Influence of Paraffin on the Structure and Pore Characteristics of Solid Organic Matter

Abstract

Paraffin, a naturally occurring macromolecular organic compound, is ubiquitously distributed in geological formations such as shale and coal seams. Its interaction with solid organic matter exerts a significant influence on the microstructure and pore architecture of host rocks, representing a critical geological process. To elucidate the underlying mechanisms of this interaction, this study systematically investigated the structural incorporation and pore-modifying behavior of paraffin within three representative high-molecular-weight organic geological substrates: kerogen, solid bitumen, and coal. A combined experimental approach integrating nitrogen adsorption and X-ray diffraction techniques was employed to characterize these interactions. The findings reveal that paraffin predominantly resides within the amorphous carbon fraction of host organic matter, where it enhances local crystallinity. However, its impact on pore networks varies markedly across different substrates: in kerogen, progressive pore reduction occurs via organic matrix swelling and paraffin precipitation. In contrast, excessive paraffin accumulation triggers pore segmentation in solid bitumen and, most notably, induces pore fracturing or splitting within coal, severely compromising pore connectivity. This study concludes that paraffin exerts the least influence on the pore connectivity of solid bitumen, and the most significant impact on coal. These insights bear direct implications for unconventional hydrocarbon systems. In shale gas reservoirs, the presence of asphaltene and solid bitumen may sequester paraffin, thereby mitigating damage to the porous matrix of kerogen. Conversely, in coalbed methane systems, paraffin effectively reduces the volume and connectivity of coal seam pores, resulting in significant adverse effects on gas production. Therefore, heating treatments may be required during exploitation to mitigate the detrimental impacts of paraffin accumulation.

1. Introduction

Paraffin is an important product derived from the cracking of macromolecular organic matter, and its formation is generally believed to be jointly controlled by basin type and stratigraphic sequence. Traditionally, paraffin has been considered to be of nonmarine origin. In conventional oil and gas geochemical studies, C₂₂₊ alkanes are broadly defined as paraffin wax, and crude oil with a wax content exceeding 5% is classified as high-wax oil. Based on Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance (NMR) analyses, Musser and Kilpatrick classified petroleum waxes into two types: microcrystalline wax, which is rich in isoparaffins and cycloparaffins, and paraffin wax, which is dominated by normal alkanes. However, subsequent studies have shown that high-molecular-weight alkanes are also abundant in some marine basins. − Tegelaar suggested that marine or lacustrine algal remains, as well as macerals other than cutinite and vitrinite derived from higher plants, may act as potential precursors for waxy hydrocarbons in crude oil. Paraffin can form during hydrocarbon evolution as long as the parent molecules contain abundant long-chain alkyl moieties, either in free states or covalently bound forms. To date, large amounts of waxy crude oil and paraffin have been discovered in major petroliferous basins worldwide, including the North Sea Basin, Barito Basin, Sabatayn Basin, Songliao Basin, and South Yellow Sea Basin, as well as in coal seams and asphalt occurrences. −

As a key product of hydrocarbon generation, paraffin exerts a significant influence on the development of associated oil and gas resources. Musser and Kilpatrick confirmed that different types of paraffin have distinct effects on the physical properties of waxy crude oil. Meanwhile, paraffin precipitation and deposition play a critical role in crude oil migration and production. Zhu divided the paraffin deposition-dissolution process into five stages: (1) diffusion of wax molecules from the wax layer into light oil components; (2) diffusion of the bottom wax layer toward the wax-oil interface; (3) dissolution of wax crystal particles; (4) weakening of the structural strength of the wax layer; and (5) shear stripping of the wax layer. They further established an empirical model to predict this process. Oualid explored the interaction mechanism between paraffin and asphaltenes, indicating that relatively strong paraffin-asphaltene interactions in crude oils with high aromatic contents may weaken paraffin-aromatic interactions. This negatively affects wax crystal morphology, oil viscosity, and the quality of deposited wax. Their study also proposed that asphaltene molecules possess a much wider chemical space than waxes due to differences in molecular size, number and size of polycyclic aromatic cores, oxygen/nitrogen/sulfur functional groups, and isoelectric points, whereas waxes vary only in alkyl chain length, branching degree, and cyclization mode.

