Compositional Characterization of Expelled and Residual Oils in the

May 7, 2019 - pyrolysis temperature, °C, % Ro, residual oil g/100 g rock, expelled ..... to the real hydrocarbon generation–expulsion process of so...
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Article Cite This: ACS Omega 2019, 4, 8239−8248

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Compositional Characterization of Expelled and Residual Oils in the Source Rocks from Oil Generation−Expulsion Thermal Simulation Experiments Yahe Zhang,*,† Yifeng Wang,‡ Wei Ma,‡ Jincheng Lu,†,§ Yuhong Liao,∥ Zhisheng Li,‡ and Quan Shi*,† †

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China CNPC State Key Laboratory of Gas Reservoir Formation and Development, Research Institute of Petroleum Exploration and Development, PetroChina, Langfang 065007, China § China Nuclear Power Engineering Co., Ltd., Beijing 100000, China ∥ State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China

ACS Omega 2019.4:8239-8248. Downloaded from pubs.acs.org by 5.188.217.23 on 05/07/19. For personal use only.



S Supporting Information *

ABSTRACT: Petroleum generation−expulsion thermal simulation experiments on a low-maturity type I source rock were carried out. The composition of hydrocarbons and heteroatom-containing compounds in both expelled and residual oils was characterized by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry and gas chromatography-mass spectrometry. A thorough analysis of fractionation effects was presented between expelled and residual oils. The oil composition strongly depended on the pyrolysis temperature. A significant difference in the molecular composition was found between expelled and residual oils at various pyrolysis temperatures. The difference in maturity for expelled and residual oils could be revealed clearly by the difference in the molecular composition of heteroatom-containing compounds. Carboxylic acids in the residual oil cracked quickly with the pyrolysis temperature above 350 °C (% Ro = 1.0) but part of them would survive if they were expelled out of the source rock timely. The variations between the relative abundance of oxygen compounds and nitrogen compounds indicated that the thermostability of oxygen-containing nitrogen compounds was lower than that of neutral nitrogen compounds. The variation trends in double bond equivalents and carbon number distributions of O1, O2, and N1 class species were similar. The aromaticity of heteroatom-containing polar species increased with maturity.



INTRODUCTION

compounds in the petroleum generation−expulsion thermal simulation process. Petroleum generation−expulsion thermal simulation experiments have been carried out in a semi-closed simulation system1,12−14 to study the petroleum generation−expulsion efficiency under geological conditions. The simulation experimental system can be used to reveal the characteristics of petroleum generation and expulsion under geological conditions, such as overburden pressure, temperature, and fluid pressure. The system can objectively reflect the characteristics and controlling factors of hydrocarbon generation−expulsion in the thermal evolution of source rocks.15

Thermal simulation is an effective method to study the petroleum generation and expulsion process and its mechanism.1 Numerous studies have been conducted with an emphasis on the kinetics of oil cracking,2 the mechanism of gas generation or the thermal stability of crude oils,3−9 and oil−expulsion efficiency.1 It has been recognized in oil shale retorting and natural petroleum generation that oil generation is a simultaneous two-step process, that is, kerogen−bitumen− oil.10,11 High molecular weight hydrocarbons and heteroatomcontaining compounds were transformed progressively into low molecular weight compounds (condensate and gas) and pyrobitumen during oil cracking.7 However, the effects of hydrocarbon generation−expulsion from source rocks on the molecular composition of petroleum are still not well characterized, especially for heteroatom-containing polar © 2019 American Chemical Society

Received: January 29, 2019 Accepted: April 12, 2019 Published: May 7, 2019 8239

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Table 1. Yields of Expelled and Residual Oils from the Pyrolysis Simulation at Various Temperatures pyrolysis temperature, °C

% Ro

residual oil g/100 g rock

expelled oil g/100 g rock

expelled oil/total oil %

300 350 400 450 500

0.7 1.0 1.6 2.2 2.7

0.63 2.36 2.70 0.21 0.09

0.02 0.44 0.69 0.35 0.28

3.1 15.7 20.4 62.5 75.7

Table 2. SARA Composition of Expelled and Residual Oils residual oil, wt %

expelled oil, wt %

pyrolysis temperature, °C

% Ro

sat.

aro.

res.

asp.

sat.

aro.

res.

asp.

