Effect of thermal maturity on carbazole distributions: insights from compaction pyrolysis experiment of coal

doi:10.21203/rs.3.rs-3392413/v1

Carbazole compounds are widely used in determining the direction of hydrocarbon migration, but the effect of thermal maturity on carbazoles remains unclear. In this paper, using compaction pyrolysis simulation experiments, artificial mature samples with vitrinite reflectance (R_o) range from 0.38–3.0% were acquired. And the content and composition change characteristics of carbazole compounds were analyzed. The results showed that thermal maturity had a significant influence on the content of carbazole compounds. Compared with the low mature stage, the content of carbazole compounds was about 10 to 100 times higher in the mature stage. With the increasing maturity, in the coal sample, the content of carbazole compounds showed a trend of first increasing and then decreasing. The formation of carbazole compounds was later than generation of hydrocarbon, suggesting carbazoles mainly produced by the thermal degradation of kerogen. In derivatives of carbazole, the corresponding maturity for the maximum generation of ethylcarbazole (EC), dimethylcarbazole (DMCA), methylcarbazole (MCA), carbazole (CA) and benzocarbazole (BCA) performed the increasing sequence. The timing of reaching the maximum content of carbazole compounds is related to the number and type of substituents. The more substituents and longer branches result earlier formation peak. With the increasing maturity, the relative abundance of 2-MCA, 1,7-DMCA and benzo[a]carbazole increased with the increasing maturity, the relative abundance of 1,5-DMCA remained almost unchanged, while 4-MCA, 1,4-DMCA and benzo[c]carbazole gradually decreased. The commonly used parameters as indicators of hydrocarbon migration were greatly affected by maturity. Therefore, when using carbazole compounds as geochemical parameters, it is necessary to fully consider the impact of thermal maturity on them, and avoid transport fractionation or other factors masking the true maturity effect.

carbazole compounds

thermal maturity

coal

compaction pyrolysis experiment

hydrocarbon migration

Organic nitrogen-containing compounds with heterocyclic aromatic structures are representative components of non-hydrocarbon in crude oil [1, 2]. Heterocyclic aromatic nitrogen-containing compounds in petroleum can be divided into two categories: neutral pyrrole and basic pyridine compounds [3, 4]. Carbazole and its derivatives, as important neutral pyrrole compounds, are effective molecular markers for tracking the migration distance and charging pathway of hydrocarbon, and their content and composition change regularly during oil and gas transportation processes. According to the migration fractionation mechanism of carbazole compounds, with the increase of migration distance, the total concentration of carbazole nitrogen-containing compounds gradually decreases, the relative content of shielded compounds increases, and the content of exposed compounds gradually decreases. Therefore, carbazole compounds are commonly used as indicators of oil and gas transport, it has been widely used in petroleum geochemical research in the past 20 years [5–11]. However, the abundance and relative composition of nitrogen-containing compounds in crude oil are not only affected by the migration fractionation effect, but also by various factors such as sedimentary facies and environment [12–15], thermal maturity [16–25] and biodegradation [26–31]. The understanding of the effect of thermal maturity on the content changes of carbazoles is still controversial. Some researchers believe that the effect is limited. For example, Li et al. [5] believed in the early stage that primary and secondary oil migration had a great influence on the abundance and distribution of carbazole derivatives in reservoir oil, while organic matter source input, sedimentary environment and thermal maturity did not seem to be the main control factor. Hallmann et al. [32] studied the distribution of carbazoles in raw oil in Gidgealpa oilfield, and believed that the distribution of benzocarbazole has no relationship with lithofacies, organic facies and maturity, but the composition of alkyl carbazole has obvious correlation with lithofacies and organic facies. There are also studies that the effect of thermal maturity is very significant [15, 18, 19, 20, 21, 23, 24, 25]. For example, the study of carbazoles in the extracts of marine carbonate rocks with %R_o range of 0.45–1.30% by Li et al. showed that the total concentration of carbazoles and benzo[a]carbazole/(benzo[a]carbazole + benzo[c]carbazole) all increase with the increasing of maturity within a certain maturity range [17]. In shale samples from the Jurassic Posidonia in northern Germany, with the increasing maturity, the ratios of 1-methylcarbazole/ (1-+2-methylcarbazole) and benzo[a]carbazole/ (benzo[a]carbazole + benzo[c]carbazole) all increased [13]. Zhang et al. [21] studied the distribution of carbazoles in lacustrine source rocks in the Qaidam Basin, suggested that the relative abundance of 1-MCA (methylcarbazole) showed a decreasing trend at immature stage and an increasing trend in the mature stage, while the relative abundance of the other three isomers (2-, 3- and 4-MCA) showed opposite changes, showing a "two-stage" change. Over the entire maturity range, the benzocarbazole ratio benzo[a]carbazole/ (benzo[a]carbazole + benzo[c]carbazole) increased with maturity, but the ratio changed less, only between 0.52 and 0.61.

According to the studied objects from previous researches, natural evolution samples are mainly used [15, 16, 21, 32]. Based on the correlation analysis for samples with different depths and maturity, the influence of thermal maturity on carbazole compounds were discussed. Carbazole compounds distribution are also affected by biogenesis, sedimentary environment, biodegradation and other factors. It is difficult to fully reveal the influence of thermal maturation on the content and composition changes of carbazole compounds. In order to overcome the constraints of other influencing factors, the compaction pyrolysis method is often used to study the influence of thermal maturity as single factor. For example, the pyrolysis simulation experiment of the monomer compound carbazole was used to reveal the change law of carbazole formation and cleavage [33, 34]. Through artificial maturation experiments, Horsfield and Clegg found that thermal maturity plays an important role in the changes of content and composition of carbazoles, and increases with maturity within a certain maturity range [18, 20], however, due to the limitation of experimental conditions, the temperature is low and the maturity range is also small, which cannot completely reveal the effect of thermal maturity on carbazoles.

Based on the above reasons, on the basis of previous carbazole pyrolysis simulation experiments [22, 33, 34], this paper selects coal-measure source rocks with low maturity to carry out compaction pyrolysis experiments, and obtains maturity from relatively complete maturity sequence ranging from 0.38%R_o to 3.02%R_o. The effect of thermal maturation on the distribution of carbazoles were analyzed, in order to explore the formation and transformation law of carbazoles under the action of thermal maturation, and provide a more solid theoretical basis for further development. The application of nitrogen-containing compounds in hydrocarbon migration may provide a more solid theoretical basis for further study.

2.1. Samples

The Longkou lignite samples were used for pyrolysis simulation experiment in this study. The basic information of the coal sample is as follows: TOC = 78.5%, HI = 350mg/g, T_max = 431℃, R_o = 0.38%, which is an ideal sample for compaction pyrolysis experiments.