Although numerous studies have been conducted on paraffin, its effects on the structure and porosity of solid organic matter remain poorly understood. In particular, the following scientific questions remain unresolved: (1) In what form does paraffin occur with solid organic matter and occur within its structural framework? (2) How does paraffin affect the pore structure of solid organic matter? These issues are closely related to hydrocarbon expulsion efficiency during thermal maturation and cracking. Notably, pore structure is a key controlling factor in the migration of coalbed methane (CBM) and shale gas. −

Gas adsorption experiment (N₂/CO₂) is the most critical testing technique for pore structure research. − Ni et al. identified three distinct groups of equivalent pore diameters in coal samples (6.7, 3.9, and 3.8 nm) using nitrogen adsorption experiments, with corresponding methane adsorption capacities of 26.7 cm³/g, 22.6 cm³/g, and 22.9 cm³/g²². Their work, supported by both experimental observations and numerical simulations, confirmed that methane adsorption in coalbed methane systems is mainly controlled by the equivalent pore size of coal. Xing et al. systematically investigated the effects of kerogen chemical structure and pore structure on the water vapor adsorption (WVA) mechanism of kerogen by combining adsorption experiments with multiple characterization techniques including Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). Finally, a chain-type response framework of “chemical structure-pore structure-WVA” was established, which deepens the understanding of the water uptake behavior of graphitized organic matter. Yuan and Rezaee further demonstrated via nitrogen adsorption tests that improved pore connectivity represents a key mechanism for enhancing coalbed methane recovery. In shale gas studies, combined low-field nuclear magnetic resonance (NMR) and nitrogen adsorption analyses revealed that clay-bound water, which displays behavior similar to paraffin, significantly complicates the fractal characteristics of nanoscale pore networks. Such complexity can hinder efficient fluid transport in shale reservoirs and thus reduce shale gas mobility. Experimental investigations into the adsorption behavior of Longmaxi Formation shale indicated that the desorption of adsorbed gas into free gas within coal microcavities is a critical process during coalbed methane development, whereas shale gas migration from the matrix to natural or artificial fractures is primarily regulated by pore connectivity. X-ray diffraction (XRD) has become an effective technique for exploring interaction mechanisms among various organic matter components. Liang et al. applied this method to clarify the occurrence space of liquid organic matter within solid bitumen, suggesting that the amorphous carbon region dominated by aliphatic carbon acts as the main host for small-molecular-weight compounds. This type of interaction significantly promotes volume expansion in solid organic matter. In recent years, computer technology has also been widely applied in the pore structure research of solid organic matter. − Liang et al. carried out a gas adsorption study on kerogen from the Longmaxi Formation in the Sichuan Basin based on molecular simulation technology. The study suggests that with the increase of thermal maturity, the aromatic structure undergoes condensation and rearrangement, which effectively reduces the adsorption sites in the structure, and further decreases the CO₂/CH₄ selective adsorption coefficient. Liu et al. carried out pore characterization on the pyrolysis products of Type III kerogen via N₂ adsorption experiments, and then adopted two machine learning algorithms, namely random forest (RF) and extra trees (ET), to predict the pore structure evolution during thermal maturation. The results of intelligent modeling show that the random forest algorithm outperforms the extra trees algorithm in estimating the N₂ adsorption/desorption volumes of the samples. Finally, the sensitivity analysis results indicate that during the thermal evolution process, water exerts a significantly stronger influence on shale pore development than temperature.

In this study, solid bitumen, kerogen, and coal three types of natural solid organic matter were selected as research objects to explore their interactions with refined natural paraffin samples. To reveal the pore-modifying effects of paraffin on solid organic matter and its occurrence mechanisms in deep geological settings, two major analytical methods were adopted: nitrogen adsorption measurements and X-ray diffraction (XRD) analysis. The research focuses on two main objectives: (1) To characterize the changes in pore properties of different solid organic matter types induced by paraffin, and establish a conceptual model for evaluating pore modification; (2) To clarify the occurrence state and structural modification effects of paraffin within solid organic matter, and explore the corresponding interaction mechanisms.

