, #10, controlled by the Guodishan fault is the smallest, while #12, #14, #28, #31 and #39) display an orientation of ENE the largest of maximum horizontal principal stresses is with an azimuth ranging from 56.25° to 78.75°; these points found in tectonic zonation III close to the axis of the occupy 19.51% and 24.39% of the total, respectively. In Likou syncline. Moreover, the maximum horizontal prin- addition, data measured at four measurement points cipal stress in tectonic zonation II varies between them. (9.76%) including points #24, #30, #32 and #38 indicate The maximum horizontal principal stresses of tectonic that the maximum horizontal principal stress is in the EW zonations II and III increase with depth, while that of direction (azimuths are 78.75–90° and 270–281.25°). A tectonic zonation I decreases with depth’s increasing. The maximum horizontal principal stress is also found in the overburden depths of measurement points of in-situ WNW direction with an azimuth of 281.25–303.75° stresses in three zonations are 645–844 440–965 and according to the measurement data of 11 measurement 459.4–1,123 m. Their maximum horizontal principal points (#1, #5, #6, #19, #22, #23, #33, #35–#37 and #40, stresses are 12.8–16.84, 14.02–31.7 and 19.6–65.5 MPa, 26.83% of the total). Two measurement points (#27 and with the average of 14.14, 24.2 and 38.18 MPa, respec- #34, 4.88% of the total) and one measurement point (#25, tively. At the same depth, the maximum horizontal prin- 2.44% of the total) reveal that the maximum horizontal cipal stress in tectonic zonation III is 1.2–1.4 times that of principal stresses are also in the NW direction (azimuth tectonic zonation II and 1.6–2.5 times that of tectonic varies from 303.75° to 326.25°) and the NWN direction zonation I. (azimuth is 326.25–348.75°). According to the distribu- The ratio of maximum horizontal principal stress to tions, the maximum horizontal principal stress is mainly vertical principal stress is the lateral pressure coefficient, distributed in the NE–ENE, E–W and WNW directions, which is divided into two parts: one part of lateral pressure accounting for 80.49%. coefficients are larger than 1 and the other are smaller than In the analysis in figure 4b–d as well as table 1, there are 1 in the coalfield (figure 3b). The lateral pressure coeffi- 18 measurement points in tectonic zonation III. Data cients of tectonic zonations III, II and I are 1.09–2.83, measured at 14 (77.78%), 3 and 1 points show that the 1.05–2.10 and 0.62–0.92, with the average of 1.83, 1.46 and maximum horizontal principal stresses are distributed in the 0.76, respectively. In other words, in the coalfield, tectonic NE–ENE, WNW and NEN directions, respectively. zonations III, II and I are ranked from large to small Therefore, the orientation of maximum horizontal principal according to the lateral pressure coefficient of in-situ stresses is shown in the NE–ENE direction, which is nearly stresses (figure 3b). Horizontal stresses are dominant in vertical to the trend of the main geologic structures Sådhanå (2020) 45:47 Page 7 of 17 47

