Molecular Transformation of Crude Oil in Confined Pyrolysis System

Article pubs.acs.org/EF

Molecular Transformation of Crude Oil in Confined Pyrolysis System and Its Impact on Migration and Maturity Geochemical Parameters Yahe Zhang,†,‡ Yuhong Liao,*,§ Shaohui Guo,‡ Chunming Xu,† and Quan Shi*,† †

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China College of Science, China University of Petroleum, Beijing 102249, China § State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China ‡

S Supporting Information *

ABSTRACT: The compositions of crude oils can vary significantly during thermal maturation through cracking and aromatization. In this study, an immature high sulfur crude oil was pyrolyzed in a closed gold-tube system with heating rates of 20 °C/h and 2 °C/h, respectively, to simulate the thermal maturation process of crude oil. The molecular compositions of heteroatom-containing compounds in crude oil were investigated by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) while the impact of thermal maturation on migration and maturity geochemical parameters were investigated by gas chromatography−mass spectrometry (GC-MS). Based on the analysis of pyrolysis products, the compositional variations of heteroatom-containing aromatic compounds and the validity of the aromatic geochemical parameters during thermal maturation were investigated. When the equivalent vitrinite reflectance value (Easy %Ro) was greater than ca. 0.85, alkyl chain cracking was the major reaction and led to the producing of a large amount of 1.3. The decrease should be the result of more and more secondary cracking at higher levels of maturity. The yields of C2, C3, C4, C5, and C6−14 hydrocarbons would decrease when their generation rates are less than their cracking rates. Basically, the yields of H2S kept increasing with the increasing of maturity at both heating rates of both 20 °C/h and 2 °C/h when Easy %Ro < 1.8, but decreased slightly thereafter (Table 1). The maximum yield of H2S for the two heating rates reached 52 mL/g, which was ∼15% of its C1−C5 gas yield. High H2S contents in gas products should be related to the high sulfur content of crude oil. The CO2 is a common product of the thermal treatment of organic matter due to decarboxylation of organic acids and/or esters. Similar to that of H2S, the yields of CO2 increases quickly when Easy %Ro < 2.0, but decreased slightly at higher levels of maturity. Characterization of Liquid Products by GC-MS. Figure 2 shows the total ion chromatograms from GC-MS of crude oil and its products over the pyrolysis temperature sequence at pyrolysis heating rates of (a) 20 °C/h and (b) 2 °C/h, respectively. The total ion chromatogram (TIC) of the crude oil shows that longchain normal alkanes were abundant with an odd−even predominance and the most abundant peak was phytane. This indicates that the crude oil was derived from deoxidized lacustrine environment with relatively low maturity. During the

initial stage (Easy %Ro < 1.1), the abundance of higher MW hydrocarbons decreased continuously, while the abundance of C1−14 increased with the increasing of maturity. The crude oil was almost depleted of hopanes and steranes when the Easy %Ro value was >0.95. Pristane and phytane were undetectable when the Easy %Ro value reached 1.1. Most of the hydrocarbons with a carbon number of >30 were cracked when the Easy %Ro value reached 1.35 and were cracked thoroughly when the Easy %Ro value reached 1.46. Synchronously, polycyclic aromatic hydrocarbon compounds (PAHs) such as phenanthrenes, pyrenes, chrysene, and their alkyl substituted homologues became dominant. When the Easy %Ro value reached 2.1, the alkylsubstituted naphthalenes were decomposed, while dibenzothiophene and phenanthrene became dominant. When the Easy %Ro value was >3.0, the alkyl-substituted naphthalenes disappeared, while S8, biphenyl, fluorene, and phenylnaphthalene were detected. Longer alkyl-substituted carbazoles and dibenzothiphenes, PAHs, disappeared, along with the increasing Easy %Ro value (see Figure 2). Molecular Compositions of Heteroatom-Containing Class Species in Crude Oil and Its Pyrolysis Products by Negative-Ion ESI FT-ICR MS. GC-MS results revealed the detailed molecular composition of crude oil, while some highmolecular-weight (HMW) hydrocarbons and heteroatoms cannot be eluted through the GC column and detected via MS analysis. In addition, the contents of most heteroatomcontaining compounds generally are very low, below the detection limit of the GC-MS system. ESI is a sensitive ionization technique for the selective ionization of heteroatom-containing compounds in hydrocarbon matrix. Coupled with highresolution FT-ICR MS, ESI has been widely used for petroleum analysis, especially for heavy petroleum fractions. Negative-ion ESI FT-ICR MS broadband (150−650 Da) mass spectra of the crude oil and its pyrolysis products are shown in Figure S1 in the Supporting Information. The relative abundance of heteroatom class species for crude oil and its pyrolysis products are shown in Figure 3. A total of 10 class species were assigned. The most abundant classes in the crude oil were N1, O1, and O2. Note that 6926