2. Material and Method

2.1. Materials

2.1.1. Solid Organic Matter

In this study, two solid bitumen samples, two kerogen samples, and two coal samples were used, with their relevant geochemical parameters presented in Figure .The solid bitumen samples were collected from Wuerhe natural bitumen (WEH) in the Junggar Basin, Karamay, Xinjiang Uygur Autonomous Region (northwest China), and Huoshiling natural bitumen (HSL) in the Sichuan Basin, Guangyuan, Sichuan Province (southwest China). After sampling, surface impurities were removed by washing with deionized water. The samples were then crushed to 120 mesh, followed by density separation using zinc bromide solution (density = 1.7 g/cm) to eliminate mineral components. The floating solid fraction was collected and rinsed with deionized water to complete sample preparation.

1.

Basic geochemistry parameter of samples.

The kerogen samples were derived from Lucaogou Formation source rocks (LCG) in the Jimsar Depression, Junggar Basin, Changji Hui Autonomous Prefecture, Xinjiang Uygur Autonomous Region (northwest China), and Xiamaling Formation source rocks (XML) in the Yanliao Basin, Hebei Province (eastern China). The source rock samples were ground to 120 mesh, after which inorganic minerals were sequentially removed using hydrochloric acid and hydrofluoric acid. Residual mineral impurities were further eliminated via density separation with zinc bromide solution (1.7 g/cm). Soluble organic matter was then extracted using a dichloromethane/methanol mixture (93:7, v/v), and the residues were dried to obtain purified kerogen samples.

The coal samples were collected from the Fushun Basin, Fushun, Liaoning Province (northeast China, FS), and the Hunchun Basin, Hunchun, Jilin Province (HC). The collected samples were crushed to 120 mesh and treated by density separation with zinc bromide solution (1.7 g/cm) to remove minerals. Purified coal samples were obtained after extracting soluble organic matter with a dichloromethane/methanol mixture (93:7, v/v) and subsequent drying.

Notably, the solid bitumen samples were not subjected to acid treatment (hydrochloric acid or hydrofluoric acid) to avoid potential structural damage. Similarly, acid treatment was not required for the coal samples due to their naturally high organic purity.

2.1.2. Paraffin

Natural solid paraffin shall be selected as the paraffin sample, and the melting point ≥ 65 °C (ASTM D 87).

2.2. Experiment

The experimental procedure is illustrated in Figure and consists of the following steps:

• 1Sample Preparation: Two sets of solid organic matter samples and two sets of paraffin samples were weighed. The total mass of paraffin in each set was adjusted to 10 and 20 wt % of the corresponding solid organic matter, respectively.
• 2Paraffin Dissolution: Paraffin was mixed with excess n-hexane and heated at 70 °C to achieve complete melting and full miscibility with the solvent.
• 3Incremental Mixing: The paraffin-n-hexane mixture was added dropwise to the solid organic matter powder in multiple increments. During the addition process, the mixture was continuously shaken and stirred to ensure uniform distribution of paraffin.
• 4Solvent Removal: The blended sample was placed on a heating plate at 50 °C to allow complete evaporation of n-hexane. It should be noted that the heating plate temperature was controlled below the paraffin melting point (65 °C in this study) to avoid remelting and reprecipitation of paraffin.
• 5Characterization: Following complete removal of n-hexane, the samples, including pristine solid organic matter, 10% paraffin-loaded samples, and 20% paraffin-loaded samples, were analyzed using nitrogen adsorption and X-ray diffraction (XRD) techniques to assess the effects of paraffin on the pore structure and structural characteristics of solid organic matter.

2.

Technology roadmap of research.

2.3. Analysis

2.3.1. N₂ Adsorption

Low-pressure nitrogen adsorption analysis was performed using a Micromeritics ASAP-2460 Accelerated Surface Area and Porosimetry System. Prior to measurement, the powdered samples were degassed in a vacuum chamber at 50 °C for 12 h to remove residual volatiles and free moisture (the temperature must not exceed the melting point of paraffin; otherwise, paraffin will melt and reprecipitate). Strict temperature control was critical during this pretreatment, as the paraffin must be maintained below its phase transition temperature to avoid structural alterations.

