The evolution of acids and neutral nitrogen-containing compounds during pyrolysis experiments on immature mudstone

Marine and Petroleum Geology 115 (2020) 104292

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Research paper

The evolution of acids and neutral nitrogen-containing compounds during pyrolysis experiments on immature mudstone

T

Gang Yana,b, Yaohui Xua,b,∗, Yan Liuc,e,f, Wenxiang Hea,b, Xiangchun Changd, Penghai Tanga,b a

Key Laboratory of Exploration Technologies for Oil and Gas Resources (Yangtze University), Ministry of Education, Wuhan, 430100, China College of Resources and Environment, Yangtze University, Wuhan, 430100, China c Institute of Mud Logging Technology and Engineering (Yangtze University), Jingzhou, 434023, China d College of Earth Science and Engineering, Shandong University of Science and Technology, Qingdao, 266590, China e State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing, 102249, China f Shandong Provincial Key Laboratory of Depositional Mineralization & Sedimentary Mineral, Shandong University of Science and Technology, Qingdao, 266590, China b

ARTICLE INFO

ABSTRACT

Keywords: Pyrolysis experiments ESI− FT-ICR MS NSO polar compounds Evolution characteristics

Pyrolysis experiments were conducted on immature(Ro = 0.47%) organic-rich (TOC = 6.0%)mudstone, and the liquid products at 10 different temperature stages were detected by negative ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI − FT-ICR MS). Seven classes of acids and neutral nitrogen-containing compounds, namely, N1, N 2, O 1, O2, O3, O4 and N1O 1, were detected in all the products. The O 2 class accounted for the largest proportion (50.56 %–91.65%) among all the products, followed by the O 1 class (1.70 %–28.80%). The % O1 and % N1 classes showed a clear upward trend with the increase in experimental temperature due to their organic resources, and the % O2 class showed the exact opposite trend, which could be explained by the chemical reactions that occurred during the maturation process, such as decarboxylation and dehydration. In addition, the relative a bundances of the classes with high double bond equivalents (DBE) (≥9) and low DBE (≤7) showed a significant upward trend and a clear downward trend, respectively. The main reasons were aromatization and condensation during thermal evolution. This series of thermal evolution characteristics indicated that a set of new thermal parameters, such as ∑%DBE ≥9/∑%DBE≤8 (O 2 compounds) = 0.0007T 2 0.372T + 48.594(R 2 = 0.9088) may be used as a new maturity parameter after further verified with crude oils.

1. Introduction The composition and distribution of polar NSO compounds have a large influence on the properties of crude oil and are particularly closely related to the source material, evolutionary conditions, and depositional environment. Therefore, it is very useful to study polar NSO compounds to better understand the process of oil formation and the laws of evolution. In recent years, polar NSO macromolecule compounds have received special attention because increasing evidence has shown that these compounds have broad application prospects in petroleum exploration and development (Li et al., 2001). Scholars have conducted many successful studies on nitrogen-containing compounds (Bastow et al., 2003; Bechtel et al., 2013; Song et al., 2016), oxygen compounds (Simoneit et al., 1996; Innes et al., 1997; Marynowski et al., 2002; Peters et al., 2018; Thiel and Hoppert, 2018) and sulfur compounds (Rospondek et al., 2007; Li et al., 2008;

∗

Fang et al., 2016; Meshoulam and Amrani, 2017); hence, the study of polar NSO compounds has received increasing attention in the field of geochemistry. Scholars have conducted extensive studies on the volatile small molecular compounds of petroleum, including saturated hydrocarbons, aromatic hydrocarbons and polar NSO compounds via geochemical analytical determination methods such as gas chromatography (GC), gas chromatography-mass spectrometry (GC-MS), and gas chromatography-mass spectrometry-mass spectrometry(GC-MSMS) (van Aarssen et al., 1999; Armstroff et al., 2007; Kashirtsev et al., 2010; Dawson et al., 2013; Elfadly et al., 2016; El-Sabagh et al., 2018). However, the study of polar NSO macromolecules using these conventional geochemical analytical methods is limited due to the wide mass distribution and complex structure and properties of these compounds. With technological advancements, electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI

Corresponding author. College of Resources and Environment, Yangtze University, Wuhan, 430100, China. E-mail address: [email protected] (Y. Xu).

https://doi.org/10.1016/j.marpetgeo.2020.104292 Received 13 June 2019; Received in revised form 13 January 2020; Accepted 8 February 2020 Available online 13 February 2020 0264-8172/ © 2020 Elsevier Ltd. All rights reserved.

