The Chemical Composition of Bitumen in Pyrolyzed Green River Oil

Article pubs.acs.org/EF

The Chemical Composition of Bitumen in Pyrolyzed Green River Oil Shale: Characterization by 13C NMR Spectroscopy Yesu Feng,†,‡ Tuong Van Le Doan,† and Andrew E. Pomerantz*,† †

Schlumberger-Doll Research, Cambridge, Massachusetts 02139, United States Duke University, Durham, North Carolina 27708, United States

‡

ABSTRACT: During oil shale pyrolysis at high temperature, the conversion of kerogen to oil and gas proceeds dominantly through a mechanism involving bitumen as a reaction intermediateat low maturities bitumen is formed from decomposition of kerogen, while at high maturities bitumen is transformed primarily to oil and gas. Here, we study the chemical composition of Green River bitumens of a range of maturities by high-field 13C nuclear magnetic resonance (NMR) spectroscopy. Numerous trends in the evolution of bitumen with maturity are observed, some of which are similar to trends previously observed in kerogen while others are not. As found previously for kerogen, bitumen becomes more aromatic (less aliphatic) with increasing maturity. However, in contrast to kerogen, which is primarily consumed during maturation and was found previously to have aliphatic chains that become shorter and/or more branched with maturity, aliphatic chains in bitumen lengthen with maturity in the low maturity regime where bitumen is primarily being formed but then shorten with maturity in the high maturity regime where bitumen is primarily being consumed. The structure of aromatic rings in bitumen is essentially unchanged with maturity, as their size, alkyl substitution, and heteroatom substitution are found to be independent of maturity; in contrast, the size of aromatic rings in kerogen generally increases with maturity. These measurements of the chemical composition of the bitumen intermediate enhance understanding of petroleum generation by oil shale pyrolysis at high temperature.

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asphaltenes plus resins in the bitumen12and traditionally, the detailed structure of asphaltenes has been difficult to measure. Recently, experimental techniques such as mass spectrometry,13−17 fluorescence spectroscopy,18 Raman spectroscopy,19 X-ray spectroscopy,20−22 and nuclear magnetic resonance spectroscopy23 have been successfully developed to characterize asphaltenes, enabling detailed characterization of bitumen and other heavy but soluble hydrocarbon fractions. NMR is particularly noteworthy owing to its ability to describe in detail the carbon backbone of these materials. Quantitative 13C NMR spectroscopy has been successfully applied to study fossil fuels since the 1980s.23−29 Compared with 1H NMR spectroscopy, 13C NMR spectroscopy has a lower signal-to-noise ratio but a chemical shift range an order of magnitude larger. This is especially advantageous in studying complex mixtures of high molecular-weight compounds, such as bitumen and kerogen, because the NMR signals of carbon in different chemical environments are spread out over a large range, avoiding overlap. Moreover, 50 years of development of NMR pulse sequence techniques has enabled the differentiation of different types of carbons based on their connectivity to protons. One successful example is the distortionless enhancement by polarization transfer (DEPT) sequence,23,30 which can selectively suppress or enhance 13C signals according to their connectivity to protonsCH, CH2, CH3, or quaternary carbons (defined here as carbons without directly attached protons). By combining DEPT with inverse-gated pulse acquire 13 C spectra, satisfactory quantification of different carbon types,

INTRODUCTION Oil shales can be defined as fine-grained sedimentary rocks containing immature organic material called kerogen, from which shale oil and combustible gas can be generated through pyrolysis. In contrast to natural petroleum generation, which requires millions of years at burial temperature (100−150 °C), under significantly higher temperature (near 400 °C) the time required can be shortened to hours or days. It is estimated that the size of the oil shale resource in the U.S. alone is approximately 2 trillion equivalent barrels, nearly triple the proven oil reserves of Saudi Arabia.1,2 As a result, oil shale may become a significant source of transportation fuel in the coming decades. However, oil and gas production from oil shale is complex, and large-scale production methods are not yet optimized. In particular, at the high temperatures required for economic petroleum generation, the dominant mechanism appears to involve two steps: first, kerogen (organic matter that is nonvolatile under pyrolysis conditions and insoluble in conventional organic solvents) is converted to bitumen (nonvolatile under pyrolysis conditions and soluble in conventional organic solvents); second, bitumen is converted to oil and gas (volatile under pyrolysis conditions) plus nonvolatile and insoluble material (referred to commonly as kerogen but occasionally also as pyrobitumen and/or coke).3−8 Thus, a detailed understanding of the petroleum generation mechanism at high temperature requires a thorough characterization of the bitumen intermediate.9−11 Despite the importance of bitumen in shale pyrolysis, its chemical composition has not been investigated in as much detail as the produced oil or the residual kerogen. Bitumen contains a much greater content of asphaltenes compared to oiloften no asphaltenes in the pyrolysis oil and above 50% © 2013 American Chemical Society

Received: August 21, 2013 Revised: November 1, 2013 Published: November 7, 2013 7314

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and accordingly, basic structural parameters of these compounds can be derived without knowledge from other sources.23,24,31 Here, we present 13C NMR measurements of bitumens obtained from a sequence of laboratory semi-open pyrolysis experiments on shale from the Green River formation. Validation of the experimental technique using a mixture of model compounds is presented. Results show that the composition of bitumen evolves in a systematic way with increasing severity of pyrolysis. The trends in this evolution mirror the trends observed in the evolution of the kerogen phase in some aspects but are opposite in other aspects.

