Structural and Maceral Controls on Biogenic Methane Generation: Insights from Vitrain–Durain Separation across the First Coalification Jump

Abstract

Biogenic coalbed methane resources are widely distributed in high-volatile bituminous coal areas, yet the controlling effect of the first coalification jump on biogas generation remains unclear. To explore the combined controlling effects of coal macerals and metamorphic degree on biogenic methane production potential, five coal samples with different metamorphic grades near the first coalification jump were selected. Vitrain and durain fractions were separated from the samples to conduct microbial degradation and methanogenesis experiments. The results show that the methane production yield of vitrain (11.96–15.75 μmol/g) is generally higher than that of durain (4.14–15.49 μmol/g). The methane production of both vitrain and durain presents a general N-shaped variation trend with increasing coalification degree, suggesting that coalification exerts a notable controlling effect on methane generation. The methane yield of vitrain is negatively correlated with stacking height (L _c), while that of durain is negatively correlated with carbon lamellar spacing (d ₀₀₂). The liptinite-rich characteristic of durain can offset the adverse impact of metamorphism. Essentially, the biogenic methanogenesis of coal refers to the biochemical transformation of aliphatic structures mediated by anaerobic microorganisms, and the gas production scale is directly determined by the abundance of available aliphatic components. The aliphatic structure of vitrain is mainly controlled by coalification, whereas that of durain is jointly regulated by coalification and macerals, with macerals playing a dominant role. Vitrinite serves as the major contributor to methane production, while inertinite has a low contribution rate, which is the primary reason for the superior gas-producing capacity of vitrain over durain.

1. Introduction

As a renewable and low-carbon energy resource, biogenic coalbed methane has considerable potential to strengthen national energy security and support efforts to reduce carbon dioxide emissions. Coalbed methane bioengineering is an advanced technological approach for achieving this objective, with its core principle being the use of microbial metabolic activities to convert organic matter within coal into methane. This technology can not only significantly improve the methane recovery rate of hard-to-mine coal resources and extend the service life of coalbed methane fields but also promote the clean and efficient utilization of coal while yielding remarkable carbon emission reduction benefits. , Among various types of coal reservoirs, low-rank coal is widely regarded as an ideal geological matrix for implementing bioengineering and enhancing methane generation, owing to its loose molecular structure and high bioavailability of organic matter.

The coal matrix serves as the material basis for biogas formations. Its inherent physicochemical properties, such as coal metamorphic degree, maceral characteristics, mineral content, trace elements, and pores/fractures, all influence biogas generation, with the first two factors being particularly impactful. , Prior research has shown that biomethane can be produced from coals of different ranks. However, in comparison with medium- and high-rank coals, low-rank coal possesses a higher content of branched-chain structures and oxygen-containing functional groups in its molecular composition, which makes it more easily degradable by microorganisms. As a result, with increasing coalification, microbial degradability of coal gradually decreases, and methane production shows a clear negative correlation with coal rank. ,, Nevertheless, Fallgren et al. conducted biogas production experiments on coal of different ranks using indigenous bacteria from coalbed water in the Powder River Basin. Their results indicated that biogas production gradually decreased with decreasing coalification degree, and they proposed that volatile organic compounds are the key factors affecting biogas production from high-volatility bituminous coal. The discrepancy between the two studies may primarily stem from inconsistent classification criteria for the coal metamorphic ranks. The first study clearly categorized coal samples into three distinct ranks: low, medium, and high metamorphism, with substantial differences in metamorphic degree among each group. In contrast, the second study only covered samples ranging from lignite to bituminous coal, featuring a narrow span of metamorphism without an obvious hierarchical differentiation. Moreover, they adopted a more refined analytical framework for testing. Collectively, these factors led to divergent conclusions in the two investigations.

Further laboratory simulation studies have confirmed that low-rank coal exhibits greater biogas production potential than higher-rank coal. ,,, Currently, there is a general consensus on the overall trend between biogas production and coal rank, but controversies remain regarding the specific stages of coal evolution. Bao et al. analyzed the results of numerous biogas simulation experiments and found that the high-value zone of microbial methane production occured in the high-volatility bituminous coal stage. This finding is consistent with the experimental results of Fallgren et al. and aligns with the metamorphic stage of coal in the Jurassic coalbed methane enrichment areas of the northwestern China. In a previous study, Lin et al. discovered that the pyrolysis products of coal exhibit a distinct response to the first coalification jump (FCJ). Notably, the metamorphic grade of coal in favorable Jurassic coalbed methane development areas of northwest China generally ranges from 0.6% to 0.7% and is significantly influenced by the FCJ. However, the mechanism by which the FCJ affects biogas generation, as well as its geological controls, remains unclear. In-depth research is therefore required to address this specific geological phenomenon.

Due to differences in chemical structure, coal macerals exhibit varying susceptibility to microbial degradation. Vitrinite is rich in C–H and CO bonds and has a high H/C atomic ratio; when decomposed by fermentative bacteria, it easily produces CO₂ and H₂. In contrast, inertinite is characterized by a high level of aromatization and structural condensation along with a limited number of oxygen-containing functional groups, which makes it more resistant to microbial degradation. Consequently, the total yield and concentration of biomethane from vitrinite are much higher than those from inertinite-rich materials. Generally, vitrinite is more readily utilized by microorganisms than inertinite, demonstrating greater methane generation potential. − Liptinite is mainly composed of lipids. Although liptinite exhibits a certain degree of stability in nature, it serves as a high-quality substrate for biogas generation due to its rich aliphatic side chains and alicyclic structures that are easily broken. The bioavailability of coal is collectively influenced by various functional groups, solvent extractability, and water-soluble characteristics, while the extractable fractions in coal are primarily dominated by vitrinite. Meanwhile, macerals also influence the composition of biogas: an increase in vitrinite (or huminite) content leads to a higher methane concentration, while inertinite shows the opposite relationship with methane concentration. However, Jurassic coal seams in northwestern China are characterized by high inertinite content (typical inertinite-rich coal), and these areas are rich in biogenic coalbed methane resources. The mechanism underlying the generation and accumulation of biogas in such regions remains unclear. Additionally, both the degree of coalification and the coal composition together control the pathway and yield of biomethane generation. Most existing studies have primarily examined the influence of individual factors, such as coal rank or maceral composition, whereas the combined synergistic effects between these factors have not yet been clearly defined. ,

Biogenic coalbed methane resources are widely distributed in high-volatility bituminous coal zones. However, the controlling mechanisms of the first coalification jump and the influence of maceral variations on the biogas formation remain poorly understood. Therefore, this study selected high-volatile bituminous coals with different metamorphic degrees from the Jurassic Huanglong Coalfield in the Ordos Basin as research samples and conducted microbial degradation and gas production experiments on different coal lithologies. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) were used to characterize structural changes and the evolution of the chemical structure of the raw coal samples. This study primarily investigates the synergistic regulatory effect of coalification degree and coal maceral composition on biogenic gas production. The results are expected to provide a solid theoretical basis for evaluating biogenic methane resources in low-rank coal reservoirs and to further advance both the theoretical understanding and the practical application of coalbed methane bioengineering technologies.