DOI: 10.1021/acs.energyfuels.6b00841 Energy Fuels 2016, 30, 6923−6932

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Figure 4. DBE versus carbon number distribution of N1 class species in pyrolysis products of the crude oil with pyrolysis heating rate of 20 °C/h. The values shown in the brackets are the Easy %Ro values.

acids, phenols, and carbazoles, respectively. With the increasing of pyrolysis temperature, carboxylic acids were cracked first, resulting in the quick decrease of O2 species. The relative abundance changes shown in Figure 3 implies that decomposition of O1 was much faster than that of N1 class species; carboxylic acids first cracked to smaller acid molecules and/or hydrocarbons, the small molecular acids were further decarboxylated at higher temperatures. The changes in CO2 concentrations were corresponding to the carboxylic acids decomposition (as shown in Table 1). DBE and carbon number distributions of N1, O1 class species of 20 °C/h pyrolysis products are shown in Figures 4 and 5, respectively. The results of 2 °C/h pyrolysis are shown in Figures S3 and S4 in the Supporting Information). The variation trends in DBE and carbon number distributions of N1 and O1, as well N1S1 class species (shown in Figures S2 and S5 in the Supporting Information) of different pyrolysis heating rates were similar. The N1 series of DBE = 9 (corresponding to carbazoles) with

the C16 and C18 fatty acids (O2 class species) in the mass spectra of products with Easy %Ro > 1.68 should be contaminants.48,49 As the polar species disappear with increased thermal maturation, the mass peaks of C16 and C18 fatty acids from the solvent or contaminated in the processing became dominant. With the increase of pyrolysis temperature, the relative abundance of the O2 and O1 class species decreased, while the N1 class species became the most abundant. Although no quantitative data of polar compounds can be obtained from the ESI MS, Figure 3 indicates that the conversion ratios of various heteroatom-containing polar species in 2 °C/h pyrolysis products are higher than those in 20 °C/h pyrolysis products at the same temperature. It is consistent with the commonly accepted viewpoint that the cracking of crude oil follow pyrolysis kinetics and the cracking is controlled not only by temperature but also by reaction time.1,10 The O2, O1, and N1 class species in the crude oil by negativeion ESI FT-ICR MS should mainly correspond to carboxylic 6927


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Figure 5. DBE versus carbon number distribution of O1 class species in pyrolysis products of the crude oil with pyrolysis heating rate of 20 °C/h. The values given in the brackets are Easy %Ro values.

carbon numbers of 12−45 was the most abundant N1 class species in the samples with Easy %Ro < 1, which were detectable over the Easy %Ro range of 0.57−3.67 (Figure 4). However, carbon number of alkyl side chains decreased with the increase of Easy %Ro, which was also verified by GC-MS results. In addition, the DBE values of N1 species increased as the Easy %Ro value increased. This indicates that the aromaticity of N1 species increases with maturity. Similar variations can be observed in O1 series (Figure 5). At low maturity (Easy %Ro < 1.0), O1 series were dominated by phenols with DBE values of 4. Phenols with DBE value of 4 and carbon number of C27 and C28 showed higher relative abundance, corresponding to isoprenoidyl phenols.27 Isoprenoidyl phenols seem to be much more unstable than normal-alkyl phenols. Carbon number distribution of alkyl phenols shifted to lower value, while average DBE values of O1

species increased as the the Easy %Ro value increased. A group of O1 class species with ∼17−24 DBEs and 25−30 carbons were abundant in samples with high maturity. These compounds are likely highly condensed phenolic compounds. Generally, N1, O1, N1S1 species showed similar variations with maturity in the distributions of DBE values. The degree of condensation of polar heteroatom species in pyrolysis products increased with the increasing of maturity, which was consistent with those reported in literatures with geological samples.25,28,32,50 Also note that the yields of liquid product were very low when the Easy %Ro value is greater than ca. 1.5 and liquid petroleum could not be obtained in high degrees of maturity in geological environments. Crude Oil Migration Parameters Based on the Composition of Carbazoles. Carbazoles and benzocarbozoles 6928


