Examination of the Structure and Definition of the Mechanism of Formation of Products by Pyrolysis of Tarim Crude Oil

Abstract

Pyrolysis of crude oil is an important way to generate natural gas. However, the current analysis of pyrolysis gas carbon isotopes and the study of gas generation dynamics are not unified, and the genesis and accumulation of gas reservoirs are needed to conduct in-depth discussions. Therefore, Tarim crude oil samples in China were selected to perform thermal simulation experiments using an autoclave. The pyrolysis hydrocarbon production yield, carbon isotope characteristics and gas-generation process of crude oil samples in Tarim Basin were studied by GC-MS, FT-IR and carbon isotope analysis, respectively. The compositions of the Tarim Oilfield were determined, including the 83.69% content of hydrocarbons, the 14.08% content of aromatic compounds, and lower than 3% content of heteroatom compounds. The non-monotonic linear relationship of C_2-5 isotopes may be due to the complexity of crude oil, and the formation of gaseous hydrocarbons can be divided into three stages. The results showed that the δ¹³C distribution range of C_2-5 hydrocarbons was −40.5% to −10.5%, and the δ¹³C distribution of methane was −53.3% to −27.4%. The lowest δ¹³C value for methane occurs at 350 °C, and the corresponding carbon isotope value is −53.3%. When the pyrolysis temperature range is 250–300 °C, crude oil undergoes volatilization and preliminary pyrolysis, and the C_4-5 output exceeds 95%. When the temperature rises to 300–500 °C, the aliphatic hydrocarbon chain in crude oil begins to crack, the side and branch chains of aromatic and heteroatomic compounds are broken, and C₄-₅ begins to crack to form C_1-3. Finally, the temperature rises to 500–600 °C, and C_3-5 begins to deeply crack into C_1-2, and eventually all is converted to methane.

1. Introduction

Pyrolysis of crude oil is an important way to generate natural gas. It is the mainstay of large oil and gas reserves as an important material basis for the formation of late gas reservoirs [1,2]. Many scholars have specialized discussions on the process of crude oil pyrolysis to produce natural gas. The general trend of organic matter evolution is to evolve from large molecules with complex structures to small molecules with stable structures with the increase of temperature and pressure in the formation. The pyrolysis of crude oil under high temperature and high pressure is in line with the evolution trend of organic matter, which is a process of reducing free energy and contributes greatly to the formation of natural gas [3,4,5,6,7]. Natural gas is an important chemical raw material and fuel, which can be used for power generation, fuel, etc., and to produce propane and butane, important raw materials for modern industry.

Crude oil development technology at home and abroad has been very advanced. China has perfect self-injection and mechanical oil production technology [8,9]. Scientists from Iran, for example Fetisov V. G., proposed a vapor recovery device to improve the economic efficiency of hydrocarbons [10]. Ilyushin, Y.V. proposed the development of a Process Control System for the Production of High-Paraffin Oil [11].

Studying the formation of crude oil pyrolysis gas has great value and significance for the exploration and development of crude oil and natural gas and has become a key research object for scholars at home and abroad. In recent years, the research on pyrolysis kinetics and the analysis and evaluation of crude oil pyrolysis gas has made some progress. However, the current analysis of pyrolysis gas carbon isotopes and the study of the formation mechanism of gaseous hydrocarbons lack the unity of the two theories, and this makes it impossible to deeply explore the causes and development of gas reservoirs.

1.1. Pyrolysis Mechanism of Crude Oil

Crude oil pyrolysis is a dynamic process with multiple parallel reactions and complex reaction mechanisms [12,13,14,15]. It is based on the kerogen pyrolysis process with free radical reaction mechanisms, including an initiation of free radical excitation reactions, hydrogen transfer reactions between molecules and free radicals, fracture reactions between free radicals, free radical addition reactions, and termination reactions, etc. [16].

The crude oil pyrolysis process is as follows:

(1)

Depolymerization reaction: chain break in recombinant branches is the mainstay, mainly the break of C-O and C-S bonds, mainly generating soluble unsaturated organic matter;

(2)

C-C bond breaking reaction: C₆₊ saturated hydrocarbons crack to generate small-molecule saturated hydrocarbons and C₁₄₊ aromatic hydrocarbons;

(3)

Demethylation reaction: C₁₄₊ aromatic hydrocarbon demethylation to generate gaseous hydrocarbons and coke;

(4)

C-C bond breaking reaction: C-C bond breaking in C_3-5 produces CH₄ and C₂.

