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. 2022 Dec 1;7(49):45708–45718. doi: 10.1021/acsomega.2c06932

Evolution of Composition and Methane Occurrence of Bituminous Coal after Igneous Intrusion: A Case Study of Daxing Coal Mine, Tiefa Basin, China

Yiming Huang †,‡,§, Jingyu Jiang †,‡,§,*, Yuanping Cheng †,‡,§, Huazhou Huang ∥, Lei Zhang ⊥
PMCID: PMC9753176  PMID: 36530316

Abstract

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Since 1987, there have been 10 coal and gas outbursts in Daxing Coal Mine, Tiefa Basin. It was reported that these outbursts were related to igneous intrusion. To study the influence of igneous intrusion into bituminous on coal bed methane (CBM) occurrence, eight coal samples were collected at different distances (0.2–10 m) from an intrusion in Daxing Coal Mine. The petrographic and chemical data, pore characteristics, and adsorption properties of unaltered and heat-affected coals were tested and compared. Approaching the dike, vitrinite reflectance (Ro) increases from 0.56 to 1.14%, while moisture decreases from 6.4 to 3.1%. According to Ro, with the thermal evolution of igneous rocks, the temperature of heat-affected coal seams generally increases from ∼78 to ∼169 °C. The bituminous coal contained high moisture content before the intrusion, and after the igneous intrusion, partial heat may be used for moisture gasification; the other heat was used to improve coal metamorphism, resulting in little increase in the coal metamorphism as expected. The synergistic effect of igneous thermal evolution and moisture content change improves the adsorption capacity of coal. Affected by the igneous intrusion, the gas content of the heat-affected coal seams No. 4 (above the sill) and No. 7 (below the sill) is higher than that of the unaltered No. 12 coal seam, and the gas content of No. 7 coal seam (beneath the sill) is higher than that of No. 4 coal seam. The low permeability sill has a sealing effect on the gas in No. 7 coal seam. Therefore, the evolution process of CBM occurrence in the Daxing Coal Mine can be speculated, that is, after magma intrusion, the temperature increased, the metamorphic degree of coal increased, and water gasification occurred at the same time. The original adsorption site occupied by moisture was vacated, which improved the methane adsorption capacity of the coal and provided favorable conditions for CBM occurrence.

1. Introduction

With the exploitation of coal resources, the conditions encountered in the mining process of coal around the world are more complex and variable. Many coal bed methane (CBM) basins have undergone thermal evolution or contact metamorphism related to igneous intrusions, such as the Piceance basins in America,1 the Bowen basins in Australia,2,3 the Qinshui basins in China,4 the Raniganj basins in India,5 the Karoo basin in South Africa,6−8 the Donets basin in Ukraine and Russia,9 and the Sverdrup basin in Canada10 (Table 1). Igneous intrusions have significant effects on the coal rank, petrographic properties, and the mineralogical and chemical composition of coal.10−21 Due to the igneous intrusion, the inorganic constituents and minerals in coal have changed.15,22 Affected by igneous intrusions, vitrinite reflectance (Ro), ash yield, fixed carbon, and carbon content of coal increase, whereas the in moisture, volatile matter (VM), nitrogen, and hydrogen content of coal decrease. Meanwhile, it causes the loss of liptinite, and the formation of nature coke, pyrolytic carbon, and graphitized coals within the thermal aureole.7,11,12,23 However, the synergistic effect of igneous intrusion and internal moisture change on CBM occurrence characteristics has not attracted much attention.