Nitrogen adsorption isotherms were acquired at 77 K over a relative pressure range of 0.005–0.998, where P denotes the equilibrium pressure and P_o represents the saturation pressure. For the characterization of pore structures, the composite nitrogen nonlocal density functional theory (NLDFT) method was employed to analyze the gas physisorption data. This method was selected due to its optimal detection capability for pore sizes ranging from 0.33 to 100 nm, along with its high reliability and precision. , In this study, the NLDFT method was utilized to assess the specific surface area and pore volume distribution across different pore size regimes of the samples. Since the average diameter of paraffin molecules is larger than the kinetic diameter of CO₂ molecules, the CO₂ adsorption experiment was not carried out.

2.3.2. XRD Analysis

X-ray diffraction (XRD) analysis was conducted using an XRD-6100 diffractometer, which was operated with Cu Kα radiation at 40.0 kV and 30.0 mA. The instrument was configured with a scanning range of 10–80° (2θ) and a step rate of 2.0°/min. This technique was employed to investigate the structural modifications of macromolecular geological organic matter induced by their interaction with paraffin.

3. Result and Discussion

3.1. Paraffin-Solid Solution of Solid Organic Matter

The XRD results of this study are presented in Figure . The paraffin sample exhibits two distinct crystallization peaks at 21.74° and 24.18°, respectively, which are attributed to its crystalline nature. XRD analysis of the solid organic matter reveals two primary structural components: the γ band peak, indicative of amorphous aliphatic carbon, and the 002 peak, corresponding to crystallites in condensed aromatic rings, with diffraction angles at approximately 20° and 26°, respectively. −

3.

Results of XRD.

Preliminary analysis of the XRD data showed no detectable paraffin peak in the HSL sample group with 10% paraffin content, while the paraffin peak intensity in the 20% group was significantly reduced. In the 20% paraffin-content group, the intensity of the paraffin peak was lower than that of the organic peaks. The WEH bitumen samples exhibited a similar trend, with diminished paraffin peak intensity in the sample with 10% paraffin content. In contrast, the LCG and XML kerogen samples showed detectable paraffin peaks, albeit with relatively low intensity, particularly in the 10% paraffin-content sample. However, the FS and HC coal samples displayed clearly visible paraffin peaks in the 10% paraffin content group, with the paraffin peak intensity significantly exceeding that of the organic peaks.

This phenomenon suggests that solid bitumen samples can effectively interact with paraffin, dispersing it within the solid organic matter structure as a “solid solution” and thereby preventing the formation of detectable paraffin crystallites at low paraffin concentrations. Among the three sample types, solid bitumen exhibits the highest structural similarity to paraffin and demonstrates the greatest capacity to “dissolve” paraffin. Kerogen, which contains numerous paraffin-like structural units, can also incorporate a portion of paraffin into its framework. In contrast, coal is predominantly composed of aromatic clusters with limited functional groups available for interaction with paraffin, resulting in the formation of observable paraffin crystallites even at a 10% paraffin content.

Based on the structural characteristics and XRD findings, solid bitumen demonstrates superior paraffin solubility, enabling substantial paraffin incorporation into its structure without surface crystallization. Coal exhibits the poorest solubility, as paraffin is unable to penetrate its tightly packed interlayer structure, leading to surface crystallization and detectable paraffin peaks. Kerogen occupies an intermediate position, being capable of retaining a portion of paraffin within its structure.

3.2. Occurrence and Structure Modification Capacity of Paraffin

Figure illustrates the variations in the microcrystalline stacking structures of solid bitumen, kerogen, and coal under different paraffin contents. As shown in the figures, the γ band of all three types of solid organic matter undergoes a significant contraction in the presence of paraffin, with the magnitude of this effect correlated with paraffin content. Specifically, the γ band peak becomes more concentrated and narrower in shape. This structural modification is directly associated with enhanced crystallinity of the amorphous carbon component in the solid organic matter, suggesting that paraffin effectively arranges the aliphatic chain structures in the solid matrix, promoting a more ordered distribution and improved crystallinity. In contrast, the 002 band does not exhibit this characteristic, indicating that paraffin exerts a negligible influence on aromatic carbon clusters, with which it has low structural similarity.

