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Article

Structural Characteristics of the Turning End of the Kaiping Syncline and Its Influence on Coal Mine Gas

1
Key Laboratory of Coalbed Methane Resources and Reservoir Formation Process, Ministry of Education, China University of Mining and Technology, Xuzhou 221008, China
2
School of Resources and Geoscience, China University of Mining and Technology, Xuzhou 221116, China
*
Authors to whom correspondence should be addressed.
Appl. Sci. 2024, 14(24), 12035; https://doi.org/10.3390/app142412035
Submission received: 17 October 2024 / Revised: 21 December 2024 / Accepted: 22 December 2024 / Published: 23 December 2024
Figure 1
<p>Kaiping syncline structure diagram (<b>a</b>) and No.1 section diagram (<b>b</b>).</p> ">
Figure 2
<p>Tectonic outline map of the study area (<b>a</b>); grid division diagram (<b>b</b>); contour map of stratigraphic dip angle at the turning end of the Kaiping syncline (<b>c</b>); scatter plot of stratigraphic dip angle in each structural zone (<b>d</b>). Notes: <math display="inline"><semantics> <mrow> <mstyle scriptlevel="0" displaystyle="true"> <mfrac> <mrow> <mi mathvariant="normal">x</mi> <mo>~</mo> <mi mathvariant="normal">y</mi> </mrow> <mrow> <mi>z</mi> </mrow> </mfrac> </mstyle> </mrow> </semantics></math>, x: minimum value; y: maximum value; z: average value.</p> ">
Figure 3
<p>Ideal stratigraphic distribution model diagram (<b>a</b>) and strata dip angle calculation model diagram (<b>b</b>).</p> ">
Figure 4
<p>Gas content contour map of the Kaiping syncline turning end. (<b>a</b>) No.7 coal seam gas content contour map; (<b>b</b>) No.9 coal seam gas content contour map; (<b>c</b>) No.12 coal seam gas content contour map.</p> ">
Figure 5
<p>The absolute gas emission and relative gas emission line diagram of the mine. (<b>a</b>) Zhaogezhuang mining area; (<b>b</b>) Linxi mining area.</p> ">
Figure 6
<p>The contour map of gas emission at the turning end of the Kaiping syncline. (<b>a</b>) No. 7 coal seam gas emission contour map; (<b>b</b>) No. 9 coal seam gas emission contour map; (<b>c</b>) No. 12 coal seam gas emission contour map.</p> ">
Figure 7
<p>Burial–hydrocarbon history of the Kaiping oblique coal seam (according to Huang and Li, 2023, modification [<a href="#B26-applsci-14-12035" class="html-bibr">26</a>]). (<b>a</b>) Sedimentary and burial history diagram of the Kaiping syncline; (<b>b</b>) “Three Histories” configuration diagram of the Kaiping syncline. Notes: C: Carboniferous; P: Permian; T: Triassic; J: Jurassic; K: Cretaceous; E: Paleogene; N: Neogene.</p> ">
Figure 8
<p>Geologic sketch of coal mine gas in coal—endowed areas of North China (according to Wang et al., 2021, modification [<a href="#B31-applsci-14-12035" class="html-bibr">31</a>]).</p> ">
Figure 9
<p>Schematic diagram of gas distribution in the Kaiping Xiangxi mine.</p> ">
Figure 10
<p>Plane distribution map of gas content at the turning end of Kaiping syncline: (<b>a</b>) No. 7 coal seam gas content plane distribution map; (<b>b</b>) No. 9 coal seam gas content plane distribution map; (<b>c</b>) No. 12 coal seam gas content plane distribution map.</p> ">
Figure 11
<p>The relationship diagram of structure, buried depth, and gas content of No. 9 coal seam: (<b>a</b>) Zhaogezhuang mining area; (<b>b</b>) Linxi mining area.</p> ">
Versions Notes

Abstract

:
Frequent coal mine gas disasters pose significant threats to the safety of miners and the continuity of coal mining operations. Understanding and mastering the patterns of gas occurrence is the foundation for controlling gas outbursts. This study, drawing on previous theories, research, and practical coal mine production data, analyzes the structural characteristics of the Kaiping syncline, with particular emphasis on the structural differentiation at its northeastern uplifted end. The study examines how gas generation and storage are influenced by progressively layered structures and their effect on coal mine gas management. The results indicate that the Kaiping syncline has a NE-SW axial orientation, which gradually shifts to an asymmetric syncline with a nearly EW trend, rising towards the northeastern end. At the turning end, the strata on the northwest limb are steep—locally vertical or overturned—gradually transitioning into the gentler southeast limb with dips of 10° to 30°, further complicated by a series of sub-parallel secondary folds. The gas formation process in coal seams has undergone multiple stages, regulated by structural burial and thermal evolution. The current gas storage characteristics result from the combined effects of these structural factors. The Kaiping syncline can be divided into two gas zones: a high-gas zone in the northwest limb and a shallow low-gas zone paired with a deep high-gas zone in the southeast limb. At the turning end, structural differentiation results in significant variations and gradations in the gas storage conditions of the coal seam. This differentiation directly causes a transition from coal and gas outburst mines in the northwest limb to low-gas mines in the southeast limb, highlighting the significant influence of structural factors on gas generation, preservation, and mine gas emissions. This study integrates theoretical analysis with measured data to enhance the understanding of structural evolution and its influence on gas storage. It offers guidance for preventing coal seam gas disasters and ensuring the safe production of coal mines in the Kaiping coalfield.