Table 1. World-Wide Coal Basins/Coalfields Investigated for Igneous Intrusion.

basin coalfield country sill/dike intrusion geological time reference
Piceance   USA     (1)
  Midland Valley Scotland sill Carboniferous (29)
Bowen   Australia     (2,3)
Gunnedah   Australia sill Permian (17,30,31)
South Sumatra   Indonesia sill Quaternary (32)
Raton   USA sill, dike Cretaceous-Paleocene (16)
Illinois   USA dike Cretaceous (11,12,33)
San Juan   USA     (4)
Sydney   Australia   Permian to Tertiary (24,34,35)
Donets   Ukraine, Russia   Permo-Triassic (9)
  Jharia Indian sill, dike Lower Cretaceous (36,37)
Gondwana   Indian dike Cretaceous? (14)
Raniganj   Indian dike Permian (5)
  Handan China sill, dike Permian (4,13,15)
  Highveld South Africa sill Jurassic (7)
Karoo   South Africa dike Jurassic (6,8)
Sverdrup   Canada dike Jurassic, Cretaceous (10)
  Huaibei, China sill, dike Jurassic, Cretaceous (4,13,28)
  Hongyang China sill, dike Cretaceous (4,13)
  Huainan China sill, dike Cretaceous (19)
  Hanpo’ao China sill, dike Permian, Triassic (23)
Qinshui   China   Jurassic, Cretaceous (4)
Fuxin   China   Tertiary (4,38)
Tiefa   China sill Tertiary (39,40)
Moatize   Mozambique sill Permian (41)
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Dikes are important for driving hydrothermal circulation, but not for generating large volumes of methane through direct conductive heating.16 According to the survey results of gas outburst events in the Highveld coalfields of South Africa, the sites of high gas emissions are substantially in the adjacent of the igneous intrusions. The porosity properties and adsorption capacity of the heat-affected coals have been improved, thus the CBM storage capacity has improved.24,25 The study of the Hoskissons coalfield in Gunnedah basin shows that the mineralization effect is not the key factor affecting the gas flow capacity. Complex stress orientation reduces the connectivity of fractures and affects the flow characteristics of gas.26,27 As the intrusion is closer to the coal seam, the thermal effect is more obvious. The high gas content shows the great potential of commercial development of CBM. On the other aspect, it also indicates a serious safety problem in coal mines.9 In the process of coal mining, igneous intrusions can cause coal and gas outburst in coal mines.7,9,28

The influence of magma intrusion on coal seam is related to the initial temperature of magma, coal pore moisture, formation maturity before magma intrusion, and thermal conductivity of formation.29 The remarkable characteristic of bituminous coal is high moisture.42 After magma intrusion, the moisture in coal decreases, which destroys the competitive adsorption equilibrium of gas and water molecules in the coal matrix.21,43 When the moisture increases from 0 to 8.5%, the macropore diffusion coefficient decreased by 82%, and the micropore diffusion coefficient decreased by 88%.44 Therefore, it is judged that the water adsorption of coal is the key mechanism to adjust methane adsorption. Most adsorption isotherm experiments are carried out with dry coal, however, the water in coal significantly affects CBM occurrence.45−48 As for the competitive adsorption on coal, CH4 and CO2 are forced to reduce the energy level in the presence of moisture.49 The experiments showed that the invasion water can displace the adsorbed methane in the matrix micropores of coal and result in the increase of the free gas content.50 The magma intrusion caused abnormal gas emission during coal mining in many underground coal mines. Therefore, it is necessary to study the influence of igneous intrusion and internal moisture variation on the CBM occurrence characteristics.

Since 1987, ten outbursts have occurred in Daxing Coal Mine, Tiefa Basin. It was reported that these outbursts were related to igneous intrusion. However, there is still a lack of systematic research on the influence of igneous intrusion on the CBM occurrence and adsorption characteristics of bituminous coal in Daxing Coal Mine, Tiefa Basin.