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EXPERIMENTAL SECTION

Sample Preparation. The oil shale sample and pyrolysis procedure used here are described in detail elsewhere.12 Briefly, the

Table 1. Pyrolysis Conditions and Vitrinite Reflectance (EASY%R0) of Different Bitumen Samples sample EASY%R0

heating rate α (°C/h)

final temp. Tf (°C)

pressure (atm)

plateau duration (h)

0.48 (native state) 0.74 0.90 0.92 0.92 0.95 0.95 1.19 1.28 1.65

6 20 20 20 20 63 63 63 20

333 359 362 362 359 363 394 394 425

40 30 30 30 30 20 40 20 30

7.5 10 10 10 15 12.5 7.5 12.5 10

Figure 2. Yield of organic products from various pyrolysis experiments, normalized to 100%. Bitumen is found to be a reaction intermediate under the conditions studied here.

Table 2. Chemical Shift Ranges for Evaluating Aromatic and Aliphatic 13C Signal Intensity, Taken from Ref 22a carbon NMR regions

begin (ppm)

end (ppm)

phenolic C substituted aromatic C bridgehead aromatic C bridgehead/protonated aromatic C protonated/hetero aromatic C aliphatic C

160 150 133 129.5 124 60

150 133 129.5 124 108 5

a These integration ranges are used to quantify different types of carbons in Tables 3 and 4.

reported as EASY%R0,33 which is a synthetic vitrinite reflectance scale based on the calculated time−temperature history. Samples in this study ranged from EASY%R0 = 0.48 (not pyrolyzed in the laboratory, but matured to a small extent naturally in the subsurface),12 to EASY%R0 = 1.65. Two samples resulting in EASY%R0 = 0.92 were pyrolyzed and analyzed under nominally identical conditions, and the similar bitumen yields and compositions attest to the experimental reproducibility. Figure 2 shows the yields of organic products as a function of maturity for the pyrolysis experiments reported here. The role of bitumen as a reaction intermediate is evident: at low maturities the organic matter is mostly kerogen, with little bitumen, oil, or gas; at intermediate maturities the organic matter is mostly bitumen, generated from decomposition of kerogen; at high maturities the bitumen has been consumed and the organic matter is mostly produced oil and gas with some kerogen (the composition of which differs from the kerogen at lower maturity but still is nonvolatile and insoluble). The composition of the other phases is described elsewhere;12 here, we focus on the bitumen. Bitumen is defined as the organic matter that remained in the pyrolysis vessel (not volatile under pyrolysis conditions) and dissolves in organic solvent. After pyrolysis, bitumen was extracted from the spent shale with a Soxhlet extractor using a solvent mixture of dichloromethane (DCM) and methanol (volume ratio 9:1). The solvent was removed with a rotary evaporator followed by a stream of nitrogen. The dried samples were then dissolved (∼60 mg) into 500 μL 1,1,2,2-tetrachloroethane-d2 mixed with 100 μL polyethylene glycol (PEG) and 0.05 M Cr(AcAc)3. PEG was used as an internal reference of the absolute signal strength for bitumen. Elemental

Figure 1. Pyrolysis parameters. Samples were rapidly heated to 180 °C, then heated at rate α to plateau temperature Tf, and held for varied duration periods (time/h). sample was drill cuttings collected from a well drilled in the R-1 zone of the illite-rich Garden Gulch member of the Green River formation in Piceance Basin, Rio Blanco County, Colorado. The well was drilled with a reverse circulation drilling method using naturally occurring groundwater and air to transport the cuttings to surface, and the roomdry cuttings were used as received. The samples were homogenized, crushed to 100−200 μm particles, and split into approximately 100 g replicates. Each replicate was pyrolyzed in a semiopen (isobaric) system under different conditions (heating rate, plateau temperature, plateau duration, and pressure), as shown in Table 1 and Figure 1. These pyrolysis experiments were designed to simulate in situ processing of oil shale.32 The maturity of the pyrolyzed shales is 7315