(figure 2). In addition, among the eight measurement points Therefore, EW-trending structures were undeveloped at this in tectonic zonation I, four (50%) uncover a maximum stage (figure 6a). In Yanshanian, under the controls of the horizontal principal stress in the WNW direction, and the Qinling–Dabie orogenic and the tectonic domain of the other four imply those in the E–W, NW, NE and ENE West Pacific Ocean, compressive tectonic stress fields were directions. Therefore, the maximum horizontal principal formed in a near NW–SE direction in the Pingdingshan stress is found in the WNW direction. Furthermore, 15 coalfield, producing NW-trending strike-slip faults in the measurement points are shown in tectonic zonation II. Data coalfield (figure 6b), which damaged coal seams to some measured at 2, 2, 4, 2, 3, 1 and 1 points indicate that the extent [6, 52, 55]. For example, the NW- and WNW- maximum horizontal principal stresses are in the S–N, trending Xiangjia fault in the north of the coalfield is shown ENE, WNW, NEN, E–W, NW and NWN directions, as a strike-slip normal fault. During the Sichuan period, due respectively, indicating the complexity of the orientation of to strong push actions of the Indian plate on the southwest maximum horizontal principal stresses. If the S–N, NEN side, compressional tectonic stress fields in a near NEN– and NWN orientations are identified as a near S–N direc- SWS direction were formed [56]. Thus, folds and faults tion while E–W, ENE and WNW are considered in terms of occurred in the coalfield on a large scale. A series of NW- a near E–W direction, the main orientations of maximum and WNW-trending folded structures and thrust faults, such horizontal principal stresses of the coalfield are in the near as the Likou syncline, Xiangjia anticline, Jingjiawa syn- E–W and NE directions, which separately account for, cline and Luye anticline as well as the Jiulishan, Guodishan respectively, 60.98% and 19.51%. The maximum horizon- and Baishigou–Huoyan faults, were formed in the coalfield tal principal stresses of tectonic zonation III are in the near and surrounding areas [52]. Particularly, more structures E–W direction (55.56%) and near NE direction (38.89%), were developed in the No. 10, No. 11, No. 8, Shoushan No. while 75% and 60% of those are in the E–W direction in 1 mine and No. 13 mines in the eastern part of the coalfield. tectonic zonations I and II, respectively. The NW- and WNW-trending fault–fold belt comprising the Lingwushan syncline, Baishishan anticline, Guozhuang 3.2c Distribution of in-situ stresses with overburden depth: anticline and Niuzhuang syncline as well as the reverse As shown in figure 5a, the maximum horizontal principal faults of Niuzhuang and the original No. 11 mine was stress, vertical principal stress and minimum horizontal formed. In this way, the fold and fault structures trending principal stress in the coalfield all show a linear increasing primarily in the NW–WNW direction of the Pingdingshan relationship with overburden depth, with certain discrete- coalfield were formed. With the strengthening of the ness in local areas. It can be seen from figure 5b and c that compression stresses of the Indian plate, the North China maximum horizontal principal stresses, vertical principal plate experienced constant diffusion creep to the east, thus stresses and minimum horizontal principal stresses in tec- forming NEN-trending normal faults like the Jiaxian fault tonic zonations II and III linearly increase with overburden and Luogang fault in the Pingdingshan coalfield (figure 6c). depth. Although vertical principal stress in tectonic zona- In the North China period, owing to the subduction of the tion I also increases linearly with overburden depth, hori- Pacific Kula plate changing to the WNW direction, nearly zontal principal stress decreases with the increase of horizontal compression actions in the WNW–ESE direction overburden depth (figure 5d). The specific correlations are were produced. Under the setting of an overall uplifting of displayed in table 2. the coalfield, the pre-existing NW- and WNW-trending faults such as the Guodishan fault, Luye fault and Xiangjia fault reversed into normal faults under the stress fields and 4. Discussion gravity sliding effects during this stage. The pre-existing NE- and ENE-trending faults showed compressive-shear 4.1 Influence factors of in-situ stresses activities and were still normal faults [56]. In this period, a banded block formed due to the depression of the sur- 4.1a Influence of tectonic stress field evolution: Since the rounding areas (the Luye fault in the south, Xiangjia fault in formation of the Carboniferous–Permian coal-bearing for- the north, Jiaxian fault in the west and Luogang fault in the mation, the multi-stage tectonic movements affected east), and the uplift of the central part was formed in the importantly the distribution of in-situ stress in the coalfield (figure 6d). In the Himalayan period, tectonic Pingdingshan coalfield [56]. During the Indosinian period, stress fields were nearly in the SN direction. The pre-ex- due to the collision between the North China plate and the isting NW- and WNW-trending faults showed compressive- Yangtze plate, the southern margin of the North China plate shear activities, and NE–NEN-trending faults showed ten- was involved in the Qinling–Dabie orogenic belt [50, 52]. sional-shear activities in this period (figure 6e), further As a result, the horizontal compressional tectonic stress damaging coal seams. In the Neotectonic period, the pre- field in the near S–N direction decreased to the interior of sent tectonic stress fields are being subjected to compres- the plate, and strong tectonic deformation mainly occurred sion in a near EW direction, slightly influencing tectonic in the Qinling–Dabie orogenic belt and its sides, which deformation in the coalfield and not forming new fold and slightly affected the Pingdingshan coalfield [52, 57]. fault structures (figure 6f). 47 Page 8 of 17 Sådhanå (2020) 45:47