1. Introduction

Coal is currently one of the most abundant, widely distributed, and economically viable energy resources worldwide. Given China’s energy resource endowment, coal is expected to remain the country’s primary energy source in the foreseeable future [1]. Gas, as a major byproduct of coal formation, is a geological entity primarily composed of methane and is widely stored in coal seams. While it is a potential clean energy source, it also represents one of the greatest hazards to coal mine safety [2,3,4,5,6]. The presence of gas reduces coal extraction efficiency, and its flammable and explosive properties pose a serious threat to coal mine safety [4,5,6,7,8].
The formation and storage of coal seam gas are controlled by the geological conditions of the study area. Its enrichment and reservoir characteristics are influenced by a combination of geological structure, coal body structure, coal seam thickness, surrounding rock porosity and permeability, magmatic intrusions, and hydrogeological conditions [4,5,6,9,10,11]. Geological structure is the primary factor controlling the storage and distribution of coal seam gas [8,12,13,14]. It not only influences the entire process of gas generation, migration, and enrichment, but also determines gas content, pressure, and emission rates in coal seams [4]. Therefore, in-depth research on the mechanisms by which geological structures control coal seam gas storage is crucial for ensuring safe and efficient coal mining operations.
Numerous scholars have conducted extensive research in the field of gas geology. However, due to the complexity and diversity of geological structures, as well as the unique characteristics of structural forms and origins, the influence of geological conditions on gas generation, migration, enrichment, and distribution varies across different regions [4,6,8,15]. Some scholars, through data collection and analysis of fold structures and gas distribution characteristics in various study areas, suggest that folds significantly influence gas distribution and outburst conditions, with syncline structures providing favorable conditions for gas accumulation and storage [4,6,7,16]. The controlling effect of faults on local gas content and outburst conditions in coal seams has also been confirmed by scholars. Some studies have shown that the open nature of faults facilitates gas dissipation and migration [4,17,18,19]. Therefore, analyzing the influence of geological structures and their evolutionary characteristics on coal and gas storage in specific areas, and uncovering the distribution patterns of coal and gas outbursts, as well as the controlling role of geological structures, is of great significance for preventing and mitigating coal and gas outbursts in mining areas.
This study focuses on the Zhaogezhuang and Linxi coal mines, located on either side of the turning end of the Kaiping syncline. Drawing on geological exploration data and coal mining practices, it examines the structural distribution and evolution on both sides of the turning end. It further analyzes the characteristics of gas content and emissions on both sides of the hinge and explores the mechanisms by which geological structures and their evolution control coal seam gas storage. This study contributes to enriching and advancing gas geology theories and guides, ensuring the safe and efficient production of coal mines in the Kaiping coalfield.

2. Geological Overview of the Study Area

The Kaiping coalfield is located in the Kailuan mining area of Hebei Province, China, and lies in the northeastern part of the North China Basin. The strata on the western limb are steeply inclined and locally overturned, while those on the southeastern limb are relatively gentle and characterized by more folds. The coal-bearing Paleozoic strata in and around the coalfield have undergone multiple phases of tectonic changes, experiencing various processes of geological burial and deformation. The current structural state is the result of the superposition of these tectonic phases, which can be divided into four stages of tectonic evolution: the initial stage (Hercynian-Indosinian period), the development stage (early to mid-Yanshanian period), the maturity stage (late Yanshanian to the first phase of the Himalayan period), and the final stage (second phase of the Himalayan period to the present) [20,21,22]. Several folds have developed in the coalfield from north to south, including the Nianzizhuang West Anticline, Gaogezhuang North Syncline, Xiaoqigongzhuang West Anticline, and Yangjiapu Anticline. The overall distribution of faults in the coalfield predominantly trends in the northeast direction, with a secondary trend in the northwest direction (Figure 1a). The study area includes two mining sites at the turning end of the Kaiping syncline, namely the Zhaogezhuang mine and the Linxi mine, with their locations shown in Figure 1a.
The Kaiping coalfield is part of the North China stratigraphic region, with widespread Paleozoic strata. The upper Carboniferous-Permian system forms the coal-bearing strata, with most formations in the coalfield in either unconformable or paraconformable contact with one another. While most of the coal-bearing strata are covered by Quaternary loess, scattered outcrops are found throughout the area. Based on drilling and stratigraphic profiles within the mining area, the strata, from oldest to youngest, include the Ordovician, Carboniferous, Permian, and Quaternary systems (Figure 1b).
The coal-bearing strata belong to the Carboniferous and Permian systems, with the coal measured underlain by Ordovician limestone (Figure 1b). The total thickness of the coal measures in the two mining fields is approximately 500 m, occurring within the Carboniferous and Permian systems, with a total coal seam thickness ranging from 20 to 28 m. The main minable coal seams are Nos. 5, 7, 8, 9, 11, and 12, which are located in the Lower Permian Damiaozhuang and Zhaogezhuang Formations, with a combined thickness of about 10.74 m.