Using C₁₅₊ pyrolysis as an example, the reaction process is as follows in Figure 1:

Uguna et al. [17] selected the n-hexadecane rich crude oil to conduct pyrolysis experiments at high temperatures and pressures. The experimental results showed that the free radical reaction mechanism could be used to predict the pyrolysis product composition of pure n-hexadecane. According to the kinetic theory of crude oil pyrolysis proposed by Behar et al. [18], the pyrolysis process of crude oil at high temperatures and pressure can be divided into the depolymerization reaction, C-C bond breaking, and the demethylation reaction. Hill et al. [19] proposed that the conservation of elements would affect the formation of products during pyrolysis, among which methane was the most stable form. The conservation of elements requires that the product should be produced in the direction of a higher hydrocarbon ratio or higher carbon content.

1.2. Crude Oil Analysis Technology

The relatively complex composition of crude oil contains abundant macromolecular fingerprint fossil information, which can be separated and characterized with the aid of GC-MS, NMR, FT-IR, etc. Riley et al. [20,21] used FT-IR to characterize the asphaltenes extracted from crude oil and to classify the oil samples through the ratios of the peak heights in two wavelengths to identify oil samples from different sources. Oudot et al. [22] used GC-MS to analyze biomarkers such as steranes, terpenes, and pinane in crude oil, and to study asphaltenes and biomarkers in the Amoco-Cardis spill. Wang et al. [23] used full two-dimensional gas chromatography time-of-flight mass spectrometry and standard samples to characterize 28 C₉-C₁₀ light hydrocarbon compounds in crude oil. Mair et al. [24] separated C₂ alkyl-substituted naphthalenes, alkyl-substituted naphthalenes, biphenyls, methyldihydrofluorene and benzothiophene in crude oil. Qualitative analysis was performed with the aid of GC-MS, NMR, and FT-IR. Mühlen et al. [25] and Flego et al. [26] applied full two-dimensional gas chromatography time-of-flight mass spectrometry to separate and characterize nitrogen compounds such as indole, carbazole, quinoline, and iso-quinoline in crude oil. Li et al. [27] detected a variety of sulfur compounds including dibenzothiophene and its alkyl substituents in crude oil and performed qualitative and quantitative analysis. Ioppolo-Armanios et al. [28,29] used chromatography and chromatography-mass spectrometry to identify and quantify phenol, alkylphenol compounds, and isopropyl methylphenol compounds in Australian crude oil samples. Wang Qing et al. [30] found through FT-IR that the main volatiles in the pyrolysis process are C₂₊ aliphatic hydrocarbons, CO₂, CO, CH₄, C₂H₄, and C-O and C=O. Iskenderova et al. [31] and Kaanagbara et al. [32] applied spectroscopic techniques to discover that the gaseous volatiles produced by the pyrolysis of transformer oil are methane (CH₄), ethane (C₂H₆) and small hydrocarbons.

Compared with crude oil, gaseous products of crude oil pyrolysis with relatively single components and structures do not contain a macromolecular organic structure. The carbon isotope characteristic information of gas components from crude oil pyrolysis can be used to analyze rich fingerprint information such as the evolution and migration of crude oil source rocks and a geological accumulation environment [33], which is of great significance to the exploration and development of large oil and gas fields. Carbon isotope fractionation was accompanied during the evolution of primitive organic matter into oil and gas resources in the complex geological environment in the Middle Ages [34,35]. Therefore, the difference in bond energy between ¹²C-¹²C and ¹²C-¹³C leads to fractionation of carbon isotopes of methane containing homologous matter generated by organic matter during the maturation process of geological migration [36], and the δ¹³C value of organic matter also increases with the maturity of organic parent material. The maturity of organic matter varies regularly from the biochemical stage to the oil generation stage to the maturity stage, and the composition of pyrolysis gas also varies regularly from dry gas to wet gas to dry gas. According to the exploration data and carbon isotope fractionation research, the δ¹³C value of methane, including homologues in coal type gas and oil type gas, will increase as the organic matter maturity increases, which means that the methane of pyrolysis gas evolves towards the δ¹³C isotope enrichment [37]. This isotope characteristic theory can be used to infer the origin of pyrolysis gas and the maturity of organic source rock [38,39,40].