2.2. Experimental

The compaction pyrolysis simulation experiment adopts the high-temperature and high-pressure hydrocarbon generation and expulsion simulation system. The experimental scheme is as follows: placed 11 parallel samples into the reaction kettle respectively, heated up at rate of 20 ℃/h to the corresponding temperature and stopped heating. The temperature setting for the pyrolysis simulation is 320 ℃, 340 ℃, 360 ℃, 400 ℃, 414 ℃, 440 ℃, 460 ℃, 480 ℃, 500 ℃, 550 ℃, 600 ℃, with the hydrocarbon expulsion pressure as 30Mpa. Vitrinite reflectivity for all the residual samples were measured after heating at various temperatures. The remaining coal samples were ground into a fine powder and then extracted with CH₂Cl₂ in a Soxhlet apparatus for 72 h. The extracts were deasphaltened and then separated into saturate, aromatics and nitrogen-enriched fractions by column chromatography. The nitrogen-containing fraction was subsequently isolated into pyrrolic nitrogen fraction, amine fraction and basic nitrogen fraction [5, 35].

Gas chromatography-mass spectrometry (GC–MS) analysis of the pyrrolic nitrogen fractions was carried out using an HP 7890 gas chromatograph equipped with an HP 5975 mass selective detector. A fused silica capillary column (30 m×0.25 mm) coated with HP-5MS (film thickness 0.25um) was used with He as carrier gas at a constant flow of 1.0 ml min^− 1. The gas chromatograph oven temperature was programmed from 50℃ to 100℃ at 20℃ min^− 1, then to 315℃ at 4℃ min^− 1. The mass spectrometer was operated in multiple ion detection (MID) mode.

Qualitative analysis of carbazole (CA), methylcarbazole (MCA), dimethylcarbazole (DMCA), ethylcarbazole (EC), and benzocarbazole (BCA) were based on literature [12]. In order to quantify the absolute concentration of carbazole, N-phenyl carbazole was added as an internal standard, and then the absolute quantification of each carbazole compound was carried out. The absolute content of carbazole compounds and their derivatives were calculated by the following formula: Compound content (µg/g) = (area of compound peak/area of internal standard peak) × weight of standard sample (µg)/weight of sample (g).

3.1. Basic experimental data

The content of carbazole compounds at the different pyrolysis temperature was shown in Table 1. Carbazole, methylcarbazole, dimethylcarbazole, ethylcarbazole and benzocarbazole are the main carbazole compounds. Figure 1 showed the distribution characteristics of carbazoles at different maturity levels.

3.1.1. Carbazole compounds

There is a close relationship between the content of carbazoles and the maturity. With the increasing maturity, the content of carbazoles showed a trend of first increasing and then decreasing. In the low mature stage of R_o < 0.7%, the total amount of carbazole compounds increased slowly with the increasing maturity, and the total content was less than 160 µg/g (Table 1). From 0.7%R_o, the total content of carbazole compounds rapidly increased during the mature stage (0.7%< R_o <1.3%) and high mature stage (1.3%< R_o <2.0%), and reached its maximum (10668.5µg/g) at 1.48%R_o. With the maturity continued to increase, the total content of carbazole compounds began to decrease rapidly after reaching the maximum. In the over-mature stage (R_o >2.0%), the total content of carbazole compounds was only 48.42µg/g at 2.26% R_o. In the stage of R_o >3.0%, carbazole compounds were hardly detected in the samples.

The content of various derivatives of carbazole compounds also exhibited similar changes, including carbazole (Fig. 2a), methylcarbazole isomers (Fig. 2b), dimethylcarbazole isomers (Fig. 2c, d), ethylcarbazole (Fig. 2e), and benzocarbazole isomers (Fig. 2f). In the low mature stage, the contents of carbazole, methylcarbazoles, dimethylcarbazoles, ethylcarbazole and benzocarbazoles were all lower than 70µg/g (Fig. 2a). Among all the carbazole compounds, dimethylcarbazoles and ethylcarbazole showed the highest (maximum 67.32µg/g) and lowest (lower than 2µg/g) content, respectively. Ethylcarbazole, dimethylcarbazoles and methylcarbazoles were rapidly generated in the mature stage, with maximum values of 19.53µg/g (1.2%R_o), 1233.47µg/g (1.33%R_o) and 2878.16µg/g (1.48%R_o), respectively. Carbazole and benzocarbazoles were rapidly generated in the high mature stage, and both reached the maximum values at 1.48%R_o, and the corresponding maximum of their content were 6686.10µg/g and 355.48µg/g, respectively. With the increasing maturity, the content of carbazole compounds rapidly decreased after reaching their maximum. At 2.26%R_o, except for carbazole, with content of 25.95µg/g, the content of other carbazole compounds were below 10µg/g. In derivatives of carbazole, the corresponding maturity for the maximum generation of ethylcarbazole, dimethylcarbazoles, methylcarbazoles, carbazole and benzocarbazoles performed the increasing sequence. The timing of reaching the maximum content of carbazole compounds is related to the number and type of substituents. The more substituents and longer branches result earlier formation peak. The content of ethylcarbazole and dimethylcarbazoles began to decrease after reaching the maximum earlier. Thermal stress may cause the alkyl side chain to crack, and even converted into methylcarbazoles or the product of intramolecular rearrangement. From the perspective of chemical stability, the more and longer the branches connected by carbon sites, the more likely to break. The carbazole content increased sharply at 1.33%R_o, indicating that the carbazole may be partially derived from the product formed by the demethylation of ethylcarbazole and dimethylcarbazoles. When the content of carbazole compounds reached its maximum, the content showed a trend of carbazole > methylcarbazoles > dimethylcarbazoles > benzocarbazoles > ethylcarbazole, alkylcarbazoles > alkylbenzocarbazoles. However, both dimethylcarbazoles and methylcarbazoles were higher than carbazole in the early mature stage (R_o<1.20%), which may be a result of the initial exfoliation of hydrocarbon in the samples rather than related to thermally-induced selective alkylation and dealkylation.

The change law of the content of carbazole compounds depends on the rate of formation and cleavage of carbazole compounds with the increasing maturity. When the formation rate is higher than the cleavage rate, the content increases, otherwise it decreases. The main formation stage of carbazoles is later than that of hydrocarbon generation, but its formation curve is close to that of hydrocarbon generation. The content of carbazole compounds is relatively low in the low mature stage, and rapidly increases in the mature and high mature stages. However, carbazole compounds are almost undetectable in the samples after the over mature stage. It can be seen that carbazole compounds may not be completely inherited from organic sediments, and their content increases significantly with the appearance of the main hydrocarbon generation period. According to the theory of kerogen oil generation, the beginning of oil generation is accompanied by certain maturity conditions. Therefore, maturity has a significant impact on the content and distribution of carbazole compounds. The mass generation of carbazole compounds is also based on a certain maturity, and they may also occur during cracking, which is synchronous with the generation of hydrocarbon.