2. Materials and Methods

2.1. Sample Collection and Basic Properties

The Ordos Basin, situated in central China, is among the most significant Jurassic coal-bearing basins and represents an important area for coalbed methane (CBM) accumulation. Located in the southwestern part of this basin, the Huanglong Coalfield is a large inland depression with a relatively stable tectonic setting and thick sedimentary sequences from the Paleozoic to Mesozoic eras. The Middle Jurassic Yan’an Formation constitutes the principal coal-bearing unit, featuring laterally continuous coal seams, large cumulative thicknesses, and strong regional correlation. Among these, the No. 2 and No. 4 coal seams represent the main mineable seams, with typical thicknesses of 5–10 m and local maxima of up to 20–30 m. Structurally, the region is dominated by a gently west-dipping monocline, which is overprinted by late-stage tectonic deformation that generated near east–west and northwest–southeast trending faults and broad, gentle folds. These structural elements exert important controls on coal seam distribution, coalbed methane enrichment, and hydrogeological conditions. , Previous studies have shown that biogenic coalbed methane in certain areas of the Huanglong Coalfield, especially within the Binchang Mining Area, is mainly produced through the CO₂ reduction pathway. − Coal samples were collected from freshly exposed underground working faces at five operating mines: Guojiahe (GJH), Cuijiagou (CJG), Dafosi (DFS), Huangling No. 1 (HL), and Gaojiabu (GJB). To minimize oxidation and contamination, only intact lump coals with well-developed vitrain bands were selected. After transport to the laboratory under sealed conditions, vitrain-rich (V) and durain-rich (D) fractions were manually separated based on macroscopic lithotype characteristics. The selected samples span a narrow but critical coalification interval (R_o, max = 0.50–0.80%), encompassing the first coalification jump. Sampling locations are shown in Figure (c).

1.

Schematic diagram illustrating regional structure and sampling locations: (a) geographical location of the Ordos Basin; (b) tectonic position of the Huanglong Coalfield; (c) sampling sites of the coal samples used in this study (modified from Lin et al.).

Proximate analysis, coal petrography, and vitrinite reflectance measurements were conducted following the international standards ISO 17246 (2024), ISO 17247(2020), and ISO 7404–5 (2009) to ensure consistency in characterizing coal rank and maceral composition.

2.2. Microbial Sources and Experimental Methods

Microorganisms used for biomethane generation experiments were obtained from the produced water of the surface coalbed methane well M143 in the Dafosi Coal Mine, Huanglong Coalfield. This source was selected to ensure geological relevance and compatibility between indigenous microbial communities and the Jurassic coal matrix. The microbial inoculum was enriched through two successive cultivation cycles using coal as the sole carbon source, promoting the selection of coal-degrading and methanogenic microorganisms. To minimize external interference and isolate the effect of coal properties on methane production, no additional organic substrates or trace elements were introduced beyond those required for microbial survival. All enrichment and degradation experiments were conducted under strictly anaerobic and standardized conditions. Coal samples were crushed to a particle size of 60–80 mesh, sterilized by ultraviolet irradiation, and incubated with a defined mineral medium in sealed serum bottles. The medium composition was designed to maintain neutral pH buffering and reducing conditions without contributing exogenous carbon. Anaerobic conditions were established by nitrogen purging, and all manipulations were performed under sterile conditions to prevent contamination. Successful microbial enrichment was confirmed by the detection of biogas during preliminary incubation at 37 °C. Microbial coal degradation experiments were conducted in 500 mL serum bottles, each containing 40 g of coal, 380 mL of culture medium, and 20 mL of enriched inoculum, and were then incubated statically at 37 °C for 80 days. For each coal sample, three parallel setups and one blank control (without coal) were prepared to ensure reproducibility and to account for background gas production. During the incubation period, headspace gas samples were collected at regular intervals and analyzed by gas chromatography (Agilent 7890A) to quantify methane and carbon dioxide concentrations, while the cumulative gas production was determined using the water displacement method. This experimental design ensured that observed variations in methane production reflected intrinsic differences in coal rank, maceral composition, and molecular structure rather than microbial or environmental variability. The standardized microbial consortium and controlled conditions provide a robust basis for linking biomethane generation to coalification-driven structural evolution, as explored in subsequent sections. The overall experimental workflow is summarized in Figure .

2.

Technical roadmap.

The calculation method for methane production per unit mass of coal sample is described as follows.

$$Y_{{\mathrm{x\; CH}}_4}=\frac{(V_{\mathrm{n}}-V_{\mathrm{n\; blank}})\times {\phi }_{{\mathrm{n\; CH}}_4}\times 1000}{22.4\times m}$$ 1

$$Y_{{\mathrm{total\; CH}}_4}=Y_{{\mathrm{1\; CH}}_4}+Y_{{\mathrm{2\; CH}}_4}+...+Y_{{\mathrm{6\; CH}}_4}$$ 2

where V _n is the volume of gas produced at the n-th sampling time (mL), n = 1, 2, 3, 4, 5, 6;

φ_n CH4: Volume concentration of CH₄ measured at the n-th sampling time (%);

m: mass of coal sample (g);

Y _total CH4: Total methane yield per unit mass of the coal sample (μmol/g).