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Fig. 1. Tectonic units of Hesigewula Sag and the stratigraphic bar graph of Well HD2. Table 1 Geochemical parameters of the source rock sample. Depth (m)

Ro (%)

TOC (%)

Tmax (°C)

S1+S2 (mg/g)

IH (mg/g)

Kerogen type

Maceral components

108.6

0.47

6.0

430

17.29

284

II1

Exinite and Sapropelinite

FT-ICR MS) has recently been introduced into the petroleum industry as a new type of mass spectrometry technology. This technology enables the direct detection of polar NSO compounds in petroleum. Because of its superior mass resolving power and accuracy, the resolution can be as high as hundreds of thousands or even millions in the relative molecular mass range of the petroleum components (200–1000 Da). Furthermore, this method can be used to achieve the complete separation of different polar compounds according to their elemental composition and accurately calculate the number of atoms of C, H, O, N, and S in the corresponding molecular compounds (Shi et al., 2008b; Liu et al., 2010; Zhang et al., 2016). The ESI ionization source can be used to selectively ionize polar heteroatom compounds in petroleum. In general, trace amounts of basic compounds (mainly nitrogen-containing compounds) and sulfur compounds can be selectively ionized in positive ion mode, while neutral nitrogen-containing compounds (mainly nitrides containing pyrrole) and acidic compounds such as phenols, carboxylic acids, and naphthenic acids usually appear on negative ion mass spectra (Shi et al., 2008b; Liu et al., 2014; Jiang et al., 2014). In recent years, many scholars have employed this method in a range of studies. Lu et al. (2014) studied

the molecular composition of high-sulfur crude oil; Shi et al. (2007) analyzed the molecular types of naphthenic acids in crude oil from Liaohe Oilfield; Kim et al. (2005) studied the effect of microbial degradation on the behavior of acidic polar NSO compounds in crude oil; Hughey et al. (2004) studied the distribution of neutral and acidic polar NSO compounds at different thermal maturities of the Smackover crude oils; Hughey et al. (2002) successfully identified the types of acidic NSO compounds in crude oil from different organic matter sources; Dias et al. (2014) successfully explained the process of degradation and corrosion of naphthenic acid using this method. All these studies have shown that FT-ICR MS has clear advantages in the study of polar NSO macromolecular compounds and provides a new tool for geochemical research. In the present research, a pyrolysis experiment on immature organic-rich mudstone was conducted and the liquid products at 10 different temperature stages were detected by negative ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI− FT-ICR MS). Then, we analyzed the thermal evolution characteristics of the polar NSO macromolecules in the products and their geochemical importance. 2

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Fig. 2. System of simulated pyrolysis experiments.

developed in house, as shown in Fig. 2. The experimental conditions are listed in Table 2. These experiments mainly included the following five steps:

Table 2 Parameters of the simulated pyrolysis experiments. Temperature (°C)

Simulated stratum pressure (MPa)

Fluid pressure (MPa)

280 310 340 360 380 400 435 460 485 540

51 63 78 84 93 99 108 129 144 162

6.55 1.69 1.42 1.36 1.42 1.67 2.04 1.90 1.88 7.46

I) The experimental system was cleaned with dichloromethane. II) The sample column was placed into the sealed pyrolysis vessel. We first evacuated the system using a vacuum pump and then filled it with sufficient helium. III) The initial experimental temperature and pressure were set (Table 2), the reactor kettle was heated using a thermowell, and the temperature and pressure were kept unchanged for 24 h. IV) The temperature was allowed to decrease rapidly to 150 °C, and then the liquid and gaseous hydrocarbons were collected. V) The next temperature and pressure were set, and experimental steps III and IV were repeated. All 10 experimental products at different temperature stages were collected.