3. Structural and Gas Distribution Characteristics of the Kaiping Syncline Turning End

3.1. Geological Structural Distribution Characteristics at the Turning End

The Kaiping syncline has a NE-SW axial orientation, transitioning to an asymmetric syncline near the EW direction and becoming uplifted at the northeastern end. The geology on the northern side of the turning end in the Zhaogezhuang mine area is more complex, with faulting as the dominant structural form. Moving northward from the hinge, the faults shift from being primarily normal faults to predominantly reverse faults. On the southern side of the turning end, the Linxi mine has been affected by multiple phases of tectonic activity, with folding as the dominant structural feature. Major folds in the area include the Dujunzhuang Anticline and Heiyazi Syncline, accompanied by normal faults and dike formations (Figure 2a).
The dip angle is a quantitative parameter that characterizes the structural features of a mining area and reflects the spatial orientation of strata. The magnitude of the dip angle can indicate the intensity of tectonic deformation. This study uses contour maps of the No. 7 coal seam floor to calculate dip angles in different areas of the study region in order to analyze and evaluate the structural distribution of the Kaiping syncline turning end.
A grid of 2 km × 2 km was superimposed on the contour map of the No. 7 coal seam floor (Figure 2b), and node data were extracted from the grid. Using an ideal dip angle calculation model, the dip values at the nodes were calculated (Figure 3) to reflect the extent of structural deformation and the degree of stratal inclination in the area. The dip angle at any given point (D) is calculated as follows:
D = a r c t a n D c l V d
In the formula, D is the dip angle value at a specific point on the coal seam floor, Dcl is the contour line difference of the coal seam floor, and Vd is the vertical distance between the contour lines of the coal seam floor.
Using the contour data of the No. 7 coal seam floor at the turning end, this method was applied to quantitatively calculate the dip angles at various points on the floor. The contour map data for the No. 7 coal seam floor indicate that the contour distribution in the study area is characterized by “north-south differentiation, with significant differences”. Structurally, deformation decreases from north to south, exhibiting a “stepped” change (Figure 3a). The calculation results show that the dip angles at the turning end range from 12.6° to 82.3°, with an average of 29.3°. A scatter plot of dip angles across different areas of the turning end indicates that the strata on the northwest limb are steep, locally upright, or overturned, with dip angles generally ranging from 60° to 85°. The strata gradually transition to gentler dips of 10° to 30° on the southeast limb, which are further complicated by a series of parallel secondary folds (Figure 2c).
Based on geological data from the Linxi and Zhaogezhuang mines at the turning end, as well as the degree of structural development and dip angles, the two mining areas at the Kaiping syncline turning end are divided into six structural zones: west wing fault structure area, wellhead open syncline area, Kaiping syncline structural zone, monoclinal tectonic zone, Dujunzhuang Anticline structure area, and Heiyazi Syncline structure area (Figure 2c).
The dip angles in the northwest limb fault zone range from 45° to 82.3°, with an average of 65.6°; in the wellhead wide syncline zone, they range from 21.3° to 36.5°, with an average of 28.2°; in the Kaiping syncline structural zone, from 12.6° to 21.3°, with an average of 15.8°; in the monocline structural zone, from 17.1° to 24.1°, with an average of 19.9°; in the Dujunzhuang Anticline zone, from 13.2° to 15.9°, with an average of 13.2°; and in the Heiyazi Syncline zone, from 26.3° to 27.2°, with an average of 26.4° (Figure 2d).
The northern limb of the turning end shows significant variation in dip angles, with a gradual decrease. The northwest limb fault zone exhibits the steepest dip angles, indicating that this area has experienced the greatest compressive stress. In contrast, the core of the turning end and the southern limb structural zones (Kaiping syncline, monocline, and Dujunzhuang Anticline) exhibit smaller variations in dip angles, indicating relatively lower compressive stress in these areas. Additionally, the distribution of dip angle contour lines exhibits “north-south differentiation”, which correlates well with the distribution of faults and fold axes in the study area, demonstrating a strong coupling relationship between dip angle variations and the development of faults and folds (Figure 2a,c).

3.2. Gas Content Distribution Characteristics at the Turning End

Due to significant differences in coal thickness, stratal dip angles, and hydrogeological conditions between the northern and southern limbs of the Kaiping syncline turning end, the Kaiping syncline structural zone can be further divided into Kaiping Syncline Structural Zone I (Zhaogezhuang mine), located on the northern limb, and Kaiping Syncline Structural Zone II (Linxi mine), located on the southern limb.
Based on the actual geological conditions and measured gas content data from the Zhaogezhuang mine, combined with gas analysis from the Linxi mine, the gas content shows a significant decreasing trend from Zone I to Zone II. Additionally, influenced by the turning end of the Kaiping syncline, the gas content shows a marked increasing trend from Kaiping Syncline Structural Zone I to the wellhead wide syncline zone and the northwest limb fault zone. Similarly, from Kaiping Syncline Structural Zone II to the monocline structural zone, the Dujunzhuang Anticline zone, and the Heiyazi Syncline zone, there is also an upward trend in gas content, although the increase is much smaller (Figure 4).
According to the gas content contour maps of the No. 7, No. 9, and No. 12 coal seams, the overall trend indicates that gas content increases with coal seam depth. Gas content is relatively lower near the axis of the Kaiping syncline, but increases with distance from the axis on both limbs. However, the rate of increase differs significantly between the two limbs, which is particularly evident in the main coal seams of the Linxi and Zhaogezhuang mines.
According to gas content statistics from the Zhaogezhuang and Linxi mines, the gas content in the No. 7 coal seam of Zhaogezhuang ranges from 4.0 to 8.0 m3/t, while in Linxi it ranges from 2.8 to 5.6 m3/t. In the No. 9 coal seam of Zhaogezhuang, the gas content ranges from 4.0 to 9.2 m3/t, whereas in Linxi, it ranges from 2.8 to 5.6 m3/t. In the No. 12 coal seam of Zhaogezhuang, the gas content ranges from 4.8 to 12.0 m3/t, while in Linxi it ranges from 3.2 to 6.6 m3/t. A comparison of gas content on both limbs of the turning end shows that the gas content in the coal seams of the Zhaogezhuang mine, located on the northwest limb of the Kaiping syncline, is higher than that in the Linxi mine on the southeast limb. Notably, the maximum gas content in the No. 12 coal seam of Zhaogezhuang is 1.8 times higher than that in Linxi.