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Figure 3. 45°-acquire spectrum of the model system: volume ratio 1:1:1 mixture of PEG, toluene and methylcyclohexane (MCH). Structures of toluene, PEG, and MCH are drawn close to the peaks of each compound. Aromatic carbons of toluene resonate in the range 120−140 ppm; the methyl carbon of toluene resonates near 21.4 ppm, closest to the TMS reference peak. All other peaks in 20−40 ppm range are from MCH. PEG 13C peaks resonate between 61 and 73 ppm. analysis of bitumen samples provided the mass percentage of carbon, which was used to estimate the carbon detection percentage of bitumen samples, assuming a 100% detection of PEG carbon. Cr(AcAc)3 reduces the 13C T1 relaxation times, accelerating the measurement. To further shorten the T1, especially for bitumen samples, which have slow molecular tumbling rates, all scans were acquired at 60 °C, allowing pulse repetition rates of 2 s. Spectral resolution is also improved due to enhanced T2 at this temperature. Lastly, a few drops of TMS were added to calibrate the spectrum. NMR Methods. NMR spectra were acquired with a Bruker 400 MHz magnet equipped with a 5 mm broadband probe. A spectral width of 300 ppm was used, and 7000 averages were acquired for each of four spectra collected: one inverse-gated (proton decoupling during acquisition) 45°-acquire spectrum and three DEPT-θ spectra, where θ is the flip angle of the last proton pulse and can be 45°, 90°, and 135°. Detailed descriptions of DEPT-θ sequences can be found in previous studies.24,30,31 In principle, DEPT-θ sequences detect only protonated carbons whereas inverse-gated 45° (4 μs)-acquire detects all types of carbons. DEPT-45° detects all protonated carbons with the same phase, while DEPT-135° inverses the CH2 carbon peak without affecting the phase of CH and CH3 peaks. DEPT-90° on the other hand, should detect only CH carbon signal while suppressing signal from CH2 and CH3. Experimentally, due to the inhomogeneity of magnetic field, the suppression is not perfect, but the artifact is stable and can be corrected by spectral editing.24 Then, subspectra of CH, CH2, and CH3 carbons can be extracted using equations24 briefly described below, where S stands for each individual spectrum and parameter values were determined experimentally (a = 0.11 ± 0.05; x = 0.24 ± 0.02; y = 1.17 ± 0.08; and z = 0.80 ± 0.05), which deviate slightly from theoretical values (a = 0; x = 0; y = 1; and z = 0.707) due to field inhomogeneity.24

spectra, because both aromatic CH and aromatic quaternary carbons (bridgehead and substituted aromatic carbons) are detected in 45°acquire whereas DEPT detects only aromatic CH carbons. Accordingly, the difference of the two aromatic/aliphatic ratios is the ratio of quaternary carbons over aliphatic carbons. Multiplying by the fraction of total carbons that are aliphatic (measured in the 45°acquire spectrum) quantifies the quaternary carbons, which in combination with the DEPT experiments yields a quantification of all carbon types considered here. Detailed chemical shifts ranges for different carbons can be found in a previous study23 and are listed in Table 2. A mathematical model described in detail previously26 is briefly reviewed here and used to estimate the aromatic cluster size based on the ratio of bridgehead carbon (defined as carbon connecting different aromatic rings) to total aromatic carbon, χb. The model includes upper/lower bounds set by two limiting conditions: linear catenation of aromatic rings, which gives the lower limit of χb (noted as χb′ in eq 2) and circular catenation of aromatic rings, which gives the upper limit of χb (noted as χb″). In these limits, the fraction of bridgehead carbons is related to the number of carbons per aromatic cluster, C, according to χb′ = 1/2 − 3/C and χb″ = 1 − (6/C)1/2. As described previously,23,26 a self-consistent optimization then relates the measured χb to C: 1 − tanh((C − 19.57)/4.15) χb ′ 2 1 + tanh((C − 19.57)/4.15) + χb ″ 2

χb =

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RESULTS Method Validation. To validate the method applied here, a mixture of polyethylene glycol (PEG), methylcyclohexane (MCH), and toluene in volume ratio of 1:1:1, dissolved in 1,1,2,2-tetrachloroethane-d2, was prepared and scanned with this methodology. This mixture contains significant amounts of each carbon type (CH, CH2, CH3, and quaternary carbon). Again, PEG was used as an internal reference of the absolute signal strength. Cr(AcAc)3 at 0.05 M and a drop of TMS were also added. Twenty scans were acquired for each spectrum at 60 °C, with a repetition delay of 2 s. Figure 3 shows the 45°acquire spectrum, and peaks from all four carbon types are

SCH = (S90 ° − aSCH2) − x(S45 ° + yS135 °); SCH2 = S45 ° − yS135 °; SCH3 = (S45 ° + yS135 °) − z(S90 ° − aSCH2)

(2)

(1)

Quaternary carbons can be quantified by combining these subspectra together with the inverse-gated 45°-acquire spectrum. Quaternary carbons exist typically as aromatics and rarely as aliphatics, as shown previously in similar samples by quaternary-only carbon NMR spectroscopy (QUAT).24 The aromatic/aliphatic carbon ratios detected in the 45°-acquire spectra should be larger than in the DEPT 7316