300 350 400 450 500

0.7 1.0 1.6 2.2 2.7

23.2 17.6 46.2 18.1 11.1

13.7 16.3 10.9 29.2 42.5

60.6 58.0 38.5 47.5 41.2

2.5 8.1 4.4 5.2 5.3

43.8 20.0 43.8 3.2 6.3

8.8 8.8 16.4 61.4 71.8

41.5 47.4 35.2 27.3 16.2

5.9 23.8 4.6 8.2 5.7

Figure 1. Total ion chromatograms of (a) residual oils and (b) expelled oils at various pyrolysis temperatures. (Phe: phenanthrene; P: pyrene; BP: benzopyrene; C: chrysene; Pr: pristane; Ph: phytane). The values in the brackets are % Ro.



RESULTS AND DISCUSSION Yields and Saturates, Aromatics, Resins, and Asphaltenes (SARA) Composition of Expelled and Residual Oils. The total yields of expelled and residual oils are shown in Table 1. % Ro was used to show the thermal maturities at various pyrolysis temperatures. Total oil yields increased with % Ro in the range of 0.7−1.6 and reach a maximum value at a % Ro value of 1.6 and decreased at higher maturity. Most oils were not expelled when % Ro ≤ 1.6. With the increase of % Ro, the percentage of expelled oil on the total oil reached a maximum value of 75.7% at % Ro 2.7, whereas the amount of residual oil decreased due to the expulsion, secondary cracking into gas, and the carbonization of heavy components. The weight distribution for SARA fractions of expelled and residual oils at various pyrolysis temperatures was shown in Table 2. The percent of saturates in both expelled and residual oils accounted for more than 43 wt % at 400 °C

Both the residual and expelled oils can be obtained through the system. Electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) enabled the detailed analysis of heteroatom-containing polar compounds in crude oil and rock extracts.16−18 Molecular transformation of crude oil in the confined gold-tube pyrolysis system has been well characterized.19 In this study, hydrocarbon generation− expulsion thermal simulation experiment was conducted on a lacustrine source rock of type I oil-prone kerogen with low maturity and high total organic carbon (TOC) content. Gas chromatography-mass spectrometry (GC-MS) and negativeion ESI FT-ICR MS were used to investigate molecular compositional changes during the hydrocarbon generation− expulsion process. 8240

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Figure 2. Broadband (m/z 200−600) negative-ion ESI FT-ICR mass spectra of (a) residual oils and (b) expelled oils at various pyrolysis temperatures. (IS: stearic acid-d35; FA: fatty acid; the values in the brackets are % Ro.).

(corresponding to % Ro 1.6), then decreased quickly at higher maturity. The percent of aromatics in expelled oil kept increasing, whereas resins roughly kept decreasing. Characterization of Oils by GC-MS. Figure 1 shows the total ion chromatograms (TICs) of (a) residual oils and (b) expelled oils at various pyrolysis temperatures. Although most of the light hydrocarbons were evaporated during experiments, fractionation effects can be presented between expelled and residual oils. The TIC of the 300 °C residual oil showed a bimodal distribution, in which long chain normal alkanes were abundant with an odd−even predominance and the most abundant peak was phytane. This implies that the shale was derived from an anoxic lacustrine environment with relatively low maturity. However, the expelled oil showed a normal distribution of n-alkanes. Phytane was also most abundant in the 300 °C expelled oil but no odd−even predominance of nalkanes was observed. The TICs of residual and expelled oil from 350 °C were similar with those from 300 °C, except for the ratio of pristane to phytane (Pr/Ph). Pr/Ph ratios of residual oils and expelled oils at various pyrolysis temperatures were as follows: 0.6:0.2, 1.4:1.6, 1.9:1.8, 1.9:0.8, and 0.6:0.7. However, it is meaningless in both residual oils and expelled oils above 400 (Ro 1.6) as shown in Figure 1. At the pyrolysis temperature of 400 °C, normal alkanes were dominant in the saturates, whereas isoprenoid alkanes and hopanes almost disappeared. No odd−even predominance of n-alkanes was

