Geochemistry Characteristics and Coal-Forming Environments of Carboniferous–Permian Coal: An Example from the Zhaokai Mine, Ningwu Coalfield, Northern China

Abstract

In order to study the geochemical characteristics of coal in the Ningwu Coalfield of Shanxi Province and the coal-forming environments reflected by it, a detailed geochemical study was carried out on the No. 5 coal of the Zhaokai Mine. The results show that the content of major-element oxides SiO₂ and Al₂O₃ is high. The trace elements Ni, Nb, Mo, Cd, Sn, Hf, Ta, W, Th, and U are slightly enriched, while the elements Li and Zr are enriched, indicating an overall LREY enrichment type in the samples. Elemental parameters suggest that the sedimentary environment in the study area is continental sedimentary, and the whole environment is reductive. The macerals in the coal samples are mainly vitrinite, with an average vitrinite reflectance (R_o) of 0.744%. The distribution range of n-alkanes in the coal samples is from n-C₁₄~n-C₃₂, with the main peak carbons being n-C₂₄ and n-C₂₅, showing the post-single-peak type distribution pattern. The average odd–even predominance index (OEP) is 0.40, the average of the light and heavy hydrocarbons ratio (${\displaystyle \sum {\mathrm{C}}_{21}^{-}}/{\displaystyle \sum {\mathrm{C}}_{22}^{+}}$) is 0.42, and the average of Pr/n-C₁₇ and Ph/n-C₁₈ are 1.08 and 0.23, respectively. The coal samples also contain various aromatic hydrocarbons, mainly from the naphthalene- and phenanthrene-series compounds. Biomarker parameters indicate that the parent material of the coal samples in the study area is mainly continental higher plants. The maturity is low, and the coal-forming environment is a reduction environment. This study of the No. 5 coal’s geochemical characteristics has laid a foundation for the efficient, green, and comprehensive exploitation of coal resources in this region, and has also provided an important basis for the sustainable development of coal resources.

1. Introduction

As China’s economy continues to grow, the contradiction between energy supply and demand has become more pronounced. We must pay attention to improving the utilization and recovery rate of resources and to promoting the sustainable development of energy development and utilization. Therefore, strengthening resource management and geological exploration and improving the resource recovery rate play important roles in promoting the sustainable development of energy resources.

Coal is the most important fossil fuel in China. As a major province in terms of coal resources, Shanxi’s coal production accounts for about 25% of the country’s total coal production [1]. The Ningwu Coalfield is a significant coal generation base in China and holds vast coal reserves. The Carboniferous–Permian period is globally recognized as the most crucial period for coal accumulation, leading to the formation of abundant coal resources in North China. The organic geochemical characteristics of coal can effectively reflect the source of coal-forming materials, the coal-forming climate, the sedimentary environment, and the maturity of the organic matter. For example, the petrological characteristics of coal can reflect the coal-forming swamp environment [2], and the saturated hydrocarbons and biomarkers can reflect the source, thermal evolution, and degradation of the organic matter [3,4]. The polycyclic aromatic hydrocarbons (PAHs) in coal can reflect the thermal maturity and paleoclimate information of coal-forming [5]. In recent years, many experts and scholars have conducted studies on the Carboniferous–Permian coal in the Ningwu Coalfield [6,7,8], primarily focusing on the distribution [9], occurrence state [10], and enrichment mechanism [11,12,13] of trace elements in coal, while the study of the organic geochemical characteristics has been relatively limited.

This research showcases the elemental geochemical traits and organic geochemical features of the Zhaokai Mine located in the Xuangang Mine Area within the Ningwu Coalfield, Shanxi Province. We aim to address the geological significance of the coal-forming material source, the coal-forming environment, and organic matter maturity. This study provides a valuable reference for further research on the paleoclimate and paleoenvironment of the Carboniferous–Permian coal-forming period in North China. It also provides a reference for the recycling and environmental protection of the strategic resources of key trace elements in coal.

2. Geological Setting

The Ningwu Coalfield is located in the north-central part of Shanxi Province, extending from Ningwu to Jingle. It spans about 160 km in length, is 20 km wide, and covers an area of 3000 km² (Figure 1). The primary coal-bearing strata in this region consist of the Jurassic Datong Formation, the Carboniferous–Permian Taiyuan Formation, and the Permian Shanxi Formation. The coalfield is segmented into four mining areas: Pingshuo, Shuonan, Xuangang, and Lanxian, extending from the northern to the southern part. The Zhaokai Mine, which is the central subject of this investigation, is situated within the Xuangang Mining Area in the Ningwu Coalfield. The coal resources in this area are abundant and contain trace elements. Studying the geochemical characteristics of coal is crucial to understanding and optimizing its utilization.

The Zhaokai Mine is situated in the north-central region of the Ningwu Basin, an area characterized by a multifaceted geological history and intricate structural composition. According to surface and borehole data, the stratigraphic sequence in the study area encompasses, in ascending stratigraphic order, the Upper Majiagou Formation from the Middle Ordovician (O₂s), the Benxi Formation from the Middle Carboniferous (C₂b), the Taiyuan Formation from the Upper Carboniferous (C₃t), the Shanxi Formation from the Lower Permian (P₁s), the Lower Shihezi Formation from the Lower Permian (P₁x), and the Upper Pleistocene from the Quaternary (Q₃). The primary coal-bearing strata in this mine are the Upper Carboniferous Taiyuan Formation (C₃t), with the Lower Permian Shanxi Formation (P₁s) being secondary. The Taiyuan Formation ranges in thickness from 83.76 to 106.90 m, averaging 91.85 m, and contains five coal seams: No. 2, No. 3, No. 4, No. 5, and No. 6 coal. The No. 5 coal seam has an average thickness of 11.24 m.

3. Samples and Analytical Procedures

A total of eight specimens from the No. 5 coal seam of the Taiyuan Formation were gathered from the Zhaokai Mine. According to the Chinese GB/T 482-2008 standard [15], the specified thickness is taken separately from each natural stratification of coal and gangue in the mining face. The samples, designated CK501 through CK508 from top to bottom, included a rock sample labeled as CK507 (Figure 2). To prevent contamination and oxidation, the samples were promptly sealed in bags. The initial samples were allowed to dry naturally, after which, they were split into two portions: one for geochemical analysis and the other was reserved for future use.

Samples with particle sizes in the range of 18~40 mesh were processed into polished blocks. In accordance with the Chinese Standard Method GB/T 8899-2013 [16], these polished sections were examined under oil-immersion reflected light for coal petrography. The primary macerals in the coal were identified and quantified. Random vitrinite reflectance (R_o, _ran) was measured using a Leica DM2500P reflected-light microscope fitted with a halogen lamp (calibrated with a standard of yttrium–aluminium–garnet; Ro = 0.89%). For proximate analysis, the ash content, volatile matter, and moisture (for 200-mesh samples) were assessed following ASTM D3174-12, ASTM D3175-17, and ASTM D3173/D3173M-17a [17,18,19], respectively. The total sulfur content (for 200-mesh samples) was determined as per the International Standards ASTM D3177-02 [20]. Major-element oxide concentrations were analyzed via X-ray fluorescence spectrometry (ARL9800 XRF, Thermo Fisher Scientific, Waltham, MA, USA) after the samples were ashed at 815 °C in a Muffle furnace. Trace element concentrations in the 200-mesh samples were quantified using inductively coupled plasma mass spectrometry (ICP-MS, Thermo Fisher Scientific, Waltham, MA, USA). Prior to the ICP-MS analysis, the fine powder samples were microwave-digested in a mixed acid. For coal and ash samples, 50 mg samples were microwave-digested following standard procedures [21,22].