3.3. Gas Emission Characteristics at the Turning End

The gas emission volumes on the northern and southern limbs of the Kaiping syncline turning end differ significantly. According to data from 2001 to 2014 on gas emissions in the Zhaogezhuang mine, the relative gas emission ranged from 5.68 m3/t to 12.63 m3/t, with an average relative gas emission of 7.41 m3/t. The absolute gas emission ranged from 19.19 m3/min to 35.1 m3/min, with an average absolute gas emission of 25.68 m3/min (Figure 5). According to gas emission data from the Linxi mine between 2012 and 2022, the absolute gas emission ranged from 9.272 m3/min to 14.864 m3/min, with an average of 12.25 m3/min. The relative gas emission ranged from 3.736 m3/t to 6.865 m3/t, with an average of 5.13 m3/t. Both the relative and absolute gas emissions in the Linxi mine were lower than those in the Zhaogezhuang mine.
To provide a clearer representation of the distribution patterns of gas emissions on both limbs of the turning end, data on gas emissions from the No. 7, No. 9, and No. 12 coal seams in the Zhaogezhuang mine were systematically collected. These data were then combined with gas emission data from the Linxi mine to create a contour map of gas emissions at the turning end for both mines (Figure 6). In the study area, the relative gas emissions from the No. 7, No. 9, and No. 12 coal seams generally increase with burial depth, consistent with trends in gas content. Gas emissions are relatively lower near the axis of the Kaiping syncline, but increase with distance from the axis on both limbs. A comparison of relative gas emissions on both limbs of the turning end shows that emissions in the Linxi mine are generally lower than those in the Zhaogezhuang mine. At the same burial depth, the relative gas emissions in Zhaogezhuang are typically higher than in Linxi. Among the coal seams, the relative gas emissions from the No. 12 seams are higher than those from the No. 7 and No. 9 seams. The relative emissions from the No. 7 and No. 9 seams are generally consistent, with only slight local variations, likely due to geological structural differences and other factors.

4. Structural Control Mechanisms on Coal and Gas Occurrence in Mines

4.1. Structural Evolution and Gas Generation

To further explain the relationship between the structural evolution of coal-bearing strata and coal seam gas generation and dissipation in different mining areas on both limbs of the Kaiping syncline, PetroMod 1D (2012.2) software was used to simulate and reconstruct the structural burial history, thermal evolution history, and hydrocarbon generation history of coal-bearing strata in different mines on both limbs of the Kaiping syncline. The burial history of coal-bearing strata in different mines on the two limbs of the Kaiping syncline is shown in Figure 7. This burial history was compiled based on lithological data, stratigraphic layering, and thickness from actual exploration drilling records in the mining area, as well as stratigraphic development and thickness data from the northern North China Basin. This history also accounts for the timing and extent of uplift and erosion during several key tectonic periods in the northern North China Basin [23,24,25].
The burial history curve of the Late Paleozoic coal seams in the Kaiping syncline forms a “V” shape, with a turning point in the Late Indosinian period. Combined with the hydrocarbon generation evolution of the coal seams, it shows that the Kaiping coalfield strata began to subside from the Late Carboniferous until the Middle Triassic, when the coal seams reached their maximum burial depth of approximately 3300 m. During this time, the coal seams were heated, generating large amounts of methane (first gas generation). Most of the methane dissipated into the surrounding rock, while some remained adsorbed in the coal seams. Subsequently, crustal uplift caused the previously adsorbed gas to dissipate. After the Triassic, tectonic activity intensified, gradually forming the Kaiping syncline. This was accompanied by regional magmatic activity, which reheated the coal seams, leading to secondary gas generation and replenishing the coal gas, primarily in adsorbed and free states. As the crust continued to uplift, the gas in the coal seams gradually dissipated, eventually reaching its present state (Figure 7).
Controlled by regional tectonic evolution, the coal-bearing strata underwent a sequence of structural burial, thermal evolution, and hydrocarbon generation processes. Subsequent preservation conditions determined the final characteristics of gas occurrence in the coal. By analyzing the stratigraphic composition of coal-bearing formations, the characteristics of coal seams and roof and floor rock layers, and the evolution of the “three histories”, the dynamic changes in coal seam gas generation, occurrence, and migration were quantitatively assessed. This enabled an assessment of the geological controls on gas accumulation, as well as an analysis of the entire process of gas generation, occurrence, accumulation, and dissipation in the main coal seams of the study area. Figure 7 illustrates the configuration of the “three histories” in the Kaiping syncline, divided into five stages.
The first stage, from the Late Carboniferous to the Early Triassic (initial stage of gas generation), involved the slow subsidence of the crust in the Kaiping coalfield during the Hercynian and Indonesian movements, forming stable coal-bearing strata. The coal seams and dark mudstone laid the foundation for coalbed methane, but hydrocarbon generation during this stage was weak. The second stage, during the Triassic (deep burial and gas enrichment stage), saw intensified subsidence due to the Indosinian movement. The coal-bearing strata underwent metamorphism, organic matter began to mature, and hydrocarbons were generated for the first time, increasing coal seam gas content. The third stage, from the Late Triassic to Early Jurassic (stagnation and gas dissipation stage), was marked by crustal fluctuations during the early Yanshan movement, causing variations in temperature and burial depth in the Kaiping coalfield. As temperatures dropped, hydrocarbon generation ceased, and the strata experienced uplift and erosion, leading to large-scale gas dissipation. The fourth stage, from the Middle Jurassic to Cretaceous (intense gas accumulation and reservoir formation stage), saw significant tectonic uplift and widespread magmatic activity during the middle to late Yanshan period. A high ancient geothermal field was formed, causing secondary hydrocarbon generation in the coal seams and leading to large-scale gas production and accumulation, particularly in the coal seam roofs and floors.