70 (a) 3.00 a

/MPa (b) 60 P M

/ 2.50 t n e

50 i c

i 2.00 f f

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30 r u s s

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15 20 15 10 10 5 5

Vertical principal stresses /MPa 0

Minimum horizontal stresses/MPa 0 400 600 800 1000 1200 400 600 800 1000 1200 Overburden depth/m Overburden depth/m

Tectonic zonation III Tectonic zonation II Tectonic zonation I Tectonic zonation III Tectonic zonation II Tectonic zonation I

Figure 3. Comparison of in-situ stresses in the three tectonic zonations with (a) maximum horizontal principal stresses, (b) the lateral pressure coefficient, (c) minimum horizontal principal stresses and (d) vertical principal stresses.

In-situ stress test data are the result of superposition of tectonic zonations on the SW side of the Likou syncline in-situ stresses in each period, and the test results for in-situ have shown dissimilar tectonic types and inheritance abil- stresses reflect tectonic principal stress fields in each period ities for the paleo-tectonic stress fields in their evolution. to some extent. The maximum principal stress in tectonic This has resulted in differences in maximum horizontal stress fields of the coalfield in the Indosinian (S–N direc- principal stresses, dominant stresses and orientation of tion), Sichuan (NEN–SWS direction) and Himalayan peri- maximum horizontal principal stresses in each tectonic ods (near S–N direction) was in a near S–N direction zonation. (measurement points #11, #20, #21, #25, #26 and #29). In Tectonic zonation III is close to the Likou syncline and Yanshanian, the maximum principal stress of the tectonic shows tectonic types with a series of folded structures stress field was shown in the NW–SE direction (measure- composed of WNW- and NW-trending synclines and anti- ment points #27 and #34). In comparison, in the North clines. According to previous tectonic stress fields and China period (the WNW–ESE direction) and at present tectonic evolution, after such tectonic types were formed (nearly in E–W direction), the maximum principal stress of under the effects of strong thrust-nappe structures in the the tectonic stress field is found to be in a near E–W Sichuan period, rock masses had been bent and deformed direction (measurement points #1, #5, #6, #19, #22–24, but not broken, showing a closed state throughout. More- #30, #32–38 and #40). The orientation of maximum hori- over, such tectonic traces were not damaged in subsequent zontal principal stresses is shown to be a NE–ENE direction tectonic evolution, so that compressional tectonic stresses according to the other measurement points, which is the in the NEN–SWS direction over this period were not result of the superposition of present tectonic stresses and effectively released and therefore remained in the subse- tectonic stresses in the Sichuan period. quent tectonic evolution. The remaining tectonic stresses are superposed with the present tectonic stresses in a near 4.1b Effects of tectonic types on distribution of in-situ EW direction. Furthermore, such folded structures with stresses: Based on the afore-discussed analysis, different syncline and anticline tend to form a stress concentration. Sådhanå (2020) 45:47 Page 9 of 17 47

(a) e e e (b) e e e e e e e e e e e e e e e e e e e

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e e e e  

e e e e e e e e e e (c) e e (d) e e e e e e e e e e

e e e e

e e e e

e e e e

e e e e 

Figure 4. Rose diagram of orientation of maximum horizontal principal stresses in the three tectonic zonations: (a) in the whole research area, (b) in tectonic zonation III, (c) in tectonic zonation II and (d) in tectonic zonation I.