4.

Effect of paraffin on microcrystalline stacking structures of solid organic matter.

To more precisely assess the impact of paraffin on the amorphous carbon fraction within solid organic matter, the d_γ (interlayer spacing of amorphous carbon) and d₀₀₂ (interlayer spacing of aromatic carbon clusters) parameters were calculated for the samples:

$${\mathrm{d}}_{\gamma }=\lambda /({\mathrm{2sin}\theta }_{\gamma })$$ 1

$${\mathrm{d}}_{002}=\lambda /({\mathrm{2sin}\theta }_{002})$$ 2

where dγ and d002 represent the interlayer spacing of amorphous aliphatic carbon and the microcrystalline planes in polycondensed aromatic rings, respectively. The X-ray wavelength (λ) was 0.15405 nm, with θγ and θ002 denoting the diffraction angles of the γ-band and 002 reflection,respectively. ,

The calculated stacking data of the solid sample structures are presented in Figure . Accurate identification of the peak positions for solid organic matter is hindered by the overlap between the γ-band and 002-band of HC and FS coal samples and the two characteristic peaks of paraffin. To ensure analytical accuracy, Figure only displays the structural parameters derived from 12 sample groups of kerogen and solid bitumen. The results indicate that paraffin distributes within both the “non-crystalline carbon γ band” and “aromatic clusters 002 band” of kerogen and solid bitumen, acting as a solid-soluble macromolecular organic compound that leads to a significant reduction in the layer spacing of both the γ-band (d_γ) and the 002-band (d₀₀₂). Among these parameters, d₀₀₂ undergoes limited transformation, with change rates of only 0.51% for LCG and 1.95% for XML samples, whereas the corresponding d_γ change rates are 1.33% and 3.18%, respectively. In contrast, solid bitumen exhibits more pronounced changes, with d₀₀₂ change rates of 2.04% for WEH and 7.29% for HSL, and d_γ change rates of 6.16% and 9.51%, respectively. These structural alterations are consistent with the observed experimental phenomena: (1) solid bitumen possesses a superior capacity to “dissolve” paraffin, accommodating substantial quantities within both its aliphatic and aromatic carbon structural units; (2) the noncrystalline carbon fraction, predominantly composed of aliphatic chains, can incorporate more paraffin molecules, resulting in greater structural deformation. During this interaction, paraffin molecules comb and align the aliphatic chains within the solid organic matter, thereby enhancing the crystallinity of the γ-band.

3.3. Porosity Modification of Solid Organic Matter by Paraffin

The nitrogen adsorption results indicate significant changes in the pore-specific surface area and total pore volume of the samples following paraffin treatment. The detailed distributions of these changes are presented in Figures and . The three types of organic matter are discussed individually based on their porosity characteristics and paraffin occurrence behavior. The model in this study is discussed based on idealized spherical pores.

5.

Change of cumulative volume (cm³/g) of solid organic matter under the action of paraffin.

6.

Change of cumulative area (m²/g) of solid organic matter under the action of paraffin.

(1) Kerogen

The pore size distribution within kerogen is broad, ranging from 0.33 to 57.73 nm. With the addition of paraffin, both the specific surface area and pore volume of the two kerogen sample groups decreased simultaneously. This synchronous reduction in area and volume suggests that paraffin influences the pore parameters of kerogen by either expanding the structure (thereby leading to pore constriction or infilling) or adhering to the pore surfaces (thus reducing pore size). Based on these experimental observations and integrating the two previously proposed mechanisms, we established a model illustrating the alteration of pore characteristics in kerogen under the influence of paraffin (Figure ). As depicted, following the interaction, the internal pores of kerogen exhibit a systematic shrinking trend, which correlates with changes in the crystallinity of certain aliphatic components in the structure.

7.

Schematic diagram of modification of kerogen structure and pores by paraffin.