4.2. Stepwise Structural Control of Gas Occurrence

4.2.1. North China Craton’s Control on Regional Gas Generation, Migration, and Preservation

The evolution of the North China Plate and its interactions with surrounding plates influence the formation and deformation of coal-bearing basins. This, in turn, controls the formation and distribution of tectonic coal, thereby regulating the generation, migration, and preservation of coal mine gas [27,28,29,30,31]. The marginal zones of the coal-rich areas in North China experienced prolonged and intense plate collisions and compressions, leading to significant structural deformation in the coal-bearing strata. During this process, as compressive stress weakened from the plate margins toward the interior, the intensity of coal-bearing strata deformation also decreased. This created a stark contrast in structural deformation, with intense compressional deformation at the margins and more differential deformation toward the interior [30]. Previous research has shown that near the northern margin of the North China Basin, thrusting from the northern Hebei orogeny created a high-gas-pressure zone (gas outburst zone), and the study area is situated within this unique geological belt. In this region, the mechanisms of gas generation, migration, and storage have been profoundly influenced by the tectonic activity of the North China Plate. The compressional structures of the Yanshan orogen and the northern margin of the North China Plate jointly played a decisive role in the deformation of coal seams in the study area, thereby influencing the spatial distribution pattern of tectonic coal. The differential spatial distribution of tectonic coal and the associated tectonic stress field have led to a distinct belt-like pattern of regional gas distribution [31] (Figure 8).

4.2.2. Kaiping Syncline’s Control on Gas Occurrence in the Kailuan Mining Area

The Kaiping syncline is a large, complex, asymmetric coal-bearing syncline with a near-NE axial orientation, a steep northwest limb, and a gentle southeast limb. According to the principles of geological structural zoning, it is classified as a fourth-order structural unit nested within the framework of the North China–Korea paraplatform. The main structure of the Kaiping syncline plays a dominant role in determining the gas distribution characteristics within the Kailuan mining area. In the northwest limb of the Kaiping syncline, the strata exhibit steep, even overturned dip angles, with numerous vertical compressional and transpressional structures, creating a high-pressure environment favorable for gas accumulation, and resulting in generally high gas content. In contrast, the southeast limb has gentle dip angles, complicated by secondary folds and high-angle normal faults that cut across the strata. These extensional faults and their associated fracture networks have significantly damaged the roofs and floors of the coal seams, weakening sealing conditions and, consequently, affecting gas preservation.
From a macro perspective, the gas distribution pattern of the Kaiping syncline exhibits a clear “high in the west, low in the east” characteristic. The closed Majia’ao and Zhaogezhuang mines on the northwest limb are classified as coal and gas outburst mines, while the Tangshan mine is categorized as a high-gas mine (Figure 9). Together, these mines form a high-gas (or coal and gas outburst) zone on the northwest limb. On the southeast limb of the Kaiping syncline, the currently operating Linxi and Fangezhuang mines are classified as low-gas mines, creating a region with a relatively low gas content. The latest assessments indicate that gas content in the Lüjiatuo mine has risen to a high level, and the occurrence of gas dynamic phenomena in the −850 main roadway area of the Qianjiaying mine has led to its classification as a coal and gas outburst mine, further complicating the gas geological characteristics of the Kaiping syncline.