observed for residual oils or expelled oils. Polycyclic aromatic hydrocarbons such as phenanthrenes, pyrenes, chrysene, and their alkyl-substituted homologs became dominant with the pyrolysis temperature of 450 °C. The carbon number of alkyl chains attached to the aromatic core decreased with the increase in the pyrolysis temperature. The compositional variation trend in residual oils was similar to that in expelled oils with the increase of the pyrolysis temperature, although there are significant differences in composition between residual and expelled oils. Expelled oils exhibited lower maturity than that of the corresponding residual oils. Molecular Compositions of Heteroatom-Containing Class Species in Residual and Expelled Oils. Broadband (m/z 200−600) negative-ion ESI FT-ICR mass spectra of residual and expelled oils at various pyrolysis temperatures are shown in Figure 2. Mass peaks with higher relative abundance in the oils at the simulation temperature of 300 °C were corresponding to fatty acids that have an even carbon number predominance. A C32H54O2 compound with the double bond equivalents (DBE) value of 6 in the residual oil was inferred as hopanoic acid. The molecular composition of 350 °C residual and expelled oils was very close to each other and both have a higher relative abundance of fatty acids. Mass spectra of the residual oil at 400 °C were quite different from the corresponding expelled oil. The molecular composition of the 400 °C expelled oil was similar to those of the residual and 8241

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Figure 3. Relative abundance of heteroatom classes assigned from the negative-ion ESI FT-ICR mass spectra of (a) residual and (b) expelled oils at various pyrolysis temperatures. Values in the brackets are % Ro.

expelled oils at 350 °C. However, the relative abundance of fatty acids in the 400 °C residual oil decreased more significantly. The highest mass peak in the mass spectrum of the residual oil was the internal standard (stearic acid-d35), which exhibits much lower relative abundance in other mass spectra. This indicates that most carboxylic acids cracked at a higher temperature (e.g., 400 °C, % Ro = 1.6), whereas the expelled oils should have experienced less severe thermal alteration because they could escape the hot reactor when they were expelled. The mass peaks with higher relative abundance were neutral nitrogen compounds in the 450 °C expelled oil. The mass peaks with higher relative abundance are neutral nitrogen compounds in the 450 and 500 °C expelled oil, such as benzocarbazoles, dibenzocarbazoles, and benzonaphthocarbazoles. Carboxylic acids were also presented in the 450 and 500 °C residual oils, in which C16 and C18 fatty acids were dominant. These compounds were more likely contaminants from the experimental process instead of pyrolysis products. However, no corresponding odd-over-even predominance was seen in the alkane distributions according to GC-MS analyses

(Figure 1), indicating that the contribution of fatty acids to the production of alkanes is limited. The contribution from the cracking of alkyl side chain of N1, N1Ox, and O1 species to the production of alkanes is nonnegligible. The relative abundance of heteroatom class species in residual and expelled oils at different pyrolysis temperatures are shown in Figure 3. A total of 16 class species were assigned. The O2, O1, and N1 class species by negative-ion ESI FT-ICR MS generally corresponded to carboxylic acids, phenols, and carbazoles, respectively.19 With the increase of the pyrolysis temperature, carboxylic acids cracked quickly, resulting in the quick decrease of O2 species, except for the contaminants of C16 and C18 fatty acids in residual oils at high temperatures. The relative abundance of nitrogen-containing species increased largely with the increase of the pyrolysis temperature, supposedly because of the joint control by the generation of nitrogen compounds and the decomposition of acids. The increase in the relative abundance of O1 class species in expelled oils of 450 and 500 °C indicated that the decomposition of O2 was much faster than that of O1 class 8242