3.1.2. C₁-carbazoles

In addition to carbazole, methylcarbazoles is also the main component of carbazole compounds. Methylcarbazoles can be divided into four isomers according to the different conditions of carbon site substitution by methyl, namely 1-MCA, 2-MCA, 3-MCA and 4-MCA. The GC/MS data of carbazole compounds showed that the content of methylcarbazole isomers also changed regularly with the increasing maturity (Fig. 2b). In the low mature stage, the content of the four methylcarbazole isomers were all below 20µg/g, with 1-methylcarbazole as the highest(17.55µg/g), followed by 4-MCA, 3-MCA and 2-MCA. The distribution characteristics of 2-, 3-, and 4-MCA isomers showed an asymmetric "∨" type in the low mature stage (Fig. 3). In the mature stage, the content of methylcarbazoles increased rapidly, with 1-MCA (38µg/g to 680µg/g) and 2-MCA (16µg/g to 600µg/g) increasing rapidly. The content of 1-MCA had an advantage at the stage of R_o <1.3%, while the content of 4-MCA gradually decreased and became the lowest. After entering the high mature stage, 2-MCA generated at a faster rate, and the content gradually exceeded 1-MCA. At this time, the distribution characteristics of the three isomers of 2-, 3-, and 4-MCA were asymmetric "∧" type (Fig. 3). The content of the four methylcarbazole isomers all reached the maximum at 1.48%R_o. Among the methylcarbazole isomers, the content of 2-MCA was the highest (maximum 986.31µg/g), followed by the content of 1-MCA (744.85µg/g), and the content of 4-MCA was lowest (505.24µg/g). With the maturity continued to increase, the content of the four isomers decreased rapidly at the stage of R_o > 1.48%, and the content of the four isomers were all lower than 4µg/g at 2.26%R_o. The methylcarbazole isomers were almost undetectable at 3.0%R_o. Zhang et al. [21] studied the Tertiary source rocks in the salt lacustrine facies environment of the Qaidam Basin, suggesting that 1-MCA in methylcarbazoles is the isomer with the highest content among the four isomers regardless of maturity. Li et al. [5] believed that 1-MCA is the isomer with the lowest content and 4-MCA is the isomer with the highest content in the shale from Liaohe Basin. Source rock type and depositional environment have a great influence on the content and composition distribution characteristics of methylcarbazoles.

With the increasing maturity, the relative abundance of 2-MCA gradually increased from 19–37%, with a large change range. The relative abundance of 4-MCA gradually decreased from 28–17% (Fig. 4). The relative abundance of 1-MCA gradually increased from 31–45%, then gradually decreased to 27%, and the maximum relative abundance appeared at 1.11%R_o. The relative abundance of 3-MCA varied only between 16% and 22%, and the variation range was relatively small (Table 1). The content and distribution characteristics of methylcarbazole isomers are controlled by thermodynamics and other factors. This change with the increasing maturity may be related to the alkylation reaction, isomerization reaction and thermal stability between methylcarbazole isomers.

3.1.3. C₂-carbazoles

Dimethylcarbazoles can be divided into shielded isomers (the H atoms at the 1-carbon position and the 8-carbon position are substituted by alkyl groups, referred to as G1 type), semi-shielded isomers (only the H atoms at the 1-carbon position are substituted by alkyl groups, referred to as G2 type), and exposed isomers (the H atoms at the 1-carbon position and the 8-carbon position are not substituted by alkyl groups, referred to as G3 type) based on the position of carbon substituted by alkyl groups.

With the increasing maturity, the total content of shielded isomers, semi-shielded isomers and exposed isomers all first increasing and then decreasing (Fig. 5a). In the low mature stage, the total content of the three types of isomers were all lower than 40µg/g. After entering the mature stage, the content of the three isomers increased rapidly, and the content all reached the maximum at 1.33%R_o (maximum 76.94µg/g, 664.90µg/g and 498.95µg/g, respectively). With the maturity continued to increase, the content of the three isomers began to decrease rapidly, and the content of the three isomers were all below 4µg/g at 2.26%R_o. The content of semi shielded isomers and shielded isomers were the highest and lowest within the entire maturity range. The exposed isomers show higher abundance compared to the shielded isomers, which is consistent with the thermodynamic equilibrium principle. From a chemical perspective, the chemical stability of the exposed isomer is higher than that of the shielded isomers.

In the dimethylcarbazoles, the ratio of shielded isomers to semi-shielded isomers content remained essentially unchanged, varying only between 0.09 to 0.12 over the entire maturity range (Fig. 5b). G1/G3 and G2/G3 showed a large change in the maturity stage, which first increasing and then decreasing, and reached maximum at 1.11%R_o.

The content of each monomer isomer of dimethylcarbazoles was lower than 10µg/g in the low mature stage (Table 1), and all increased rapidly in the mature stage. Except that the content of 1-EC reached the maximum at 1.20%R_o, the rest of the dimethylcarbazole isomers are either exposed isomers (2,6-DMCA and 2,4-DMCA), shielded isomers (1,8-DMCA), or semi-shielded isomers (1,7-DMCA and 1,3-DMCA), with maximum at 1.33%R_o. At this stage, the content of 1,7-DMCA is the highest (153.80µg/g) and the content of 3,4-DMCA is the lowest (18.96µg/g) (Table 1). With the maturity continued to increase, the content of dimethylcarbazole isomers decreased rapidly. The content of each dimethyl isomer is below 1µg/g, and dimethylcarbazole isomers were almost undetectable in the samples at 2.26%R_o (Fig. 2c, d, e).

Over the entire maturity range, the content of 1,7-DMCA, 1,6-DMCA, and 2,6-DMCA were higher than those of 1,2-DMCA, 1,3-DMCA, and 2,3-DMCA replaced by symmetric carbon sites, while the content of 2,4-DMCA and 2,5-DMCA were basically the same (Table 1). 1,7-DMCA and 2,7-DMCA existed with the highest abundance in semi-shielded and exposed isomers, respectively, followed by 1,6-DMCA and 2,6-DMCA. However, those isomers with adjacent substituents such as 1,2-DMCA, 2,3-DMCA, and 3,4-DMCA were always detected with less abundance. The general distribution of C₂-carbazole is similar to the reported studies [36, 39].

With the increasing maturity, the relative abundance changes of the semi-shielded isomers and the exposed isomers were completely different. The relative abundance of 1,7-DMCA gradually increased from 7–16%. The relative abundance of 1,4-DMCA gradually decreased from 15–4%. The relative abundance of 1,5-DMCA is between 9% and 11% over the entire maturity range, and remains basically unchanged. The relative abundance of 1,8-DMCA showed a significant increasing and then decreasing trend during the mature stage, with maximum (8.25%) at 1.11%R_o. However, the relative abundance remains basically unchanged after 1.5%R_o (Fig. 6a). The relative abundance of 2,4-DMCA, 2,5-DMCA, 2,6-DMCA, and 2,7-DMCA showed a significant decrease and then increase during the mature stage, and the minimum also appeared at 1.11%R_o. Their relative abundance remained basically unchanged after 1.5%R_o. The relative abundance of 2,3-DMCA gradually decreased from 4.23–2.53% (Fig. 6b).

Over the entire maturity range, 1,3-/1,6-DMCA remains basically unchanged, ranging between 0.79 to 0.88 (mean value of 0.83, the standard deviation of 0.03) (Fig. 7). Zhang et al. [38] found that the neutral nitrogen-containing compounds in crude oil from the Pearl River Mouth Basin also had a similar situation.