70 (b) 70 (a) 60 60

50 50

40 40

30 30

20 20 Geostress /MPa Geostress /MPa

10 10 0 0 400 600 800 1000 1200 400 600 800 1000 1200 Overburden depth/m Overburden depth/m 30 (d) 40 (c) 35 25

30 20 25 20 15 15 10 Geostress/MPa

Geostress/MPa 10 5 5 0 0 400 500 600 700 800 900 1000 600 650 700 750 800 850 900 Overburden depth/m Overburden depth/m Maximum horizontal principal stresses Vertical principal stresses Minimum horizontal principal stresses Maximum horizontal principal stresses Vertical principal stresses Minimum horizontal principal stresses

Figure 5. Relationship between in-situ stress and overburden depth in three tectonic zonations: (a) in the whole research area, (b) in tectonic zonation III, (c) in tectonic zonation II and (d) in tectonic zonation I. 47 Page 10 of 17 Sådhanå (2020) 45:47

Table 2. Relationships between in-situ stress and burial depth in different tectonic zonations of research area.

Name of tectonic zonation Relationship between rH and H Relationship between rV and H Relationship between rh and H 2 2 2 Tectonic zonation III rH = 0.033H ? 11.76, R = 0.621 rV = 0.018H ? 5.728, R = 0.403 rh = 0.021H ? 1.377, R = 0.506 2 2 2 Tectonic zonation II rH = 0.020H ? 10.81, R = 0.428 rV = 0.015H ? 6.509, R = 0.548 rh = 0.011H ? 6.875, R = 0.370 2 Tectonic zonation I Negative correlation rV = 0.01H ? 11.75, R = 0.064 Negative correlation

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Figure 6. The evolution of tectonic stress field and tectonics in the Pingdingshan coalfield during geological histories, respectively, showing the tectonic stress field and tectonics formation during (a) Indosinian, (b) Yanshanian, (c) Sichuan period, (d) North China period, (e) Himalaya period and (f) Neotectonic period.

Therefore, tectonic stress is dominant and maximum compression in the WNW–ESE direction and horizontal horizontal principal stress is in a NE–ENE direction in extension in the NEN–SWS direction were produced, tectonic zonation III. Moreover, the maximum horizontal reversing the Guodishan fault to a normal fault and further principal stress is the largest in this coalfield. releasing horizontal stresses. Therefore, tectonic zonation Tectonic zonation I is at a distance from the Likou I, controlled by the Guodishan fault, demonstrates self- syncline and displays a tectonic form dominated by the weight fields with vertical stresses as the principal in-situ WNW- and NW-trending fault structures under the control stresses, and the maximum horizontal principal stress is of the Guodishan fault. In accordance with the afore- controlled by the present tectonic stress fields in the mentioned tectonic stress fields and tectonic evolution, the WNW direction. Guodishan fault is a reverse fault formed due to the strong Tectonic zonation II has a simple structure, lacking large thrust-nappe structure in the Sichuan period. The forma- fold and fault structures. Located between tectonic zona- tion of reverse fault led to the fracturing of the formations, tions I and III, the ability of inheritance and release stress is which consumed and released tectonic stresses to some between tectonic zonations I and III, and the intensity and extent. Moreover, the horst structure was formed under orientation of maximum horizontal principal stresses also the common controls of the upper walls of the Guodishan lie between them. Horizontal tectonic stresses are dominant fault and Jiulishan fault in the southwest side, lifting the in this zonation, and the orientation of maximum horizontal whole western region and further releasing horizontal principal stresses is complex, being mainly in a near E–W stresses. During the North China period, nearly horizontal direction. Sådhanå (2020) 45:47 Page 11 of 17 47

4.2 In-situ stress partition and characteristics Based on distribution characteristics of in-situ stresses on

mean the SW side of the Likou syncline and tectonic control Lateral pressure min–max/ coefficient effects, in-situ stresses in the Pingdingshan coalfield were 1.05–2.10/1.46 divided into high tectonic stress zonation, tectonic stress zonation and vertical stress zonation. They correspond to tectonic zonations III, II and I, respectively (table 3 and figure 7), where the maximum horizontal principal stresses decrease successively. The isolines of the dominant in-situ stress trend of coal seam group F in the Pingdingshan coalfield were drawn (figure 7) based on the distribution

direction characteristics of in-situ stresses in each tectonic zonation

horizontal stress and data in tables 1 and 2. The two tables display the relationship between dominant stresses in each tectonic Orientation of maximum zonation and overburden depth.