(2) Solid Bitumen

Due to the initial precipitation of solid bitumen, its inherent pore distribution is limited, with only two distinct pore size ranges observed: 0.33–0.40 nm and 18.37–55.24 nm. During the initial interaction stage, as solid bitumen “dissolves” paraffin, this process occurs primarily within the bulk structure rather than affecting the pore network. This is evidenced by the nitrogen adsorption results for WEH bitumen, where the addition of 10 wt % paraffin led to no significant alterations in specific surface area and pore volume. This indicates that a substantial portion of the paraffin was incorporated into the solid bitumen matrix, resulting in changes to its bulk properties rather than its pore structure. However, with further paraffin addition, the excess paraffin begins to precipitate or adsorb onto the bitumen surface, rapidly filling its sparse pore network and leading to a sharp decline in both pore surface area and volume. Consequently, the model for pore evolution in solid bitumen under paraffin influence (Figure ) can be described in two key stages: (1) initial dispersion of paraffin within the solid bitumen structure, primarily affecting the crystallinity of the amorphous carbon fraction with minimal impact on pores; (2) subsequent paraffin oversaturation, which leads to a significant reduction in both pore surface area and pore volume.

8.

Schematic diagram of modification of bitumen structure and pores by paraffin.

(3) Coal

Unlike kerogen and solid bitumen, which are rich in aliphatic (paraffinic) structures, coal is structurally dominated by stacked aromatic clusters and contains only minimal paraffinic components in its native state. This fundamental material composition results in distinctly specific changes in pore evolution during its interaction with paraffin. When initially adding 10% paraffin to the raw coal samples, reductions in both pore surface area and pore volume were observed. However, as the paraffin content increased from 10% to 20%, the pore volume of both HC and FS coal samples decreased by approximately half, while the pore surface area remained largely unchanged. This behavior, in contrast to the trends observed for kerogen and solid bitumen, can be attributed to pore splitting in coal. Specifically, paraffin cannot enter and disperse within the aromatic clusters; instead, the excess paraffin precipitates on the coal surface, filling and dividing the existing pores into smaller ones. This process significantly reduces the total pore volume without drastically altering the overall surface area available for adsorption.

Assuming that the pores in the solid component are spherical, the pore volume and surface area are calculated as

$${\mathrm{V}}_{\mathrm{origin}}{=4/3\times \pi \times \mathrm{r}}^3$$ 3

$${\mathrm{S}}_{\mathrm{origin}}{=4\times \pi \times \mathrm{r}}^2$$ 4

As shown in Figure , when a sphere with radius r is split into four spheres with radius r/2, the volume and surface area are respectively:

$${\mathrm{V}}_{\mathrm{total}}=4\times [4/3\times \pi \times {(\mathrm{r}/2)}^3{]=2/3\times \pi \times \mathrm{r}}^3={\mathrm{V}}_{\mathrm{original}}/2$$ 5

$${\mathrm{S}}_{\mathrm{total}}=4\times [4\times \pi \times {(\mathrm{r}/2)}^2{]=4\times \pi \times \mathrm{r}}^2{=\mathrm{S}}_{\mathrm{original}}$$ 6

9.

Schematic diagram of modification of coal structure and pores by paraffin.

Where V_original and S_original represented the original pore volume and area; V _total and S _total represented the total volume and surface area of four pores with radius r/2. According to the calculation, the total surface area of the four pores after splitting remains unchanged, while the total volume decreases to half of the original pore, and it is consistent with the experimental phenomena.

Consequently, a pore evolution model for coal under paraffin influence was established, illustrating this distinct mechanism (Figure ). The coal pore evolution model delineates two key stages: (1) Owing to its poor solid solubility, coal primarily adsorbs paraffin onto its pore surfaces, leading to a rapid decline in both specific surface area and pore volume until a critical threshold is reached. (2) When the paraffin content within the pores continues to rise beyond this threshold, the paraffin induces the division of existing pores, essentially splitting them into multiple smaller ones. It should be noted that while the quantitative data in this study followed a pattern consistent with pores splitting into four smaller ones, this exact ratio may not be universally applicable due to the inherent complexity and specificity of our experimental conditions. Further investigation is required to confirm the universality of this phenomenon. It is this pore-splitting mechanism that ultimately accounts for the distinct outcome observed: a significant reduction in pore volume coupled with minimal alteration in overall specific surface area.