4.2.3. Structural Control of Gas Occurrence at the Turning End of the Kaiping Syncline

Based on the available measured data for predicting gas content distribution at the turning end, the gas occurrence zones are classified into low gas content zones (<4 m3/t), medium gas content zones (4–8 m3/t), and high gas content zones (>8 m3/t). According to the gas content distribution maps, high-gas zones in the No. 7, No. 9, and No. 12 coal seams are primarily located on the northwest limb of the Kaiping syncline in the southern part of the Zhaogezhuang mine. Only a tiny portion of the high-gas zones in the No. 12 seam are found on the southeast limb, near the core of the Kaiping syncline. Among the seams, the No. 12 coal seam has the largest high-gas zone, while the No. 7 seam has the smallest. The distribution of medium- and low-gas zones in the No. 9 and No. 12 coal seams follows a clear pattern, with these zones covering almost the entire southeast limb of the turning end. Moving toward the northwest limb, the coverage area decreases, with low-gas zones primarily located in shallow areas (Figure 10).
In terms of geological structure, the northwest limb of the turning end, where the Zhaogezhuang mine is located, is subjected to compressional stress from the northwest direction, forming several medium- to large-scale transpressional faults. The mine is predominantly characterized by fault development, with the western limb experiencing the most significant compression and the most developed reverse faults. The effect of this compression diminishes toward the east, where the eastern limb near the core of the Kaiping syncline has fewer faults. In the study area, the buffering effect of the syncline has prevented the northwest-directed tectonic stress field from impacting the Linxi mine. Fault development in the Linxi mine is limited, primarily occurring in the northern part of the mine. The northern compressional stress field, caused by the twisting of the Kaiping syncline’s axis, has led to the formation of a series of NWW-oriented folds within the Linxi mine. According to the collected and analyzed fault data (Table 1), the Zhaogezhuang mine predominantly features reverse faults, with a total of 13 reverse faults and 11 normal faults. In contrast, the Linxi mine primarily has normal faults, with 10 normal faults and 3 reverse faults.
In mining areas, the structural characteristics of the mining field are the primary control factor for gas occurrence. The sealing capacity of the geological structure directly influences gas occurrence, and different structural features (such as folds, faults, and igneous intrusions) lead to variations in gas occurrence (Figure 11). In the eastern limb of the Zhaogezhuang mine, structural development is less pronounced, the dip angles of the strata are smaller, and they are close to the axis of the Kaiping syncline. At the same burial depth, gas content is relatively low. In the wide syncline near the wellhead, where fault structures are densely developed, gas content and its variation gradient are high at the same burial depth. There is a lack of geological data for the western limb of the mine, but based on the structural stress field trend, tectonic activity in the west is more substantial than in the central and eastern parts, with many reverse faults. This impedes gas dissipation, leading to the hypothesis that gas content and pressure are both higher in the western region (Figure 11a). In the southern limb of the turning end, extensional faults are well developed in the northern part of the Linxi mine, near the end of the Kaiping syncline, which promotes gas dissipation, resulting in lower gas content. Moving south toward the monocline zone, Dujunzhuang Anticline zone, and Heiyazi Syncline zone, gas content tends to increase at the same burial depth. In the Heiyazi Syncline zone, where the coal seams are deeply buried and tectonic activity is intense in the syncline core, tectonic coal is well developed, leading to significantly higher gas content (Figure 11b).
Overall, the northwest limb of the turning end provides a favorable environment for gas occurrence, with gas being less likely to dissipate, resulting in higher gas content. In contrast, on the southeast limb of the turning end, factors such as smaller dip angles, more normal faults, thinner coal seams, and relatively shallower burial depths lead to more excellent gas dissipation, resulting in generally lower gas content. The structural differences and burial depths between the two limbs are the primary control factors influencing the variation in gas occurrence. Understanding the development characteristics of deep structures and the distribution of coal seams in the mine is crucial for comprehending gas occurrence patterns and guiding safe coal mining operations.

4.3. Structure-Controlled Gas Prevention and Mitigation Measures

Based on the structural development characteristics of the Kaiping syncline, coal seam distribution features, gas occurrence patterns, and controlling factors, gas outburst prevention in structurally developed areas must comply with the Regulations on the Prevention and Control of Coal and Gas Outbursts [32]. The principle of “regional comprehensive outburst prevention measures first, supplemented by local comprehensive outburst prevention measures”, must always be adhered to. In line with the requirements of “one strategy for one mine, one strategy for one face”, the goals of “drainage before construction, excavation, and mining, as well as achieving pre-drainage standards”, can be achieved. Outburst-prone coal seams must implement the two “four-in-one” comprehensive outburst prevention measures to ensure that multiple strategies are applied, safety is ensured, gas drainage is optimized, and the desired results are achieved. Failure to comply will strictly prohibit mining and excavation activities. The specific prevention and control measures are as follows:
(1)
Minimize the number of times that roadways expose (or intersect) outburst-prone coal seams. The locations for exposing (or intersecting) such coal seams should reasonably avoid geological structural zones, especially areas with compressive-shear faults and well-developed fractures.
(2)
For areas predicted to be free of outburst risk, regional prediction can be performed block by block using measured coal seam gas parameters, coal seam occurrence, and geological structures within the section.
(3)
Based on coal seam occurrence characteristics, geological structure conditions, outburst distribution patterns in the mined areas, and detection and prediction results of coal seam geological structures in the forecasted areas, the gas geological analysis method is applied to delineate outburst hazard zones. If the distribution of outburst points or locations with significant warning signs is directly associated with a structural belt, the extended position of this structure and the coal seam within a specific range on both sides are classified as outburst hazard zones. Otherwise, within the same geological unit, the coal seam extending 20 m (vertical depth) above and below the outburst point or location with significant warning signs is also classified as an outburst hazard zone.
(4)
Inspection and testing points in each study area should be located in regions with lower borehole density, wider borehole spacing, and shorter pre-drainage times. These points should be positioned as far as possible from pre-drainage gas boreholes or equidistant from surrounding boreholes, where feasible, to avoid the discharge range of excavation roadways and the pre-drainage advance distance of working faces. In geologically complex areas, the number of inspection and testing points should be appropriately increased, with special attention to closed faults (e.g., compressive or compressive-shear faults), which act as barriers, obstruct gas emissions, and seal against gas dissipation.
(5)
For every 10–50 m of working face advancement—or every 30 m in geologically complex areas or pre-drainage gas regions using non-directional drilling rigs—at least two regional verifications must be performed. Comprehensive records of engineering design, construction, and performance inspections must be meticulously preserved. Continuous regional verifications should be conducted in structurally damaged zones to analyze fault zone characteristics, including fault fillings, fracture development, and the properties of rock layers contacting the coal seam on the opposite fault block.
(6)
Before exposing outburst-prone coal seams in roadways, it is critical to determine the coal seam horizon, occurrence parameters, and geological structures, with special attention to areas where coal seam thickness varies. These areas are often locations where in situ stress varies and concentrates, resulting in elevated gas content and pressure, which makes the coal seam prone to gas outbursts or gas accumulation.
(7)
When the coal roadway heading face encounters geological structural damage zones or abrupt changes in coal seam occurrence conditions, and the original design measures cannot be applied, boreholes must be drilled to determine coal seam conditions. Gas should then be discharged through boreholes with diameters ranging from 42 to 50 mm.
(8)
Where technically feasible, an integrated surface and underground gas prevention and control approach should be implemented. Gas control primarily employs a long-term hydraulic fracturing extraction strategy through surface horizontal wells, supplemented by underground in-seam boreholes, to mitigate outburst hazards. During protective seam mining, gas extraction is conducted using surface mining-induced wells and surface L-shaped wells to drain pressure-relief gas from the protected seam. During the retreat of the working face, gas from adjacent seams is extracted through surface L-shaped wells and long underground directional boreholes.