In this manuscript, the crude oil sample from the Tarim region in China was selected, and pyrolysis experiments on the formation of gaseous hydrocarbons were conducted using an autoclave at different temperatures. The Tarim crude oil sample was characterized with the aid of GC-MS and FT-IR. Gas composition and δ¹³C distribution of gaseous products (C_1-5 hydrocarbons) of crude oil pyrolysis at different temperatures were determined by using GC and carbon isotope analysis. Based on the experimental results, the formation mechanism of gaseous hydrocarbons during Tarim crude oil pyrolysis was discussed.

2. Experimental

2.1. Material

The crude oil used in the experiment were collected from the Zhonggu No. 101 well area in the Tarim area 1 of the Xinjiang Oilfield, and the basic properties of crude oil used are shown in

Table 1. Basic properties of crude oil.

Basic Properties of Crude Oil	Crude Oil in Tarim, Xinjiang
Saturate, %	65.54
Aromatics, %	18.31
Resin, %	11.33
Asphaltene, %	4.82
Density (25 °C), g/cm3	0.8821
Wax, %	11.29
Viscosity (25 °C), g/cm3	74.40
δ13C, %	−33.00

.

2.2. Pyrolysis Experimental Apparatus and Steps

All experiments in Tarim crude oil pyrolysis illustrated in Figure 2 were carried out at high temperatures and pressures in an autoclave. The experimental device consists of an autoclave with a height of 100 mm and a diameter of 50 mm, gas circuit and sampling system. The autoclave is a vertical reactor made of a stainless-steel tube with a volume of 200 mL. The sample basket is a quartz cylinder with a height of 80 mm and a diameter of 10 mm. The quartz cylinder has a bottom to hold Tarim crude oil and its other side is left open, and it is placed on the bottom of the autoclave.

In each run, the quartz cylinder with 50 mL of Tarim crude oil was put into the autoclave, and then the autoclave was vacuumed through an outlet point. The autoclave was filled with high-purity argon to increase the pressure to 3 MPa. By using the full-load heating method, the feedstock was heated to the designed reaction temperature, and then the temperature was kept constant for a certain period of time. The reaction temperatures were 250 °C, 300 °C, 350 °C, 400 °C, 450 °C, 500 °C, 550 °C, and 600 °C, respectively. Correspondingly, constant times were 120 h, 96 h, 72 h, 60 h, 48 h, 36 h, 24 h and 12 h, respectively. At the end of the reaction, the autoclave was cooled to the room temperature in the air, and the gaseous product was collected using the drainage method.

2.3. Characterization of Feedstock and Gaseous Products

FT-IR analysis: 1 mg Tarim crude oil was evenly applied on a KBr tablet and dried by an infrared lamp. Then it was placed in a Bruker 80 v vacuum Fourier infrared spectrometer for measurement. All spectral peaks were recorded from 4000 cm⁻¹ to 400 cm⁻¹ using the Nicolet FT-IR spectrometer. The average number of scans per spectrum was 32 and the resolution was 4 cm⁻¹.

GC-MS analysis: The composition and content of organic matter in Tarim crude oil were analyzed by Trace DSQ chromatography mass spectrometer. The carrier gas was helium, the column model was HP-FFAP, the vaporization temperature was 250 °C, the bombardment voltage was 50 eV, and the scanning range was 20–300 D. The temperature was kept constant for 3 min at an initial temperature of 25 °C, then increased to 250 °C at the rate of 10 °C/min, and kept constant for 10 min.

GC analysis: Agilent 6890 GC equipped with a thermal conductivity detector, a flame ionization detecto, and five mixed columns (capillary columns and packed columns) was used to analyze the gaseous products. The temperature of the detector was 250 °C. The temperature of the oven was heated to 100 °C at a heating rate of 5 °C/min after at 50 °C for 3 min, and then to 180 °C at a heating rate of 10 °C/min, and the residence time was 3 min.