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Figure 4. DEPT-45°, 90°, 135° spectra (a−c) and reconstructed subspectra of protonated carbons (d) for the model system: volume ratio 1:1:1 mixture of PEG, toluene, and methylcyclohexane (MCH). (a) All protonated 13C have positive peaks in DEPT-45°; note the substituted aromatic carbon (the farthest downfield peak in Figure 3) is suppressed in the DEPT spectra. (b) DEPT-90° spectrum shows CH 13Cs, while CH2 and CH3 peaks are suppressed. (c) In DEPT-135°, CH2 13C peaks are inverted while CH and CH3 peaks remain positive. (d) Protonated carbon subspectra reconstructed from a−c. Integration of peaks over each of these spectra and the 45°-acquire spectrum in Figure 3 produces the fractions of different carbons listed in Table 3

Table 3. Summary of Quantitative 13C NMR Spectra of the Model Compounds theoretical carbon detection %

100

aromatic carbon % CH3 % CH2 % CH % aromatic bridgehead carbon %

46.75 14.3 32.5 45.5 7.8

observed. Only protonated carbons are detected in the DEPT-θ spectra (Figure 4a−c). Separated CH, CH 2 , and CH 3 subspectra obtained from spectral editing are shown in Figure 4d. Integration over each individual subspectrum provides the percentage of three types of protonated carbons; the relative amount of quaternary carbons can be inferred using the method described in the previous section. 100% detection of PEG carbons is assumed. Carbon detection percentages represent integrated peak areas of MCH and toluene carbons relative to the peak areas of PEG carbons (three peaks in the range 61−73 ppm, Figure 3). Detection percentages are found to be slightly less than 100%, likely resulting from experimental errors in

experimental toluene: 95.5 methylcyclohexane: 92.1 48.82 12.8 32.4 47.1 7.7

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Table 4. Structural Parameters Obtained from Elemental Analysis and 13C NMR Spectroscopy for Bitumen Samples of Different Maturity Levels Indicated by EASY%R0a sample EASY%R0 H:C ratio carbon mass % CH3% CH2% α carbon % β carbon % γ carbon % δ carbon % ε carbon % aliphatic CH% aromatic % bridgehead aromatic % phenolic % substituted aromatic % hetero aromatic % χb C

0.48 1.61 80.4 16.1 57.2 2.9 3.3 4.2 2.5 26.9 13.8 12.9 3.5 0.3 3.1 2.8 0.27 12.9

0.74 1.62 75.7 13.9 54.7 3.5 3.3 4.3 3.3 33.0 11.0 20.4 5.0 0.0 6.6 4.8 0.24 11.7

0.9 1.49 83.6 10.9 52.4 4.0 3.6 4.4 4.9 30.1 9.6 27.2 8.1 0.0 7.0 7.6 0.30 14.4

0.92 1.55 84.2 12.1 53.2 3.9 2.9 5.2 3.8 30.0 6.6 28.1 7.3 1.9 8.5 7.4 0.26 12.5

0.92 1.51 84.0 11.9 54.2 4.2 3.0 5.4 3.5 30.9 7.6 26.4 6.7 2.0 8.1 6.7 0.25 12.1

0.95 1.46 80.2 8.2 45.9 3.0 2.8 4.1 3.2 27.7 11.3 34.6 10.3 1.6 10.0 8.5 0.30 14.5

0.95 1.45 81.5 10.4 50.3 3.8 2.5 5.5 3.8 29.4 4.7 34.7 10.1 2.1 11.0 9.4 0.29 14.1

1.19 1.34 82.8 10.7 47.6 5.0 3.8 7.2 3.9 29.4 1.2 40.6 14.1 1.2 12.3 9.2 0.35 17.3

1.28 1.29 82.3 5.3 40.2 2.5 3.2 4.9 3.6 24.6 6.2 48.3 14.9 1.8 13.8 13.1 0.31 15.0

1.65 1.00 84.9 7.3 16.0 2.1 2.1 2.1 2.2 9.4 3.0 73.7 21.5 2.0 16.2 17.5 0.29 14.2

χb is the ratio of bridgehead to total aromatic carbons; C is the number of carbons per aromatic cluster, calculated according to eq 2. All other carbon percentages are calculated against total carbon content.

a

ppm). The main features of the aliphatic CH, CH2, and CH3 subspectra (Figure 6) are similar for the two bitumens, although more complexity is found in the least mature sample, potentially observable due to the somewhat greater signal-tonoise.