4.3 Influence of in-situ stress on CBM occurrence 4.3a Measurement of CBM occurrence parameter: CBM max/mean

(MPa) min– (gas) content and gas pressure are very important parame- Vertical stress ters in coal mining and CBM development, which are the main parameters that reflect CBM (gas) occurrence of coal seam. In Pingdingshan coalfield, many CBM (gas) contents and gas pressures have been tested using the direct method of determining coalbed gas content [58] and the direct

mean measuring method of the coal seam gas pressure in mine [59]. Tables 4 and 5 show the data and statistical result of principal stress (MPa) min–max/

Minimum horizontal CMB (gas) content of coal seam group F only shallower than 1,200 m underground in the research area. 4.3b Influence of in-situ stress on CBM occurrence: The relationship between permeability and stress of porous coal can be expressed as [5] mean K ¼ K0 expðÀ3C/DrÞð1Þ principal stress 19.6–65.5/38.18 5.97–31.3/19.54 12.46–38.1/20.88 NE–ENE direction 1.09–2.83/1.83 12.8–16.84/14.14 6.9–15.41/9.74 17.24–21.1/18.54 WNW direction 0.62–0.92/0.76 (MPa) min–max/ 14.02–31.7/24.2 10.05–19.03/14.34 12.66–26.23/16.82 Complex, being mainly near E–W

Maximum horizontal By analysing well test permeability of CBM drilling in China, Ye et al [60] found that in-situ stress significantly affects the permeability of coal seams. When in-situ stress

stress is larger than 20 MPa, the permeability of coal seams is stress stress stress -3 2

Dominant generally less than 0.1 9 10 lm . In cases where in-situ stress is less than 14 MPa, the coal seams show perme- ability larger than 0.1 9 10-3 lm2. Moreover, when in-situ stresses range from 10 to 20 MPa, the permeability of coal seams changes greatly. Compressional stress decreases 440–965 Tectonic 645–844 Self-weight depth (m) 459.4–1123 Tectonic

Measurement permeability of coal seams such that the migration and diffusion of CBM are inhibited, which is conducive to saving gas and easily forming CBM enrichment areas. Meanwhile, tensile stress increases the permeability of coal

zonation I seams, which is favourable for migration and release of Tectonic Tectonic zonation II Tectonic zonation III CBM. Affected by in-situ stresses and tectonic structures, CBM

In-situ stress partition and characteristics in the research area. occurrence is obviously different in the western, middle and eastern parts of the Pingdingshan coalfield, which shows zonation characteristics as well. That is, the CBM occur- High tectonic stress zonation Tectonic stress zonation Vertical stress zonation Table 3. In-situ stress partition Range rence changes in various in-situ stress zonations. From west 47 Page 12 of 17 Sådhanå (2020) 45:47

Huoyan fault  NP Baishigou reverse fault 1 Likou syncline

  NO.8

 ult fa fault

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 X

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 anticline 

 Niuzhuang syncline 



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  NO.8

 name of coal mine 

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 principal stress   vertical stress zonation NO.7  coalbed outcrop   

Figure 7. In-situ stress partition and trend isolines of dominant in-situ stress of the research area.

Table 4. Data of CMB (gas) measurement of coal seam group F in the research area.