For organic geochemistry analysis, the filter paper was extracted with dichloromethane for 48 h. The silica gel was activated at 140 °C for 4 h in a drying oven, and the alumina was activated at 450 °C for 4 h in a muffle furnace. A number of several 1 cm² copper sheets were immersed in hydrochloric acid to remove rust and impurities. A 10 g (±0.001 g) 200-mesh sample was weighed on a balance, wrapped in extracted filter paper and placed in an extractor. Before extraction, 3~4 pieces of copper were soaked in hydrochloric acid in advance into a flat-bottom flask to remove sulfur. We connected the extractor to a round-bottom flask, added dichloromethane, siphoned the solution twice, and then stored the solution in a thermostatic water bath, extracting at 45 °C, for 48 h. The solution in the round-bottom flask after extraction was filtered into a chicken heart flask. The solution was evaporated to 1~2 mL using a rotary evaporator and then completely transferred, a few times, to a 4 mL cell flask with dichloromethane. The cell bottles were placed in the ventilated cupboard to dry until a constant weight was achieved.

The appropriate amount of extract was weighed in the chicken heart flask, followed by the addition of a small amount of dichloromethane to dissolve it. Approximately 25 mg of activated silica gel was then added. The silica gel was evaporated into a quicksand shape using a rotary evaporator (45 °C). After rinsing the chromatographic column with dichloromethane and n-hexane, 3 g of activated alumina and 10 g of activated silica gel were added sequentially. The chromatographic column was continuously tapped with a washing ear ball to prevent adhesion to the inner wall.

The silica gel was washed with n-hexane, and the mixture of extract and silica gel was then washed with 40 mL n-hexane multiple times. The chicken heart flask was positioned at the bottom of the chromatographic column to collect saturated hydrocarbons. When the liquid level of n-hexane was approximately 2 cm away from the silica gel, 40 mL of a mixture of n-hexane and dichloromethane (in a ratio 3:1) was added, and the chicken heart flask was replaced. A colored ring would appear at this point. Once the ring reduced to about 2 cm from the bottom, 40 mL anhydrous methanol was added, resulting in a darker ring appearing. When this ring was reduced to about 2 cm from the bottom, another chicken heart flask was replaced. Each chicken heart flask containing a component was labeled and evaporated to 1~2 mL using a rotary evaporator. The chicken heart flasks were washed several times with dichloromethane and n-hexane, and the organic matter was completely transferred to 2 mL cell flasks. These cell flasks were placed in the fume hood to attain a constant weight, and the weight of each component was weighed.

After a constant weight was attained, each component was diluted, supplemented with a squalane, sealed, and stored in a refrigerator. The separated components were analyzed and tested using gas chromatography (Agilent 7890B) and gas chromatography–mass spectrometry (Agilent GC 7890B-5977A). Agilent Technology Co., Ltd. (Beijing, China). Agilent MassHunter Qualitative Analysis software (B.07.00) was used to process the data. The retention time in total ion current (TIC) chromatography was compared with the mass spectrum in the NIST02 spectrum library of the United States, as well as the existing related literature, to complete the identification of the material components of the chromatographic peak [23,24].

4. Results and Discussion

4.1. Coal Chemistry

Table 1. Proximate analyses (%), total sulfur (%), and vitrinite reflectance (%) of the No. 5 coal seam in the Zhaokai Mine.

Sample	Mad	Ad	Vdaf	St,d	Ro,ran
CK501	3.42	14.80	30.35	3.28	0.793
CK502	2.73	9.40	28.36	2.35	0.712
CK503	2.63	10.73	27.56	3.12	0.728
CK504	2.64	14.93	25.37	2.18	0.724
CK505	2.32	13.63	30.71	3.06	0.728
CK506	2.43	10.36	31.14	2.43	0.707
CK507	2.40	51.91	20.21	3.57	n.a.
CK508	2.15	45.76	19.42	2.78	0.818
Average	2.62	17.09	27.56	2.74	0.744

M, moisture; A, ash content; V, volatile matter; St, total sulfur; R_o, random reflectance of vitrinite; ad, as-received basis; d, dry basis; daf, dry and ash-free basis; n.a., not analyzed.

provides the proximate analysis, total sulfur content, and vitrinite reflectance for all the examined samples. The moisture levels varied between 2.15% and 3.42%, averaging 2.62%. Ash yield fluctuated from 9.40% to 45.76%, with a mean of 17.09%. Based on the Chinese National Standards [25,26], the No. 5 coal seam is classified as having low moisture and ash content. The total sulfur content ranged from 2.18% to 3.57%, with an average of 2.74%, and is categorized as medium-to-high-sulfur coal according to GB/T 15224.2-2010 [27]. Volatile matter content spanned from 19.42% to 31.14%, with an average of 27.56%. The mean vitrinite reflectance of 0.744% suggests that it is a medium bituminous coal, as defined by ASTM Standard D388-12 [28].

4.2. Characteristics of Macerals and Coal Facies

4.2.1. Characteristics of Macerals

Table 2. Maceral composition of the No. 5 coal seam in the Zhaokai Mine.

Sample		CK501	CK502	CK503	CK504	CK505	CK506	CK508
Vitrinite	Telinite	0	0.36	2.96	0	3.73	0.65	0
	Telocollinite	1.28	2.73	2.3	1.36	1.02	2.19	1.25
	Desmocollinite	63.08	65.71	61.07	71.63	62.06	58.64	62.49
	Corpocollinite	4.16	2.12	4.43	3.28	0	0	1.68
	Gelocollinite	1.03	2.43	1.89	1.67	3.03	3.59	2.87
	Vitrodetrinite	2.12	1.7	2.61	2.48	3.57	7.52	6.15
Inertinite	Fusinite	5.06	7.83	5.34	6.61	6.62	7.93	4.63
	Semifusinite	6.32	5.59	4.78	3.53	5.61	6.38	3.03
	Sclerotinite	0	0.54	0.58	0	0	0.27	0
	Macrinite	0.23	0.73	0.49	0.94	0.32	0.87	0.68
	Micrinite	2.55	2.43	1.04	2.67	1.29	3.25	2.07
	Inertodetrinite	5.01	4.14	6.62	2.21	4.9	4.71	5.78
Liptinite	Cutinite	0.96	0.17	0	0.09	0.67	0	0
	Sporinite	2.52	1.43	1.26	2.78	3.34	1.92	3.74
	Barkinite	0	0.51	0.28	0.06	0.74	1.07	0.28
Mineral	Clay mineral	3.31	1.58	1.94	0.69	2.14	0.95	5.12
	Pyrite	2.37	0	0.89	0	0.96	0.06	0.23
	Calcite	0	0	1.52	0	0	0	0

presents the quantitative statistics of maceral components, including mineral components, and calculates each parameter. The vitrinite content was the highest (ranging from 71.67% to 80.42%, with a mean of 74.69%), followed by inertinite (ranging from 15.96% to 23.41%, with a mean 19.08%) and liptinite (ranging from 1.54% to 4.75%, with a mean 3.12%). Minerals accounted for 0.69% to 5.68%, with an average of 3.11%.

4.2.2. Characteristics of Coal Facies

The microstructure of coal is a crucial indicator for identifying coal facies, which plays a significant role in determining the coal-forming environment and the plant species involved [29]. In this study, parameters such as the gelation index (GI), plant structure preservation index (TPI), groundwater flow index (GWI), vegetation index (VI), the value of V/I and F/M were utilized as the main indicators to investigate the sedimentary environment during the coal-forming period in this area.