2. Geological Setting

2.1. Geological Stratigraphy of Tiefa Basin

The Tiefa Basin is a continental sedimentary basin39 and one of the main coal mining areas in Northeast China, located in Tieling City, Liaoning Province, China. The map of the study area, data locations for stratigraphic characteristics, and profile from Tiefa Basin are shown in Figure 1. The minerable coal seams are Nos. 2, 4, 7, 9, 12, 13, 15, and 16, respectively. The stratigraphic column of the Daxing Coal Mine is shown in Figure 2. The bedrock of the Tiefa Basin is composed of Cretaceous basal beds of continental sedimentary strata and quaternary sediments. The coal-bearing units are the Fuxin group of the lower Cretaceous, and the four lithologic members of this group, respectively, are: the glutenite section (K1f1), the lower coal-bearing member (K1f2), the sand–mudstone member (K1f3), and the top coal-bearing member (K1f4). The coal seams are staggered with other geological units (including mudstones, shales, or clays).CBM is extracted from the lower coal-bearing segment (K1f2) and the top coal-bearing segment (K1f4).

Figure 1.

Figure 1

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Map showing study area, data locations for stratigraphic characteristics, and section from Tiefa Basin (elevation of the ground is +73 m).

Figure 2.

Figure 2

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Stratigraphic column of Daxing Coal Mine.

There are three stages of igneous activity in the Tiefa Basin. The first and second stages were dominated by eruptive rock. At the third stage, Himalayan (36.5–53 Ma) exhibited igneous activity mainly by intrusive rocks. The lithology of igneous rock is diabase, rich in iron and magnesium. The deep magma upwelled along the Gangtun fault and invaded the coal seams and strata along the relevant secondary faults (Figure 1).

2.2. Burial and Geological History of Coal Measuring Strata of Tiefa Basin

As presented in Figure 3, the burial history of the Fuxin Formation in the coal-bearing strata can be roughly divided into six stages. The first stage is early Fuxin Formation–Jurassic–early Cretaceous. During this period, the in situ stress field dominated by the north–northeast–south–eastern eastward extension of the Tiefa Basin was weakened twice, and the coal seams experienced a deep metamorphic stage. The second stage is late Fuxin Formation–early Cretaceous. During this period, the tensile stress field continued to weaken. The third stage is from late Early Cretaceous to the Upper Cretaceous. During this period, the fault of the western Gangtun fracture was transformed into the reverse fault of the eastern side rising. The fourth stage is the middle and late period of the Late Cretaceous in which most of the basins were deposited with Quantou Formation. The fifth stage is the Tertiary, since Paleogene. During this stage, the Himalayan movement raised the Tiefa Basin as a whole, and a large amount of magma intruded the coal-bearing strata, causing widespread erosion, and the coal-bearing strata were again subjected to erosion and weathering. The sixth stage is the Neogene–Quaternary period during which the surface folds of the basin and the formation of coal-bearing strata eventually formed.

Figure 3.

Figure 3

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Stratigraphic evolution and burial history of the coal measure strata of Tiefa Basin.

3. Methodology

3.1. Sample Preparation

To investigate the influence of different occurrence of magma on the porosity, coal rank, and adsorption characteristics of coals, eight coal samples (named D1–D8, 0.2–10 m from the dike) were taken from the No. 9 coal seam in Daxing Mine, Tiefa coalfield.

To study the desorption capacity of coal samples with different moisture contents, 1–3 mm coal particles with different moisture contents were prepared as coal samples. The dry coal samples were weighed and placed in a vacuum tank for vacuum treatment and later weighed again. Then the coal samples were made to absorb water vapor until saturation, and the moisture content of coal was calculated from the change in weight. Then, the saturated water coal samples were evacuated and the moisture content of the coal samples was obtained based on the vacuum time (2.35, 4.52, 6.63, and 8.64%, respectively). Finally, methane gas was introduced into them to achieve adsorption equilibrium. To study the effects of igneous intrusion on the gas and moisture content of coal seams, 20 coal samples at different buried depths were collected from the No. 4 coal seam (above the sill), the No. 7 coal seam (below the sill), and the No. 12 coal seam (normal) in the Daxing Mine.