2. Sampling and experimental methods 2.1. Sample The Hesigewula Sag is a Mesozoic sedimentary basin located in the northeastern Erlian Basin, China. The sag belongs to the Daxinganling stratigraphic subregion. There are many strata in the periphery of this sag, including Jurassic, Cretaceous and Upper Paleozoic. The sample for these experiments was collected from the Lower Cretaceous stratum of drill core HD2(Fig. 1). This drill core was collected from the Hesigewula Sag near the western edge of Daxinganling Prefecture. The source rock sample is a brown gray mudstone, and the geochemical parameters of this sample are listed in Table 1. In the experimental preparation phase, the source rock sample was crushed to 2–4 mm. Then, the sample was pressed into a cylindrical shape with a pressure of 50 MPa. The weight, height and diameter of the sample column were 145.22 g, 76.92 mm and 4.00 cm, respectively.

2.3. ESI− FT-ICR MS analysis Firstly, each product (net weight: 2.50 mg, 4.40 mg, 14.50 mg, 25.80 mg, 30.70 mg, 14.90 mg, 5.70 mg, 2.10 mg, 1.10 mg, and 0.90 mg) was dissolved in toluene to obtain a mother liquor whose concentration was equal to 10 mg/ml. Secondly, we diluted 20 μl of the mother liquor to 1 ml with methanol and toluene (V:V = 3:1). Thirdly, we spiked the final solution with 20 μl NH4OH and shook it gently to facilitate deprotonation of the acids and neutral nitrogen compounds in the experimental products. The purpose was to promote the generation of ions [M−H]-. Finally, the injection pipeline was cleaned and the instrument was calibrated before the injection. Then, we injected the solutions and started collecting data. Mass analyses were performed using a Bruker Apex IV FT-ICR mass spectrometry which was equipped with a 9.4 T magnet. The ESI conditions were as follows: negative ion electrospray ionization mode; the polarization voltage was 3.5 kv; the capillary inlet voltage was 4.0 kv,

2.2. Pyrolysis experiments We carried out the pyrolysis experiments in an experimental system 3

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Fig. 3. Negative-ion ESI FT-ICR spectrum of liquid hydrocarbons at various temperatures.

and the outlet voltage was 0.4 kv; quadrupole Q1 = 200 Da; the collision energy was 1.5 v; the time of source accumulation was 0.001 s, and the collision cell accumulation time was 1.0 s; the flight time of ion introduction was 1.1 ms; the mass acquisition was from 200 Da to 800 Da; the acquisition points were 4 M; the excitation attenuation was 13.5 dB; the front panel voltage was −0.9 V, and the rear voltage was −0.95 V; the speed of injection was 180 μl/h; and the temperature of dry gas was 200 °C. We scanned the spectra 64 times in total.

ESI FT-ICR MS analysis was conducted in negative ion mode. We performed ESI− FT-ICR MS on the liquid products collected from 10 different temperature stages to obtain the corresponding ESI FT-ICR mass spectrum (Fig. 3). As shown in Fig. 3, the mass distribution of each product was narrow, ranging from m/z 200 to 350 with a mass center at approximately 250 and 280 in the mass spectrum. The mass spectrum showed a distinct unimodal pattern below 360 °C, with no obvious peak pattern above this temperature. Moreover, with the increase in the experimental temperature, the relative abundance of high-mass compounds reduced significantly, while the low-mass compounds had a tendency to increase. This change can be explained by a series of thermochemical reactions that occurred during thermal evolution process, such as decarboxylation (elimination of carboxyl), deamination (removal of an amine), dehydration (loss of a water molecule), and demethylation (removal of a methyl group) and the breaking of C–C bonds (Price, 1993; Seewald, 1994; Behar et al., 1997). Furthermore, a greater diversity of compounds was found in the high temperature stage (> 435 °C). This phenomenon may be attributed to the release of these compounds from the insoluble organic matrix (kerogen) (Noble et al., 1987; Zhao et al., 2018) above this temperature. We used the types and numbers of the heteroatoms contained in the molecule to indicate the type of compounds. In this experiment, we detected and identified a total of seven classes of heteroatom compounds: N1, N1O1, N2, O1, O2, O3, and O4 (N1 stands for the compound containing one nitrogen atom, N1O1 stands for the compound containing one nitrogen atom and one oxygen atom, and so on). Fig. 4 shows the corresponding polar compound types and their relative abundance at different temperatures. The different colors in Fig. 4 indicate different DBE, which was obtained by adding the number of rings and the number of double bonds (Zhang et al., 2011; Ke et al., 2017, 2018). Fig. 4 shows that seven classes of polar compounds (N1, N2, O1, O2, O3, O4 and N1O1) could be detected in every product at different experimental temperatures, and the main classes were O1 (1.70 %–28.80%) and O2 (50.56 %-91.65%) in all products, in which the proportion of the O2 class was particularly prominent (Table 3). This phenomenon was consistent with that of Rocha et al. (2018) regarding acid and neutral polar NSO compounds in the simulated thermal evolution of a Type-I source rock. In their study about the Type-I source rock pyrolysis products, whether in the 11 expelled oil sample or residual bitumen samples, the Ox compounds dominated the acidic compound classes and comprised approximately 75–100%. Fig. 4 also