transferring viscous PEG quantitatively into the NMR tube. Relative percentages of each carbon type are summarized in Table 3. Experimentally observed carbon percentages agree well with those calculated for the model compounds mixture; the largest discrepancy is the percentage of CH, which has an error of 1.6% absolute. This is mostly likely due to the imperfect suppression of CH3 and CH2 peaks in DEPT-90° spectrum (Figure 4b). Bitumen Composition. Extracted bitumens from the pyrolysis experiments were analyzed using the method described above. The detection percentage varied between 81% and 90%. Results for all samples (quantitative fractions of different carbon types) are summarized in Table 4. Additionally, the hydrogen and carbon contents of the bitumens were measured by standard combustion elemental analysis and are also listed in Table 4. Here, we present spectra of the least mature (native state; EASY%R0 = 0.48) and most mature (EASY%R0 = 1.65) bitumens, which serve to illustrate the compositional trends observed in these samples. The most striking difference between these samples is their aromaticity. Both 45°-acquire (CH and quaternary carbons, Figure 5) and DEPT spectra (CH only, Figure 6) show much greater aromatic signal in the most mature bitumen compared with least mature sample. For instance, Figure 5c shows a broad peak in the aromatic region of the most mature bitumen, while that broad peak is barely observed in the aromatic region of the native state bitumen (Figure 5a). Similarly, in the edited DEPT spectra (Figure 6), a broad CH peak exists in the aromatic region of the most mature bitumen that is almost unobservable in native state bitumen. For the mature bitumen, the aromatic peak is broader in the 45°-acquire spectrum than in the edited CH subspectrum due to quaternary aromatic carbons in the 45°-acquire spectrum. For both samples, the aliphatic region is dominated by ε carbons (29.6 ppm), which are carbons at least 4 carbons away from chain ends or branch points, meaning they represent carbon in straight chains at least 9 carbons long.23 Carbons other distances from ends or branches are also observed: terminal carbons (α, 14.1 ppm), carbons one atom removed (β, 22.7 ppm), two (γ, 31.9 ppm), and three (δ, 29.3

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DISCUSSION Given the structural parameters in Table 4, we have investigated the trends in bitumen composition with thermal maturity. As discussed above, the correlation between thermal maturity and aromaticity is evident. Figure 7a shows a monotonic relationship where the aromaticity increases with increasing synthetic vitrinite reflectance. A similar trend of increasing aromaticity with maturity has been observed previously in insoluble kerogen.34−36 Increasing aromaticity is typically correlated with decreasing H:C ratio, as hydrogen-rich aliphatics are replaced by hydrogen-poor aromatics.37 This trend holds for these bitumen samples, as shown in Figure 7b. As described previously,38 extrapolating this plot to the limits of 0 and 100% aromaticity yields the H:C ratios of purely aliphatic and purely aromatic end member components of bitumen. Extrapolation to zero aromaticity produces H/C = 1.80, suggesting that saturated ring structures are significant. Saturated ring structures are often found in the GC-amenable fraction of petroleum, and this result suggests that similar structures persist into the nonvolatile fraction.39 Extrapolation to 100% aromaticity produces H/C = 0.73, close to that of a three-ring fused aromatic cluster. Figure 8 presents the variations with maturity of the composition of aliphatic carbons in bitumen. The CH2/CH3 ratio is a common measure of aliphatic chemistry, as smaller CH2/CH3 ratios indicate shorter and/or more branched aliphatic chains.40 Infrared spectroscopy and microscopy have been used previously to study the CH2/CH3 ratio of kerogen, and it was found that the CH2/CH3 ratio decreases with maturity, consistent with cleaving long chains from kerogen during maturation.40,41 Here, we find the speciation of aliphatic carbon in bitumen has a more complicated trend with maturity. The CH2/CH3 ratio of bitumen is found to be nonmonotonic 7318

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Figure 5. 45°-acquire spectra of the least mature (EASY%R0 = 0.48, (a and b)) and most mature bitumen sample (EASY%R0 = 1.65, (c and d)). Both spectra are split into the aromatic region (a and c) and aliphatic region (b and d) so that the off-scale solvent peak (∼80 ppm) as well as PEG peaks (61−72 ppm) are not shown. The sharp peak at 120 ppm is suspected to be an impurity. Because the peak is extremely narrow, its contribution is small (approximately 0.3% of the total signal). The aromatic carbon in part a is almost unobservable on this scale, but the broad peak still accounts for 12.9% of total signal (Table 4).

the ε carbon form. The abundance of CH, indicative of branched chains, is nearly independent of maturity. The abundance of CH3 shows the opposite trend as CH2as required by the normalizationwith intermediate maturities having the least CH3. Previous measurements of sulfur chemistry identified a significant difference in the compositions of kerogen and bitumen,42 and these results further suggest that kerogen and bitumen evolve differently with thermal maturity. Figure 9 presents the variations with maturity of the composition of aromatic carbons in bitumen. The bitumens