5. Conclusions

In recent years, gas outburst accidents in structurally complex areas have become increasingly frequent. This study analyzes the geological characteristics of the present-day Kaiping syncline hinge zone, current gas content, and the generation and occurrence characteristics of gas to explore the controlling effects of regional and local structural patterns on coal seam gas occurrence. The main results are summarized as follows:
(1)
The structural distribution of the Kaiping syncline hinge zone shows pronounced north–south differentiation. The strata on the northwestern limb are steeply inclined, with locally vertical or overturned sections, and dip angles generally ranging from 60° to 85°. Conversely, the strata on the southeastern limb are relatively gentle, with dip angles ranging from 10° to 30°, further complicated by a series of subordinate folds trending in the same direction. Based on the geological data of the syncline hinge zone, the degree of structural development, and the dip angles of the strata, the area can be roughly categorized into six structural zones: west wing fault structure area, wellhead open syncline area, Kaiping syncline structural zone, monoclinal tectonic zone, Dujunzhuang Anticline structure area, and Heiyazi Syncline structure area. Additionally, the variation in the dip angles of the strata shows a strong correlation with the development of faults and folds within the study area.
(2)
The Kaiping syncline hinge zone is classified into Structural Zone I (Zhaogezhuang Mine) and Structural Zone II (Linxi Mine) based on variations in coal thickness, strata dip angles, and hydrogeological conditions. The study reveals that in Structural Zone I, the gas content ranges from 4.8 to 12.0 m3/t, the relative gas emission ranges from 5.68 to 12.63 m3/t, and the absolute gas emission ranges from 19.19 to 35.1 m3/min. In Structural Zone II, the gas content ranges from 2.8 to 6.6 m3/t, the relative gas emission ranges from 3.74 to 6.87 m3/t, and the absolute gas emission ranges from 9.27 to 14.86 m3/min. Overall, both gas content and gas emissions show a significant increase with coal seam depth. Gas content and gas emissions on the northern limb of the hinge zone are notably higher than those in the southern structural zone, suggesting that structural complexity plays a significant role in influencing gas distribution and emission.
(3)
The coal-bearing strata in the Kailuan mining area have undergone structural burial, thermal evolution, and hydrocarbon generation, with subsequent preservation conditions ultimately determining the characteristics of gas occurrence in the coal. The burial history of the coal seams on both limbs of the Kaiping syncline is approximately “V-shaped”, with a turning point in the Late Indosinian period. The coal seams in the study area have undergone multiple cycles of subsidence, hydrocarbon generation, and dissipation, resulting in varied current states of gas occurrence. Through quantitative analysis of the evolution of the “three histories” and coal seam characteristics, the complete process of gas generation, occurrence, and migration in the study area has been revealed. This process is divided into five stages: the initial generation stage, the deep burial and enrichment stage, the stagnation and dissipation stage, the intense accumulation and reservoir formation stage, and the dissipation and stabilization stage.
(4)
Gas occurrence in the study area is stepwise, controlled by geological structures. North China Craton governs regional gas generation, migration, and preservation, while the Kaiping syncline plays a dominant role in controlling gas occurrence in the Kailuan mining area. The structural differences and burial depths on the two limbs of the turning end are the main factors influencing the variation in gas occurrence between the two limbs.