2.4. Data processing We conducted data processing with the Petro MS software developed by China University of Petroleum (Beijing). The software was used to determine the formula attribution and recalibration of known homologous series on the basis of the measured m/z values of polar compounds in crude oils. According to the m/z values, elemental formulas could be calculated and assigned easily. Firstly, the MS peaks with signal-to-noise ratio > 3 were output to an Excel spreadsheet, and the combination of C, H, O, N, and S atoms was calculated for each molecule. Then, the formula for each corresponding mass peak (CcHhOoNnSs, where c, h, o, n, and s represent the number of atoms of carbon, hydrogen, oxygen, nitrogen, and sulfur, respectively) was obtained. Finally, the molecular composition information of all compound types in the products and their corresponding double bond equivalents (DBE, calculated based on the formula DBE = c−h/2 + n/2 + 1) could be obtained. All the assigned compounds could be also grouped into different classes using the software based on the relative abundances of whole heteroatom classes, DBE distributions and carbon number distributions within a heteroatom class. The detailed calculation method of the software was described by Shi Q et al. (Shi et al., 2008a; Liu et al., 2010). 3. Results and discussions 3.1. Spectrum characteristics and polar compound types Sulfur compounds must undergo methyl derivatization due to their low polarity and could only be analyzed by using positive ion mode (Liu et al., 2010; Li et al., 2013). There were no sulfur compounds such as thiophene in the 10 experimental products according to the conventional GC-MS analysis before the ESI− FT-ICR MS test. Therefore, this 4

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Fig. 4. The classes and relative abundance of the polar compounds at various temperatures.

3.2. Changing characteristics of the relative abundance of different classes of polar compounds with increasing temperature

Table 3 The relative abundance of different classes of polar compounds at different temperatures. Experimental temperature (°C)

%N1

%N1O1

%N2

%O1

%O2

%O3

%O4

280 310 340 360 380 400 435 460 485 540

0.22 0.29 0.27 0.42 1.21 1.42 2.33 4.09 5.98 10.31

1.21 1.96 3.82 5.58 4.91 5.92 3.92 3.26 3.58 4.92

0.21 0.10 0.20 0.07 0.19 0.94 0.65 0.49 0.45 0.73

3.85 1.70 3.58 15.52 24.18 17.35 21.47 28.57 23.96 28.80

84.04 91.65 82.84 73.82 62.76 70.91 66.87 55.32 59.43 50.56

7.67 3.34 7.70 3.53 5.20 2.50 3.84 6.57 5.30 3.69

2.81 0.98 1.60 1.07 1.55 0.95 0.93 1.70 1.29 0.98

Oxygen compounds in crude oil mainly include two major categories: acidic oxygen compounds (including fatty acids, phenols, aromatic acids, and naphthenic acids) and neutral compounds (including aldehydes, ketones, esters, and furans). However, their content is generally very low (Niu et al., 2013). Because neutral oxygen compounds are difficult to ionize in the negative ion ESI mode (Jiang et al., 2014), the oxygen compounds identified by ESI− FT-ICR MS in this experiment are mainly acidic oxygen compounds. In addition, Fig. 4 clearly shows that the DBE values of O1 compounds in all products were generally ≥4, and phenolic compounds (DBE ≥ 4 and containing one oxygen atom) are commonly found in crude oil. Therefore, it is reasonable to conjecture that the O1 class identified in this experiment is most likely alkylphenol (Cheng et al., 2010; Liu et al., 2014; Wan et al., 2015). Similarly, the minimum DBE of the O2 compounds is 1, and the proportion of O2 compounds is the largest. According to the degree of condensation, we conclude that the O2 compounds with DBE = 1 are likely to be predominantly a saturated fatty acid containing a carboxyl. The nitrogen-containing compounds in crude oil are mainly classified into basic and neutral compounds, of which the basic nitrogen-

shows that the major DBE distribution ranged from 0 to 15, with DBE = 1 (29.26%–78.61%) as the main type, followed by DBE = 2 (7.18%–18.76%) (Table 4). With the progression of thermal evolution, the relative abundance and DBE of various classes of polar NSO compounds were substantially changed. These specific characteristics will be described in detail in the following sections.