Among them, the following equations apply.

$$GI={\displaystyle \frac{\mathrm{v}\mathrm{i}\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}+\mathrm{m}\mathrm{a}\mathrm{c}\mathrm{r}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}}{\mathrm{s}\mathrm{e}\mathrm{m}\mathrm{i}\mathrm{f}\mathrm{u}\mathrm{s}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}+\mathrm{f}\mathrm{u}\mathrm{s}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}+\mathrm{i}\mathrm{n}\mathrm{e}\mathrm{r}\mathrm{t}\mathrm{o}\mathrm{d}\mathrm{e}\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}}}$$

(1)

$$TPI={\displaystyle \frac{\mathrm{t}\mathrm{e}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}+\mathrm{t}\mathrm{e}\mathrm{l}\mathrm{o}\mathrm{c}\mathrm{o}\mathrm{l}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}+\mathrm{s}\mathrm{e}\mathrm{m}\mathrm{i}\mathrm{f}\mathrm{u}\mathrm{s}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}+\mathrm{f}\mathrm{u}\mathrm{s}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}}{\mathrm{d}\mathrm{e}\mathrm{s}\mathrm{m}\mathrm{o}\mathrm{c}\mathrm{o}\mathrm{l}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}+\mathrm{m}\mathrm{a}\mathrm{c}\mathrm{r}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}+\mathrm{i}\mathrm{n}\mathrm{e}\mathrm{r}\mathrm{t}\mathrm{o}\mathrm{d}\mathrm{e}\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}}}$$

(2)

$$GWI={\displaystyle \frac{\mathrm{g}\mathrm{e}\mathrm{l}\mathrm{o}\mathrm{c}\mathrm{o}\mathrm{l}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}+\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{p}\mathrm{o}\mathrm{c}\mathrm{o}\mathrm{l}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}+\mathrm{c}\mathrm{l}\mathrm{a}\mathrm{y}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{e}\mathrm{r}\mathrm{a}\mathrm{l}+\mathrm{v}\mathrm{i}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{e}\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}}{\mathrm{t}\mathrm{e}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}+\mathrm{t}\mathrm{e}\mathrm{l}\mathrm{o}\mathrm{c}\mathrm{o}\mathrm{l}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}+\mathrm{d}\mathrm{e}\mathrm{s}\mathrm{m}\mathrm{o}\mathrm{c}\mathrm{o}\mathrm{l}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}}}$$

(3)

$$VI={\displaystyle \frac{\mathrm{t}\mathrm{e}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}+\mathrm{t}\mathrm{e}\mathrm{l}\mathrm{o}\mathrm{c}\mathrm{o}\mathrm{l}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}+\mathrm{f}\mathrm{u}\mathrm{s}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}+\mathrm{s}\mathrm{c}\mathrm{l}\mathrm{e}\mathrm{r}\mathrm{o}\mathrm{t}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}}{\mathrm{d}\mathrm{e}\mathrm{s}\mathrm{m}\mathrm{o}\mathrm{c}\mathrm{o}\mathrm{l}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}+\mathrm{i}\mathrm{n}\mathrm{e}\mathrm{r}\mathrm{t}\mathrm{o}\mathrm{d}\mathrm{e}\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}+\mathrm{v}\mathrm{i}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{e}\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}+\mathrm{c}\mathrm{u}\mathrm{t}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}}}$$

(4)

$${\displaystyle \frac{V}{I}}={\displaystyle \frac{\mathrm{v}\mathrm{i}\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}}{\mathrm{i}\mathrm{n}\mathrm{e}\mathrm{r}\mathrm{t}\mathrm{o}\mathrm{d}\mathrm{e}\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}}}$$

(5)

$${\displaystyle \frac{F}{M}}={\displaystyle \frac{\mathrm{t}\mathrm{e}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}+\mathrm{t}\mathrm{e}\mathrm{l}\mathrm{o}\mathrm{c}\mathrm{o}\mathrm{l}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}+\mathrm{f}\mathrm{u}\mathrm{s}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}+\mathrm{s}\mathrm{e}\mathrm{m}\mathrm{i}\mathrm{f}\mathrm{u}\mathrm{s}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}}{\mathrm{d}\mathrm{e}\mathrm{s}\mathrm{m}\mathrm{o}\mathrm{c}\mathrm{o}\mathrm{l}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}+\mathrm{m}\mathrm{a}\mathrm{c}\mathrm{r}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}+\mathrm{i}\mathrm{n}\mathrm{e}\mathrm{r}\mathrm{t}\mathrm{o}\mathrm{d}\mathrm{e}\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}+\mathrm{v}\mathrm{i}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{e}\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}}}$$

(6)

Figure 3 illustrates the coal facies diagram for No. 5 coal in the study region. The moisture level of the peat swamp during the coal-forming process can be quantified using the gelation index (GI). A high GI signifies a wet, deeply submerged peat swamp, whereas a low GI indicates a drier, less-submerged environment [30]. The structural preservation index (TPI) reflects the extent of plant tissue degradation and the preservation of plant cell structures. A TPI below 1 suggests significant degradation and poor cell structure preservation, while a TPI above 1 indicates minimal decomposition and well-preserved cell structures. In the study area, the TPI values for No. 5 coal samples are low, ranging from 0.13 to 0.27, indicating the extensive degradation and poor preservation of plant cell structures. According to the GI-TPI coal phase diagram (Figure 3A), the samples are classified as originating from a low-peat swamp, suggesting a humid environment with shallow water coverage.

In the coal-forming period, the groundwater flow index (GWI) is frequently employed to indicate the impact of subsurface water on the peat swamp and its mineral composition. A GWI of less than 1 suggests weak groundwater activity, while a GWI of greater than 1 indicates strong groundwater activity. The vegetation index (VI) reflects the different plant species. A VI of less than 1 suggests that coal-forming plants are more closely related to herbs or aquatic plants, while a VI of greater than 1 suggests that coal-forming plants have an affinity with woody plants [31]. The No. 5 coal samples in the study area fall within the humid swamp, and the results are consistent with the GI-TPI coal phase diagram (Figure 3B).

The level of oxidation in peat swamps is strongly linked to the extent of water coverage. In environments with limited oxygen, conditions favor the creation of vitrinite, whereas in settings with ample oxygen, inertinite formation is more likely. Consequently, the V/I ratio serves as an indicator of both the water coverage and the climatic conditions prevailing during the coalification process. When V/I is greater than 4, it is a wet forest swamp; when 4 > V/I > 1, it is a deep-water forest swamp; when 1 > V/I > 0.25, it is a shallow-water dry forest swamp; when V/I < 0.25, it is a dry forest swamp. During coal-forming, the skeleton components of plants are difficult to decompose or cement, while the matrix components are easily decomposed and broken down. The mobility of water affects the destruction of plants. The stronger the fluidity of the water, the more oxygen it carries, and the higher the degree of decomposition and destruction of the plant. Therefore, the coal-forming plant type can be studied by the ratio of the content of the skeleton component of the plant to the matrix component (including debris) (F/M). The smaller the F/M, the higher the degree of decomposition and destruction of plants, and the stronger the hydrodynamic effect of the swamp. In this study area, the F/M of No. 5 coal samples is less than 1, indicating strong hydrodynamic conditions during the coal-forming period; while alternating, it indicates changes in the depth of the overlying water, primarily in the shallow-water swamp (Figure 3C).

4.3. Geochemistry

4.3.1. Major-Element Oxides

Table 3. The percentages of major element oxides (%) of the No. 5 coal samples from the Zhaokai Mine.