2.3. Experiments on the Physical and Chemical Properties of Coal

2.3.1. Gas Component Tests

Aseptic anaerobic sampling was performed on serum bottles to collect headspace gas. The collected gas was manually injected into an Agilent 7890 gas chromatograph using a Hamilton 1710 gastight syringe. This instrument was fitted with a 1177 column paired with a flame ionization detector (FID) and a thermal conductivity detector (TCD). The TCD operating temperature was maintained at 80 °C; the column temperature was set to 50 °C, and helium was adopted as the carrier gas. ,

2.3.2. X-ray Diffraction Experiment

X-ray diffraction (XRD) analysis was used to describe the microcrystalline structure of coal organic matter and to quantify structural parameters related to the coalification degree and aromatic ordering. The measurements were performed using a D/Max-RINT 2500 X-ray diffractometer with Cu Kα radiation (λ = 0.15406 nm). The operating conditions were set to 40 kV and 100 mA. The divergence slit and antidivergence slit widths were both 1.0 mm, and the receiving slit width was 0.2 mm. Diffraction patterns were recorded over a 2θ range of 5°–80° in continuous scanning mode at a rate of 2° min^–1. The diffraction profiles were processed using Origin software. After background subtraction and smoothing, the characteristic γ, 002, and 100 peaks were deconvoluted by peak fitting. These reflections correspond to aliphatic structures (γ band), aromatic layer stacking (002), and lateral aromatic extension (100), respectively. Based on the fitted peak positions and full width at half-maximum (fwhm), several microcrystalline structure parameters were calculated to quantify the degree of aromatic ordering and structural evolution during coalification. The interlayer spacing between aromatic carbon layers (d ₀₀₂) was calculated using Bragg’s law (eq ), which reflects the distance between stacked aromatic lamellae and decreases with increasing structural ordering. The stacking height of aromatic layers (L _c), representing the vertical coherence length of stacked aromatic sheets, was calculated using the Scherrer equation (eq ). The lateral extension of aromatic layers (L _a), which characterizes the in-plane size of aromatic clusters, was derived from the 100 reflection (eq ). Finally, the average number of crystal stacking layers (Nave) was calculated using eq , providing an integrated indicator of aromatic stacking development. ,

$$d_{002}=\lambda /(2\times \sin {\theta }_{002})$$ 3

$$L_c=0.89\lambda /({\beta }_{002}\times \cos {\theta }_{002})$$ 4

$$L_a=1.84\lambda /({\beta }_{100}\times \cos {\theta }_{100})$$ 5

$$N_{\mathrm{ave}}=L_c/d_{002}$$ 6

where: β₀₀₂ and β₁₀₀ are the full width at half-maximum (fwhm) of the 002 peak and 100 peak, respectively, in radians (rad); θ₁₀₀ is the diffraction angle corresponding to the maximum intensity of the 100 peaks.

2.3.3. Fourier Transform Infrared Spectroscopy Experiment

FTIR spectroscopy was used to characterize the distribution and evolution of functional groups in coal and to evaluate chemical structures associated with microbial methane generation. The analyses were carried out using a Nicolet 6700 FTIR spectrometer. Before testing, the coal samples were dried at a constant temperature of 105 °C for 12 h in a forced-air drying oven to remove moisture, then homogenized with spectroscopic-grade KBr at a mass ratio of 1:100. The mixtures were finely ground to a particle size of <200 mesh and compressed into translucent pellets with thicknesses of approximately 0.1–1.0 mm using a hydraulic press. The obtained infrared spectra were processed using Origin software. After baseline correction, spectral deconvolution and curve fitting were performed to separate overlapping absorption bands, and the integrated peak areas were calculated for semiquantitative comparison across different samples. Functional group assignments for individual absorption bands followed established interpretations reported by Ibarra et al. Beyond individual functional group contents, semiquantitative infrared parameters derived from stable and representative absorption regions provide more robust indicators of coal chemical structure than single peak intensities. − Accordingly, four FTIR-derived structural parameters were calculated to characterize aliphatic and aromatic features relevant to biomethane generation (eqs -). The aliphatic hydrogen fraction (H_al/H), calculated using eq , reflects the relative contribution of aliphatic hydrogen and is indicative of the overall abundance of hydrogen-rich, microbially labile components. The ratio of aliphatic to aromatic structures (A_al/A_ar; eq ) represents the relative enrichment of aliphatic chains compared with aromatic frameworks. The degree of aromatic ring condensation (DOC; eq ) quantifies the extent of aromatic structural ordering, which typically increases with coalification and reduces biodegradability. The CH₂/CH₃ ratio (eq ) describes the average length and branching degree of aliphatic side chains, providing insight into molecular accessibility for microbial degradation, ,,

$${\mathrm{H}}_{\mathrm{al}}{/\mathrm{H}=\mathrm{A}}_{(\mathrm{2800‐3000})}/[{\mathrm{A}}_{(\mathrm{700‐900})}{+\mathrm{A}}_{(\mathrm{2800‐3000})}]$$ 7

$${\mathrm{A}}_{\mathrm{al}}{/\mathrm{A}}_{\mathrm{ar}}{=\mathrm{A}}_{(\mathrm{2800‐3000})}{/\mathrm{A}}_{(\mathrm{1650‐1420})}$$ 8

$${\mathrm{DOC}=\mathrm{A}}_{(\mathrm{700‐900})}{/\mathrm{A}}_{(\mathrm{1650‐1420})}$$ 9

$${\mathrm{CH}}_2{/\mathrm{CH}}_3{=\mathrm{A}}_{2925}{/\mathrm{A}}_{2955}$$ 10

where H_al/H represents the aliphatic hydrogen fraction; A_al/A_ar represents the abundance of aliphatic structures; DOC represents the degree of aromatic ring condensation; and CH₂/CH₃ represents the length and branching degree of aliphatic chains.

2.3.4. Fluorescent Maceral Analysis Method

Fluorescence microscopy is commonly used for identifying coal macerals. In this study, fluorescence microscopy images were collected at China University of Mining and Technology using a Leica DM4P upright fluorescence microscope, paired with a FLEXACAM C1 color imaging camera and CRAIC GeoImage microscopic spectroscopic testing system. The image acquisition control software is LAS X 3.0.14.23224. The observation uses an N PLAN 50 × /0.85 oil immersion objective lens, with a refractive index of 1.518 for the immersion medium. The fluorescence path uses Leica I3 standard fluorescence filter blocks (excitation band 470/40 nm, dichroic mirror 510 LP, and long wave emission cutoff 515 nm), with a fluorescence light source intensity set at 215 and a single channel exposure time of 1 s. The original imaging resolution of the camera is 2880 × 2160, and the image sharpening and multiframe average noise reduction functions were not turned on during the acquisition process. The original fluorescence microscopy images were directly saved for subsequent analysis of microscopic component fluorescence characteristics.