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Figure 4. DBE versus carbon number distribution of O2 class species in (a) residual and (b) expelled oils at various pyrolysis temperatures. The values in the brackets are % Ro.

species (Figure 3) and the decomposition of O1 was faster than that of N1 class species. In other words, O1 class species had

higher thermostability than O2 class species, such as fatty acids, but lower thermostability than N1 class species, such as 8243

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Figure 5. DBE versus carbon number distribution of O1 class species in (a) residual and (b) expelled oils at various pyrolysis temperatures.

compounds, implied that the thermostability of oxygencontaining compounds as well as oxynitrides was lower than that of neutral nitrogen compounds. As a result, the ratio of

carbazoles. Additionally, the decrease in the relative abundance of oxygen-containing compounds, as well as oxynitrides and the increase in the relative abundance of neutral nitrogen 8244

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Figure 6. DBE versus carbon number distribution of N1 class species in (a) residual and (b) expelled oils at various pyrolysis temperatures.

N1O1/N1 decreased gradually with the increase of Ro (0.7− 2.7%) in both residual and expelled oils. It may be used as a

potential maturity parameter for source rocks from lowmaturity stage to over-maturity stage. 8245

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demonstrated. At a low-maturity stage and the principal oilgeneration stage, the main nonpolar components of residual and expelled oils were normal alkanes, pristane, and phytane. The polar molecules were mainly carboxylic acids with even carbon number predominance. At a high-maturity stage, the main nonpolar components of residual and expelled oils were normal alkanes and aromatics. The polar molecules in expelled oil were mainly carboxylic acids. The polar molecules in residual oil were mainly N1, N1Ox, and Ox species. At the over mature stage, the main nonpolar components of residual and expelled oils were polycyclic aromatic hydrocarbon. The polar molecules were mainly N1, N1Ox, and Ox species with higher DBE and shorter alkyl side chains. The compositional differences between residual oils and expelled oils at identical temperatures could be attributed to both the geological chromatography-like effect26 and maturity differences between residual oils and expelled oils. The geological chromatography effect could lead to compositional differences between residual oils and expelled oils. The compounds with higher polarity were preferentially retained in the source rock as residual oil, whereas those with lower polarity were preferentially expelled from the source rock as expelled oil. Based on the results and advancements in petrochemistry by ESI FT-ICR MS, fractionation of petroleum fractions occurs in this sequence: asphaltenes > polars, resins > aromatics > alkanes; the latter being preferentially expelled relative to the former groups.27 Furthermore, in this semi-closed system, there were significant differences in maturity between residual oils and expelled oils because the hydrocarbons generated earlier were expelled out of the heating system and avoided further cracking.