The positive correlation between the content of 1,3-DMCA and 1,6-DMCA is shown in Fig. 7b, with R² of 0.9988, and showing no change with the increasing maturity. This shows two points: ①1,3-DMCA and 1,6-DMCA may come from a common precursor; ②1,3-DMCA and 1,6-DMCA form in the same mechanism and at the same rate. Considering the structural characteristics of 1,3-DMCA and 1,6-DMCA, the kerogen matrix with the structural characteristics of 1-methylcarbazole is obviously the most likely to be the common precursor of both [39]. That is, a formation mode of 1,3-DMCA and 1,6-DMCA is formed by the methylation of 1-methylcarbazole at the 3,6 carbon sites, respectively. Theoretically, 1,4-/1,5-DMCA should exhibit the same characteristics as 1,3-/1,6-DMCA. However, over the entire maturity range, 1,4-/1,5-DMCA gradually decreased from 1.44 to 0.39, which was greatly affected by maturity. This may be due to the co-elution of 3-EC and 4-EC at the 5-carbon and 4-carbon positions. As shown in Fig. 1, the content of 1,4-DMCA is higher than that of 1,5-DMCA, and the content of 1,4-+1,5-DMCA is higher than that of 1,3-+1,6-DMCA in the early mature stage. With the increasing maturity, the content of 1,5-DMCA has a stronger advantage over 1,4-DMCA, while the content of 1,3-+1,6-DMCA has a stronger advantage over 1,4-+1,5-DMCA (Fig. 7c). The ratio of 1,3-+1,6-DMCA/1,4-+1,5-DMCA gradually increases from 0.64 to 1.36. (Fig. 7d).

The exposed isomers exhibited a different variation pattern from the semi-shielded isomers. The relative abundance between 2,4-DMCA and 2,5-DMCA remained almost unchanged with the increasing maturity, with the 2,4-/2,5-DMCA ratio ranging between 0.99 and 1.10, with a mean of 1.03 (≈ 1) (Fig. 8a). The ratio did not change due to changes in the content and maturity of the two isomers.

Figure 8b showed the correlation between the content of 2,4-DMCA and 2,5-DMCA (R² = 0.9994), and 2,4-/2,5-DMCA also did not change with the increasing maturity. Similarly, 2,4-DMCA and 2,5-DMCA were formed at the same rate, based on the same formation mechanism and a common precursor. Kerogen matrices with 2-methylcarbazole are most likely to be the precursors of the two, namely, a formation mode of 2,4-DMCA and 2,5-DMCA is formed by methylation of 2-MCA at the 4-carbon and 5-carbon sites, respectively (Fig. 9). 2,4-/2,5-DMCA is almost 1 throughout the entire maturity range, indicating the consistency of 4-carbon and 5-carbon methylation in the further methylation process of 2-MCA. The consistency of 4-carbon and 5-carbon chemical activity, which has not been confirmed in semi shielded isomers. The relative abundance of ortho-substituted 2,3-DMCA was always relatively low over the entire maturity range, so the 2,3-/2,6-DMCA ratio decreased with the increasing maturity, indicated that the content of 2,6-DMCA was more dominant than 2,3-DMCA (Fig. 8c).

2,3-+2,6-/2,4-+2,5-DMCA only varied in a small range of 0.63 to 0.75, which indicated that 2,4-+2,5-DMCA was more dominant than 2,3-+2,6-DMCA over the entire maturity range (Fig. 8d). The inconsistency of their contents may indicate that 2-MCA has a competitive reaction mechanism at the 3- and 6-carbon, 4-carbon and 5-carbon sites during further methylation. The patterns observed in dimethylcarbazoles maybe due to thermally induced selective alkylation and dealkylation, and the preferred carbon site selection of 1-MCA and 2-MCA in further methylation will affect the changes in their content and ratio.

3.1.4 Benzocarbazoles

Most studies have shown that linear benzo[a]carbazole and subspherical benzo[c]carbazole typically exist in high abundance in benzocarbazole isomers, while benzo[b]carbazole is usually detected in low abundance. Except for coal samples containing high abundance of benzo[b]carbazole, the distribution of benzocarbazole isomers in most samples is similar [16, 40].

Coincidentally, with the increasing maturity, the content of the three benzocarbazole isomers showed a pattern of first increasing and then decreasing (Fig. 2f). In the low mature stage, the content of the three benzocarbazole isomers was all lower than 3g/g, the content of benzo[c]carbazole was slightly higher than that of benzo[a]carbazole, and the content of benzo [b] carbazole was the lowest (1.17g/g). With the increasing maturity, benzo[a]carbazole was produced at a larger rate than benzo[c]carbazole, and the content gradually exceeded benzo[c]carbazole. Molecular mechanics data on these two benzocarbazoles indicate that benzo[a]carbazole is more unstable than benzo[c]carbazole, thus indicating that the prominence of benzo[a]carbazole is caused by specific precursors within the kerogen [16]. The content of benzo[a]carbazole, benzo[b]carbazole and benzo[c]carbazole reached their maximum at 1.48%R_o, with maximum values of 158.62g/g, 75.37g/g and 121.29g/g, respectively. With the maturity continued to increase, the content of benzocarbazole isomers rapidly decreased. The content of all three isomers were below 3g/g at 2.26%R_o. The relative abundance of benzo[a]carbazole and benzo[c]carbazole was greatly influenced by maturity, ranging from 30–50% and 31–54%, respectively. With the increasing maturity, the relative abundance of benzo[a]carbazole continued to increase (Fig. 10), while the relative abundance of benzo[c]carbazole continued to decrease (Table 1). The relative abundance of benzo[b]carbazole showed a trend of first increasing and then decreasing with the increasing maturity, ranging from 15–31%, with maximum at 1.11%R_o.

3.2 Changes in parameters of carbazole compounds with the increasing maturity

According to the migration and fractionation mechanism of carbazole compounds, the concentration changes of carbazole compounds have been widely applied in hydrocarbon migration. The above has proven that thermal maturity has a significant impact on the distribution of carbazole compounds. Coincidentally, this study also found that thermal maturity has an impact on some indicators indicating hydrocarbon migration, but the degree of influence varies. When applying carbazole compound parameters in situations with significant differences in maturity, it is necessary to treat them differently and fully consider the impact of thermal maturity.

3.2.1. Methylcarbazole isomers

In the methylcarbazole isomers, with the increasing maturity, the ratios between the isomers showed different changes (Fig. 11). In the mature stage, 1-/4-MCA and 1-/3-MCA undergo wide range of 1.4 to 3.0, with a pattern of first increasing and then decreasing. The maximum appeared around 1.11%R_o, and the ratio remained basically unchanged after 1.5%R_o. 2-/3-MCA and 2-/4-MCA linearly increased with the increasing maturity and showed a good correlation with maturity, which may be potential maturity indicators (Fig. 11a).