CBM (gas) content (m3/ CBM (gas) content (m3/ CBM (gas) content (m3/ Number t) H (m) Number t) H (m) Number t) H (m) 1 3.91 955 25 16.64 693 49 27.2 831 2 4.31 959 26 10.48 600 50 10.2 530 3 3.67 917 27 19.58 985 51 14.9 590 4 10.38 1096 28 12.14 612 52 18 680 5 7.15 984 29 14.08 853.4 53 19.8 1117 6 9.43 1109 30 10.49 612 54 19.55 1003.58 7 7.29 1080 31 15.13 1079 55 19.27 1003.73 8 4.77 878 32 14.22 1056 56 18.42 1043.08 9 9.24 890 33 14.07 1039.5 57 20.35 1115.84 10 4.56 870 34 15.06 1106 58 18 795 11 6.42 900 35 12.12 869 59 16.5 490 12 13.16 1050 36 6.7 630 60 25.64 504 13 10.99 1101 37 11.88 935 61 13.09 490 14 3.32 848 38 9.8 951 62 12.29 494 15 11.02 1027 39 10.34 986.2 63 9.24 505 16 12.12 1180 40 6.16 846 64 16.69 537 17 14.47 1101 41 10.62 891 65 7.87 489 18 10.17 716 42 4.77 747 66 11.1 495 19 8.49 612 43 5.85 851 67 10.87 490 20 9.81 427 44 9.41 792 68 13.58 545 21 10.48 475 45 11.25 890 69 15.28 605 22 11.77 435 46 20.04 850 70 12.17 535 23 13.55 550 47 12.37 843 71 13.02 505 24 14.05 577 48 26 993 72 10.6 565

to east, CBM content and gas pressure generally increase contents and pressures of vertical stress zonation II in the first, then decrease and increase again (figure 8, table 5 and west are the lowest. The gas contents and pressures of figure 9c). Also, affected by burial depth, CBM content and tectonic stress zonation in the middle are between those of gas pressure have an overall trend of increasing with the vertical stress zonation II and those of vertical stress increase of burial depth in the same stress zonation (fig- zonation I (figures 8 and 9 and table 5). ures 8 and 9b). In general, at the same burial depth, high Specifically, in the high tectonic stress zonation, CBM tectonic stress zonation in the eastern part of the coalfield (gas) preservation conditions are the best because of the demonstrates the highest CBM contents and gas pressures, highest tectonic stress and complex extrusion structures. followed by vertical stress zonation I in the west. The CBM This zonation demonstrates that gas weathering zones are Sådhanå (2020) 45:47 Page 13 of 17 47

Table 5. Statistics of CBM (gas) measurement results of coal seam group F in the research area.

Name of stress Range of CBM content Range of CBM content (m3/t) Depth range of CBM content measurement zonation number min–max/mean (m) min–max/mean Vertical stress 1–17 3.32–14.47/8.01 848–1180/996.8 zonation II Vertical stress 18–30 8.49–19.58/12.44 427–985/626.7 zonation I Tectonic stress 31–45 4.77–15.13/10.49 630–1106/911.2 zonation High tectonic stress 46–72 7.87–27.2/16.00 489–1117/690.5 zonation

 (a)( b)

 

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Figure 8. Comparison of CBM (gas) data of coal seam group F in different in-situ stress partitions: (a) comparison of CBM (gas) and (b) comparison of gas pressure. very shallow in coal seams; CBM contents and gas high tectonic stress zonation, and the depth of coalbed gas pressures are very high. CBM content of the coal seam weathering zone is deep, which is about 500 m. CBM group F shallower than 1,200 m underground generally content and gas pressure have an overall trend of increasing ranges from 10 to 20 m3/t and reaches even about 30 m3/t; with the increase of in-situ stress (figures 5c and 8). In the while gas pressure generally is 1–3 MPa, the depth of vertical stress zonation, CBM (gas) preservation conditions coalbed gas weathering zone is approximately 200 m. CBM should be bad because of low tectonic stress. Due to the content and gas pressure have an overall trend of increasing influences of formation and evolution of the Guodishan with the increase of in-situ stress (figures 5b and 8). Fur- fault, this vertical stress zonation is divided into two dis- thermore, from 1984 to 2016, 98 coal and gas outbursts tinctive zonations, called vertical stress zonations I and II. have occurred in coal seam groups E and F, which are the The latter is in the western part of the coalfield at some areas showing the highest CBM (gas) content and most distance from the Guodishan fault, consisting of the west- severe gas disasters in the coalfield. In the tectonic stress ern part of the No. 11 mine and Xsh mine. In comparison, zonation, CBM (gas) preservation conditions are relatively the former covers the area in the vicinity of the Guodishan good because of relatively high tectonic stress and simple fault, including the western part of the No. 5 (No. 6) mine geological structures. This zonation has relatively high as well as the eastern part of the No. 9 and No. 11 mines CBM content and significant gas pressure. Coal seam group (figure 8). Under the controlling of the Guodishan fault, in F demonstrates 5–15 m3/t of CBM content, and gas pres- vertical stress zonation I, the geological structures are very sure ranges from 0.4 to 1.5 MPa. When the overburden complex, coal seams are seriously damaged and deformed depth is larger than 1,100 m, gas pressure increases greatly, coal is very developed. Furthermore, adsorption ability and and gas pressure reaches 1.69–2.6 MPa, leading to mines hold capacity of coal are relatively strong, and gas pres- that are prone to coal and gas outburst. However, the fre- sures and CBM contents in coal seams are generally higher quency and strength of outburst are smaller than those of than those of vertical stress zonation II and tectonic stress 47 Page 14 of 17 Sådhanå (2020) 45:47