3.2. Analytical Methods

According to ASTM standards (ASTM: 2007), the proximate analysis of coal was carried out using a 5E-MAG6600 automatic industrial analyzer. Based on the international standards (ISO 7404-5: 1984), vitrinite reflectance (Ro) was measured using (Zeiss, Germany) a microscope photometer. The maximum paleo temperature (Tpeak, the coal/intrusion contact temperature) can be calculated by the following formula established by Barker and Pawlewicz:51

3.2. 1

Referring to Taylor and Glick,52 the maceral composition of the coals can be obtained through incident light microscopy and oil immersion. The porosity of the coal was measured by the mercury injection method of Auto Pore IV 9510 (Micromeritics, USA). During the methane adsorption experiment, 50 g crushed coal with a particle size of 0.2–0.25 mm were set at 30 °C, and the adsorption equilibrium gas pressure reached 6 MPa. The determination of gas content adopts the direct method and follows the Chinese national standard (State Administration of Coal Mine Safety of China: 2008). The gas desorption experiment of coal samples with different moisture contents was carried out for 120 min. The gas desorption amount of coal samples with various moisture contents at different times can be obtained by the drainage method.

4. Results and Discussion

4.1. Influence of Igneous Intrusion on the Geochemistry and Coalification of Bituminous Coal

The moisture, ash, VM, vitrinite, inertinite, exinite, mineral, and vitrinite reflectance of coal samples are listed in Table 2. Approaching the dike, vitrinite increases from 89.9 to 97.9% and inertinite decreases from 8.2 to 0.9%. The change law of exinite and mineral is not obvious. The relationship curves of moisture, ash, VM, and Ro of the coal samples with different distances from the dike are shown in Figure 4.

Table 2. Geochemical and Petrographic Analyses of Daxing Coal Samples near Sill/Dike Intrusionsa.

coal seam number sample Dis (m) proximate analysis (wt %)
 macerals (vol %)
 Ro (%) T (°C)
Mois Ash VM V I E M
9 D1 0.2 3.1 36.6 17.5 97.9 0.9 0.6 0.6 1.14 169
9 D2 0.5 3.4 34.3 21.6 97.1 0.6 0.8 1.5 0.82 127
9 D3 1.0 4.4 20.0 29.0 96.4 1.2 2.1 0.3 0.80 124
9 D4 2.0 4.4 16.0 32.7 96.0 2.1 1.1 0.8 0.74 114
9 D5 3.5 6.3 6.5 36.1 95.2 1.9 0.8 2.1 0.66 99
9 D6 5.0 6.2 4.7 37.0 94.6 2.3 1.4 1.7 0.60 87
9 D7 7.0 6.4 4.7 38.4 93.5 5.7 0.2 0.6 0.56 78
9 D8 10.0 5.8 4.0 37.4 89.9 8.2 0.6 1.3 0.57 80
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a

Dis = distance from sill/dike; Mois = moisture; Ash is on a dry basis; VM = volatile matter; on dry ash free (daf) basis; V = vitrinite, I = inertinite, E = exinite, M = mineral; Ro = random vitrinite reflectance (%, oil); T = maximum paleo temperature.

Figure 4.

Figure 4

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Moisture (Mois), ash (Aad), VM, and vitrinite reflectance (Ro) of coal at different distances from the dike boundary.

The range of Ro of the coal samples (D1–D8) at different distances from the dike (about 2.5 m thick) is 0.56–1.14% (Table 2). Based on the international coal classification standard, the eight coal samples are bituminous coals. With decreasing distance to the dike, the Ro increases from 0.56% (D7, 7 m from the dike) to 1.14% (D1, 0.2 m from the dike) (Figure 4). The ash yield gradually increases from 4.02% (D8, 10 m from the dike) to 36.6% (D1, 0.2 m from the dike). The VM content gradually decreases from 38.4% (D7, 7 m from the dike) to 17.5% (D1, 0.2 m from the dike) (Figure 4). The decrease of the VM implies the improvement of coal metamorphic degree, which has a good negative correlation with Ro (Figure 4). According to the variation of Ro, the thermal aureole of the dike is determined to be about 5 m.