5

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Table 4 The relative abundance of different DBE polar compounds at different temperatures. Experimental temperature (°C)

280

310

340

360

380

400

435

460

485

540

%(DBE %(DBE %(DBE %(DBE %(DBE %(DBE %(DBE %(DBE %(DBE %(DBE %(DBE %(DBE %(DBE %(DBE %(DBE %(DBE

0.00 0.12 0.18 0.28 0.00 0.26 0.36 1.09 1.71 1.77 1.56 1.35 2.22 15.42 73.66 0.00

0.11 0.05 0.06 0.00 0.04 0.35 0.26 0.47 1.40 1.17 1.45 0.86 1.82 13.31 78.61 0.04

0.15 0.06 0.12 0.13 0.38 0.22 0.19 0.67 0.71 0.83 0.99 2.55 2.77 18.76 71.41 0.05

0.07 0.00 0.00 0.23 0.82 1.13 3.29 2.28 1.54 1.60 2.33 6.53 1.61 14.20 64.38 0.00

0.27 0.13 1.55 0.91 1.66 3.50 7.06 3.65 1.38 1.77 2.67 10.08 1.30 11.52 52.50 0.05

0.25 0.26 2.01 1.08 1.71 3.50 6.12 4.06 1.12 1.13 1.85 6.53 1.52 15.23 53.45 0.19

0.10 0.40 2.56 3.59 2.97 4.60 9.55 3.95 0.88 0.64 1.77 5.29 1.40 11.82 50.30 0.17

0.97 1.66 4.24 6.89 4.89 5.55 9.60 4.41 0.99 0.57 1.10 3.49 0.96 9.58 35.89 0.07

1.29 2.80 8.80 10.76 5.41 4.39 9.74 2.52 0.50 0.27 0.83 1.83 1.14 9.15 40.43 0.13

3.33 4.64 13.13 15.89 6.49 5.11 9.67 1.81 0.40 0.26 0.52 1.52 0.73 7.18 29.26 0.07

= = = = = = = = = = = = = = = =

15) 14) 13) 12) 11) 10) 9) 8) 7) 6) 5) 4) 3) 2) 1) 0)

containing compounds cannot be ionized in the negative ion mode. Therefore, the N1 classes identified in this experiment were mainly neutral nitrogen compounds containing one pyrrole ring (Li et al., 2010; Cheng et al., 2010). To more intuitively observe the thermal evolution characteristics of various classes of polar compounds, we defined the relative abundance of a class polar compounds as the ratio of the abundance of this class to the sum of the abundances of all polar compounds at that temperature (for instance, % O1 = class O1/∑class (N1, N1O1, N2, O1, O2, O3, O4)). As seen from Fig. 5 and Table 3, with the continuous increase in the experimental temperature, the relative abundances of the seven polar compound classes presented different changes. The %N1 class (carbazoles) and %O1 class (alkylphenols) showed a significant upward trend with the increasing experimental temperature, indicating that the formation mechanisms of phenolic compounds and nitrogen compounds containing pyrrole have both commonalities and differences. In addition, this result indicated that phenolic compounds could be derived from the degradation of high-mass heteroatom compounds, i.e., the important source of phenolic compounds was the thermal degradation of kerogen (Li et al., 2000). The %O2 class showed the exact opposite change trend, which was similar to the conclusions of the study by Hughey et al. (2004) on Smackover crude oils with different maturities. Their research revealed that with increasing maturity, the relative percentage of oxygen compounds decreased and the length of alkyl and saturated side chains decreased, indicating that substantial decarboxylation reactions occurred during thermal evolution (Rocha et al., 2018). In their opinion, thermal maturation can promote aromatization and condensation of acidic polar compounds (Ox compounds). Within a given formula class, the relative distribution shifts toward compounds with a higher degree of unsaturation and favors fully aromatized species relative to those with partially unsaturated naphthenic rings. In addition, the other four classes (N1O1, O3, O4, and N2) did not show clear patterns of change with the increasing experimental temperature, indicating that the relative percentage of these four classes may be affected by various factors.