(Figure 8a), reaching a maximum near moderate maturity. This result implies that the chains are longest and/or least branched at medium maturity. Consistently, the fraction of ε carbon (carbon in straight chains at least 9 carbons long) reaches a maximum at similar maturity (Figure 8a), again suggesting that long straight chains are most important at intermediate maturity. Figure 8b presents the relative abundance of the individual CH, CH2, and CH3 groups normalized to all aliphatic carbon. The abundance of CH2 has a similar trend as the ε carbon, which is expected because CH2 carbon is dominated by 7319

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Figure 6. CH(green), CH2(red), and CH3(blue) subspectra of the least mature (EASY%R0 = 0.48, (a and b)) and most mature bitumen sample (EASY%R0 = 1.65, (c and d)). Both spectra are split into the aromatic region (a and c) and aliphatic region (b and d).

the fraction of aromatic carbon in bitumen increases with maturity, the internal structure of the aromatic clusters seems to be independent of maturity. This invariance may reflect a constraint on composition resulting from the definition of bitumen as being soluble but nonvolatile, as larger and/or more heteroatom-rich ring systems are likely to be insoluble while smaller and/or more heteroatom-lean ring systems are likely to be volatile.

are observed to have similar aromatic ring size with increasing maturity (Figure 9a), varying from 12 to 17 carbons per cluster, which corresponds to 3 or 4 fused rings and agrees with estimate from the extrapolation of Figure 7. There might be a small increase in ring size in the low maturity regime, yet the trend is small compared with what is observed previously with kerogen.34,43 Moreover, the ring size is smaller than found for a typical petroleum asphaltene (30 carbons per cluster),23 likely reflecting the contribution of aromatic and resin classes in addition to the asphaltenes in the measured bitumens. Additionally, the fractions of aromatic carbon connected to alkyl substitutions or heteroatoms in bitumen are found to be nearly independent of maturity (Figure 9b). Overall, although

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CONCLUSION

Structural changes in bitumen extracted from pyrolyzed Green River oil shale are examined quantitatively using 13C NMR 7320

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Figure 7. (a) Aromaticity versus thermal maturity (EASY%R0) (b) H:C ratio versus aromaticity. Extrapolation from the best linear fit (R2 = 0.97) predicts a H:C ratio of 1.8 at 0% aromaticity and a ratio of 0.73 at 100% aromaticity. The aromaticity of bitumen increases monotonically with maturity.

Figure 8. (a) CH2/CH3 carbon ratio versus vitrinite reflectance (top) and percentage of ε carbons within the aliphatic carbon portion (bottom). (b) Percentages of CH2, CH3, and CH within the aliphatic carbon portion. Increasing maturity causes the chain length of bitumen to increase initially and subsequently decline.

spectroscopy. The method is validated using model compounds and then applied to investigate the correlation between thermal maturity and bitumen structure. The dynamics of bitumen are complex, as under the pyrolysis conditions studied here bitumen represents a small fraction (70%) at intermediate maturities. Thus, the dominant reaction mechanism at low maturities is the formation of bitumen from kerogen and at high maturities is the consumption of bitumen to form oil and gas as well as insoluble material. Bitumen consistently increases in aromaticity with increasing maturity, suggesting both the successive

generation of more aromatic bitumen (dominant at low maturity) and consumption of less aromatic bitumen (dominant at higher maturities). A similar trend was observed previously in kerogen, indicating that both nonvolatile phases enrich in aromatics while mostly aliphatics are released as volatile oil and gas during maturation. Aliphatic chains in bitumen lengthen with maturity in the low maturity regime (suggesting the bitumen generated by pyrolysis contains longer chains than the initial bitumen) then shorten with maturity in the high maturity regime (suggesting preferential consumption of longer chains in bitumen). Chains were observed previously to become monotonically shorter and/or more branched with 7321

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Figure 9. (a) Correlation between aromatic cluster size (number of aromatic carbons per cluster, blue dots) and thermal maturity (EASY%R0). Two reference lines are added (red dashed lines): one is benzene, which has only 6 carbons, and the other is UG-8 petroleum asphaltene,23 which on average has 30 aromatic carbons per cluster. (b) Fractions of substituted aromatic carbons (top) and hetero/protonated aromatic carbons (bottom) within the aromatic carbon portion, plotted against maturity. Essentially no changes with maturity are observed, indicating that the structure of the aromatic core of bitumen is independent of maturity.