Tpeak of coal seam in the Daxing Coal Mine can be estimated by Ro from formula 1, and the estimated Tpeak is listed in Table 2. The paleo temperature of No. 9 coal seam near the dike rises from 78 to 169 °C. Therefore, under the thermal evolution of igneous, the paleo temperature of the Daxing Coal Mine heat-affected coal seam increases from ∼78 to ∼169 °C on the whole.

Due to the different background coal rank before igneous intrusion, the influence of igneous intrusion on the coal was obviously different. Ro of coal samples approaching the intrusion of Telkwa coal seam increases from 0.79% (4.2 m from the intrusion) to 7% (0.05 m from the intrusion),10 which increases 6.21% by igneous intrusion. Ro of the coal samples approaching the intrusion of Raton basin increased from 0.9% (11.25 m from the intrusion) to 4.6% (0.75 m from the intrusion),16 which increased 3.7% by igneous intrusion. Ro of coal samples approaching the intrusion of the Huainan coalfield increased from 0.87% (4 m from the intrusion) to 5.16% (0.5 m from the intrusion),53 which increased 4.29% by igneous intrusion. The Ro of coal samples approaching the dike of Tiefa Basin increases from 0.57% (10 m from the intrusion) to 1.14% (0.2 m from the intrusion), which increases 0.57% by igneous intrusion. Although the magma in the Tiefa Basin belongs to the basic magma, the temperature of the basic igneous intrusion can reach ∼1000 °C. The impact of igneous intrusion on the coal samples of the Tiefa Basin is less than that of the coal samples with a higher background coal rank before intrusion. It is probably because the coal samples in the Tiefa Basin were bituminous coal before igneous intrusion. Due to the high moisture content of the bituminous coal, the moisture in the coal can absorb part of the heat of magma, and the heat used to heat coal was relatively reduced.

With decreasing distance to the dike, the moisture content of the coal samples gradually decreases from 6.4% (D7, 7 m from the dike) to 3.1% (D1, 0.2 m from the dike). This is mainly due to the heating of the high-temperature igneous intrusion near the dike, which leads to the gasification of moisture in the coal.

There is a competitive adsorption relationship between the methane and water molecules in the coal matrix. When the moisture decreases, the methane adsorption quantity will increase.44 Igneous intrusion reduces the moisture of coal in the thermal aureole, so methane replaced the adsorption site of the water molecules, which may theoretically improve the methane content and methane adsorption capacity of the heat-affected coal. In addition, the Tiefa Basin is rich in bituminous coal. The bituminous coal has the characteristics of high natural moisture.45,54 Igneous intrusion caused the sudden decrease of the moisture in bituminous coal, resulting in the vacancy of the water molecular adsorption site and the significant increase of methane adsorption capacity. In terms of methane adsorption capacity, high-rank coal is less affected by water compared with low-rank coal.49

4.2. Influence of Igneous Intrusion on Adsorption/Desorption Properties and Moisture Content

The test results of Langmuir volume (VL), Langmuir pressure (PL), and the porosity of coal samples are listed in Table 3.

Table 3. Porosity Characteristics and Methane Adsorption Capacity of Coal Samplesa.

sample Dis (m) VL (m3/t) PL (kPa) porosity (%)
D1 0.2 18.4 827.5 4.3
D2 0.5 23.6 1167.7 7.8
D3 1.0 28.7 977.4 9.4
D4 2.0 25.3 1822.2 8.8
D5 3.5 24.7 1117.3 7.1
D6 5.0 24.4 993.4 8.4
D7 7.0 22.7 999.0 7.7
D8 10.0 23.7 1117.3 na
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a

Dis = distance from sill/dike; VL = Langmuir volume; PL = Langmuir pressure; na = not analyzed.