3. Results and Discussion

3.1. Coal Petrography and Coal Quality Analysis

The coal petrography and proximate analysis results of the collected coal samples are presented in Table . The proximate analysis results indicate that among coal samples from the same study area, the air-dried moisture (M _ad) and dry ash-free volatile matter (V _daf) of vitrain are generally higher than those of durain, while the air-dried ash yield (A _ad) is lower. Coal petrographic analysis shows that vitrain is predominantly composed of vitrinite, with contents ranging from 65.5% to 85.9%. By contrast, durain is dominated by inertinite, accounting for 57.2% to 77.7%. Figure illustrates that the dry ash-free volatile matter gradually decreases with increasing coalification degree. Meanwhile, vitrain exhibits distinctly higher V _daf values than durain, demonstrating that vitrain contains significantly more chemically active and volatile components. These components provide a favorable material foundation for the biogenic gas generation.

1. Proximate, Petrographic, and Reflectance of Macerals .

	Proximate Analysis (wt,%)				Maceral Composition (%)
Sample ID	M ad	A ad	V daf	FCad	Vitrinite	Inertinite	Liptinite	Ro,max (%)
GJH-V	9.31	4.71	43.94	48.45	76	19.7	1.8	0.50
GJH-D	8.66	5.3	37.19	54.33	28	64.6	2.3
CJG-V	4.83	5.15	42.49	51.91	85.9	12.5	0.2	0.55
CJG-D	3.76	21.38	33	50.7	9.1	77.7	1.3
DFS-V	5.19	6.71	39.64	53.39	77.6	18.5	0.4	0.63
DFS-D	2.62	25.18	36.1	46.56	21.2	57.2	4.7
HL-V	3.28	5.76	36.41	57.96	65.5	30.5	1.4	0.73
HL-D	2.42	19.43	33.69	52.13	35.8	58.4	1.6
GJB-V	3.04	5.16	38.51	56.55	71.3	22.8	1.9	0.80
GJB-D	2.27	9.72	31.71	60.25	32.2	59.7	2.2

^a M _ad and A _ad represent the moisture content and ash yield on an air-dried basis, respectively. V _daf denotes the volatile matter yield on a dry-ash-free basis, while FC_ad refers to the fixed carbon content on an air-dried basis.

3.

Correlation between R _o,max and V _daf of coal samples.

The fluorescent maceral micrographs are presented in Figure . The bright yellow fluorescent substances observed under a fluorescence microscope were identified as bitumen in coal. Overall comparison reveals that, for vitrain samples (Figure a–e), both the distribution area and fluorescence intensity of bitumen were significantly lower than those of durain samples (Figure f–j), indicating that durain contains a higher bitumen content than vitrain. Meanwhile, the fluorescent bright regions in durain after the FCJ exhibited a distinct increasing trend in area, suggesting that massive bitumen formation was initiated in coal subsequent to the FCJ. This phenomenon implies that more hydrocarbon components in the system became available for further transformation and utilization, thereby facilitating methane generation.

4.

Fluorescent maceral micrographs.

3.2. Characterization of Gas Production

After 80 days of microbial degradation incubation of coal samples, quantitative analysis was performed to characterize the gas production properties. Cumulative gas production data are presented as mean values. The total methane yield was calculated by adding the products of the gas volume produced at each stage and the corresponding methane concentration in the headspace gas mixture. The statistical results of gas production are summarized in Table .

2. Gas Production Statistics Table.

Sample	Cumulative gas production (mL)	Cumulative CO2 production (mL)	Cumulative CH4 production (mL)	Methane yield (μmol/g,coal)	Average CH4 concentration (%)
GJH-V	156.35 ± 8.51	14.48	26.81(95% CI: 21.56, 32.06)	13.63(95% CI: 10.82, 16.44)	17.15
GJH-D	144.07 ± 23.12	10.74	18.46(95% CI: 10.90, 26.02)	4.31(95% CI: 0.29, 8.32)	12.81
CJG-V	126.37 ± 7.94	10.06	27.83(95% CI: 22.16, 33.49)	14.77(95% CI: 13.13, 16.40)	22.02
CJG-D	120.50 ± 1.61	9.12	21.87(95% CI: 20.07, 23.67)	8.11(95% CI: 2.40, 13.83)	18.15
DFS-V	135.90 ± 6.35	17.50	25.83(95% CI: 22.82, 28.85)	12.53(95% CI: 8.25, 16.82)	19.01
DFS-D	127.90 ± 7.71	13.57	18.31(95% CI: 14.40, 22.22)	4.14(95% CI: 0.71, 7.57)	14.32
HL-V	133.03 ± 5.65	10.75	25.32(95% CI: 22.82, 27.82)	11.96(95% CI: 6.81, 17.11)	19.03
HL-D	123.00 ± 6.53	9.99	23.26(95% CI: 19.66, 26.85)	9.67(95% CI: 5.22, 14.10)	18.91
GJB-V	142.70 ± 7.15	14.69	28.71(95% CI: 25.95, 31.47)	15.75(95% CI: 10.97, 20.52)	20.12
GJB-D	133.53 ± 3.19	12.43	28.48(95% CI: 27.39, 29.57)	15.49(95% CI: 9.13, 21.84)	21.33
K	118.03 ± 17.81	10.83	14.60(95% CI: 7.84, 21.36)	/	12.37

The results show that the biogenic gas derived from microbial degradation of coal is predominantly composed of CH₄ and CO₂. Trace amounts of H₂, CO and C₂–C₅ hydrocarbons were detected in individual coal samples, with their concentrations generally below 0.1%. No C₂–C₅ gaseous hydrocarbons were identified in the blank control groups. In this study, the total biogas production ranged from 120.50 to 156.35 mL, while the cumulative methane output varied between 18.31 and 28.71 mL. The specific methane production (blank methane production subtracted) reached 4.14–15.75 μmol/g, and methane accounted for 12.81%–22.02% of the total biogas volume.

Notably, no further gas production was observed from any of the serum bottles at the end of the 80-day incubation period. Headspace gas detection revealed that the CH₄ concentration ranged from 20.57% to 40.50%, which was slightly higher than that measured on the 60th day. This evidence suggests that the methane concentration increased continuously during the biodegradation process. Nevertheless, the gas production rate declined progressively and ultimately ceased, demonstrating that the metabolic activity of gas-producing microorganisms was gradually limited in the late incubation stage.