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shown in Figure 7). In the Easy %Ro range of 1.0−2.5, 4MDBT/1-MDBT (where MDBT = methyldibenzothiophene),

have been used as molecular indicator of secondary oil migration.51 Liu et al.26 found an enrichment of neutral nitrogen compounds with lower DBE values and higher carbon numbers with increasing migration distance. However, the influence of maturity on the relative abundance of these neutral nitrogen compounds has not been well-investigated. Relationships between migration parameters of carbazoles and Easy %Ro values of pyrolysis products from crude oil with two heating rates are shown in Figure 6. The upper limit of carbon number of alkyl

Figure 7. Relationships between maturity parameters of methylated dibenzothiophenes and Easy %Ro of pyrolysis products of the crude oil. (Legend: 4-/1- = 4-methyldibenzothiophene/1-methyldibenzothiophene (4-MDBT/1-MDBT); 2,4-/1,4- = 2,4-dimethyldibenzothiophene/1,4-dimethyldibenzothiophene (2,4-DMDBT/1,4-DMDBT); and 4,6-/1,4- = 4,6-DMDBT/1,4-DMDBT.)

2,4-DMDBT/1,4-DMDBT, and 4,6-DMDBT/1,4-DMDBT increased with the increasing of the Easy %Ro. DMDBT parameters are not available because the DMDBT was cracked, forming DBT and MDBT when Easy %Ro > 2.5. Methylated dibenzothiophenes are expected to be efficient molecular maturity indices of marine carbonate source rocks and their oils or gases at mature, highly mature, and overmature stages.53 Relationships between maturity parameters of methylated DBTs and Easy %Ro can be achieved at highly mature and overmature stages, based on the data at two heating rates using the mathematical regression method with high linear correlation as follows: for 4-MDBT and 1-MDBT:

Figure 6. Relationships between migration parameters of carbazoles and Easy %Ro of pyrolysis products of the crude oil. Note: 1-, 2-, 3-, and 4refer to 1-, 2-, 3-, and 4-methyl carbazoles; 1,8-, 1,3-, 1,6-, 1,7-, and 2,7refer to the 1,8-, 1,3-, 1,6-, 1,7-, and 2,7- dimethylcarbazoles; NEXs represents the adduct of nitrogen-exposed isomers and includes 2,6-, 2,7-, 2,4-, 2,5-, 2,3-, and 3,4-dimethylcarbazoles; NPEs refers to the adduct of nitrogen partially shielded isomers and includes 1-ethylcarbazole, as well as 1,3-, 1,6-, 1,7-, 1,4-, 1,5-, and 1,2-dimethylcarbazoles; [a] = benzo[a]carbazole; [c] = benzo[c]carbazole.

side chains on N1 species decreased as the Easy %Ro values increased; however, the DBE value of N1 species increased as the Easy %Ro values increased (recall Figure 4). The results indicated that thermal maturity has a significant impact on migration parameters, based on the composition of the carbazoles.52 In an Easy %Ro range of 1.25−2.5, 1-MC/4-MC increased slowly at first, then decreased slightly, while 1-MC/(3MC + 2-MC) continued to decrease slowly. Generally, 1,8DMC/1,3-DMC, 1,8-DMC/1,6-DMC, 1,8-DMC/2,7-DMC, 1,8-DMC/1,7-DMC, 1,8-DMC/NEXs-DMC, and 1,8-DMC/ NPEs-DMC all changed to various extents at one heating rate or both when Easy %Ro < 2.5. Therefore, variations in thermal maturity can significantly affect the ratios based on nitrogenshielded isomers to nitrogen-exposed isomers, which could lead to wrong interpretations on oil migration. However, benzocarbazole (BC) ratios [a]/([a] + [c]) remained almost unaltered in the Easy %Ro range of 1.25−2.5 at both heating rates, indicating that benzocarbazole ratio [a]/([a] + [c]) can be used to investigate petroleum migration, even for the crudes with highly different maturities. Thermal Maturity Parameters of Methylated Dibenzothiophenes. Thermal maturity has a significant impact on maturity parameters of methylated dibenzothiophenes (as

⎧ ⎛ 4‐MDBT ⎞ ⎟ + 1.2383 ⎪ R c = 0.546 ln⎝⎜ 1‐MDBT ⎠ ⎪ ⎨ 2 ⎪ R = 0.9445 ⎪1.0 ≤ Easy %Ro ≤ 2.5 ⎩

for 2,4-DMDBT and 1,4-DMDBT: ⎧ ⎛ 2,4‐DMDBT ⎞ ⎪ R c = 0.5014 ln⎜ ⎟ + 2.0028 ⎝ 1,4‐DMDBT ⎠ ⎪ ⎨ ⎪ R2 = 0.9474 ⎪ ⎩1.0 ≤ Easy %Ro ≤ 2.5 6929