Sample	Al2O3	SiO2	Fe2O3	CaO	MgO	TiO2	P2O5	K2O	Na2O	MnO
CK501	40.51	38.12	14.71	1.95	1.03	0.67	0.327	0.218	0.0704	0.0071
CK502	52.84	37.63	2.10	2.18	1.06	0.77	0.464	0.111	0.0414	0.0032
CK503	52.46	35.60	3.36	2.48	1.19	1.11	0.247	0.299	0.0582	0.0120
CK504	59.35	23.71	1.51	1.47	0.90	9.85	0.223	0.260	0.0545	0.0024
CK505	54.01	36.86	1.13	2.00	1.10	1.55	0.385	0.247	0.0531	0.0025
CK506	57.29	32.33	1.45	2.33	1.43	1.24	0.245	0.324	0.0531	0.0029
CK507	41.87	45.00	9.55	0.09	0.48	1.66	0.083	0.619	0.0266	0.0216
CK508	47.51	44.67	3.71	0.20	0.43	1.92	0.595	0.175	0.0608	0.0011
Average	50.73	36.74	4.69	1.59	0.95	2.35	0.321	0.282	0.0523	0.0066
China	5.98	8.47	4.85	1.23	0.22	0.33	0.09	0.19	0.16	0.02

displays the concentrations of major-element oxides in No. 5 coal, contrasting them with the average values for typical Chinese coals [32]. The No. 5 coal exhibits elevated levels of Al₂O₃, SiO₂, CaO, MgO, TiO₂, P₂O₅, and K₂O. Notably, the amounts of Al₂O₃, TiO₂, SiO₂, MgO, and P₂O₅ are 8.48, 7.12, 4.34, 4.32, and 3.57 times the Chinese value, respectively. The content of Fe₂O₃ is equivalent to the Chinese value.

The coal-forming environment and sedimentary medium will affect their content and composition of major elements in coal. Therefore, the ash composition parameters of coal are often determined by calculating the content and composition characteristics of major-elements in coal to assess the coal-forming environment.

The value of (Fe₂O₃ + MgO + CaO)/(SiO₂ + Al₂O₃) can be used to approximate the conditions of peat accumulation [33]. A value greater than 0.23 indicates that the sedimentary environment is influenced by sea water; otherwise, it is considered to be a continental peat swamp. When the content of SiO₂ + Al₂O₃ in coal ash is high, the coal-forming environment is a weak reduction environment; when the content of Fe₂O₃ + MgO + CaO is high, this suggests a strong reducing environment [34]. The CaO/MgO ratio reflects the influence degree of seawater, and the ratio is negatively correlated with the influence degree of seawater [35]; that is, the greater the ratio, the lower the influence degree of seawater. The ratio of CaO/(CaO + Fe₂O₃) can reflect the salinity of the sedimentary water medium, and they are positively correlated [36]; that is, the larger the ratio, the greater the salinity of the sedimentary water medium. As to the value of No. 5 coal, the (Fe₂O₃ + MgO + CaO)/(SiO₂ + Al₂O₃) value falls between 0.05 and 0.22, with an average value of 0.09, indicating a continental peat swamp environmental (

Table 4. Ash composition parameters of coal gathering environment in the Zhaokai Mine.

Sample	SiO2 + Al2O3 (%)	Fe2O3 + MgO + CaO (%)	(Fe2O3 + MgO + CaO)/ (SiO2 + Al2O3)	CaO/ MgO	CaO/ (CaO + Fe2O3)
CK501	78.63	17.69	0.22	1.89	0.12
CK502	90.47	5.34	0.06	2.06	0.51
CK503	88.06	7.03	0.08	2.08	0.42
CK504	83.06	3.88	0.05	1.63	0.49
CK505	90.87	4.23	0.05	1.82	0.64
CK506	89.62	5.21	0.06	1.63	0.62
CK507	86.87	10.12	0.12	0.18	0.01
CK508	92.18	4.34	0.05	0.46	0.05
Min	78.63	3.88	0.05	0.18	0.01
Max	92.18	17.69	0.22	2.08	0.64
Average	87.47	7.23	0.09	1.47	0.36

). The average value of SiO₂ + Al₂O₃ is 87.47, which is absolutely dominant. It is speculated that the coal-forming environment is a weak reduction environment. The value of CaO/MgO is between 0.18 and 2.08, with an average value of 1.47, and the ratio is small, indicating that the No. 5 coal seam is greatly affected by seawater, which is consistent with the geological background of the coal-forming period. The ratio of CaO/(CaO + Fe₂O₃) ranges from 0.01 to 0.64, with an average of 0.36. This may be due to the influence of epigenetic minerals, resulting in major-element anomalies. It can be seen that the difference in the coal-forming microenvironment will lead to a significant difference in the content of major-elements. These results highlight the significant impact of the coal forming microenvironmental on major element content.

4.3.2. Trace Elements

Through an examination of the trace element composition in coal, and by applying the concentration coefficient (CC) metric introduced by Dai et al. [37], we determined the enrichment factor for trace elements in No. 5 coal. According to Figure 4, Li and Zr exhibit significant enrichment, while Ni, Nb, Mo, Cd, Sn, Hf, Ta, W, Th, and U show minor enrichment. The enrichment coefficients for Be, Sc, V, Cr, Co, Cu, Zn, Ga, Sr, and Pb fall between 0.5 and 2, indicating a normal range. On the other hand, the enrichment coefficients of Rb, Cs, Ba, Tl, and Bi are all less than 0.5, showing a loss.

The unique geological–geochemical characteristics of trace elements make them extremely sensitive to changes in the sedimentary environment [39]. The composition characteristics of coal can indicate the conditions under which it was formed [40]. The content of Sr is more abundant in marine settings, while Ba is more prevalent on land. The Sr/Ba ratio serves as an indicator of the depositional environment of coal; when Sr/Ba is less than 1, it is continental freshwater sediment, and an Sr/Ba ratio exceeding 1 typically indicates marine sediment [41]. In freshwater environments, the Sr/Ba ratio sometimes exceeds 1, which may be related to lake salinity caused by climate change [42]. As shown in

Table 5. Indicative parameters in the No. 5 coal of the Zhaokai Mine.

Sample	Sr/Ba	V/Zn	Cu/Zn	Sr/Cu
CK501	10.97	0.17	0.37	13.80
CK502	10.09	0.19	0.20	28.53
CK503	6.34	0.15	0.09	27.58
CK504	5.49	1.19	0.50	9.65
CK505	8.07	0.34	0.28	23.35
CK506	4.85	0.73	1.31	12.87
CK507	0.81	0.90	0.27	1.04
CK508	11.14	1.16	0.57	30.38
Average	7.22	0.60	0.45	18.40

, the Sr/Ba values in the No. 5 coal samples from the study area range from 0.81 to 11.14, showing significant variability. This suggests that the depositional environment was heavily impacted by seawater at the time. The V/Zn ratio can also indicate changes in the sedimentary environment. When V/Zn is less than 1.33, it belongs to continental deposits; when the value of V/Zn is between 1.33 and 3.59, it belongs to a transitional-phase deposition; and when V/Zn is greater than 3.59, it belongs to a marine deposition [43]. In the No. 5 coal, the V/Zn ratios vary between 0.15 and 0.19, with a mean of 0.60, which is below 1.33. It is judged that the overall sedimentary setting in the research area is a continental deposit. The Cu/Zn ratio serves as an indicator of the redox conditions during coal-forming: a Cu/Zn ratio below 0.21 signifies a reducing environment; when Cu/Zn is between 0.21 and 0.38, it indicates a weakly reducing environment; a ratio between 0.38 and 0.50 points to a mixed reduction–oxidation environment; when the value of Cu/Zn is between 0.50 and 0.63, this indicates a weakly oxidizing environment; and a ratio above 0.63 denotes an oxidizing environment [44]. The average Cu/Zn value in the No. 5 coal is 0.45, indicating a reducing environment. The Sr/Cu ratio can indicate the paleoclimate: when the value of Sr/Cu is between 1 and 10, it suggests a warm and moist environment, while a ratio above 10 points to a hot and arid climate. The value of Sr/Cu in No. 5 coal samples is between 1.04 and 30.38, with a mean of 18.40, signaling that the depositional setting was predominantly dry and hot.