Carbon isotope analysis: Carbon isotope analysis of gaseous products was conducted on a Thermo Scientific MAT-253 isotope mass spectrometer equipped with a Trace GC chromatograph. Gas components were separated on the gas chromatograph in a stream of helium, converted to CO₂ in a combustion reactor at 941 °C and then introduced into the mass spectrometer. Individual hydrocarbon components (C₁-C₅) were initially separated using a fused silica capillary column. The GC oven temperature was kept constant for 5 min at an initial temperature of 50 °C, and increased to 150 °C at the rate of 5 °C/min.

3. Results and Discussion

3.1. FT-IR Analysis of Tarim Crude Oil

Figure 3 shows the infrared spectra of Tarim crude oil, and

Table 2. Main vibrational peaks in infrared spectra.

Wave Number/cm−1	Absorption Peak Attribution	Functional Groups
2975–2800	Absorption peak of -CH3	Aliphatic and side chain
1706–1604	Double bond telescopic vibration peak of C=O and C=C	Unsaturated hydrocarbon
1460–1370	Methylene variable angle vibration Methyl asymmetric variable angle	Aliphatic compound structure
868	Benzene toroidal flexural vibration	Monocyclic benzene
783–746	Amino characteristic peak	Amidogen

lists the analysis results of the main vibration peaks. Strong absorption peaks are observed at 2930–2850 cm⁻¹ and 1460–1370 cm⁻¹, and they are determined as methyl and methylene absorption peaks according to the characteristics of the peaks, including the variable-angle vibration peaks of alkane -CH₂ and the asymmetric variable-angle vibration peaks of -CH₃. There is an obvious strong peak at 1462.73 cm⁻¹, indicating that there are more -CH₂, which might be long-chain hydrocarbon and aliphatic compounds. The peak at 1604–1706 cm⁻¹ indicates the stretching vibration of the C-O double bond and the C-C double bond. The peak at 868 cm⁻¹ represents the characteristic absorption peak of bending vibration outside the C-H surface of a benzene ring, which may contain a benzene ring or benzene derivatives. The peak at 746 cm⁻¹ is the characteristic peak of an amino group. The peaks at 871–863 cm⁻¹ are characteristic peaks of the S-C-S stretching vibrations, suggesting that they may also contain sulfides. This is consistent with the study of Tarim heavy oil by Ping Wen et al. [41].

Aliphatic carbon absorption peaks of Tarim crude oil are obvious, indicating that Tarim crude oil is mainly composed of long-chain aliphatic hydrocarbons. Single-bond stretching vibration absorption peaks are stronger than double-bond stretching vibration peaks, indicating that aliphatic hydrocarbons have more saturated hydrocarbons than unsaturated ones. The C-O double bond stretching vibration peak is relatively obvious, which means that the content of oxygenated compounds in heteroatom compounds is relatively high. FT-IR characterization can only provide functional group information for structural analysis of organic compounds; other characterization methods such as GC-MS need to be combined.

3.2. GC-MS Analysis of Oil Fraction of Tarim Crude

The total ion chromatogram of an oil fraction of Tarim crude was demonstrated in Figure 4. The carbon chain length of organics is about C₆-C₂₉, and most of the organics are distributed between C₇-C₂₂. According to the differences of mass-to-charge ratios, a detailed analysis can be conducted on the organic compounds and quantitative composition of the oil fraction of Tarim crude. Taking a m/z of 85 and 91, the ion current spectra of normal paraffins and aromatic hydrocarbons were separated, as shown in Figure 5. Obviously, it can be seen that n-alkanes are mainly distributed in the range of C₉-C₂₁, and the content of the remaining carbon chain length is less. Aromatic hydrocarbons mainly have peaks between C₆-C₉, and they are mainly phenols, ketones, aldehydes and their derivatives, while fused ring aromatic hydrocarbons such as naphthalene, anthracene and their derivatives have no obvious characteristic peaks.

If the mass-to-charge ratios are 79, 93 and 129, the ion current spectra of nitrogen compounds of pyridine, aniline, indole, and quinoline derivatives were separated, as shown in Figure 6. The pyridine spectrum mainly has a slight peak between C₆-C₉, and the aniline spectrum has no obvious characteristic peaks. The quinoline spectrum has slight characteristic peaks between C₇-C₁₃.