3.4. Influence of Structure of Solid Organic Matter

To further elucidate the structural characteristics from a molecular perspective, we incorporated nuclear magnetic resonance (NMR) data from six sample groups reported by Liang into our analysis (Figure and Table ). A synthesized view of the structural information reveals the following:

10.

Analysis of structural composition of solid organic Matter samples .

1. Chemical Structure Parameters of Samples Based on NMR analysis .

		Aliphatic (%, 5–95 ppm)						Aromatic (%, 95–150 ppm)
Sample		$$f_{{\mathrm{CH}}_3\mathrm{al}}$$	$$f_{{\mathrm{CH}}_3\mathrm{ar}}$$	$$f_{{\mathrm{CH}}_2}$$	f CH	f C	f O	$$f_{\mathrm{a}}^{\mathrm{H}}$$	$$f_{\mathrm{a}}^{\mathrm{B}}$$	$$f_{\mathrm{a}}^{\mathrm{S}}$$	$$f_{\mathrm{a}}^{\mathrm{O}}$$	f al %	f ar %
Solid Bitumen	WEH	2.48	16.96	18.57	25.68	9.82	1.91	7.02	8.68	6.66	2.23	75.42	24.59
	HSL	9.02	13.28	19.53	12.11	4.79	1.39	9.46	17.51	10.91	2.00	60.12	39.88
Kerogen	LCG	6.91	16.49	30.10	10.55	4.44	2.27	7.26	12.40	7.09	2.49	70.76	29.24
	XML	4.28	13.69	12.90	12.47	6.16	2.51	10.85	20.36	13.87	2.91	52.01	47.99
Coal	FS	4.12	8.53	6.89	6.10	3.90	2.12	13.74	25.86	21.36	7.40	31.65	68.35
	HC	3.22	7.89	12.38	3.81	2.09	0.47	13.10	25.34	24.57	7.13	29.86	70.14

^aThe amount of aliphatic carbon $f_{\mathrm{al}}=f_{{\mathrm{CH}}_3\mathrm{al}}+f_{{\mathrm{CH}}_3\mathrm{ar}}+f_{{\mathrm{CH}}_2}+f_{\mathrm{CH}}+f_{\mathrm{C}}+f_{\mathrm{O}}$ ; the amount of aromatic carbon $f_{\mathrm{ar}}=f_{\mathrm{a}}^{\mathrm{H}}+f_{\mathrm{a}}^{\mathrm{B}}+f_{\mathrm{a}}^{\mathrm{S}}+f_{\mathrm{a}}^{\mathrm{O}}$ .

Kerogen possesses a chemically heterogeneous and porous structure. Its inherent structural composition includes numerous waxy constituents and small aromatic clusters, which results in complex molecular stacking. The kerogen structure contains the highest proportion of methylene components $\left(f_{{\mathrm{CH}}_2}\right)$ . As a consequence, paraffin does not readily ingress into its pores; instead, the observed pore reduction stems primarily from organic matrix expansion and limited surface precipitation.

Solid bitumen is composed of smaller molecules. As shown in Table , NMR analysis confirms an abundance of short-chain structures within its framework, which confers high solid solubility. Compared with kerogen and coal samples, solid bitumen structure contains more methine groups (f _CH). Consequently, paraffin can effectively diffuse into both the matrix and the pores, thereby altering the specific surface area and pore volume within the measured ranges.

Coal is characterized by a high aromatic carbon content, featuring the most extensive aromatic ring clusters and the lowest structural similarity to paraffin. This hinders the formation of a solid solution. Instead, its pore network is most susceptible to paraffin-induced disruption. In the later stages of interaction, excessive paraffin induces the splitting of existing pores into numerous smaller ones, which drastically reduces pore connectivity and volume while leaving the overall specific surface area largely unchanged.