Judd, MaryEllen Loan, Lisa Meeks, and Marina Polyakov. We also thank Warren Warren and John Edward for helpful discussions.

maturity in kerogen as kerogen is consumed during pyrolysis, suggesting the preferential cleavage of longer chains in that phase as well; a mechanism involving cleavage of the C−C bonds between carbons α and β to a ring has been proposed to explain this result and may also apply to bitumen.40,44−49 However, the degree of aliphatic branching as well as the structure of the aromatic core (size, heteroatomic substitution, and alkyl substitution) in bitumen are mostly independent of pyrolysis severity, while the size of aromatic cores in kerogen typically increases with maturity. The results shed some light on the composition of bitumen and its variation with maturity. Dramatic changes in the composition of bitumen are observed at different stages in the pyrolysis. Trends observed in the compositional evolution of bitumen are found to be similar to those observed in kerogen in some aspects but different in others, as has been found in measurements of the sulfur chemistry of kerogen and bitumen.42 Because bitumen plays the role of a reaction intermediate under some pyrolysis conditions, these and other measurements of bitumen composition may yield a more thorough understanding of pyrolysis chemistry and hence a more efficient utilization of oil shale resources.

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REFERENCES

(1) Allix, P.; Burnham, A.; Fowler, T.; Herron, M.; Kleinberg, R.; Symington, B. Coaxing Oil from Shale. Oilfield Rev. 2011, 22 (4), 4− 15. (2) Biglarbigi, K.; Crawford, P.; Carolus, M.; Dean, C. Rethinking World Oil-Shale Resource Estimates. In SPE Annual Technical Conference and Exhibition, Society of Petroleum Engineers: Florence, Italy, 2010; p 135453. (3) Miknis, F. P.; Turner, T. F. The Bitumen Intermediate in Isothermal and Nonisothermal Decomposition of Oil Shales. In Composition, Geochemistry and Conversion of Oil Shales; Snape, C., Ed.; Kluwer Academic Publishers: Dordrecht/Boston, 1995; Vol. 455, pp 295−311. (4) Burnham, A. K. Comment on “Experiments on the Role of Water in Petroleum Formation” by M.D. Lewan. Geochim. Cosmochim. Acta 1998, 62 (12), 2207−2210. (5) Behar, F.; Lorant, F.; Lewan, M. Role of NSO Compounds During Primary Cracking of a Type II Kerogen and a Type III Lignite. Org. Geochem. 2008, 39 (1), 1−22. (6) Behar, F.; Roy, S.; Jarvie, D. Artificial Maturation of a Type I Kerogen in Closed System: Mass Balance and Kinetic Modelling. Org. Geochem. 2010, 41 (11), 1235−1247. (7) Lewan, M. D. Experiments on the Role of Water in Petroleum Formation. Geochim. Cosmochim. Acta 1997, 61 (17), 3691−3723. (8) Lewan, M. D. Reply to the Comment by A.K. Burnham on “Experiments on the Role of Water in Petroleum Formation”. Geochim. Cosmochim. Acta 1998, 62 (12), 2211−2216. (9) Hemelsoet, K.; Van Speybroeck, V.; Van Geem, K. M.; Marin, G. B.; Waroquier, M. Using Elementary Reactions to Model Growth Processes of Polyaromatic Hydrocarbons under Pyrolysis Conditions of Light Feedstocks. Mol. Simul. 2008, 34 (2), 193−199. (10) Hemelsoet, K.; Van Speybroeck, V.; Waroquier, M. Bond Dissociation Enthalpies of Large Aromatic Carbon-Centered Radicals. J. Phys. Chem. A 2008, 112 (51), 13566−13573.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Alan Burnham of American Shale Oil LLC (AMSO), Pierre Allix of TOTAL, and Bob Kleinberg and Abby Matteson of Schlumberger for technical discussions and encouragement. Samples were provided by Roger Day of AMSO. Supporting analytical work at Schlumberger was ably performed by Kyle Bake, Alyssa Charsky, Paul Craddock, Kamilla Fellah, Andrew 7322