DBE and carbon number distributions of O2 class species in residual and expelled oils at various pyrolysis temperatures are shown in Figure 4. The size of point in each figure denoted the relative response to the internal standard (stearic acid-d35), which was spiked into each sample to show the variations in the relative concentration of various compounds. Mass peaks with higher relative abundance in the residual and expelled oils at 300 °C were fatty acids. Except for the contaminants of fatty acids with low DBEs in residual oils at high temperatures, the aromaticity of O2 class species increased with the increase of maturity, implying that the O2 class species with high DBE may be aromatic carboxylic acids, phenols, or ketones. The DBE and carbon number distributions of O1 class species in residual and expelled oils at various pyrolysis temperatures are shown in Figure 5. The O1 compound with DBE = 4 and carbon number of C28 in the residual oil of 300 °C and the expelled oil of 350 °C, which showed very high relative abundance, should be isoprenoidyl phenols.20 Isoprenoidyl phenols were more unstable than normal alkyl phenols, which was consistent with the previous results of crude oil pyrolyzed in the confined pyrolysis system.19 With the increase of the pyrolysis temperature, in general, carbon number ranges of alkyl phenols shifted to a lower value, whereas average DBE values of O1 species increased. In general, the O1 class species with higher DBE values were abundant in the samples with higher maturity. Based on the planar limits,21−23 these compounds were likely highly condensed phenolic compounds or aromatic ketones. O1 class species with DBE < 4 were likely alcohols or aliphatic ketones, which should be the products from the decomposition of Ox or N1Ox species. Considering that the DBE = 0 species (alcohols) were not detected, these O1 class species with DBE < 4 should be aliphatic ketones. A standard compound, 2heptanone, was proved to be ionized in the negative-ion mode ESI with the presence of ammonium hydroxide. DBE and carbon number distributions of N1 class species in residual and expelled oils at various pyrolysis temperatures are shown in Figure 6. The N1 series with DBE = 9 corresponded to carbazoles. N1 class species in the 300 °C expelled oil was undetectable. The carbon number of alkyl side chains in N1 class species decreased with the increase of pyrolysis temperature, whereas the DBE value of N1 species increased. This indicates that the aromaticity of N1 species increased with increasing maturity. Similar variations could also be observed in O2 and O1 class species (Figures 4 and 5). In general, the variation trends in DBE and carbon number distributions of O2, O1, and N1 class species were similar, which was consistent with the results of crude oil and its products pyrolyzed in the confined pyrolysis system.19 The aromaticity of the polar heteroatom species increased with maturity. The results were also consistent with those reported in the literature with geological samples.16,17,24,25 Distribution of Hydrocarbon Generation−Expulsion from Source Rocks. The variation trend between the hydrocarbon expulsion efficiency of the source rock and thermal evolution degree was plotted elsewhere.1 The results in the literature1 showed that the hydrocarbon expulsion efficiency of type I source rock is usually lower than 20% at a low-maturity stage, 20−50% at the principal oil-generation stage (Ro = 0.8−1.3%) and 50−90% at the high-maturity stage (Ro = 1.3−2.0%). Based on the results of GC-MS and ESI FTICR MS, the distribution of hydrocarbon generation− expulsion from source rocks on the molecular level were



CONCLUSIONS



EXPERIMENTAL SECTION

Petroleum generation−expulsion simulation experiments were conducted on a low mature lacustrine shale source rock. A thorough analysis of fractionation effects was presented between expelled and residual oils. Significant compositional differences were found between residual oils and expelled oils. The expelled oils usually showed lower polarity and maturity. The difference in maturity for expelled and residual oils could be revealed clearly by the difference in the molecular composition of hydrocarbons and heteroatom-containing compounds. Most oils were not expelled when the pyrolysis temperature was ≤400 °C. In general, the variations in molecular composition with the increase of the pyrolysis temperature in residual oil were similar to that in expelled oil. The contribution of fatty acids to the production of alkanes was limited. The contribution from the cracking of the alkyl side chain of N1, N1Ox, and O1 species to the production of alkanes was nonnegligible. The variation trends in DBE and carbon number distributions of O1, O2, and N1 class species with maturity were similar, which was consistent with the results of crude oil and its products pyrolyzed in the confined pyrolysis system.

Source Rock Sample. The source rock sample used in the simulation experiment was a grayish black shale, which was collected from Qikou Sag of Bohai Bay Basin, China. The shale was of lacustrine origin and low maturity (TOC = 7.71%, % Ro = 0.67%, Tmax = 438 °C, S0 = 0.24 mg/g, S1 = 0.95 mg/g, S2 = 52.01 mg/g, (S1 + S2) = 52.96 mg/g, HI = 675 mg/g). Based 8246