Further analysis (Figure a) shows that both total gas production and CH₄ production in vitrain are higher than those in durain. However, there is no correlation between the total biogas and CH₄ production across different coal samples, indicating that microbial gas production is a complex process influenced by multiple factors. The error bar chart of gas production for different samples reveals that the variation in gas production for Guojiahe durain is larger than that of other coal samples (Figure b). As shown in Figure c, both the average CH₄ concentration and the final concentration at the end of the vitrain experiment are higher than those of durain. Since biogenic methane is the focus of this study, the subsequent part of this paper will discuss the relationship between methane production and coal composition.

5.

(a) Gas production and methane yield from microbial degradation of coal samples with different metamorphic degrees; (b) error bars of gas production from vitrain and durain with different metamorphic degrees; (c) average CH₄ concentration and node concentration at the end of the experiment.

3.3. Law of Biogenic Methane Production under the Influence of Coalification

Coalification fundamentally governs biogas yield by modifying the physicochemical structure of coal. Based on previous studies by Lin et al., the first coalification jump in the Huanglong Jurassic Coalfield has been reported to occur at approximately R _o, max ≈ 0.6%, though the precise threshold may vary depending on coal type and local geological conditions. As shown in Figure a, both vitrain and durain exhibit a general N-shaped variation pattern in methane production that is tentatively consistent with structural reorganization associated with the FCJ. Before the inferred FCJ interval (R _o,max < 0.6%), methane production tends to increase with increasing R _o, max; a decline is observed near R _o, max ≈ 0.6%, which may be associated with the FCJ; after this interval (R _o, max > 0.6%), methane production appears to continue increasing as R _o, max rises. Meng et al. found that methane production gradually decreases with increasing coal rank. The coal samples in ref. cover an R _o,max range of 0.48–2.07, which differs significantly from the R _o,max range of 0.5–0.8 for the coal samples adopted in this study. Namely, the coalification degree range of representative samples in this work is narrower. These results suggest that coalification exerts a notable control on biogenic methane generation. Figure b illustrates the relationship between V _daf and methane production for vitrain and durain. The methane production of vitrain shows a weak positive linear correlation with increasing V _daf, while durain exhibits a negative linear correlation. This phenomenon indicates that although total volatile matter contributes to biomethane generation, it is not the dominant factor. Instead, the types and structures of specific functional groups within the volatile matter are critical in determining whether they can be effectively utilized by microorganisms and converted into methane.

6.

(a) Relationship between R _o,max and methane production of coal samples with different metamorphic degrees; (b) relationship between methane production and coalification degree V _daf.

3.4. Law of Biogenic Methane Production through Maceral Components’ Influence

The proportion of microscopic macerals determines the macroscopic coal lithotypes. Typically, vitrain contains over 50% vitrinite, while durain has more than 50% inertinite. The correlations between vitrinite/inertinite contents in vitrain and durain and methane production are illustrated in Figure a and Figure b. Figure a reveals a positive correlation between vitrinite content and methane production, indicating that vitrinite content contributes to methane production to a certain extent. Figure b shows a negative correlation between inertinite content and methane production efficiency, indicating that microorganisms are unable to effectively utilize inertinite during methane generation. Consequently, vitrain, dominated by vitrinite, yields more methane than durain, which is dominated by inertinities. This is consistent with previous findings that vitrinite is more readily degradable by microorganisms and exhibits a higher methane-generating potential. Figure c indicates that the correlation between liptinite content and methane production is more complex, with an overall low correlation.

7.

Correlation diagrams between methane production and maceral contents: (a) vitrinite; (b) inertinite; (c) liptinite.

Figure shows the correlation between V _daf and maceral content. This indicates that ash yield is positively correlated with vitrinite content in vitrain, showing a significant relationship. Moreover, its variation trend is consistent with the correlation observed between methane production and maceral content. In addition, Figure c indicates that organic volatile matter is positively correlated with liptinite content in durain, suggesting that both macroscopic coal lithotypes and microscopic macerals control volatile matter beneficial to methane production.

8.

Direct correlation diagrams between V _daf and maceral contents: (a) vitrinite; (b) inertinite; (c) liptinite.

3.5. Gas Generation Mechanism of High-Volatile Bituminous Coal under the Coupling Effect of Coalification and Macrolithotypes

Microbial coal degradation for gas production refers to the process in which organic matter within coal is transformed into gases such as CH₄ and CO₂ through microbial activity. Coalification plays a key role in controlling microbial degradation efficiency and gas generation by modifying the physical and chemical structures of coal. This study mainly focuses on the influence of the coal’s chemical structure on biogenic methane production.

3.5.1. X-ray Diffraction Analysis

The XRD patterns of the raw coal samples are presented in Figure . It should be noted that since this study focuses on analyzing the order parameters of the organic macromolecular structure of coal and does not involve the crystal structure of mineral phases, the samples were not subjected to deashing treatment before testing. Several sharp diffraction peaks observed in the spectra originate from these unremoved mineral impurities. After smoothing and peak fitting, the calculated parameters are listed in Table . Diffraction peaks corresponding to aliphatic carbon microcrystalline structures (γ band) and aromatic carbon microcrystalline structures (002 band) were observed. The overlap of their peak areas results in an asymmetric peak shape, and the intensity of the 100-diffraction peak is significantly weaker.

9.

XRD spectra of raw coal samples: (a), (b); typical XRD peak-fitting diagrams for vitrain (c) and durain (d).

3. Table of XRD Test Parameter Results.

Sample	R o, max	2θ 002 (°)	d 002 (Å)	L c (Å)	2θ100 (°)	L a (Å)	Nave
GJH-V	0.50	23.277	3.818	8.946	42.368	19.1	3.343
CJG-V	0.55	23.281	3.818	8.643	43.948	17.801	3.264
DFS-V	0.63	23.304	3.814	8.947	43.788	15.953	3.346
HL-V	0.73	23.315	3.812	9.358	44.377	16.826	3.455
GJB-V	0.80	23.333	3.809	8.504	44.646	16.735	3.232
GJH-D	0.50	23.569	3.772	9.67	43.714	17.594	3.564
CJG-D	0.55	23.605	3.766	10.021	43.607	14.835	3.661
DFS-D	0.63	23.697	3.751	10.432	42.192	14.184	3.781
HL-D	0.73	24.019	3.702	11.204	43.789	16.323	4.027
GJB-D	0.80	24.700	3.601	10.444	42.629	18.232	3.900

Relevant studies have demonstrated that the area of the γ-peak located on the left side of 20° in the XRD pattern is associated with aliphatic substances, whereas the 002 peak is primarily correlated with aromatic structures, ,, Data analysis indicates that with the increase in R _o, max in raw coal samples, the d ₀₀₂ value decreases gradually (Figure a), and the reduction is more pronounced in durain. This phenomenon suggests that the interlayer spacing of aromatic lamellae decreases with increasing coalification degree, which is consistent with the evolutionary law of coalification, and the decreasing trend in durain is more distinct than that in vitrain, These results are consistent with those reported by Wang et al. The stacking height of aromatic lamellae (L _c) may exhibit a general three-stage variation with the increase in R _o, max, with a notable inflection observed in the vicinity of the inferred FCJ interval (Figure b); notably, the L _c value of durain is consistently higher than that of vitrain. In contrast, the diameter of aromatic lamellae (L _a) shows a general trend of first decreasing and then increasing with the increase in R _o, max, also with a notable inflection observed in the vicinity of the inferred FCJ interval (Figure c), and vitrain has a larger L _a value than durain.