abundances of all polar compounds (for instance, % DBE1 = ∑DBE 1/ ∑DBE 0 to 15). Fig. 6 and Table 4 show the varying pattern of the relative abundance of different DBE polar compounds with increasing experimental temperature. Fig. 6 and Table 4 show that the DBE = 1 compounds accounted for the largest proportion, followed by DBE = 2, DBE = 4, and DBE = 8 to 15, while DBE = 3 and DBE = 5 to 7 compounds accounted for a relatively small proportion. The proportion of DBE = 0 compounds was the smallest, close to zero. With the increase in experimental temperature, compounds of different DBEs showed different evolution rules, which could be roughly divided into the following four scenarios: I) The relative abundances of the DBE = 9 to 15 compounds showed a significant upward trend with the increasing experimental temperature. Among them, the increasing rates of the DBE = 12 and DBE = 13 compounds were the largest, followed by those of the DBE = 11, DBE = 14 and DBE = 15 compounds. However, the DBE = 9 and DBE = 10 compounds did not change significantly in the last stage, and there was a tendency for these compounds to be stable. II) The changes in the relative percentage of the DBE = 5 to 7 compounds could be divided into three stages. With the increasing experimental temperature, there was a small decrease initially, followed by an upward trend and then a downward trend. III) The relative abundances of the compounds with DBE = 4 and DBE = 8 showed a clear upward trend first and then decreased with increasing temperature. In addition, the corresponding experimental temperature of the turning point was 360–380 °C. IV) The relative abundances of the DBE = 1 to 3 compounds showed a significant downward trend with the increase in experimental temperature, and the DBE = 1 compounds showed the most significant downward trend. In summary, with the increase in the experimental temperature, the relative abundance of high-DBE (DBE ≥ 9) compounds exhibited a significant upward trend, while the relative abundance of low-DBE (DBE ≤ 7) compounds had a tendency to decrease significantly, which indicated that, except for the reactions (such as decarboxylation, demethylation, and dehydration) that could lead to a downward trend in the molecular weight of the compounds, there were other reactions (such as aromatization and condensation) that could lead to an increase in DBE. This is consistent with the studies of Rocha et al. (2018), who found that the increase of high DBE value compound (O2 compounds) abundances with ongoing maturity. More precisely, the abundance of compounds with DBE values from 9 to 17 (class O2) present in the expelled oil samples increase significantly from the immature oil to gas window, but the lower DBE (5–7) value compounds (O2 compounds)

3.3. Changing characteristics of the relative abundance of polar compounds in different DBE with increasing temperature As noted in the previous section, the DBE can be calculated based on the formula DBE = c-h/2 + n/2 + 1. Then, we can subdivide the same polar compound class into additional categories according to the DBE. For example, we could classify N1 into 0N1, 1N1, 2N1...15N1 according to the value of DBE. To better study the thermal evolution pattern of different DBE polar compounds, we defined the relative abundance of a DBE polar compound as the ratio between the sum of the abundances of the seven classes corresponding to this DBE and the sum of the 6

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Fig. 5. Evolution of different classes of polar compounds with increasing temperature.

abundances showed completely opposite characteristics. In order to find one or two meaningful maturity parameters, a more detailed evaluation of the O2 compounds is addressed in this section because it accounts for the major proportion detected with increasing temperature, just like the work of Rocha et al. (2018) about the Type-I source rock hydrous pyrolysis experiment. Fig. 7 shows the maturityrelated changes in DBE distribution of O2 compounds measured in ESI negative ion mode. The relative abundance of O2 compounds with DBE

values from 9 to 15 increase significantly (∑%DBE ≥9 = 0.0732T 22.889, R2 = 0.8964). Simultaneously, Fig. 7(a) illustrates the decrease of low DBE (≤8) value compound abundances with ongoing maturity (∑%DBE≤8 = −0.2242T + 152.97,R2 = 0.8928. what is more, the relationship between the relative abundance sums for the O2 compounds with DBE values that increased versus decreased with the increasing experiment temperature was evaluated. The corresponding equation is: ∑%DBE ≥9/∑%DBE≤8 (O2 compounds) = 0.0007T2 7

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Fig. 6. Evolution of polar compounds with different DBE with increasing temperature.