Author Contributions

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

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Acknowledgments

We would like to thank the Key Laboratory of Coalbed Methane Resources and Reservoir Formation Process for all the support provided in this research. We are also very grateful to Bin Xing and Xiangjun Cai for their significant contributions to this research.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Figure 1. Kaiping syncline structure diagram (a) and No.1 section diagram (b).
Figure 1. Kaiping syncline structure diagram (a) and No.1 section diagram (b).
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Figure 2. Tectonic outline map of the study area (a); grid division diagram (b); contour map of stratigraphic dip angle at the turning end of the Kaiping syncline (c); scatter plot of stratigraphic dip angle in each structural zone (d). Notes: x ~ y z , x: minimum value; y: maximum value; z: average value.
Figure 2. Tectonic outline map of the study area (a); grid division diagram (b); contour map of stratigraphic dip angle at the turning end of the Kaiping syncline (c); scatter plot of stratigraphic dip angle in each structural zone (d). Notes: x ~ y z , x: minimum value; y: maximum value; z: average value.
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Figure 3. Ideal stratigraphic distribution model diagram (a) and strata dip angle calculation model diagram (b).
Figure 3. Ideal stratigraphic distribution model diagram (a) and strata dip angle calculation model diagram (b).
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Figure 4. Gas content contour map of the Kaiping syncline turning end. (a) No.7 coal seam gas content contour map; (b) No.9 coal seam gas content contour map; (c) No.12 coal seam gas content contour map.
Figure 4. Gas content contour map of the Kaiping syncline turning end. (a) No.7 coal seam gas content contour map; (b) No.9 coal seam gas content contour map; (c) No.12 coal seam gas content contour map.
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Figure 5. The absolute gas emission and relative gas emission line diagram of the mine. (a) Zhaogezhuang mining area; (b) Linxi mining area.
Figure 5. The absolute gas emission and relative gas emission line diagram of the mine. (a) Zhaogezhuang mining area; (b) Linxi mining area.
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Figure 6. The contour map of gas emission at the turning end of the Kaiping syncline. (a) No. 7 coal seam gas emission contour map; (b) No. 9 coal seam gas emission contour map; (c) No. 12 coal seam gas emission contour map.
Figure 6. The contour map of gas emission at the turning end of the Kaiping syncline. (a) No. 7 coal seam gas emission contour map; (b) No. 9 coal seam gas emission contour map; (c) No. 12 coal seam gas emission contour map.
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Figure 7. Burial–hydrocarbon history of the Kaiping oblique coal seam (according to Huang and Li, 2023, modification [26]). (a) Sedimentary and burial history diagram of the Kaiping syncline; (b) “Three Histories” configuration diagram of the Kaiping syncline. Notes: C: Carboniferous; P: Permian; T: Triassic; J: Jurassic; K: Cretaceous; E: Paleogene; N: Neogene.
Figure 7. Burial–hydrocarbon history of the Kaiping oblique coal seam (according to Huang and Li, 2023, modification [26]). (a) Sedimentary and burial history diagram of the Kaiping syncline; (b) “Three Histories” configuration diagram of the Kaiping syncline. Notes: C: Carboniferous; P: Permian; T: Triassic; J: Jurassic; K: Cretaceous; E: Paleogene; N: Neogene.
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Figure 8. Geologic sketch of coal mine gas in coal—endowed areas of North China (according to Wang et al., 2021, modification [31]).
Figure 8. Geologic sketch of coal mine gas in coal—endowed areas of North China (according to Wang et al., 2021, modification [31]).
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Figure 9. Schematic diagram of gas distribution in the Kaiping Xiangxi mine.
Figure 9. Schematic diagram of gas distribution in the Kaiping Xiangxi mine.
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Figure 10. Plane distribution map of gas content at the turning end of Kaiping syncline: (a) No. 7 coal seam gas content plane distribution map; (b) No. 9 coal seam gas content plane distribution map; (c) No. 12 coal seam gas content plane distribution map.
Figure 10. Plane distribution map of gas content at the turning end of Kaiping syncline: (a) No. 7 coal seam gas content plane distribution map; (b) No. 9 coal seam gas content plane distribution map; (c) No. 12 coal seam gas content plane distribution map.
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Figure 11. The relationship diagram of structure, buried depth, and gas content of No. 9 coal seam: (a) Zhaogezhuang mining area; (b) Linxi mining area.
Figure 11. The relationship diagram of structure, buried depth, and gas content of No. 9 coal seam: (a) Zhaogezhuang mining area; (b) Linxi mining area.
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Table 1. The number distribution and gas content parameters of large faults on the two wings of the turning end of the horizontal syncline.
Table 1. The number distribution and gas content parameters of large faults on the two wings of the turning end of the horizontal syncline.
MineReverse Fault
(Article)
Normal Fault
(Article)
The Proportion of Reverse FaultsThe Proportion of Normal Faults
Zhaogezhuang Mine131154.17%45.83%
Linxi Mine31023.08%76.92%
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Chen, Z.; Zhu, Y.; Zhang, H.; Li, J. Structural Characteristics of the Turning End of the Kaiping Syncline and Its Influence on Coal Mine Gas. Appl. Sci. 2024, 14, 12035. https://doi.org/10.3390/app142412035

AMA Style

Chen Z, Zhu Y, Zhang H, Li J. Structural Characteristics of the Turning End of the Kaiping Syncline and Its Influence on Coal Mine Gas. Applied Sciences. 2024; 14(24):12035. https://doi.org/10.3390/app142412035

Chicago/Turabian Style

Chen, Zhenning, Yanming Zhu, Hanyu Zhang, and Jin Li. 2024. "Structural Characteristics of the Turning End of the Kaiping Syncline and Its Influence on Coal Mine Gas" Applied Sciences 14, no. 24: 12035. https://doi.org/10.3390/app142412035

APA Style

Chen, Z., Zhu, Y., Zhang, H., & Li, J. (2024). Structural Characteristics of the Turning End of the Kaiping Syncline and Its Influence on Coal Mine Gas. Applied Sciences, 14(24), 12035. https://doi.org/10.3390/app142412035

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