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accumulated for 0.1 s. The delay was generally set to 1.1 ms to transfer the ions to the ICR cell by the electrostatic focusing of transfer optics. The RF excitation was attenuated at 13 dB and used to excite ions over the range of 150−1000 Da. The data set size was set to 4 M words with a transient length of 2.3 s. Time-domain data sets of 64 acquisitions were co-added. Mass calibration and the details of data processing have been described elsewhere.16,28

on the high HI index and maceral compositions, the organic type of the source rock was verified as type I (sapropel type). Petroleum Generation−Expulsion Thermal Simulation Experiment. To make the experimental conditions similar to the real hydrocarbon generation−expulsion process of source rocks under geological conditions, the experiment was optimized in operating parameters including temperature, time, water addition, pore fluid pressure, and hydrocarbon generation space.1 The simulation experiments were carried out at five temperature/pressure conditions: 300 °C/40 MPa, 350 °C/45 MPa, 400 °C/50 MPa, 450 °C/55 MPa, and 500 °C/60 Mpa, respectively. The pressure value is the upper limit of the pressure in the reactor, if the fluid pressure in the reactor increases higher than this value, the valve of the reactor could open to allow the generated hydrocarbons partially expelled. Each experiment at a certain temperature point was conducted individually. A portion of 150−200 g shale particle with overflowing water was compacted to the cylindrical core forced by mechanical pressure, such as lithostatic pressure. Excess water was expelled in the compacting process. Then the core was loaded in the high-pressure reactor. It was heated to the temperature point from room temperature with a heating rate of 50 °C/h. The reactor was held at the pyrolysis temperature for 24 h and then cooled to the corresponding formation expulsion temperature. The simulation temperature of 300, 350, 400, 450, and 500 °C were corresponding to the formation of expulsion temperatures of 105, 110, 115, 120, and 130 °C, respectively. Vitrinite reflectance values (% Ro) were measured to show the thermal maturities at various pyrolysis temperatures. After the simulation experiment, the expelled oil, including expelled liquid hydrocarbon in the reactor and cold trap, was collected and weighed. The residual oil was extracted from the source rock powder by chloroform for 72 h in a Soxhlet. The yields of the expelled and residual oils are listed in Table 1. Expelled and residual oils were then fractionated into saturates, aromatics, resins, and asphaltenes (SARA) fractions by column chromatography. GC-MS Analysis. An Agilent 5975C GC-MS equipped with an HP-5MS (60 m × 0.25 mm × 0.25 μm) fused-silica capillary column was used for the analysis of the expelled and residual oils. The GC injector was maintained at 300 °C in the splitless mode. The oven temperature was held at 50 °C for 1 min, increased to 120 °C at 20 °C/min, increased to 240 °C at 4 °C/min, and then increased to 310 °C at 3 °C/min and held constant at 310 °C for 30 min. The MS run with a 70 eV electron impact source. The mass scan range was from 35 to 450 Da with a 1 s interval. Negative-Ion ESI FT-ICR MS Analysis. The sample preparation procedure for ESI FT-ICR MS analysis has been described elsewhere.19,20 Samples were dissolved in toluene and diluted to 0.2 mg/mL with toluene/methanol (1:3, v/v). The same amount of stearic acid-d35 was spiked in each sample as an internal standard to quantify the relative concentration among the samples. MS analyses were performed using a Bruker apex-ultra 9.4 T FT-ICR MS. The diluted samples were injected into the spray needle with a syringe pump at a rate of 180 μL/h. The operating conditions for the negative-ion mode were: 3.5 kV spray shield voltage; 4.0 kV capillary column introduction voltage; and −320 V capillary column end voltage. Optimized mass for the quadrupole was 200 Da. An argon-filled hexapole collision pool was operated at 5 MHz and 600 Vp-p radio frequency (RF) amplitude, in which ions were



ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b00260. Time domain signals for FT-ICR MS data shown in Figure 2 (Figure S1) (PDF)



AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +86 10 89734478 (Y.Z.). *E-mail: [email protected]. Tel: +86 10 89739157 (Q.S.). ORCID

Yahe Zhang: 0000-0003-2573-568X Yuhong Liao: 0000-0002-0546-9743 Quan Shi: 0000-0002-1363-1237 Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (NSFC 41503032, 41672128, U1463207).



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DOI: 10.1021/acsomega.9b00260 ACS Omega 2019, 4, 8239−8248