This structural framework provides a coherent explanation for the distinct pore evolution behaviors previously described.

3.5. Pore Connectivity Analysis

Paraffin has the least influence on the pore connectivity of solid bitumen. A small amount of paraffin in solid bitumen only exists in the structure in the form of swelling, and will not affect the pore volume. Only when the amount of paraffin exceeds the swelling capacity of the solid component, will it precipitate out from the structure, attach to the pore surface in a layered form, thereby reducing the pore volume and decreasing the connectivity.

The pore connectivity of kerogen is more affected by paraffin than that of solid bitumen. Due to the long aliphatic chains and complex stacking of the kerogen structure, it is difficult to form a favorable solid solution structure with paraffin molecules. However, a large number of exposed long-chain alkane structures in the pores allow paraffin molecules to adhere to the pore surface of kerogen. Therefore, although there is no “swelling-precipitation” process in the kerogen structure, paraffin can still directly attach to the inner wall of kerogen pores in a layered form, resulting in the reduction of pore connectivity of kerogen.

The pore connectivity of coal is most significantly affected by paraffin. As shown in Figure and Table , the coal structure is dominated by aromatic ring units. Different from solid bitumen and kerogen, paraffin cannot form a layered adhesion on the pore surface. Under adsorption, following the formation of a thin paraffin layer in the first stage, the pore wall initiates the development of micropores under the combined effect of scarce alkane functional groups and aromatic cluster stacking, imposing a substantial influence on pore volume and pore connectivity.

It is noteworthy that all relevant research results are experimental outcomes obtained under the condition of 50 °C. When the temperature changes, it may lead to property alteration of the organic matter within pores.

4. Conclusions

This study explores the structural occurrence and pore distribution effects during the interaction between macromolecular paraffin and three types of natural organic matter (kerogen, solid bitumen, and coal), leading to three principal conclusions:

• 1The primary occurrence mechanisms of paraffin within solid organic matter are established. In kerogen and solid bitumen, a portion of paraffin can exist in the form of a solid solution. In contrast, such diffusion is minimal within coal, where surface adhesion dominates. Accordingly, distinct pore evolution models are proposed: Kerogen: Pore-specific surface area and volume decrease continuously due to the combined effects of matrix swelling and paraffin precipitation. Solid bitumen: Pores begin to contract noticeably only when the paraffin content exceeds a critical threshold, owing to its effective solid solubility. Coal: The pore network undergoes an initial reduction, followed by pore splitting into numerous smaller pores, resulting in a sharp decline in pore volume while the specific surface area remains largely unchanged. Combined with the pore variation characteristics and structural mechanism, the influence of paraffin on the pore connectivity of solid organic matter follows the order: solid bitumen < kerogen < coal.
• 2For shale gas systems, paraffin contributes to the reduction of kerogen pores, thereby diminishing the overall reservoir space. However, because shale contains abundant solid bitumen and asphaltene components, paraffin preferentially interacts with and disperses into these constituents. This protective mechanism helps preserve the original porous and permeable characteristics of kerogen. In essence, solid bitumen and asphaltene act as a buffer, protecting reservoir pores from excessive paraffin blockage and thereby maintaining effective storage and transport pathways for shale gas.
• 3Paraffin exerts a significant influence on coalbed methane operations. Due to the weak solid solubility of coal, pores are rapidly blocked during paraffin interaction. The pore splitting phenomenon further triggers a drastic loss of pore volume, which severely compromises both the reservoir space and migration channels for gas. While this might enhance gas retention to some extent, it complicates extraction by drastically reducing permeability. A potential mitigation strategy is to selectively heat the coal reservoir to melt the paraffin. This process would reopen migration pathways and could substantially improve coalbed methane recovery efficiency from paraffin-affected formations.

All data generated or analyzed during this study are included in this published article.

X.T.-W.: Methodology, Formal analysis, Writing – Original Draft T.L.: Supervision, Project administration Duan J.-B.: Conceptualization, Validation, Resources Wang J.-F.: Data Curation, Software Z.Z.-W.: Visualization, Formal Analysis.

The authors declare no competing financial interest.