3.2.2. Dimethylcarbazole isomers

In the dimethylcarbazole isomers, with the increasing maturity, the ratios of the content of shielded isomers (1,8-DMCA) to the content of exposed isomers (2,7-DMCA, 2,6-DMCA, et al.) showed a similar variation pattern (Fig. 11b). In the mature stage, the ratios all showed a significant change of first increasing and then decreaing, ranging from 0.7 to 1.8 (maximum at 1.11%R_o). After R_o>1.5%, the ratios remained almost unchanged. The ratio of the semi-shielded isomer to the exposed isomer is such as 1,4-/2,6-DMCA, and the ratio gradually decreased from 2.55 to 0.46 with the increasing maturity. 1,5-/2,6-DMCA also exhibits a significant increase and then decrease in the mature stage, while the ratios remained basically unchanged after R_o>1.5%. The ratio between semi shielded isomers, such as 1,7-/1,4-DMCA linearly increased from 0.49 to 3.8 with the increasing maturity. 1, 8-/1,8-+1-EC increased with the increasing maturity, with limit range between 0.60 and 0.82(Fig. 11c). The dealkylation of 1-ethylcarbazole was suggested to have an equivalent mechanism to 4-ethyldibenzothiophene [41], as the dealkylation of 1-ethylcarbazole leads to the formation of 1-methylcarbazole. If dealkylation occurs, an increase for this ratio can be reasonably expected. The parameters of some dimethylcarbazole isomers mentioned above are greatly affected by maturity, which may be related to thermally induced selective alkylation and dealkylation.

3.2.3. Benzocarbazole isomers

In the benzocarbazole isomers, maturity had a great influence on the abundance of benzocarbazole in source rocks and isomers ratios, but relatively limited influence on [a]/[a]+[c]. [a]/[a]+[c] and [a]/[c] gradually increased with the increasing maturity, [a]/[c] showed a large range of changes from 0.56 to 1.58. [a]/[a]+[c] only changed in a relatively small range of 0.36 to 0.61, showing limited maturity dependence (Fig. 11d), which is similar to what some scholars have found [13, 15, 17, 18, 20, 21]. Studies have shown that [a]/[c] can be used as tracers for oil-gas migration, and their absolute concentrations in crude oil decrease with the increasing migration distance [42, 43]. Therefore, the changes that occur with the increasing maturity [a]/[c] are opposite to the changes that occur with migration distance.

During the hydrocarbon migration, nitrogen-containing compounds will undergo migration fractionation effects. Therefore, the concentration and ratios of carbazole compounds can be used as indicators of hydrocarbon migration. However, when applying them, it is necessary to consider the impact of sedimentary environment and maturity factors on them. Liu et al. [44] used 1-MCA/4-MCA, 1,8-/2,7-DMCA, [a]/[c] to indicate the direction of oil-gas migration from Permian lacustrine source rock crude oil to Triassic reservoirs in the Mahu Sag, Junggar Basin. Zhu et al. [45] used 1,8-/2,6-DMCA and [a]/[a]+[c] to indicate oil-gas migration pathways of oil transport in the Qinan Slope Zone of the Bohai Bay Basin.1,4-/2,6-DMCA and 1,5-/2,6-DMCA were suggested as good indicators for migration in the TW block on the eastern wing of the Eastern Basin in Ecuador [46].

This study showed that thermal maturity does have a significant impact on the content of carbazole compounds, especially in the mature and high mature stages. In the case of significant differences in maturity, the concentration of carbazole compounds is greatly affected by maturity and no longer suitable as an indicator of hydrocarbon migration. The ratios between some carbazole isomers were affected to varying maturity degrees. 1-/4-MCA, 1-/3-MCA, 1,8-/2,7-DMCA, 1,8-/1,6-DMCA, 1,8-/2,4-DMCA all undergo relatively large increased and then decreased in the mature stage, reached maximum at 1.11%R_o and remained basically unchanged after R_o>1.5%. 1,4-/2,6-DMCA decreased continuously with the increasing maturity. [a]/[c], [a]/[a]+[c], 2-/3-MCA, 2-/4-MCA, 1,7-/1,4-DMCA all linearly increased with the increasing maturity and showed good correlation with maturity. Under certain conditions, [a]/[c] may become a maturity indicator, while [a]/[a]+[c] only changed within a relatively small range of 0.36 to 0.61 throughout the maturity stage, showing limited maturity dependence. Throughout the entire maturity range, 1,3-/1,6-DMCA and 2,4-/2,5-DMCA were almost unaffected by maturity and abundance, indicating good stability and potential indicator for oil source correlation.

Therefore, the concentration variation caused by component fractionation related to migration may mask the true maturity effect. When using carbazole compounds for hydrocarbon migration and oil source correlation, it is necessary to fully consider the impact of maturity and select parameters less affected by maturity.

(1) The thermal maturity has a significant impact on the content distribution of carbazole compounds, especially in the mature and high mature stages. Compared with the low mature stage, the content of carbazole compounds was about 10 to 100 times higher in the mature stage. At over mature stage with R_o higher than 2.5%, almost no carbazole compounds could be detected. The main formation stage of carbazoles is later than the stage of hydrocarbon generation, but its formation curve is close to that of hydrocarbon generation. It can be seen that carbazole compounds may not be completely inherited from organic sediments, and their content increases significantly with the appearance of the main hydrocarbon generation period.

(2) With the increasing maturity, the relative abundance of 2-MCA, 1,7-DMCA, B[a]CA gradually increased, while the relative abundance of 4-MCA, 1,4-DMCA, B[a]CA, 2,3-DMCA gradually decreased. The relative abundance of 1,8-DMCA and B[b]CA showed a relatively significant increasing and then decreasing during the mature stage, reached its maximum at 1.11%R_o. The relative abundance of 2,7-DMCA, 2,6-DMCA, 2,5-DMCA, and 2,4-DMCA showed a relatively significant decreased and then increased in the mature stage, and also reached its maximum at 1.11%R_o. The relative abundance of 2,5-DMCA remains basically unchanged throughout the maturity stage.

(3) Within the entire maturity range, 1,3-/1,6-DMCA, 2,4-/2,5-DMCA were almost unaffected by abundance and maturity, indicating that their formation rate and mechanism are the same and they may come from common precursors. Considering their structural characteristics, the kerogen matrix with 1-MCA and 2-MCA is most likely to become their precursors, namely 1,3-DMCA and 1,6-DMCA, 2,4-DMCA and 2,5-DMCA, which are formed by methylation of 1-MCA and 2-MCA at the 3,6 and 4,5 carbon positions, respectively. 2,4-/2,5-DMCA was almost 1 throughout the entire maturity range, indicating the consistency of methylation reactions at the 4 and 5 carbon positions during further methylation of 2-MCA, the consistency of chemical activity.

(4) The parameters of carbazole compounds are affected by varying degrees of thermal maturity. 1-/4-MCA, 1-/3-MCA, 1,8-/2,7-DMCA, 1,8-/1,6-DMCA, 1,8-/2,4-DMCA, etc. all undergo relatively large increased and then decreased in the mature stage, and reached their maximum at 1.11%R_o, and remained basically unchanged after R_o >1.5%; 1,4-/2,6-DMCA decreased continuously with the increasing maturity; [a]/[c], [a]/[a]+[c], 2-/3-MCA, 2-/4-MCA, 1,7-/1,4-DMCA all linearly increased with the increasing maturity and showed good correlation with maturity. Under certain conditions, [a]/[a]+[c] may become a maturity indicator, while [a]/[a]+[c] only changed within a relatively small range of 0.36 to 0.61 throughout the maturity stage, showing limited maturity dependence.