Taking a m/z of 47 and 83, ion chromatograms of sulfur compounds such as thiols and thiophene compounds were separated, and the results are demonstrated in Figure 7. It is difficult to observe the characteristic peaks of thiols and thioethers, indicating that the content of non-thiophene heteroatom sulfides is low. The thiophene compounds are mainly widely distributed between C₆-C₂₆.

Taking a m/z of 74 and 94, the ion current spectra of oxygen compounds and their derivatives, including carboxylic acids, esters, and phenol, were isolated. The results are illustrated in Figure 8. The carboxylic acid and ester compounds have two distinct characteristic peaks, which are mainly distributed between C₆-C₉ and C₁₁-C₁₃. At the same time, it is difficult to observe obvious characteristic peaks of phenol and its derivatives.

Relative contents of each component of Tarim crude oil are summarized in

Table 3. Relative contents of each compound of Tarim crude oil.

Material Composition		Content/%
Aliphatic compound	N-alkane	68.02
	Branched alkanes	3.17
	Alpha olefin	8.32
	Non-alpha olefins	1.31
	Naphthenic	2.87
Aromatic compound	Mononuclear aromatics, PAHs	14.08
Heteroatom compounds	Oxy-compound	1.31
	Sulfur compounds	0.64
	Nitrogen compounds	0.28

. Tarim crude oil is mainly composed of hydrocarbon compounds, accounting for 83.69%. Moreover, the alkane content is 74.06% and the olefin content is 9.63%. The content of aromatic compound is 14.08%, and the content of heteroatom compounds is not exceeding 3%.

3.3. Composition Analysis of Gaseous Products

Figure 9 shows the C_1-5 composition of gaseous products at different temperatures. It can be concluded that C_1-5 is formed during the entire pyrolysis process of Tarim crude oil. At a temperature of 300 °C, the total content of C_4-5 heavy hydrocarbons in the gas component exceeds 95%, and there are very few C_1-3 light hydrocarbons. When the temperature is 400 °C, the content of C_4-5 heavy hydrocarbons decreases, while the content of C_1-3 light hydrocarbons increases, and the content of methane is higher than that of C_2-3 hydrocarbons. At the temperature of 550 °C, the heavy hydrocarbons of C_3-5 are significantly reduced, and the content does not exceed 4%, while the light hydrocarbons of C_1-2 exceed 95%. When the temperature is 600 °C, all C_3-5 will be converted into C₂ and methane. The content of C_3-5 is less than 1%, and the content of methane is higher than 93%.

3.4. Carbon Isotope Composition of Gaseous Products

Figure 10 indicates the carbon isotope composition of gaseous products at different temperatures. The δ¹³C distribution range of C_2-5 hydrocarbon is from −40.5‰ to −10.5‰, and that of methane is between −53.3‰ and −27.4‰. With the increase of pyrolysis temperature, the δ¹³C of C_2-5 hydrocarbons decreases first and then increases. The inflection temperature of C_2-5 hydrocarbons is 400 °C regardless of whether this concerns a normal isotope or an isomer. Moreover, the lowest δ¹³C value of methane appears at 350 °C, and the corresponding carbon isotope value is −53.3‰. The non-monotonic linear relationship of C_2-5 isotopes may be due to the complexity of crude oil, leading to complicated products in the process of hydrocarbon pyrolysis.

The δ¹³C values of heavy hydrocarbons of C_4-5 are relatively close to each other at the same temperature, and there is no obvious change. However, the δ¹³C value suddenly becomes heavier after 400 °C. Meanwhile, the larger the molecular of C_1-3 hydrocarbon, the heavier δ¹³C, namely δ¹³C₃ > δ¹³C₂ > δ¹³C₁. The δ¹³C value of methane is much lighter than those of C_2-5 hydrocarbons. The C_1-3 hydrocarbons do not only come from the cracking of C_4-5 hydrocarbon, but also from the C-C bond rupture of C_2-3, which leads to δ¹³C₁ and δ¹³C₂ are lighter than δ¹³C₃.