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for MPI2:

⎧ ⎛ 4,6‐DMDBT ⎞ ⎪ R c = 0.6682 ln⎜ ⎟ + 1.6518 ⎝ 1,4‐DMDBT ⎠ ⎪ ⎨ ⎪ R2 = 0.9383 ⎪ ⎩1.0 ≤ Easy %Ro ≤ 2.5

R c = −1.0486 ln(MPI2) + 2.1769 R2 = 0.9453 1.0 ≤ Easy %Ro ≤ 2.5

for MPR:

Thermal Maturity Parameters of Methylphenanthrenes. Relationships between methylphenanthrene index (MPI), methylphenanthrene ratio (MPR), and Easy %Ro are shown in Figure 8. Thermal maturity has significant impact on

R c = 1.0008 ln(MPR) + 0.6924 R2 = 0.9332 1.0 ≤ Easy %Ro ≤ 2.5

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CONCLUSIONS In this study, confined gold-tube pyrolysis was conducted on immature crude oil; gas chromatography−mass spectrometry (GC-MS) and electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) were used to investigate the molecular transformation and thermal stability of hydrocarbons and polar heteroatom compounds. Higher-molecular-weight hydrocarbons were first cracked into low-molecular-weight liquid hydrocarbons (C6−C14) and gaseous products (C1, C2, C3, C4, C5), and finally mostly cracked into methane at overmature stage. The relative abundance of polycyclic aromatic compounds (PAHs), especially for nonsubstituted or less-substituted PAHs increased as the %Ro value increases, because of their high thermal stability. Maturity parameters (calculated reflectance values (Rc)) based on methylated dibenzothiophenes (DBTs) exhibited a systematic increase with Easy %Ro from mature to overmature stages. Although thermal maturation processes can have apparent effects on most of carbazole migration parameters, benzocarbazole ratio [a]/([a] + [c]) seems still valid in much wider maturity range. Maturity parameter MPR was more sensitive to maturity than MPI and showed a systematic change in the Easy %Ro range of 1.0−2.5.

Figure 8. Relationships between methylphenanthrene index (MPI), methylphenanthrene ratio (MPR), and Easy %Ro of pyrolysis products of the crude oil. (MPI1 = 1.5 × (3-MP + 2-MP)/(P + 9-MP + 1-MP); MPI2 = 3 × 2-MP/(P + 9-MP + 1-MP); MPR = (3-MP + 2-MP)/(9-MP + 1-MP).)

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b00841. Negative-ion ESI FT-ICR MS broadband mass spectra of the crude oil and its pyrolysis products, plots of DBE as a function of carbon number for different species (N1S1, N1, O1) species in pyrolysis products from crude oil at various pyrolysis heating rates (20 °C/h or 2 °C/h) (PDF)

both MPI and MPR indices. In the Easy %Ro range of 1.0−1.75, MPI1 and MPI2 did not have a linear relationship with Easy %Ro (Figure 8). However, both MPI1 and MPI2 decreased monotonously when Easy %Ro > 1.75. Relative to MPI1 and MPI2, MPR seems to be a better maturity index in a broad Easy %Ro range of 1.0−2.5. This is because MPR showed a systematic increase with Easy %Ro in this maturity range. In addition, MPR changed more significantly than MPI1 and MPI2. This means that MPR is more sensitive to variation in maturity than MPI. Therefore, MPR may be a better maturity index than MPI1 and MPI2 in the Easy %Ro range of 1.0−2.5. This is consistent with the results of Chen.54 The relationships between MPI, MPR, and Easy %Ro can be achieved at high and overmature stages, based on valid data with two heating rates using the mathematical regression method with high linear correlation as follows: for MPI1:

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y. Liao). *E-mail: [email protected] (Q. Shi). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the China Postdoctoral Science Foundation (2015M571226) and the National Natural Science Foundation of China (41503032, 21236009, 21376262, and 41172113).

⎧ R c = −1.3864 ln(MPI1) + 1.9123 ⎪ ⎨ R2 = 0.8552 ⎪ ⎩1.0 ≤ Easy %Ro ≤ 2.5 6930


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Molecular Transformation of Crude Oil in Confined Pyrolysis System

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