(5) The carbazole parameters are no longer suitable as indicators of hydrocarbon migration when there are significant differences in maturity. Therefore, when using carbazole compounds as geochemical parameters, it is necessary to fully consider the impact of thermal maturity on them, and avoid transport fractionation or other factors masking the true maturity effect.

Data availability

Data is available upon request from the corresponding author for the purpose of verifying the results in this study.

Acknowledgements

The authors are very grateful to the editors and all anonymous reviewers for their insightful comments. This work was supported by the National Natural Science Foundation of China (No.42272175,41503034,42172139) and the major national science and technology projects(2017ZX0500105).

Author Contributions

J.B. Writing—review and editing, writing—original draft preparation, conceptualization, methodology, validation, investigation, data curation. Y.L. validation, investigation, resources, writing—review and editing, supervision, project administration, funding acquisition. Y.P.F. validation, investigation, supervision. Y.H.X. validation, investigation. K.L.D. validation, investigation. Z.G.W. investigation. Y.L. investigation. Y.G. investigation. C.Y.Z. investigation. L.L. investigation. All authors have read and agreed to the published version of the manuscript.

Competing interests

The authors declare no competing interests.

Tissot, B. P. & Welte, D. H. Petroleum Formation and Occurrence. Springer, Berlin. p. 699(1984).
Tissot, B., Pelet, R. & Roucache, J. Alkanes as geochemical fossils indicators of geological environments. In: Campos, R., Goni, J. (Eds.), Advances in Organic Geochemistry 1975. Enadimsa, Madrid, pp. 117–154(1977).
Oldenburg, T. B. P., Larter, S. R. & Huang, H. Nitrogen isotope systematics of petroleum fractions of differing polarity–Neutral versus basic compounds. Organic Geochemistry. 38(10), 1789–1794. https://doi.org/10.1016/j.orggeochem.2007.05.016 (2007).
Terken, J. M. J. & Frewin, N. L. The Dhahaban petroleum system of Oman. American Association of Petroleum Geolo gists Bulletin.84, 523–544.https://doi.org/10.1306/C9EBCE41-1735-11 D7-8645000102C1865D (2000).
Li, M., Larter, S. R., Stoddart, D. & Bjoroy, M. Fractionation of pyrrolic nitrogen compounds in petroleum during migration: derivation of migration-related geochemical parameters. Geological Society Publications. Special Publ. 86, 103–123. https://doi.org/10.1144/GSL.SP.1995.086.01.09 (1995).
Larter, S. R. et al. Molecular indicators of secondary oil migration distances. Nature. 383, 593–597. https://doi.org/10.1038/383593a0 (1996).
Silliman, J. E., Li, M., Yao, H. & Hwang, R. Molecular distributions and geochemical implications of pyrrolic nitrogen compounds in the Permian Phosphoria Formation derived oils of Wyoming. Organic Geochemistry. 33(5), 527–544. https://doi.org/10.1016/S0146-6380（02）00018-9 (2002).
Zhao, F., Luo, W., Huang, J. & Teng, J. The hydrocarbon accumulation regularity and the model of hydrocarbon accumulation along the fault ridges in the slope zone of the continental fault basin. Journal of Petroleum Science and Engineering. 208, 109188. https://doi.org/10.1016/j.petr ol.2021.109188 (2021).
Duan, Y., Yuan, Y. & Qian, R. Migration features of crude oil in fluvial deposits of Maling oilfield in Ordos Basin, China. Organic Geochemistry. 58, 78–85.https://doi.org/10.1016/j.orggeoc hem.2013.02.011 (2013).
Cui J., Zhang Z., Zhang Y., Liu G. & Qi Y. Oil and gas accumulation periods and charging path of continental lake basin: A case study of the Chang 9-Chang 10 oil reservoir in the Yanchang formation in the Ordos Basin. Journal of Petroleum Science and Engineering. 219,111136. https://doi.org/10.1016/j.petrol.2022.111136(2022).
Yin J.et al. Oil origin and migration direction in Wushi sag, Beibuwan Basin, South China Sea. Marine and Petroleum Geology. 145,105848. https://doi.org/10.1016/j.marpetgeo.2022.105848 (2022).
Li, M., Fowler, M., Obermajer, M., Stasiuk, L. & Snowdon, L. Geochemical characterisation of Middle Devonian oils in NW Alberta Canada possible source and maturity effect on pyrrolic nitrogen compounds. Organic Geochemistry.30, 1039–1057. https://doi.org/10.1016/S0146-6380（99）00049-2 (1999).
Clegg, H., Wilkes, H. & Horsfield, B. Carbazole distributions in carbonate and clastic source rocks. Geochimica et Cosmochimica Acta. 61, 5335–5345. https://doi.org/10.1016/S0016-7037（97）00304-9 (1977).
Bakr, M. M. Y. & Wilkes, H. The influence of facies and depositional environment on the occurrence and distribution of carbazoles and benzocarbazoles in crude oils: a case study from the Gulf of Suez, Egypt. Organic Geochemistry. 33(5), 561–580. https://doi.org/10.1016/S0146-6 380(02)00016-5 (2022).
Zhang, C., Zhang, Y., Zhang, M., Zhao, H. & Cai, C. Carbazole distributions in rocks from non-marine depositional environments. Organic Geochemistry. 39(7), 868–878. https://doi.org /10.1016/j.orggeochem.2008.03.011 (2008).
Harrison, E. et al. Maturity Controls on Carbazole Distributions in Coals and Source Rocks.18th International Meeting on Organic Geochemistry, Maastricht, pp. 235–236(1997).
Li, M., Yao, H., Stasiuk, L. D., Fowler, M. G. & Larter, S. R. Effect of maturity and petroleum expulsion on pyrrolic nitrogen compound yields and distributions in Duvernay Formation petroleum source rocks in central Alberta, Canada. Organic Geochemistry. 26, 731–744. https://doi.org/10.1016/S0146-6380（97）00053-3 (1997).
Clegg, H., Horsfield, B., Wilkes, H., Damsté, J. S. & Koopmans, M. P. Effect of artificial maturation on carbazole distributions, as revealed by the hydrous pyrolysis of an organic-sulphur-rich source rock (Ghareb Formation, Jordan). Organic Geochemistry. 29, 1953–1960. https://doi.org/ 10.1016/S0146-6380(98)00172-7 (1998a).
Clegg, H., Wilkes, H., Oldenburg, T., Santamaría-Orozco, D. & Horsfield, B. Influence of maturity on carbazole and benzocarbazole distributions in crude oils and source rocks from the Sonda de Campeche, Gulf of Mexico. Organic Geochemistry. 29, 183–194. https://doi.org /10.1016/S0146-6380(03)00033-0 (1998b).
Horsfield, B., Clegg, H., Wilkes, H. & Santamaria-Orozco, D. Effect of maturity on carbazole distributions in petroleum systems: new insights from the Sonda de Campeche, Mexico, and Hils Syncline, Germany. Naturwissenschaften.85, 233–237. https://doi.org/10.1007/s001140050489 (1998).