4.3.3. Rare Earth Elements and Yttrium (REY)

The chemical properties of REY are relatively stable, making them valuable for assessing the sedimentary conditions of peat swamps, the properties of parent rocks in the source area, and various geological processes following coal seam formation [45,46,47,48]. As per [49] Seredin and Dai., REY can be categorized into light (LREY: La, Ce, Pr, Nd, and Sm), medium (MREY: Eu, Gd, Tb, Dy, and Y), and heavy (HERY: Ho, Er, Tm, Yb, and Lu) groups. The REY levels in No. 5 coal range from 46.59 μg/g to 187.56 μg/g, averaging 104.35 μg/g. This is lower than the mean REY content in the Ningwu Coalfield (151.17 μg/g) and for Chinese coal (135.89 μg/g). Nonetheless, it is 1.5 times greater than the global average REY content in coal (68.41 μg/g). The concentration coefficient of REY in all samples of the No. 5 coal seam is between 0.5 and 2, which falls within the normal range.

The parameters of REY in coal can effectively reflect the source and sedimentary environment of sediments [50] (

Table 6. Geochemical parameters of REY in the No. 5 coal of the Zhaokai Mine.

Sample	CK501	CK502	CK503	CK504	CK505	CK506	CK507	CK508	Average
LREY	32.69	92.7	67.55	91.72	35.31	48.57	145.52	148.73	82.85
MREY	11.63	14	18.17	29.71	9.33	11.76	35.88	13.42	17.99
HREY	2.28	2.61	3.25	6.79	2.07	2.38	6.16	2.43	3.5
L/M	2.81	6.62	3.72	3.09	3.79	4.13	4.06	11.08	4.91
L/H	14.35	35.52	20.76	13.51	17.06	20.42	23.63	61.14	25.8
M/H	5.11	5.36	5.59	4.38	4.51	4.94	5.83	5.52	5.15
(La/Yb)N	0.65	2.15	0.86	0.62	0.8	1.17	1.26	3.15	1.33
(La/Lu)N	0.7	2.45	0.97	0.61	0.91	1.36	1.3	3.46	1.47
(La/Sm)N	0.69	1.87	0.56	0.98	0.86	1.37	1.21	1.28	1.1
(Gd/Lu)N	1.24	1.66	1.82	0.78	1.25	1.28	1.34	2.99	1.54
δEu	0.76	0.78	0.93	0.83	0.79	0.76	0.94	0.79	0.82
δCe	1.02	0.99	1.01	1.03	0.98	0.98	0.98	1.09	1.01
δCe/δEu	1.34	1.27	1.09	1.25	1.24	1.29	1.04	1.38	1.24

LREY = La + Ce + Pr + Nd + Sm; MREY = Eu + Gd + Tb + Dy + Y; HREY = Ho + Er + Tm + Yb + Lu; L/M = LREY/MREY; L/H = LREY/HREY; M/H = MREY/HREY; δEu = Eu/Eu* = Eu_N/(Sm_N × Gd_N)½; δCe = Ce/Ce* = Ce_N/(La_N × Pr_N)½. REY are normalized by chondrite. Data from Wang [7].

). The REY was standardized using chondrite data, and the distribution pattern of REY was studied. In the No. 5 coal seam, the ratio of LREY to HREY ranges from 13.51 to 61.14, with an average of 25.8; the ratio of LREY to MREY ranges from 2.81 to 11.08, with an average of 4.91; and the ratio of MREY to HREY ranges from 4.38 to 5.83, with an average of 5.15. Generally, the No. 5 coal is primarily enriched in LREY, similar to late Paleozoic coal in China. It is speculated that the REY are mainly derived from terrigenous debris. The REY anomalies in coal may occur in La, Ce, Eu, Gd, and Y. When La_N/Lu_N > 1, it is a LREY enrichment type; when La_N/Sm_N < 1 and Gd_N/Lu_N > 1, it is an MREY enrichment type; and when La_N/Lu_N < 1, it is an HREY enrichment type. The average value of La_N/Lu_N in No. 5 coal is 1.47, with large ratios of La_N/Sm_N and La_N/Yb_N indicating LREY enrichment, consistent with previous conclusions. Eu and Ce are the two kinds of multivalent REY used to trace the sedimentary environment [51].

To avoid Gd interference, δEu and δCe indicate an abnormal degree of Eu and Ce levels: when δEu is greater than 1, this means that Eu is a positive anomaly; when δEu is less than 1, this indicates that Eu is a negative anomaly. Similar to Ce, when δCe is greater than 1, this means that Ce is a positive anomaly; when δCe is less than 1, this indicates that Ce is a negative anomaly. In No. 5 coal, δEu ranges from 0.76 and 0.94, with an average value of 0.82. The variation range of δCe is between 0.98 and 1.09, and the change is relatively gentle, indicating that δEu is a slightly negative anomaly. The ratio of δCe/δEu can be used to judge the redox environment; when the value of δCe/δEu is greater than 1, this indicates that the coal-forming environment is mainly dominated by reduction conditions. The No. 5 coal’s δCe/δEu ranges from 1.04 to 1.38, averaging 1.24, and indicating a reducing environment, aligning with previous conclusions.

The REY distribution pattern is unaffected by external factors, thus providing a more accurate indication of the trace-element origins in coal. In this study, chondrite data were used to standardize REY, eliminating differences in element due to atomic number increases, allowing for a more efficient study of the geochemical characteristics of REY distribution patterns (Figure 5). In the distribution pattern curve of the No. 5 coal seam, the slope of LREY is steep, indicating a high degree of fractionation. The curve, overall, slopes to the right, showing that LREY are relatively enriched compared with MREY and HREY. There is a noticeable “V”-shaped fluctuation at “Eu”, suggesting a significant negative anomaly at that point. This study proposes that the negative “Eu” anomaly is a result of terrigenous clastic materials inheritance.

In conclusion, the No. 5 coal seam primarily inherited terrigenous clastic material during deposition. The REY distribution pattern in each layer is similar, indicating a consistent evolution and deposition process, stable external factors, and a relatively stable source.

4.4. Composition Characteristics of Extractable Organic Matter in Coal

Through Soxhlet extraction and group component separation, the organic matter content in coal samples is in the range of 6.02~18.24 mg/g, averaging 13.13 mg/g. The ratio of saturated aromatic falls between 0.16 and 0.32, averaging 0.21 (

Table 7. Contents of organic extract in coal of the No. 5 coal seam in the Zhaokai Mine.

Sample	Organic Matter (mg/g)	Group Component (mg/g)			Saturated/ Aromatic
		Saturated Hydrocarbon	Aromatic Hydrocarbon	Non-Hydrocarbon + Asphaltene
CK501	14.52	1.04	5.60	7.88	0.19
CK502	14.48	0.96	5.16	8.36	0.19
CK503	15.64	1.04	6.16	8.44	0.17
CK504	12.60	0.92	4.56	7.12	0.20
CK505	18.24	1.12	6.92	10.20	0.16
CK506	17.20	1.20	6.44	9.56	0.19
CK507	6.36	0.66	2.08	3.62	0.32
CK508	6.02	0.56	2.32	3.14	0.24
Average	13.13	0.94	4.90	7.29	0.21

). The aromatic hydrocarbon concentration in the organic matter exceeds that of the saturated hydrocarbon, leading to a saturated aromatic ratio. This suggests that the No. 5 coal samples are classified as humic coal, with the primary coal-forming material being terrestrial higher plants.