10.

Correlation data analysis (a) relationship between R _o, max and d ₀₀₂; (b) relationship between R _o, max and L _c; (c) relationship between R _o, max and L_a ; (d) comparison of correlation between methane volume and d ₀₀₂; (e) comparison of correlation between methane volume and L _c; (f) comparison of correlation between methane volume and L_a .

It is inferred that the aromatic structures may undergo a “deconstruction-reconstruction” transformation in the vicinity of the inferred FCJ interval. Before the coalification jump, the macromolecular structure of low-rank coal contains abundant oxygen-containing functional groups, which create steric hindrance and restrict the orderly stacking of aromatic layers. As the coalification degree increases, the deoxygenation reactions occur preferentially, destroying the supporting structure between lamellae and leading to the disordered rearrangement of partially ordered stacked lamellae. Macroscopically, this process results in a temporary decrease in L _c. Meanwhile, small aromatic clusters preferentially form irregular macromolecules with numerous structural defects through edge condensation, thereby reducing the statistical value of the aromatic lamellae diameter (L _a). Near the inferred FCJ interval, a large amount of volatile matter is appearing to be released, and the dominant mechanism of coalification reactions may shift from “functional-group removal” to aromatization and polycondensation. After the removal of functional groups, the steric hindrance between lamellae is significantly reduced, leading to a rapid increase in the number of stacked aromatic lamellae and a marked rise in L _c. After the jump, the stacking degree of aromatic lamellae reaches a relatively high level, accompanied by a significant increase in internal structural stress and a continuous decrease in hydrogen content. The weakening of weak intermolecular interactions between lamellae causes the dislocation of some stacked lamellae and a reduction in effective stacking height, which is macroscopically reflected by the decrease of L _c with the increase of R _o, max. A large number of free radicals generated in this process provide a driving force for the lateral preferential growth of aromatic clusters, and the polycondensation reactions further promote the increase of L _a. The inherent structural differences between coal lithotypes result in consistently higher L _c values for durain than for vitrain, which is consistent with the findings of Zhou et al.

Correlation analysis reveals that for vitrain, the methane production has poor correlations with d ₀₀₂ and L _a of raw coal samples, but shows a distinct negative correlation with L _c. This indicates that a smaller L _c is more conducive to methane generation. It is inferred that for vitrain, a smaller L _c represents a higher abundance of “aliphatic precursor materials” available for methane generation, thus resulting in a higher methane yield. Therefore, L _c can serve as an effective “negative indicator” for evaluating the hydrocarbon generation potential of vitrain at the current stage. For durain, the methane production has poor correlations with L _c and L _a of raw coal samples, but presents an obvious negative correlation with d ₀₀₂, suggesting that a smaller d ₀₀₂ (i.e., a higher metamorphic degree) is more favorable for methane generation. The biogenic gas production of durain samples does not simply decrease with the increase of R _o, max; instead, it is jointly controlled by the coal metamorphic degree and maceral composition, and the latter may play a dominant role. Durain with a relatively high metamorphic degree is likely to offset the negative impacts caused by the increase of metamorphic degree due to its relatively high liptinite content.

The results of this study suggest that the impact of the first coalification jump on biomethane generation is predominantly structural rather than compositional, potentially marking a notable transition in the abundance and accessibility of biodegradable aliphatic components. While earlier investigations linked the FCJ to abrupt changes in gas yield or pyrolysis characteristics, our combined FTIR-XRD analysis provides supporting evidence that the observed decline in biomethane production near R _o, max ≈ 0.6% may be associated with a reduction in aliphatic structural domains and a reorganization of aromatic stacking, a pattern broadly consistent with FCJ-related structural reorganization reported in the literature. The observed negative correlation between methane yield and aromatic stacking height (L _c) in vitrain suggests L _c may serve as an effective structural indicator of microbial degradability, potentially reflecting the progressive loss of aliphatic precursors as coalification advances. This finding extends the application of XRD-derived parameters beyond thermal maturity assessment and establishes a mechanistic bridge between coal microstructure and biogenic gas generation. Importantly, the contrasting behavior of vitrain and durain suggests the need to distinguish between coalification-dominated and maceral-dominated control regimes in biogenic coalbed methane systems. In vitrain, aliphatic structural evolution appears to be closely associated with coalification and relatively independent of maceral variability, consistent with an FCJ-associated methane generation pattern. In contrast, the aliphatic structure of durain is decoupled from coalification alone and is strongly influenced by the relative abundance of hydrogen-rich macerals, particularly vitrinite and liptinite, which locally preserve biodegradable substrates despite overall inertinite dominance. This dual control mechanism explains the persistence of biomethane generation in inertinite-rich Jurassic coal reservoirs and highlights the importance of integrating coal rank, lithotype, and microstructural parameters when evaluating biogenic coalbed methane potential.

3.5.2. Fourier Transform Infrared Spectroscopy Analysis

Fourier transform infrared spectroscopy is an effective technique for identifying functional groups in coal, and the absorption peak areas in the spectra provide further information about the distribution of these functional groups. , The FTIR results are presented in Figure , and the calculations of the area proportions and ratios of each functional group are presented in Table . The proportion of peak areas attributed to oxygen-containing functional groups (including OH, C = O, and C–O) ranges from 48.753% to 56.248% in vitrain and from 49.433% to 66.707% in durain. Overall, durain exhibits a higher proportion of oxygen-containing functional groups than vitrain, which reflects differences in their formation environments. The total aliphatic content accounts for 12.896%–15.896% in vitrain and 10.826%–15.941% in durain. In general, vitrain exhibits slightly higher aliphatic content than durain in the same coal sample, which is consistent with the findings reported by Li et al. However, the CH₂/CH₃ ratio indicates that vitrain values are lower than or comparable to those of durain, suggesting a lower degree of aliphatic chain development in vitrain, where short alkyl groups mainly serve as side chains attached to aromatic rings.