Zhang, C., Zhang, Y. & Cai, C. Maturity effect on carbazole distributions in source rocks from the saline lacustrine settings, the western Qaidam Basin, NW China. Journal of Asian Earth Sciences. 42(6), 1288–1296. https://doi.org/10.1016/j.jseaes.2011.07.015 (2011).
Chen Z., Zhang M. & Gao L. Evolution characteristics of carbazole compounds in non-hydrocarbons under different temperature conditions in coal and mudstone. Acta Sedimentologica Sinica. 29(03),587-592. https://doi.org/10.1016/j.orggeochem.2008.03.011 (2011).
Oldenburg, T. B. P., Brown, M., Bennett, B. & Larter, S. R. The impact of thermal maturity level on the composition of crude oils, assessed using ultra-high resolution mass spectrometry. Organic Geochemistry. 75, 151–168. https://doi.org/10.1016/j.orggeochem.2014.07.002 (2014).
Han Y.et al. Fractionation of hydrocarbons and NSO-compounds during primary oil migration revealed by high resolution mass spectrometry: Insights from oil trapped in fluid inclusions. International Journal of Coal Geology. 254, 103974. https://doi.org/10.1016/j.coal.2022.103974 (2022).
Yue, H.et al. The retention of precursor biotic signatures in the organonitrogen and organooxygen compounds of immature fine-grained sedimentary rocks. International Journal of Coal Geology. 259, 104039. https://doi.org/10.1016/j.coal.2022.104039 (2022).
Zhang, C.et al. Effect of biodegradation on carbazole compounds in crude oils. Oil & Gas Geology (Oil Gas Geol).20,341–343. https://doi.org/10.1006/mcpr.1998.0211 (1999).
Huang, H., Bowler, B.F.J., Zhang, Z., Oldenburg, T.B.P. & Larter, S.R. Influence of biodegradation on carbazole and benzocarbazole distributions in oil columns from the Liaohe basin, NE China. Organic Geochemistry.34, 951–969. https://doi.org/10.1016/S0146-63 80(03)00033-0 (2003).
Huang, H., Bowler, B.F.J., Oldenburg, T.B.P. & Larter, S.R. The effect of biodegradation on polycyclic aromatic hydrocarbons in reservoired oils from the Liaohe basin, NE China. Organic Geochemistry. 35, 1619–1634. https://doi.org/10.1016/j.orggeochem.2004.05.009 (2004).
Song, H., Wen, Z. & Bao, J. Influence of biodegradation on carbazole and benzocarbazole distributions in oils from the Bongor Basin, Chad. Organic Geochemistry. 100, 18–28. https://doi.org/10.1016/j.orggeochem.2016.07.005 (2016).
Yin, M., Huang, H. & Oldenburg, T.B.P. An application of exploratory factor analysis in the deconvolution of heavy oil biodegradation, charging and mixing history in southeastern Mexico. Organic Geochemistry. 151,104161. https://doi.org/10.1016/j.orggeochem.2020.104161 (2021).
Wang D.et al. New biodegradation degree proxies based on acids and neutral nitrogen- and oxygen-containing compounds characterized by high resolution mass spectrometry. Fuel. 347, 128438. https://doi.org/10.1016/j.fuel.2023.128438 (2023).
Hallmann, C.O.E., Arouri, K.R., McKirdy, D.M. & Schwark, L. Temporal resolution of an oil charging history — a case study of residual oil benzocarbazoles from the Gidgealpa Field. Organic Geochemistry. 38, 1516–1536.https://doi.org/10.1016/j.orggeochem.2007.05.006 (2007).
Ding K., Liu Y., Xiao Q., Luo Y. & Yang H. Pyrolysis of carbazole: Experimental results and kinetic study. Journal of Analytical and Applied Pyrolysis.113(1), 370–379. https://doi.org/10.1 016/j.jaap.2015.02.029 (2015).
Ding K. & Zhang C. Interactions between organic nitrogen and inorganic matter in the pyrolysis zone of underground coal gasification: Insights from controlled pyrolysis experiments. Energy. 135, 279-293. https://doi.org/10.1016/j.energy.2017.06.130 (2017).
Li, M., Larter, S., Stoddart, D. & Bjoroy, M. Practical liquid chromatographic separation schemes for pyrrolic and pyridinic nitrogen aromatic heterocycle fraction from crude oils suitable for rapid characterization of geochemical samples. Analytical Chemistry. 64, 1337–1344. https://do i.org/10.1021/ac00037a007 (1992).
Asif, M., Alexander, R., Fazeelat, T. & Grice, K. Sedimentary processes for the geosynthesis of heterocyclic aromatic hydrocarbons and fluorenes by surface reactions. Organic Geochemistry. 41, 522–530. https://doi.org/10.1016/j.orggeochem.2009.12.002 (2010).
Smirnov, M. B. & Frolov, E. B. A complete analysis of a crude oil C₂-carbazole fraction by ¹H NMR spectroscopy. Organic Geochemistry. 29(5-7), 1091–1099. https://doi.org/10.1016/S0 146-6380 (98)00123-5 (1998).
Zhang, C., Li, S., Yang, J., Yang, S. & Wang, J. Petroleum migration and mixing in the Pearl River Mouth Basin, South China Sea. Marine and Petroleum Geology.21, 215–224. https://doi.org/10.1016/j.marpetgeo.2003.11.010 (2004a).
Zhang, C., Li, S., Yang, J., Yang, S. & Wang, J. A Formation Mechanism of 1, X-Dimethylcarbazole. Natural Gas Geoscience. 2,174-177. https://doi.org/10.11764/j.issn. 1672-1926.2004.02.174 (2004b).
Dorbon, M.et al. Distribution of carbazole derivatives in petroleum. Organic Geochemistry. 7, 110–120. https://doi.org/10.1016/0146-6380（84）90124-4 (1984).
Radke, M. & Willsch, H. Extractable alkyldibenzothiophenes in Posidonia Shale (Toarcian) source rocks: Relationship of yields to petroleum formation and expulsion. Geochimica et Cosmochimica Acta. 58(23), 5223–5244. https://doi.org/10.1016/0016-7037(94)90307-7 (1994).
Yamamoto, M. Fractionation of azaarenes during oil migration. Organic Geochemistry. 19, 389–402. https://doi.org/10.1016/0146-6380(92)90007-K (1992).
Wilhelms, A.et al. Application of Migration Indicators (Benzocarbazoles) in Unraveling of the Filling History of the Oseberg Field, North Sea. International Meeting on Organic Geochemistry. Maastricht, pp. 22–26(1997).
Liu, G.et al. Migration and accumulation of crude oils from Permian lacustrine source rocks to Triassic reservoirs in the Mahu depression of Junggar Basin, NW China: Constraints from pyrrolic nitrogen compounds and fluid inclusion analysis.Organic Geochemistry.101, 82–98. https://doi.org/10.1016/j.orggeochem.2016.08.013 (2016).
Zhu, C.et al. The preferential pathways of oil migration in the Qi-nan slope belt of Bohai Bay Basin, China: Insights from integrated analyses using geochemical and morphology tools. Journal of Petroleum Science and Engineering. 193,107451. https://doi.org/10.1016/j.petrol.2020.107451 (2020).
Ma Z.et al. Nitrogen–Sulfur–Oxygen compounds in the study of oil migration in Block TW, eastern flank of the Oriente Basin, Ecuador. Marine and Petroleum Geology.140, 105640. https://doi.org/10.1016/j.marpetgeo.2022.105640 (2022).