With decreasing distance to the dike, the porosity of the coal samples increases from 7.7% (D7, 7.0 m from the dike) to 9.4% (D3, 1.0 m) and then decreases to 4.3% (D1, 0.2 m from the dike) (Table 3). The result shows that the porosity of most coal in the thermal aureole increased. The decrease of porosity was probably caused by the frequent cleat and fracture filling by mineral, as well as the strongly decreased mesopore and micropore volumes in the altered zone.33,55

The methane isothermal adsorption curve of the coal samples near the dike is shown in Figure 5, which conforms to the Langmuir Model.56,57 The isothermal adsorption curve increases rapidly in the low-pressure zone (<2000 kPa), and gradually slows down in the high-pressure zone (>4000 kPa) (Figure 5). With decreasing distance to the dike, the Langmuir volume of the coal sample first increases then decreases. The variation relationship is shown in Figure 6. With decreasing distance to the dike, VL initially increases from 22.7 m3/t (D7, 7.0 m from the dike) to 28.7 m3/t (D3, 1.0 m from the dike) and then gradually decreases to 18.4 m3/t (D1, 0.2 m from the dike) (Table 3).

Figure 5.

Figure 5

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Comparison of methane adsorption behavior and Langmuir fitting lines of six samples.

Figure 6.

Figure 6

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Langmuir volume of the coal seam near a dike in the Daxing Coal Mine.

The coal in the thermal aureoles had special fractures and micropores. The synergistic effect may improve the methane adsorption capacity, gas desorption, and permeability.17 Some hydroxyl groups with polar sites on the surface of the coal matrix are more easily occupied by water, which reduces the adsorption capacity of the coal matrix for CH4 and CO2. Therefore, the adsorption capacity of wet coal or low-rank coal is significantly lower than that of dry coal or high-rank coal.45,49,58

With the increase of Ro, the moisture in heat-affected coal decreases (Figure 7a). A negative linear correlation between the moisture content and Ro was represented. With the decrease of moisture, VL of the coal generally has an increasing trend, except sample D2 (Figure 7b). There is a linear positive correlation between VL and Ro in dry coal, and a parabolic (U shape) relationship between VL and Ro in moisture coals.45 However, the relationship between moisture and VL in Figure 7b is relatively discrete, which may be due to the same influence of other factors on VL. The ash (daf basis) increases dramatically toward the dike, ranging from 4.0 to 36.6% (D8–D1) (Table 2). With decreasing distance to the dike, the porosity first increases from 7.7% (D7, 7 m from the dike) to 9.4% (D3, 1 m from the dike), then decreases to 4.3% (D1, 0.2 m from the dike) (Table 3).

Figure 7.

Figure 7

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Relationships between moisture and Ro (a) and moisture and Langmuir volume (b) of heat-affected coal samples.

To investigate the effect of moisture content in the coal matrix on gas desorption characteristics, the gas desorption data of four groups of coal samples with different moisture contents (moisture is 2.35, 4.52, 6.63, and 8.64%, respectively) and one group of dry coal samples under the initial equilibrium pressure of 1.00 MPa were measured. After data processing, the 120 min desorption curves of coal samples under different moisture contents were obtained, as shown in Figure 8. The change law of the gas desorption curve of five groups of coal samples is that it tends to be stable after increasing to the limit desorption capacity, which is 5.00, 3.45, 2.31, 1.37, and 0.90 cm3/g, respectively. It is obvious that with the increase of the moisture content of the coal sample, the limit desorption capacity decreases. At the same time, the lower the moisture of coal samples, the faster the early gas desorption rate. Therefore, the coal with a low moisture content has the characteristics of fast gas desorption rate and large amount of gas desorption, resulting in a higher outburst risk than the coal seam with a high moisture content.

Figure 8.

Figure 8

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Gas desorption curves of coal samples with different moisture contents in Daxing Coal Mine.

4.3. Influence of Igneous Intrusion on the Moisture and Gas Content of Coal Seams

To investigate the influence of sill on gas emission from the coalface during the coal mining process, coal samples were taken in the return airway of coalface N1408, N1708, and N21203 at different depths in Daxing Coal Mine. Twenty coal samples were collected in each coalface with a total of 60 samples. These samples are used to determine the gas content (by direct methods) and moisture (by proximate analysis).