4.5. Characteristics of Saturated Hydrocarbon in Coal—n-Alkanes, Pr and Ph

Saturated hydrocarbons are a crucial class of substances found in plants and other organisms, and they are among the most extensively studied substances [52]. Currently, studies on the properties of saturated hydrocarbons in coal, both domestically and internationally, primarily focuses on n-alkanes. These n-alkanes are a representative group of saturated hydrocarbon organic matter that mainly originates from the degradation products of terrestrial organisms or biological lipids [4,53].

Due to their relatively stable nature, the degradation rate of n-alkanes during the entire degradation process is slow; thus, its components can be a good indicator of the source of the organic matter [54]. By analyzing parameters related to saturated hydrocarbon, valuable information about the sedimentary environment of coal samples in the study area, the type of coal-forming parent material, and other related information can be obtained [55].

4.5.1. Carbon Number Distribution

The variation in the number of carbon atoms in n-alkanes and the difference in the number of carbon atoms in the main chain can indicate the source of various substances in sediments [56]. It is generally believed that organic matter with fewer than n-C₂₀ carbon atoms mainly comes from lower plankton, and the peak distribution pattern is mainly unimodal. Carbon atoms in the range of n-C₂₁~n-C₃₃ indicates that the organic matter is primarily from terrestrial higher plants, with a peak distribution pattern of the post-single-peak type. A bimodal curve suggests that the organic matter comes from both lower plankton and higher plants [57]. In the No. 5 coal samples, the carbon distribution ranges from n-C₁₄ to n-C₃₂. The main peak carbons are n-C₂₄ and n-C₂₅, with a peak distribution pattern of the post-single-peak type, indicating that the source of organic matter is mainly terrestrial higher plants (Figure 6;

Table 8. Saturated hydrocarbon related parameters in coal of the No. 5 coal seam in the Zhaokai Mine.

Sample	Carbon Range	Main Peak Carbon	OEP	${\displaystyle \sum {\mathrm{C}}_{21}^{-}}/{\displaystyle \sum {\mathrm{C}}_{22}^{+}}$	Pr/Ph	Pr/n-C17	Ph/n-C18
CK501	C14–C32	C24	0.40	0.37	2.83	1.26	0.22
CK502	C14–C32	C25	0.33	0.46	2.69	1.02	0.22
CK503	C15–C32	C25	0.35	0.32	2.31	1.08	0.26
CK504	C14–C32	C24	0.13	0.48	2.23	1.08	0.26
CK505	C14–C32	C25	0.23	0.73	3.49	1.54	0.29
CK506	C14–C32	C24	0.27	0.55	3.53	1.40	0.24
CK507	C14–C32	C25	1.30	0.23	2.04	0.81	0.23
CK508	C14–C32	C24	0.18	0.24	1.06	0.45	0.15
Average	-	-	0.40	0.42	2.52	1.08	0.23

).

4.5.2. Odd–Even Predominance Index (OEP)

The odd–even predominance index (OEP) serves as a metric for evaluating the maturity of organic matter based on the odd–even carbon predominance of n-alkanes. During the coal-forming period, the long-chain odd-numbered carbon n-alkanes are initially decomposed due to the combined effects of thermal activity, microorganisms, and bacteria, weakening the odd–even advantage [58]. An OEP value in the range of 1.0~1.2 typically indicates mature organic matter, while a value of 1.2~1.4 suggests low mature organic matter, and anything above 1.4 indicates immature organic. The mean value of OEP in the No. 5 coal samples is 0.40, which is less than 1 (Table 3). This implies that the organic matter in the No. 5 coal samples may have undergone thermal effects or microbial degradation [59].

4.5.3. Light–Heavy Fraction Ratio (${\displaystyle \sum {\mathrm{C}}_{21}^{-}}/{\displaystyle \sum {\mathrm{C}}_{22}^{+}}$)

The light–heavy fraction is the ratio of the total content of low-carbon-number n-alkanes to the total content of high-carbon-number n-alkanes (${\displaystyle \sum {\mathrm{C}}_{21}^{-}}/{\displaystyle \sum {\mathrm{C}}_{22}^{+}}$). This ratio change can be used to infer the source of the organic material and the types of depositional conditions of the coal-forming swamps [60]. The low-carbon n-alkanes before C₂₁ are mainly provided by aquatic organisms or microorganisms, while the high-carbon n-alkanes after C₂₂ are provided by terrestrial organisms. The average ${\displaystyle \sum {\mathrm{C}}_{21}^{-}}/{\displaystyle \sum {\mathrm{C}}_{22}^{+}}$ ratio of the No. 5 coal samples is 0.42, suggesting that the sedimentary environment is dominated by continental deposits.

4.5.4. Pristane (Pr) and Phytane (Ph)

Pristane (Pr) and phytane (Ph) are key indexes for studying paleoenvironmental changes and organic materials [61]. It has been found that under oxidative conditions, phytol is oxidized to phytic acid; phytic acid is dehydroxylated and hydrogenated to form pristane; otherwise, it is easy for it to form phytane [62]. This indicates that the Pr/Ph ratio is a crucial markers of the redox state [63]. When the Pr/Ph is less than 3, the sedimentary environment is strongly reducing; when it is greater than 3, it is in a weak oxidation state. Furthermore, the ratios of Pr/n-C₁₇ and Ph/n-C₁₈ can also determine the original type of organic matter and the redox of the sedimentary conditions [64]. The ratios of Pr/n-C₁₇ and Ph/n-C₁₈ in undegraded organic matter are typically very low, falling within the range of 0.1 to 0.5. When subjected to high temperatures or microbial action, n-alkanes are first, while isoprenoid alkanes remain relatively intact and preserved. The average value of Pr/Ph in the No. 5 coal in the study area is 2.52, i.e., less than 3, indicating that the sedimentary environment is a reducing–strong reducing environment. The average values of Pr/n-C₁₇ and Ph/n-C₁₈ are 1.08 and 0.23, respectively, indicating that the organic matter in the study area has been degraded by heat or bacterial microorganisms. The correlation between Pr/n-C₁₇ and Ph/n-C₁₈ can also determine the characteristics of the coal-forming environment. The coal samples in the study area are terrigenous deposits, as a whole (Figure 7).

4.6. Characteristics of Aromatic Hydrocarbon in Coal

4.6.1. Composition of Aromatic Hydrocarbon

Aromatic hydrocarbons are polycyclic aromatic hydrocarbons (PAHs) containing two or more benzene rings in their molecules, which are highly abundant in soluble organic matter. The reasons for this abundance include the biological action of coal-forming plants, thermal cracking during the coal-forming period, and peatland wildfires [65]. Thus, PAHs contain extremely important geological information. PAHs are a class of toxic pollutants with mutagenic, carcinogenic, and other hazardous properties. They are mainly derived from the use and incineration of coal [66]. Research on the content and distribution characteristics of polycyclic aromatic hydrocarbons in coal can not only supply essential information for the clean utilization of coal and environmental pollution assessment but also lay a foundation for the study of the geological evolution degree, the parent material source, and the coal-forming environment in the research area.

The aromatic hydrocarbons in No. 5 coal samples were tested (Figure 8,

Table 9. Identified aromatic hydrocarbons of the No. 5 coal in the Zhaokai Mine (μg/g).