11.

FTIR spectra of raw coal samples.

4. Structural Parameters of Functional Groups in Coal Samples .

Sample	Hydroxyl groups (%)	Aliphatic C–H stretching groups (%)	CO groups (%)	Aromatic CC (%)	Aliphatic C–H bending groups (%)	C–O groups (%)	Aromatic C–H groups (%)	Unclassified bands (%)	Total aliphatic group (%)	Hal/H	Aal/Aar	DOC	CH2/CH3
GJH-V	26.821	6.271	6.483	21.826	6.625	22.944	1.569	7.461	12.896	0.800	0.287	0.072	2.681
CJG-V	19.248	8.714	6.340	25.293	6.897	23.165	2.238	8.105	15.610	0.796	0.345	0.088	2.461
DFS-V	20.761	6.497	7.234	23.395	6.907	23.995	2.238	8.973	13.404	0.744	0.278	0.096	2.562
HL-V	22.871	8.016	4.564	22.085	6.096	24.017	3.602	8.750	14.112	0.690	0.363	0.163	2.546
GJB-V	23.329	8.966	4.317	22.973	6.930	24.428	2.427	6.630	15.896	0.787	0.390	0.106	2.326
GJH-D	18.362	4.302	7.886	24.813	8.094	25.231	1.889	9.423	12.396	0.695	0.173	0.076	2.776
CJG-D	27.236	4.693	9.980	23.419	6.133	19.634	3.888	5.017	10.826	0.547	0.200	0.166	3.247
DFS-D	32.622	9.216	9.460	17.562	2.294	24.625	3.052	1.169	11.510	0.751	0.525	0.174	3.315
HL-D	28.236	11.026	5.532	19.392	4.914	18.481	3.354	9.065	15.941	0.767	0.569	0.173	1.614
GJB-D	20.664	7.681	7.282	23.288	6.804	21.486	3.658	9.137	14.484	0.677	0.330	0.157	2.539

^aThe unclassified bands mainly include the peak areas at 1410 cm^–1 and 1350 cm^–1.

To further investigate the influence of characteristic parameters associated with different coalification degree grades and macroscopic coal lithotypes on methane production, Pearson correlation analysis was conducted using Origin software to evaluate the relationships between methane yield and variables such as functional groups, semiquantitative functional group parameters, and major macerals. As presented in Figure , the net methane output of vitrain and durain displays a prominent positive correlation solely with the total aliphatic structural content, whereas weak correlations are observed with other structural groups. In the analysis of the correlations between on correlations between semiquantitative functional group parameters and methane production, it was found that only the Hal/H and CH₂/CH₃ ratios of vitrain show strong correlations with methane production parameters, whereas correlations between other parameters and methane production are weak. For vitrain macerals, the total aliphatic content is positively correlated with the vitrinite content and negatively correlated with the inertinite content, but both correlations are weak. For durain, the correlations between total aliphatic structural content and vitrinite/inertinite contents follow the same pattern as in vitrain, but the correlations are stronger than those in vitrain.

12.

Correlation diagrams between methane production, and various functional groups, semiquantitative parameters, and other indicators: (a) vitrain; (b) durain.

Figure a shows that vitrain consistently contains a higher abundance of aliphatic structural-functional groups than durain, reflecting fundamental differences in substrate availability. Coalification appears to exert a notable control on aliphatic structures in vitrain, which display a variation pattern tentatively consistent with FCJ-related structural reorganization. Before the inferred FCJ interval (R _o, max < 0.6%), aliphatic content tends to increase with coalification, possibly due to the preferential removal of oxygen-containing functional groups while aliphatic structures are relatively preserved. Near R _o, max ≈ 0.6%, a notable decline in aliphatic content is observed, which may be associated with structural reorganization consistent with the FCJ, followed by gradual enrichment of more stable aliphatic components at higher ranks. In contrast, durain exhibits a nonmonotonic aliphatic evolution pattern that deviates from coalification-dominated behavior, reflecting additional control by maceral composition. The semi-quantitative FTIR parameters H_al/H and CH₂/CH₃ closely track aliphatic structural content (Figure b–c), confirming their effectiveness as proxies for aliphatic abundance and structural evolution in both lithotypes.

13.

Relationship diagram between the content of total aliphatic structural functional groups, H_al/H, CH₂/CH₃ and R _o, max.

The variation pattern of total aliphatic structural functional group content in durain is not entirely synchronized with that in vitrain; specifically, the variation pattern of total aliphatic structural functional group content in durain is not fully aligned with coalification. This may be because the total aliphatic structural group content in durain is controlled not only by coalification but also by vitrinite and liptinite content within durain, maceral fractions that facilitate methane generation. Figure presents the correlation between the combined content of vitrinite and liptinite (among macerals) and the total aliphatic structural content. The figure shows a strong positive correlation between the total vitrinite and liptinite content and the total aliphatic structural group content in durain, whereas the correlation between these two parameters in vitrain is not significant. This indicates that the total aliphatic structural group content in durain is significantly controlled by macerals. Specifically, the total content of hydrogen-rich components (vitrinite and liptinite) in durain is the key factor governing its aliphatic structure. which is further supported by the fluorescence micrographs shown in Figure f–j.

14.

Relationship diagram between the content of total aliphatic structural functional groups and the content of vitrinite + liptinite.

Further analysis suggests that there is no fundamental difference in microbial degradability between vitrain and durain: both use aliphatic structures as substrates to produce methane gas, and the abundance of aliphatic structures directly determines the amount of gas produced. The aliphatic structure content in vitrain is significantly controlled by coalification and exhibits a low correlation with macerals. In contrast, the aliphatic structure content in durain is controlled by the dual factors of coalification and macerals with the latter playing a dominant role. Meanwhile, the data indicate that vitrinite in different macroscopic coal lithotypes contributes only to a limited extent to methane production, whereas the inertinite content contributes less. This is also the main reason that the methane production of vitrain is higher than that of durain. The high aliphatic content of vitrain reflects its theoretical methane generation potential, which is consistent with the understanding proposed by Huang et al. Although the aliphatic content of durain is not as high as that of vitrain, durain, due to its greater thickness and larger absolute reserves, can still generate massive amounts of methane gas under favorable conditions for biogas formation. This explains why biogas accumulates in the Jurassic inertinite-rich coal reservoirs of northwestern China. Figure illustrates the mechanism of biomethanogenesis regulated by coalification and coal maceral components.