Table 1. The content of carbazole and its derivatives in coal at the different pyrolysis temperature (μg/g)

	Temperature℃	320	340	360	400	414	440	460	480	500	550	600
	R_o（%）	0.60	0.64	0.68	0.93	1.11	1.20	1.33	1.48	1.58	2.26	3.03
Ion	Name

167	CA	19.40	27.27	22.38	28.25	64.05	420.08	1,700.96	6,686.10	4,307.95	25.95	3.67
181	1-MCA	13.38	17.55	15.08	38.95	94.46	421.05	691.45	744.85	610.25	2.75	0.99
181	3-MCA	8.98	12.24	10.13	15.22	32.97	177.36	488.19	641.76	495.01	1.93	0.66
181	2-MCA	8.55	11.62	9.72	16.98	40.81	222.26	615.65	986.31	710.94	3.77	1.35
181	4-MCA	12.14	16.39	14.41	20.18	40.24	185.70	449.23	505.24	397.64	1.75	1.69
195	1,8-DMCA	2.60	3.70	3.24	9.82	22.68	68.89	76.94	45.05	36.80	0.43	/
195	1-EC	1.50	1.94	1.84	4.71	8.62	19.53	17.32	12.53	9.43	0.14	/
195	1,3-DMCA	3.89	5.03	4.12	12.31	27.50	94.75	105.77	63.41	55.31	0.60	/
195	1,6-DMCA	4.59	5.76	4.69	14.03	32.26	113.31	132.18	80.26	70.13	0.75	/
195	1,7-DMCA	3.82	5.27	4.24	14.58	35.39	123.22	153.80	108.78	86.62	1.08	/
195	1,4-DMCA+4-EC	7.85	9.40	7.93	14.66	30.56	74.12	78.02	41.55	32.91	0.28	/
195	1,5-DMCA+3-EC	5.45	7.74	6.83	14.91	31.28	102.49	128.07	76.52	59.86	0.71	/
195	2,6-DMCA	3.08	4.00	3.46	6.47	13.63	60.58	109.78	69.78	57.66	0.61	/
195	2,7-DMCA	4.81	6.10	5.88	8.88	19.03	81.72	138.62	73.67	65.58	0.62	/
195	1,2-DMCA	3.32	4.99	3.80	9.86	17.08	47.62	49.74	33.73	22.22	0.24	/
195	2,4-DMCA	3.76	4.89	4.11	7.90	16.75	59.45	101.53	57.89	48.10	0.56	/
195	2,5-DMCA	3.63	4.46	3.92	7.69	15.73	59.45	98.33	58.49	47.20	0.55	/
195	2,3-DMCA	2.18	2.82	2.46	4.09	6.94	22.41	31.72	17.74	15.33	0.19	/
195	3,4-DMCA	2.55	3.14	3.00	3.86	6.01	16.33	18.96	9.40	7.32	0.15	/
217	B[a]CA	1.30	1.93	1.57	2.56	3.12	14.93	57.68	158.82	109.16	2.49	/
217	B[b]CA	0.67	1.17	0.98	1.91	2.87	13.01	45.96	75.37	57.32	0.95	/
217	B[c]CA	2.34	2.96	2.58	3.01	3.34	13.21	49.08	121.29	79.21	1.58	/
	CA	19.40	27.27	23.38	28.25	64.05	420.08	1700.96	6686.10	4307.95	25.95	3.67
	MCA	43.05	57.80	49.34	91.33	208.48	1066.37	2244.52	2878.16	2213.84	10.60	4.69
	DMCA	51.52	67.32	57.68	129.06	274.85	924.33	1233.47	736.25	605.06	6.77	/
	BCA	4.31	6.06	5.13	7.48	9.32	41.55	152.72	355.48	245.69	5.02	/
	EC	1.50	1.94	1.84	4.71	8.62	19.53	17.32	12.53	9.43	0.48	/
	AC+BC	119.7	160.3	136.3	260.83	565.32	2,411.87	5,338.99	10,668.5	7,381.97	48.42	8.36
	1-MCA/MCA	0.31	0.30	0.31	0.43	0.45	0.42	0.34	0.26	0.28	0.27	/
	2-MCA/MCA	0.20	0.20	0.20	0.19	0.20	0.22	0.27	0.34	0.32	0.37	/
	3-MCA/MCA	0.21	0.21	0.21	0.17	0.16	0.18	0.22	0.22	0.22	0.19	/
	4-MCA/MCA	0.28	0.28	0.29	0.22	0.19	0.18	0.20	0.18	0.18	0.17	/
	B[a]CA/BCA	0.30	0.32	0.31	0.34	0.33	0.36	0.38	0.45	0.44	0.50	/
	B[b]CA/BCA	0.15	0.19	0.19	0.26	0.31	0.32	0.30	0.21	0.23	0.19	/
	B[c]CA/BCA	0.54	0.49	0.50	0.40	0.36	0.32	0.32	0.34	0.32	0.31	/

¹CA: carbazole; MCA: Methylcarbazole; DMCA: Ethylcarbazole; BCA: Benzocarbazole; B[a]CA: Benzo[a]carbazole;
B[b]CA: Benzo[b]carbazole; B[c]CA: Benzo[c]carbazole; AC+BC: Alkylcarbazole + benzocarbazole

No competing interests reported.

Effect of thermal maturity on carbazole distributions: insights from compaction pyrolysis experiment of coal

Status:

Journal Publication

Version 1

Abstract

Figures

1. Introduction

2. Samples and experimental

2.1. Samples

2.2. Experimental

3. Results and discussion

3.1. Basic experimental data

3.1.1. Carbazole compounds

3.1.2. C₁-carbazoles

3.1.3. C₂-carbazoles

3.1.4 Benzocarbazoles

3.2 Changes in parameters of carbazole compounds with the increasing maturity

3.2.1. Methylcarbazole isomers

3.2.2. Dimethylcarbazole isomers

3.2.3. Benzocarbazole isomers

4. Geological significances

5. Conclusions

Declarations

References

Table 1

Additional Declarations

Status:

Journal Publication

Version 1

Effect of thermal maturity on carbazole distributions: insights from compaction pyrolysis experiment of coal

Status:

Journal Publication

Version 1

Abstract

Figures

1. Introduction

2. Samples and experimental

2.1. Samples

2.2. Experimental

3. Results and discussion

3.1. Basic experimental data

3.1.1. Carbazole compounds

3.1.2. C1-carbazoles

3.1.3. C2-carbazoles

3.1.4 Benzocarbazoles

3.2 Changes in parameters of carbazole compounds with the increasing maturity

3.2.1. Methylcarbazole isomers

3.2.2. Dimethylcarbazole isomers

3.2.3. Benzocarbazole isomers

4. Geological significances

5. Conclusions

Declarations

References

Table 1

Additional Declarations

Status:

Journal Publication

Version 1

3.1.2. C₁-carbazoles

3.1.3. C₂-carbazoles