According to the results of borehole exposure, the samples of coalface N1408 and N1708 are within twice the thickness of the sill, and there is no igneous intrusion near the coalface N21203. The determination results of moisture, VM, and gas content of coal samples from three coalfaces are shown in Figure 9a (the data in the figure are from field measurement).

Figure 9.

Figure 9

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Moisture, volatility, and gas content of three coal seams with different buried depths (a); relationship between the gas content and moisture of the coal samples in the Daxing Coal Mine (b).

As can be seen from Figure 9a, the moisture content and VM at the sampling points of No. 4 coal seam and No. 7 coal seam in the upper layer are lower than that of No. 12 coal seam in the lower, and the gas content of No. 4 coal seam and No. 7 coal seam is higher than that of No. 12 coal seam. The smaller the VM, the greater the metamorphic degree, which indicates that the metamorphic degree of No. 4 coal seam and No. 7 coal seam was improved by thermal evolution. Figure 9b shows the relationship between gas content and moisture in coalface N1408, N1708, and N21203 in the Daxing coalfield. On the whole, with the decrease of moisture content, the gas content of all 60 measuring points in three coal seams significantly increases.

Generally speaking, the deeper the coal seam is buried, the higher the gas content and metamorphic degree are, and the smaller the water content is. The field data of the Tiefa coal mine do not conform to the above law (Figure 9a) because No. 4 coal seam and No. 7 coal seam are located in the upper and lower part of the sill, respectively. In geological history, igneous intrusion is often accompanied by secondary hydrocarbon generation, and the hydrocarbon generation quantity can reach 340 m3/t.50 Therefore, the gas content of thermal evolution coal seam is usually much higher than that of normal coal seam. At the same time, under the influence of the sill, their metamorphic degree, moisture, and pore properties changed, resulting in a stronger gas adsorption capacity than that of No. 12 coal seam. Jiang et al.28 found that the abnormally high formation pressure was created by the thermal evolution and trap effect of sill intrusion. Therefore, it can be assumed that the gas content of No. 7 coal seam is higher than that of No. 4 coal seam due to the trap effect of the sill.

Compared to the fitting lines of three coal seams, the fitting line slope of No. 4 coal seam and No. 7 coal seam is higher than that of No. 12 coal seam (Figure 9b). This sequence is completely consistent with the metamorphic degree sequence of the three coal seams. Therefore, it can be inferred that the higher the metamorphic degree, the more obvious the gas content of the coal seam affected by moisture.

4.4. Characteristics of Moisture and Methane Occurrence and Evolution after Igneous Intrusion

According to the brief burial history of Tiefa Basin (Figure 3), the evolution of CBM can be divided into two stages. At the end of Early Cretaceous, the first hydrocarbon generation (Ro = 0.41–0.45%) was experienced, and Fuxin Formation entered a mature stage. The coal seam was in the stage of uplift in Neogene, and the contact metamorphism of the Himalayan igneous intrusion resulted in the local coal rank elevation and the secondary hydrocarbon generation (Ro = 0.45–0.70%). The uplift stage from the end of Cretaceous to the present was the main stage for the generation of secondary biogenic gas.

Therefore, the CBM reservoir was formed by the first hydrocarbon generation of Yanshanian thermogenesis in Early Cretaceous,39 and the second hydrocarbon generation of the Himalayan igneous intrusion and continuous biological hydrocarbon generation were the main supplement. Due to the evolution of stratum sedimentation, the moisture and gas in coal seam were significantly affected. The variation of moisture in coal during the sedimentary evolution of coal strata in Daxing Coal Mine is presented in Figure 10.

Figure 10.

Figure 10

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Variation of moisture and gas content of bituminous coal during strata sedimentary evolution.