15.

Mechanism diagram of biomethane production controlled by coalification and macrolithotype composition.

To minimize potential confounding effects arising from microbial variability, the same enriched microbial consortium derived from the produced water of a single coalbed methane well in the Dafosi Mine was used for all degradation experiments. All incubations were performed under strictly identical conditions, including medium composition, inoculum volume, coal particle size, incubation temperature (37 °C), anaerobic conditions, and cultivation duration. As a result, differences in methane production among samples primarily reflect variations in coal properties rather than experimental or microbial inconsistencies. Although microbial community composition may evolve during incubation, its influence on intersample variability is considered secondary in this study because the inoculum source, enrichment history, and physicochemical conditions were uniform across all experiments. Moreover, the observed systematic relationships between methane yield and coal structural parameters, particularly the consistent correlations with aliphatic structural abundance and XRD-derived aromatic stacking height (L _c) suggest that coal microstructure exerts a dominant control on biomethane generation within the investigated coalification window. Previous investigations in the Huanglong and adjacent Jurassic coalfields further indicate that biogenic methane is dominantly generated via the CO₂ reduction pathway, which is relatively robust under neutral pH and strictly anaerobic conditions. The predominance of CH₄ and CO₂ in the produced gas, together with the absence of significant higher hydrocarbons, is consistent with hydrogenotrophic methanogenesis and supports the assumption of comparable methanogenic pathways across samples. While future studies incorporating quantitative PCR or high-throughput sequencing would provide additional constraints on microbial community dynamics, the consistent experimental design and convergence of structural, petrographic, and gas-generation evidence indicate that the key conclusions of this study are not primarily controlled by microbial variability.

3.6. Statistical Considerations and Limitations

It should be noted that the correlations identified in this study are based on a limited number of coal samples spanning five discrete coalification levels (n = 5), which constrain the statistical power of regression and correlation analyses. Consequently, the relationships observed among biomethane production, coalification degree, maceral composition, and structural parameters should be interpreted as indicative trends rather than definitive quantitative laws. While the consistency of variation patterns across vitrain and durain supports the robustness of the inferred mechanisms, the small sample size inherently increases uncertainty and limits the generalizability of correlation coefficients. To reduce the risk of overinterpretation, only correlations exhibiting relatively high coefficients of determination (R > 0.6) are considered to reflect strong associations, whereas relationships with lower R² values are treated as weak or suggestive and are discussed qualitatively. Pearson correlation coefficients were used to evaluate linear relationships, and the associated p-values were examined to assess statistical significance at conventional confidence levels (p < 0.05). However, given the small data set, statistical significance alone does not imply causality, and correlations with marginal significance are interpreted with caution. Accordingly, causal language has been avoided where correlations are weak, and mechanistic interpretations are supported primarily by the consistency of independent evidence from microbial experiments, FTIR functional group analysis, and XRD-derived structural parameters rather than solely by the correlation strength itself.

4. Conclusions

In this study, five coal samples with different coalification degrees before and after the FCJ were selected, with R _o,max ranging from 0.50% to 0.80%. Microbial degradation and gas production simulation experiments were conducted on typical coal lithotypes (vitrain and durain). The main conclusions are as follows:

• 1The methane yield of vitrain (11.96–15.75 μmol/g) was generally higher than that of durain (4.14–15.49 μmol/g). The absence of a significant correlation between total biogas and CH₄ yield indicates that coal biodegradation and methanogenesis are governed by multiple interacting factors rather than a single controlling variable.
• 2Both vitrain and durain exhibited a general N-shaped variation in methane production with increasing coalification degree, a trend tentatively consistent with FCJ-related structural reorganization rather than an abrupt jump. A notable decline in methanogenic capacity was observed near R _o, max ≈ 0.6%, followed by a recovery at higher ranks, suggesting that coalification exerts a notable control on biomethane generation. The weak and statistically insignificant correlation between volatile matter yield (V _daf) and methane production indicates that methanogenesis is more closely related to specific biodegradable structural components than to total volatile content. It should be noted that this interpretation is based on a limited number of samples (n = 5) and should be treated with appropriate caution pending validation with a larger and more densely sampled data set, particularly within the critical R _o, max = 0.55%–0.70% interval.
• 3With increasing coalification, the aromatic framework of coal underwent a discernible “deconstruction–reconstruction” evolution in the vicinity of the inferred FCJ interval, evidenced by notable inflections in both L _c and L _a near R _o, max ≈ 0.6%. The observed negative correlation between L _c and methane yield in vitrain suggests that a smaller L _c may indicate greater abundance of aliphatic precursors available for methanogenesis. The relatively higher exinite content in higher-rank durain, which becomes extensively bituminized near the inferred FCJ interval, may effectively offset the inhibitory effect of enhanced coalification on methanogenesis.
• 4Coal biogenic methanogenesis is essentially the anaerobic biochemical transformation of aliphatic structures, with gas production scale directly determined by the abundance of bioavailable aliphatic components. Within the studied R _o, max range (0.50%–0.80%), the aliphatic structure of vitrain is primarily controlled by coalification, whereas that of durain is codominated by coalification and maceral assemblages, with macerals as the leading factor. The higher methanogenic potential of vitrain over durain is primarily attributable to its greater vitrinite content and correspondingly higher aliphatic abundance, given the limited contribution of inertinite to biomethane generation.

The results indicate that near the first coalification jump (FCJ), the methane generation capacity of coal does not decline with an increasing coal rank, revising the conventional understanding of the evolution of methane generation potential. The methane yield rises following the FCJ, which may be attributed to the increased asphaltene content induced by the coalification jump. As vital hydrocarbon-generating precursors, asphaltenes can supply substrates for the formation of methane. This conclusion reasonably explains why coal seams enriched in commercially viable secondary biogenic methane worldwide generally exhibit an R _o,max of around 0.7%.

Data will be made available upon request.

H.Z.: Investigation, Writingoriginal draft, Software. Y.L.: Supervision, Funding acquisition. L.X.: Methodology. X.N.: Formal analysis, Data curation. Y.H.: Methodology. Y.M.: Data curation. O.D.O.: Writingreview and editing, Validation and Conceptualization. S.Z.: Methodology.

The authors declare no competing financial interest.