Huoyan fault  NP (a) Baishigou reverse fault 1 Likou syncline

  NO.8

 lt  NO.12 NO.10 ian fau d Zhangwan fault Legend

 Xin

  anticline

Jiaxian fault Guozhuang syncline

 





 Niuzhuang syncline anticline

   

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    Wzh reverse fault 

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  NO.8  

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  &Wzh

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  tectonic stress zonation trend contour line

     Xsh ᓅ

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(b) (c) High tectonic stress zonation /t /t

3 30 vertical stress zonation II vertical stress zonation I 3 25 Vertical stress zonation I 25 tectonic stress zonation high tectonic stress zonation 20 20 15 15 10 10 5 5 0 CBM contentCBM / m Vertical stress zonation II Tectonic stress zonation 300 400 500 600 700 800 900 1000 CBM / m constent 0 Burial depth of coalseam / m From west to east in research area Burial depth 500 m Burial depth 600 m Burial depth 700 m Burial depth 800 m Burial depth 900 m Burial depth 1000 m

Figure 9. Distribution of CBM (gas) content and relationship between CBM (gas) content and in-situ stress in the research area: (a) trend contour line of CBM (gas) content of coal seam group F, (b) comparison of CBM (gas) content in different in-situ stress partitions and (c) change of CBM (gas) content from west to east in the research area. zonation in the middle part. CBM content in coal seam evolution theories of geologic structure and a statistical group F ranges from 10 to 15 m3/t, and gas pressure varies analysis method. The following conclusions were obtained. from 1.2 to 2.0 MPa and even reaches 3.20 MPa. Gas 1. The research area is divided into three tectonic zona- content in coal seams increases with approaching the tions. Structures in the tectonic zonation III are the most Guodishan fault. The depth of coalbed gas weathering zone complex, mainly shown as the WNW- and NW-trending is shallow, which is about 300 m. In vertical stress zonation thrust-nappe folds and faults, especially folded struc- II far away from Guodishan fault, because of low tectonic tures. The structures in the tectonic zonation II are stress, simple structures and undeveloped deformed coal, simple. Moreover, the WNW- and NW-trending fault coal seams show low CBM contents and gas pressures that structures are dominant in tectonic zonation I, and are less than 10 m3/t and 0.7 MPa, respectively. The depth complex structures are found in the area close to the of coalbed gas weathering zone is very deep, which is about Guodishan fault. 750 m. In vertical stress zonation, CBM content and gas 2. On the whole, in-situ stress in this research area pressure have an overall trend of increasing with the increases with overburden depth and horizontal tectonic increase of vertical stress (figures 5d and 8). stress is dominant. The maximum horizontal principal stress is in a near EW direction. 3. In-situ stress distribution is mainly controlled by 5. Conclusions different tectonic types and evolution of different tectonic zonations. This research area was divided into This research investigated the distribution characteristics of three stress zonations. High tectonic stress zonation geologic structures and partition of in-situ stress as well as shows the most complex structure, where the folded the effects of in-situ stresses on CBM occurrence using structures are dominant. It belongs to the distribution Sådhanå (2020) 45:47 Page 15 of 17 47

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