Compounds	CK501	CK502	CK503	CK504	CK505	CK506	CK507	CK508	Average
Naphthalene series	344.42	194.50	296.05	183.37	225.43	181.81	112.96	70.69	201.15
N	0.21	nd	nd	nd	0.22	0.12	0.01	0.40	0.19
MN	30.37	3.88	3.75	2.46	20.05	14.67	8.46	11.33	11.87
DMN	97.73	54.56	67.50	39.66	65.44	52.19	32.92	18.52	53.56
TMN	117.34	77.74	128.33	81.90	79.89	67.00	41.20	20.61	76.75
TeMN	83.82	50.47	84.56	53.36	49.71	41.58	24.58	14.76	50.36
EMN	9.82	3.51	4.81	2.26	6.94	4.17	3.69	2.13	4.67
PhN	5.12	4.33	7.11	3.73	3.19	2.08	2.10	2.94	3.83
Phenanthrene series	159.12	150.75	193.89	123.69	96.76	96.20	68.42	59.45	118.54
P	24.08	24.05	32.53	24.13	18.78	14.55	15.24	18.54	21.49
MP	51.31	48.60	66.27	43.08	35.54	29.97	24.97	24.28	40.50
DMP	54.01	54.40	75.62	41.35	31.26	36.14	22.15	13.45	41.05
TMP	29.72	23.70	19.47	15.13	11.19	15.54	6.06	3.18	15.50
Biphenyl series	48.23	19.35	34.75	22.72	19.36	17.76	12.09	12.04	23.29
Bi	0.42	0.54	0.47	0.49	0.58	0.62	0.66	0.90	0.58
MBi	5.84	3.46	4.86	4.48	3.39	2.39	2.43	4.44	3.91
DMBi	7.97	1.34	6.24	1.50	4.56	0.63	0.74	2.35	3.17
TMBi	34.00	14.01	23.19	16.26	10.84	14.13	8.25	4.35	15.63
Fluorene series	23.36	25.87	25.69	24.88	18.28	17.06	14.70	8.17	19.75
FLU	0.65	4.44	0.49	5.82	0.23	3.38	3.29	0.13	2.30
MFLU	7.25	11.98	17.61	11.74	11.93	7.17	6.77	5.34	9.98
DMFLU	15.47	9.45	7.59	7.32	6.12	6.51	4.64	2.70	7.47
Oxygenated series	51.89	52.43	82.88	48.53	41.43	29.91	24.07	32.07	45.40
DBF	4.92	7.62	8.31	5.52	5.05	4.35	4.86	4.28	5.61
MDBF	17.95	22.86	29.12	20.77	15.35	13.24	10.06	8.59	17.24
BNF	21.10	14.94	27.15	15.86	14.88	10.05	7.31	9.92	15.15
DNF	6.69	5.70	15.66	4.91	5.82	1.51	1.36	8.92	6.32
BDNF	1.23	1.31	2.64	1.47	0.33	0.76	0.48	0.36	1.07
Sulphureous series	73.04	85.90	93.43	84.16	78.11	72.10	72.87	55.43	76.88
DBT	nd	16.06	19.79	17.05	18.63	16.70	15.93	17.31	17.35
MDBT	31.87	17.70	29.46	15.58	15.53	14.97	9.26	7.04	17.68
DMDBT	5.47	10.51	5.28	2.79	3.15	3.93	5.82	3.71	5.08
DMNT	4.46	4.15	nd	nd	nd	nd	5.33	nd	4.65
EMNT	7.05	8.32	5.78	4.39	3.14	5.65	3.28	nd	5.37
BNT	19.60	19.34	25.95	17.01	14.97	12.74	10.09	9.94	16.21
MBNT	4.59	9.82	7.17	27.34	22.69	18.11	23.16	17.43	16.29
Pyrene series	104.48	103.03	104.43	96.37	51.97	56.18	50.57	51.62	77.33
PY	7.24	7.82	8.07	9.75	4.91	4.52	6.67	9.30	7.29
MPY	44.51	46.16	61.07	43.91	25.52	22.33	23.42	21.79	36.09
DMPY	45.07	40.71	30.27	33.17	17.96	23.48	17.27	14.12	27.76
TMPY	7.10	7.06	4.50	6.08	2.36	4.15	2.03	5.26	4.82
IPY	0.58	1.28	0.53	3.46	1.21	1.70	1.17	1.14	1.38
Fluoranthene series	14.80	16.86	18.08	22.51	12.45	10.24	13.03	22.52	16.31
FLT	9.42	11.10	15.62	10.50	8.05	6.93	5.38	7.75	9.35
BFLT	5.39	5.76	2.46	12.00	4.39	3.30	7.65	14.77	6.97
Chrysene series	8.08	8.60	16.38	8.94	5.53	5.51	4.41	7.23	8.09
Ch	7.39	8.02	8.59	8.29	4.78	4.82	3.81	5.33	6.38
MCh	0.70	0.57	7.79	0.65	0.75	0.69	0.60	1.90	1.71
Perylene series	3.16	9.66	9.16	24.12	6.75	7.55	9.65	14.87	10.62
MPE	2.26	7.44	8.39	15.40	5.73	6.08	6.02	12.54	7.98
BPE	0.90	2.22	0.76	8.72	1.02	1.47	3.62	2.34	2.63
Anthracene series	6.81	5.82	6.08	4.89	4.43	3.96	1.14	3.98	4.64
MAn	6.81	5.82	6.08	4.89	4.43	3.96	1.14	3.98	4.64
Benzanthracene sreies	9.13	9.30	15.70	10.18	6.94	6.31	4.48	6.56	8.58
BAT	5.18	6.15	5.98	6.66	3.71	4.03	2.88	4.49	4.89
DMBAT	3.95	3.15	9.72	3.51	3.23	2.29	1.60	2.07	3.69
Cad	18.27	5.57	23.80	8.81	8.93	7.84	3.91	2.66	9.97
Re	11.37	31.25	29.85	10.10	9.96	9.44	2.30	10.73	14.37
MBP	13.04	13.83	5.73	12.45	2.40	8.30	6.26	2.8	8.10

N: naphthalene; MN: methylnaphthalene; DMN: dimethylnaphthalene; TMN: trimethylnaphthalene; TeMN: tetramethylnaphthalene; EMN: ethylnaphthalene; PhN: phenylnaphthalene; P: phenanthrene; MP: methylphenanthrene; DMP: dimethylphenanthrene; TMP: trimethylphenanthrene; Bi: biphenyl; MBi: methylbiphenyl; DMBi: dimethylbiphenyl; TMBi: trimethylbiphenyl; FLU: fluorene; MFLU: methylfluorene; DMFLU: dimethylfluorene; DBF: dibenzofuran; MDBF: methyldibenzofuran; BNF: benzaphthofuran; DNF: dinaphthofuran; BDNF: benzdinaphthofuran; DBT: dibenzothiophene; MDBT: methyldibenzothiophene; DMDBT: dimethyldibenzothiophene; DMNT: dimethylnaphthiophene; EMNT: ethylnaphthiophene; BNT: benzothiophene; MBNT: methylbenaphthiophene; PY: Pyrene; MPY: Methylpyrene: DMPY: dimethylpyrene; TMPY: trimethylpyrene; IPY: indenpyrene; FLT: fluoranthrene; BFLT: benzofluoranthrene; Ch: chrysene; MCh: methylchrysene; MPE: methylperylene; BPE: benzopyylene; MAn: methylanthracene; BAT: benzanthracene; DMBAT: dimethylbenzanthracene; Cad: cadalene; Re: retene; MBP: methylbenzophenanthrene.