The non-monotonic linear relationship of C_2-5 isotopes may be due to the occurrence of disproportionation during the pyrolysis process, such as the break of the aliphatic carbon chain, side chain and branch chain of aromatic and heteroatomic compounds. Some intermediate products with lighter isotopic compositions can be formed. These intermediate products further decompose to C_4-5 heavy gaseous hydrocarbons, and then break to C_1-3 light gaseous hydrocarbons. The emergence of these intermediate products makes heavy carbon and light carbon different in the reactions that generate gaseous hydrocarbons. The difference in activation energy is different, which in turn causes the non-monotonic linear relationship of C_2-5 isotopes.

3.5. Formation Mechanism of Gaseous Hydrocarbons

Combining FT-IR analysis, GC-MS analysis, and carbon isotope fractionation characteristics, the formation mechanism of gaseous hydrocarbons during pyrolysis of Tarim crude oil was discussed. The formation of gaseous hydrocarbons can be divided into three stages.

The pyrolysis process at 250–300 °C is the first stage, at which the crude oil undergoes volatilization and preliminary cracking. A small amount of crude oil is cracked to generate C_4-5 heavy gaseous hydrocarbons. At the same time, some low-carbon hydrocarbons contained in crude oils will also be volatile. More than 95% of the gas composition is C_4-5 heavy hydrocarbons, and very few are C_1-3 light hydrocarbons.

The second stage is at 300–500 °C, and crude oil accelerates to crack at this stage. The aliphatic hydrocarbon carbon chain in crude oil is gradually cracked, and aromatic and heteroatom compounds also undergo the breakage of side chains and branch chains. The C_4-5 heavy gaseous hydrocarbons generated will be further cracked into C_1-3 light gaseous hydrocarbons. C_2-3 gaseous hydrocarbons have an inflection point at 450 °C. The cracking occurs rapidly, and the cracking rate slows down slightly afterwards. It is indicated that, at this stage, C_2-3 may not only originate from direct cracking of crude oil but also from the breakage of C_4-5 heavy gaseous hydrocarbons. The pyrolysis characteristics of methane gas are slightly different from those of C_2-3 gaseous hydrocarbons, and the content is significantly increased before 400 °C. At the peak temperature of 400 °C, the content reaches approximately 35%, and then the content decreases by about 5%. It is possible that the yield of C_2-3 is higher than that of methane during the cracking process at 400–500 °C.

The pyrolysis process at 500–600 °C is the third stage; solid products are generated, and heavy gaseous hydrocarbons are deeply cracked. After 500 °C, the degree of cracking of crude oil gradually reached the highest level, and the heavy gaseous hydrocarbons of C_3-5 were gradually and deeply cracked into light gaseous hydrocarbons of C_1-2, until almost all of them were cracked, and the content was significantly reduced. At the last stage of the pyrolysis process at 550–600 °C, C₂ is gradually converted to methane.

4. Conclusions

In this manuscript, the Tarim crude oil was characterized using GC-MS and FT-IR. The pyrolysis experiments on formation of gaseous hydrocarbons were conducted. The composition and δ¹³C distribution of C_1-5 gaseous were determined using GC and carbon isotope analysis.

Tarim crude oil is mainly composed of hydrocarbon compounds, accounting for 83.69%. Moreover, the alkane content is 74.06% and the olefin content is 9.63%. The aromatic compound content is 14.08%, and the heteroatom compound content is not exceeding 3%.

C_1-5 was generated during the whole pyrolysis process of Tarim crude oil, indicating that different amounts of C_1-3 light hydrocarbons and C_4-5 heavy hydrocarbons are produced during the whole pyrolysis process. With the increase of pyrolysis temperature, the δ¹³C of C_2-5 hydrocarbons decreases first and then increases. The lowest δ¹³C value of methane appears at 350 °C, and the corresponding carbon isotope value is −53.3‰.

Based on the experimental results, the formation of gaseous hydrocarbons can be divided into three stages. The pyrolysis process at 250–300 °C is the first stage, and the crude oil undergoes volatilization and preliminary cracking. More than 95% of the gas composition is C_4-5 and very few are C_1-3. The second stage is at 300–500 °C. The carbon chain of aliphatic hydrocarbons in crude oil is gradually cracked, and aromatic and heteroatom compounds also undergo the breakage of side chains and branch chains. The C_4-5 generated will be further cracked into C_1-3. The pyrolysis process at 500–600 °C is the third stage, in which C_3-5 are gradually and deeply cracked into light C_1-2.