The moisture content in the coal seam increased rapidly during 140–100 Ma. In the period of 100–65 Ma, because of the rising geotemperature, the moisture content in the coal seam began to show an obvious downward trend, and the downward trend slowed down in the period of 65–55 Ma (Figure 10). From 55 to 40 Ma, due to the influence of contact metamorphism, high-temperature igneous intrusion led to a large amount of water to be gasified, and the moisture content of heat-affected coal was significantly lower than that of unaltered coal (Figure 10). Since 125 Ma, due to deep metamorphism, a large amount of gas has been gradually generated in the coal seam, and the amount of thermogenic methane generated reaches the maximum at 100 Ma (Figure 10). At 100–55 Ma, the coal seam was eroded due to the uplift of the stratum and exposed to the atmosphere, which led to a large amount of gas emission. Therefore, during this period, the gas content in the coal seam decreased sharply, and the gas content decreased to the lowest value at 55 Ma. During the 55–20 Ma period, the gas content in the coal seam gradually increased due to secondary hydrocarbon generation, and began to decrease slowly at 20 Ma (Figure 10).

During the middle period of 60–30 Ma, the igneous intrusion, and the temperature of the coal seam increased. In the thermal evolution area,28 the metamorphic degree of coal was improved and water was gasified. This allowed the free methane molecules to occupy some of the adsorption sites vacated by the water molecules (Figure 11), which increases the overall methane adsorption site on the coal matrix surface. Therefore, the methane adsorption capacity of the heat-affected coal increases (different from the deterioration of the adsorption capacity of the coal samples within 5 m of the intrusion reported by Mastaerz et al.). In addition, the tight roof sill and roof mudstone with good sealing can trap these gases, forming natural gas pockets. This demonstrates that after about 20 Ma, the gas content in the heat-affected coal seam was significantly higher than that of the unaltered coal seam.

Figure 11.

Figure 11

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Water and methane sorption in the open nanopore of bituminous coal after igneous thermal evolution.

5. Conclusions

In this work, the influence of igneous intrusion on bituminous coal in the Daxing Coal Mine has been systematically analyzed by means of proximate analysis, maceral and Ro determination, isothermal adsorption test, and on-site direct gas content measurement. The main results are as follows:

  • (1)

    Approaching the dike (thickness 2.5 m), Ro increased from 0.56 to 1.14% and the moisture content decreased from 6.4 to 3.1%. Under the thermal evolution of igneous intrusion, the paleo temperature of the Daxing Coal Mine heat-affected coal seam increases from ∼78 to ∼169 °C on the whole. Due to the high moisture content of bituminous coal, the coal rank elevation after igneous intrusion is lower than expected.

  • (2)

    The thermal evolution of igneous intrusion improves the porosity and adsorption capacity of coal as a whole. It is found that the lower moisture containing coal has faster gas desorption rate and larger gas desorption amount, resulting in its higher outburst risk than that of higher moisture containing coal.

  • (3)

    The gas content of No. 4 and 7 coal seams located at the upper and lower part of the sill is 6.55–10.70 and 7.16–11.79 m3/t, respectively, which is larger than 5.38–9.47 m3/t of No. 12 coal seam (the largest buried depth is not affected by sill). The moisture content of No. 4 and 7 coal seams affected by igneous intrusion is 1.92–3.79%, which is less than 3.59–5.95% of No. 12 coal seam. Combined with the VM analysis results, it is concluded that with the decrease of moisture content, the gas content significantly increases.

  • (4)

    Due to the rise of geotemperature, the moisture content in the coal seam decreased after 140 Ma, and igneous intrusion exacerbated this phenomenon. During this period, Tiefa Basin experienced two hydrocarbon generation processes, which increased the gas content. After the second hydrocarbon generation, the temperature of coal seam increased with the igneous intrusion, which gasified the water in the coal while improving the gas adsorption capacity of coal. A reasonable explanation is made for the phenomenon that the gas content of heat-affected coal seam is significantly higher than that of unaltered coal seam.

Acknowledgments

The authors are grateful to the National Natural Science Foundation of China (No. 51874298 and No. 52174217), the Qing Lan Project of Jiangsu province, and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

The authors declare no competing financial interest.

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