). Among them, the naphthalene series (mean value: 201.15 μg/g) and phenanthrene series (mean value: 118.54 μg/g) are the main ones, followed by the pyrene series (mean value: 77.33 μg/g) and the sulfurous series (mean value: 76.88 μg/g). The content of other hydrocarbon compounds is relatively low. Although the contents of different types of aromatic hydrocarbons in different samples are slightly different, the overall distribution trend is relatively consistent (Figure 9).

4.6.2. Aromatic Hydrocarbons and Coal-Forming Parent Materials

The naphthalene series detected in the No. 5 coal from the Zhaokai Mine includes N, MN, DMN, TMN, TeMN, EMN, and PhN, with average values of 0.19 μg/g, 11.87 μg/g, 53.56 μg/g, 76.75 μg/g, 50.36 μg/g, 4.67 μg/g, and 3.83 μg/g, respectively. Among these, TMN and TeMN can be formed by the decomposition and recombination of pentacyclic triterpenoids, which are abundant in higher plants. Therefore, it is speculated that the main source of the naphthalene series is higher plants [67].

In this experiment, the pyrene series (mean value: 77.33 μg/g), fluoranthene series (mean value: 16.31 μg/g), chrysene series (mean value: 8.09 μg/g), perylene series (mean value: 10.62 μg/g), anthracene series (mean value: 4.64 μg/g), benzoanthracene series (mean value: 8.58 μg/g), Cad (mean value: 9.97 μg/g), Re (mean value: 14.37 μg/g), and MBP (mean value: 8.10 μg/g) were also detected in coal samples. They are mainly derived from terrestrial higher plants, indicating the input of higher plants in coal-forming parent materials.

4.6.3. Aromatic Hydrocarbons and Maturity

Phenanthrene-series compounds are frequently utilized as markers of organic matter maturity. Radke [68] defined organic matter maturity using MPI-1 and MPI-2: the MPI value increases as the thermal evolution stage deepens; then, it gradually decreases after reaching its peak [69]. However, a multitude of studies have shown that MPI-1 and MPI-2 are not accurate in assessing high-maturity samples. Some scholars suggests that the relative content of phenanthrene-series compounds is closely related to the spatial distribution of their isomers and has nothing to do with maturity. After studying the samples with vitrinite reflectance in the range of 0.53%~1.2%, Kvalheim et al. [70] proposed the methylphenanthrene indices R₁, R₂, F₁, and F₂: F₁ < 0.40 and F₂ < 0.27 indicate a low-maturing area; 0.40 < F₁ < 0.55 and 0.27 < F₂ < 0.35 indicate a mature area; and F₁ > 0.56 and F₂ > 0.35 indicates a high-maturity area. The F₁ of No. 5 coal in the research area ranges from 0.42 to 0.52, with a mean of 0.47; the F₂ is between 0.07 and 0.12, with an average of 0.10 (

Table 10. Methylphenanthrene index of the No. 5 coal in the Zhaokai Mine (μg/g).

Sample	MPI-1	MPI-2	R1	R2	F1	F2
CK501	0.30	0.33	0.91	0.50	0.48	0.09
CK502	0.29	0.30	0.93	0.48	0.48	0.08
CK503	0.27	0.30	0.82	0.45	0.45	0.08
CK504	0.26	0.25	0.90	0.45	0.47	0.07
CK505	0.38	0.42	0.72	0.40	0.42	0.11
CK506	0.45	0.46	0.83	0.43	0.45	0.12
CK507	0.46	0.45	1.11	0.54	0.52	0.12
CK508	0.35	0.35	0.91	0.45	0.48	0.09
Average	0.35	0.36	0.89	0.46	0.47	0.10

$\mathrm{M}\mathrm{P}\mathrm{I}-1={\displaystyle \frac{1.5\times (2-\mathrm{M}\mathrm{P}+3-\mathrm{M}\mathrm{P})}{\mathrm{P}+1-\mathrm{M}\mathrm{P}+9-\mathrm{M}\mathrm{P}}}$; $\mathrm{M}\mathrm{P}\mathrm{I}-2={\displaystyle \frac{3\times (2-\mathrm{M}\mathrm{P})}{\mathrm{P}+1-\mathrm{M}\mathrm{P}+9-\mathrm{M}\mathrm{P}}}$; ${\mathrm{R}}_1={\displaystyle \frac{2-\mathrm{M}\mathrm{P}+3-\mathrm{M}\mathrm{P}}{1-\mathrm{M}\mathrm{P}+9-\mathrm{M}\mathrm{P}}}$; ${\mathrm{R}}_2={\displaystyle \frac{2-\mathrm{M}\mathrm{P}}{1-\mathrm{M}\mathrm{P}+9-\mathrm{M}\mathrm{P}}}$; ${\mathrm{F}}_1={\displaystyle \frac{2-\mathrm{M}\mathrm{P}+3-\mathrm{M}\mathrm{P}}{1-\mathrm{M}\mathrm{P}+9-\mathrm{M}\mathrm{P}+2-\mathrm{M}\mathrm{P}+3-\mathrm{M}\mathrm{P}}}$; ${\mathrm{F}}_2={\displaystyle \frac{2-\mathrm{M}\mathrm{P}}{1-\mathrm{M}\mathrm{P}+9-\mathrm{M}\mathrm{P}+2-\mathrm{M}\mathrm{P}+3-\mathrm{M}\mathrm{P}}}$.

).

4.6.4. Aromatic Hydrocarbons and Sedimentary Environment

The relative content of dibenzofuran- and dibenzothiophens-series compounds can indicate the redox environment during coal-forming. In the process of coal-forming, dibenzofuran accounts for a large proportion in a weak reduction environment, while dibenzothiophens accounts for a large proportion in a strong reduction environment [71,72]. The mean of dibenzothiophen-series compounds in No. 5 coal samples is 17.35 μg/g, and the mean of dibenzofuran-series compounds is 5.61 μg/g. This indicates that the sedimentary conditions of the No. 5 coal samples in the Zhaokai Mine form a strong-reduction environment, and it is speculated that the sedimentary environment may be affected by seawater.

5. Conclusions

The No. 5 coal in the Zhaokai Mine is bituminous coal with low moisture, low ash, medium-high sulfur, and medium volatile matter. The maceral composition is mainly vitrinite, followed by inertinite, with liptinite present in the lowest amount. The parameters of coal facies suggest that the No. 5 coal formed in a moist peat swam with a thin layer of surface water. The hydrodynamic conditions of the swamp were strong, and the depth of the overlying water changes alternately, primarily consisting of mainly shallow water swamps.

The content of major-element oxides Al₂O₃ and SiO₂ is high, while trace elements, such as Ni, Nb, Mo, Cd, Sn, Hf, Ta, W, Th, U, Li, and Zr, are slightly to moderately enriched. The overall performance shows LREY enrichment. Based on the facies parameters, the coal-forming environment is characterized as a weak-reduction environment. It is a continental peat swamp influenced by seawater, with a relatively dry and hot sedimentary environment.

The ratio of saturated aromatics in the No. 5 samples is relatively low, showing typical humic characteristics. Through the calculation and analysis of saturated hydrocarbon parameters, the distribution pattern of the n-alkanes’ peak is a post-single-peak type, and the main peak carbons are n-C₂₄ and n-C₂₅, indicating that the primary coal-forming parent material of the No. 5 coal is terrestrial higher plants. With the degradation of heat, bacteria, and other microorganisms, the coal that is formed is predominantly reductive. Aromatic hydrocarbon compounds are mainly from the naphthalene and phenanthrene series. Through the study of the parameters of aromatic hydrocarbons, it is determined that there is input from higher plants in the coal-forming parent material within the research area.