Fig. 7. Maturity-related changes in DBE distribution of O2 compounds. (a) Relative abundances of sum of DBE range from 1 to 8 and sum of 9–15 associated to their linear regressions; (b) exponential behavior of the relation of the sum of higher DBE (9–15) against lower DBE numbers (1–8).

0.372T + 48.594,R2 = 0.9088. The experimental results suggest that this parameter may be used as a new maturity parameter, which needs to be further verified.

between the carbon number and DBE of polar compounds; the horizontal axis represents the carbon number, and the vertical axis represents the corresponding DBE. The size of the solid circle in the figure represents the relative content of the corresponding compound. In addition, the chemical structural formulas in the figure were all inferred based on the carbon numbers and the values of DBE. Fig. 8(a) shows that the DBE of the O1 class ranged from 4 to 8 at 280 °C to 4–13 at 400 °C, and finally reached 8–15 at 540 °C. In addition, the DBE center showed an increasing sequence (4 → 9→12) with increasing experimental temperature. The distribution of carbon numbers of the O1 species ranged from 11 to 24, mainly from 16 to 20. The O1 class in the products was mainly alkylphenols with DBE = 6 at

3.4. Evolution characteristics of the carbon number and DBE of polar compounds Figs. 4 and 5 clearly show that the relative percentage of the O1, O2 and N1 classes was significantly higher than that of the other four polar compounds. Therefore, we focused our attention on the evolution characteristics of the carbon numbers and DBE of the O1, O2, and N1 classes in this section. Fig. 8(a)–(c) are graphs showing the correlation 8

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Fig. 8. Iso-abundance plots of DBE versus carbon number for different heteroatom compounds. a: O1 class b: O2 class c: N1 class.

280 °C based on the DBE and the number of oxygen atoms. Because the DBE of a benzene ring was equal to 4, there may be two double bonds or two cyclic structures or one triple bond in the substituent branches of phenol. When the experimental temperature increased to 340 °C, the O1 class was mainly alkylphenols with DBE = 4, and the carbon numbers

ranged from 11 to 22 (mainly from 15 to 19), indicating that the branches of phenol had undergone a thermal cracking reaction in the higher temperature stage. In addition to alkylphenols with DBE = 4 from 400 °C to 435 °C, a large number of xenols with DBE = 8 (Wan et al., 2015) and phenol indane with DBE = 9 (Geng et al., 2012, 2013) 9

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Fig. 8. (continued)

Fig. 9. The possible reaction pathways occurring during maturation (Poetz et al., 2014).

appeared. This process may be accompanied by the rearrangement of alkylphenols by thermal interactions to form more stable compounds (Li et al., 2000). Above 435 °C, the content of the O1 class in the products decreased, which may indicate that the thermal degradation of kerogen was an important source of phenolic compounds (Li et al., 2000) and that most of the kerogen had undergone thermal degradation in the early stage of the experiment. At the same time, DBE distinctly increased and gradually evolved into compounds with fluorene structure with DBE = 12 above 435 °C, which may be due to the rearrangement of the alkylphenol by thermal interaction. As seen from Fig. 8(b), with the increase in experimental temperature, the O2 class in the products at each temperature stage mainly consisted of long-chain saturated fatty acids with DBE = 1, and the carbon center was C16 and C18, as determined by the original source of the compound. However, the carbon number distribution range became

increasingly narrow with increasing temperature, and the high-carbon number compounds tended to decrease gradually, indicating that the saturated fatty acids had undergone a substantial thermal cracking process with the increase in the experimental temperature. A small number of compounds with DBE = 9 (C15–C18) and DBE = 13 (C18–C21) could be detected in the products at 460 °C and 540 °C. We inferred that they may be biphenylacetic acid and polycyclic aromatic acids based on the DBE and the number of carbons. This result could be explained by the following reasons: (1) aromatic naphthenic acid containing a stable aromatic ring structure could be released from the kerogen at a high temperature stage, and (2) aromatization may occur at a high temperature stage. As seen from Fig. 8(c), the relative percentage of the N1 class in the products at each temperature stage was relatively low, but the change in DBE was very obvious. At the beginning, carbazoles with DBE = 9 10

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were the main products, it was released from the kerogen matrix via thermal cracking. And when the temperature reached 435 °C, the main products were benzoxazole with DBE = 12. Moreover, the carbazoles with DBE = 9 substantially decreased in the process of temperature increase. Finally, a small amount of dibenzocarbazole with DBE = 15 in the product could be detected above the 460 °C stage, which may be formed through cyclization and aromatization. The possible reaction pathways occurring during maturation of the N1 compounds can be learned from research of Poetz et al. (2014) (Fig. 9). In general, as the experimental temperature increased, the three classes all had the following two characteristics: (1) the relative abundance of low-DBE polar compounds decreased, while the abundance of high-DBE polar compounds increased, and (2) high-carbon number polar compounds were converted to low-carbon number polar compounds. This result was mainly due to the simultaneous occurrence of thermal cracking reactions, condensation reactions, and aromatization reactions during the thermal evolution process.

Acknowledgments We would like to express our sincere gratitude to Shi Q for his help with the ESI FT-ICR-MS test. We truly appreciate the help and support that Zhang YH provided for the data processing. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.marpetgeo.2020.104292. References Armstroff, A., Wilkes, H., Schwarzbauer, J., Littke, R., Horsfield, B., 2007. The potential role of redox reactions for the distribution of alkyl naphthalenes and their oxygenated analogues in terrestrial organic matter of Late Palaeozoic age. Org. Geochem. 38, 1692–1714. https://doi.org/10.1016/j.orggeochem.2007.06.002. Bastow, T.P., van Aarssen, B.G., Chidlow, G.E., Alexander, R., Kagi, R.I., 2003. Small-scale and rapid quantitative analysis of phenols and carbazoles in sedimentary matter. Org. Geochem. 34, 1113–1127. https://doi.org/10.1016/s0146-6380(03)00066-4. Bechtel, A., Gratzer, R., Linzer, H.-G., Sachsenhofer, R.F., 2013. 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4. Conclusions (1) A total of seven types of polar compounds (N1, N1O1, N2, O1, O2, O3, O4) could be detected in thermal evolution products at different temperature stages, and the proportion of the O2 class was the largest, followed by the O1 class. The other five polar compound classes accounted for relatively small proportions. (2) As the experimental temperature increased, the relative abundances of the N1 class (carbazoles) and O1 class (phenols) tended to increase because of their material sources, while the relative abundance of the O2 class (carboxylic acids) showed a marked downward trend due to decarboxylation and dehydration. (3) With increasing experimental temperature, the relative abundance of high-DBE (DBE ≥ 9) compounds presented a significant upward trend, while the relative percentage of low-DBE (DBE ≤ 7) compounds showed the completely opposite trend, which was mainly due to condensation and aromatization. (4) ∑%DBE ≥9/∑%DBE≤8 (O2 compounds) = 0.0007T2 0.372T + 48.594,R2 = 0.9088 could be used as a new maturity parameter after further verified with the crude oils. Funding This work was supported by the Natural Science Foundation of Hubei Province, China (Grant Number 2017CFA027); the National Natural Science Foundation of China (Grant Numbers 41672117and 41503034); the National Major Science and Technology Projects of China (Grant Number 2017ZX05001005-002-001); the funding project of Excellent Doctoral and Master's Thesis Cultivation Program of Yangtze University(Grant Number YS2018051); the open subject of Shandong Provincial Key Laboratory of Depositional Mineralization & Sedimentary Mineral (Baojun Liu Geoscience Science Foundation) (Grant Number DMSM2017084); and the open subject of the State Key Laboratory of Petroleum Resources and Prospecting (Grant Number PRP/open- 1509). CRediT authorship contribution statement Gang Yan: Writing - original draft, Formal analysis. Yaohui Xu: Conceptualization, Writing - review & editing, Supervision, Funding acquisition. Yan Liu: Methodology, Investigation. Wenxiang He: Project administration. Xiangchun Chang: Resources. Penghai Tang: Validation. Declaration of competing interest None. 11

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