dx.doi.org/10.1021/ef4016685 | Energy Fuels 2013, 27, 7314−7323

Energy & Fuels

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(32) Burnham, A. K. Oil Shale: A Solution to the Liquid Fuel Dilemma. In AMSO’s Novel Approach to in-Situ Oil Shale Recovery; American Chemical Society: Washington, DC, 2010. (33) Sweeney, J. J.; Burnham, A. K. Evaluation of a Simple-Model of Vitrinite Reflectance Based on Chemical Kinetics. AAPG 1990, 74 (10), 1559−1570. (34) Kelemen, S. R.; Afeworki, M.; Gorbaty, M. L.; Sansone, M.; Kwiatek, P. J.; Walters, C. C.; Freund, H.; Siskin, M.; Bence, A. E.; Curry, D. J.; Solum, M.; Pugmire, R. J.; Vandenbroucke, M.; Leblond, M.; Behar, F. Direct Characterization of Kerogen by X-Ray and SolidState 13C Nuclear Magnetic Resonance Methods. Energy Fuels 2007, 21 (3), 1548−1561. (35) Cao, X.; Birdwell, J. E.; Chappell, M. A.; Li, Y.; Pignatello, J. J.; Mao, J. Characterization of Oil Shale, Isolated Kerogen, and Postpyrolysis Residues Using Advanced 13C Solid-State Nuclear Magnetic Resonance Spectroscopy. AAPG 2013, 97 (3), 421−436. (36) Werner-Zwanziger, U.; Lis, G.; Mastalerz, M.; Schimmelmann, A. Thermal Maturity of Type II Kerogen from the New Albany Shale Assessed by 13C CP/MAS NMR. Solid State Nucl. Magn. Reson. 2005, 27 (1−2), 140−148. (37) Miknis, F. P.; Turner, T. F.; Berdan, G. L.; Conn, P. J. Formation of Soluble Products from Thermal-Decomposition of Colorado and Kentucky Oil Shales. Energy Fuels 1987, 1 (6), 477− 483. (38) Burnham, A. K.; Sanborn, R. H.; Crawford, R. W.; Newton, J. C.; Happe, J. A. Shale Oil Cracking 2. Effect on Oil Composition, Lawrence Livermore Laboratory Report; Lawrence Livermore National Laboratory: Livermore, CA, 1980; UCID-18763. (39) Peters, K. E.; Walters, C. C.; Moldowan, J. M., The Biomarker Guide. Vol. 1: Biomarkers and Isotopes in the Environment and Human History; Cambridge University Press: Cambridge, 2005. (40) Lin, R.; Ritz, G. P. Studying Individual Macerals Using IR Microspectroscopy, and Implications on Oil Versus Gas Condensate Proneness and Low-Rank Generation. Org. Geochem. 1993, 20 (6), 695−706. (41) Lis, G. P.; Mastalerz, M.; Schimmelmann, A.; Lewan, M. D.; Stankiewicz, B. A. FTIR Absorption Indices for Thermal Maturity in Comparison with Vitrinite Reflectance R0 in Type II Kerogens from Devonian Black Shales. Org. Geochem. 2005, 36 (11), 1533−1552. (42) Pomerantz, A. E.; Bake, K. D.; Craddock, P. R.; Kurzenhauser, K. W.; Kodalen, B. G.; Mitra-Kirtley, S.; Bolin, T. B. Sulfur Speciation in Kerogen and Bitumen from Gas and Oil Shales. Submitted for publication 2013. (43) Mao, J.; Fang, X.; Lan, Y.; Schimmelmann, A.; Mastalerz, M.; Xu, L.; Schmidt-Rohr, K. Chemical and Nanometer-Scale Structure of Kerogen and Its Change During Thermal Maturation Investigated by Advanced Solid-State 13C NMR Spectroscopy. Geochim. Cosmochim. Acta 2010, 74 (7), 2110−2127. (44) Savage, P. E.; Klein, M. T. Asphaltene Reaction Pathways 0.2. Pyrolysis of N-Pentadecylbenzene. Ind. Eng. Chem. Res. 1987, 26 (3), 488−494. (45) Savage, P. E.; Korotney, D. J. Pyrolysis Kinetics for Long-Chain Normal AlkylbenzenesExperimental and Mechanistic Modeling Results. Ind. Eng. Chem. Res. 1990, 29 (3), 499−502. (46) Billaud, F.; Chaverot, P.; Berthelin, M.; Freund, E. Thermal Decomposition of Aromatics Substituted by a Long Aliphatic Chain. Ind. Eng. Chem. Res. 1988, 27 (8), 1529−1536. (47) Watanabe, M.; Adschiri, T.; Arai, K. Overall Rate Constant of Pyrolysis of n-Alkanes at a Low Conversion Level. Ind. Eng. Chem. Res. 2001, 40 (9), 2027−2036. (48) Smith, C. M.; Savage, P. E. Reactions of Polycyclic Alkylaromatics 0.1. Pathways, Kinetics, and Mechanisms for 1Dodecylpyrene Pyrolysis. Ind. Eng. Chem. Res. 1991, 30 (2), 331−339. (49) Smith, C. M.; Savage, P. E. Reactions of Polycyclic Alkylaromatics 0.2. Pyrolysis of 1,3-Diarylpropanes. Energy Fuels 1991, 5 (1), 146−155.

(11) Hemelsoet, K.; Van Speybroeck, V.; Waroquier, M. A DFTBased Investigation of Hydrogen Abstraction Reactions from Methylated Polycyclic Aromatic Hydrocarbons. ChemPhysChem 2008, 9 (16), 2349−2358. (12) Le-Doan, T.-V.; Bostrom, N. W.; Burnham, A. K.; Kleinberg, R. L.; Pomerantz, A. E.; Allix, P. Green River Oil Shale Pyrolysis: SemiOpen Conditions. Energy Fuels 2013, http://dx.doi.org/10.1021/ ef401162p. (13) Pomerantz, A. E.; Hammond, M. R.; Morrow, A. L.; Mullins, O. C.; Zare, R. N. Two-Step Laser Mass Spectrometry of Asphaltenes. J. Am. Chem. Soc. 2008, 130 (23), 7216−7217. (14) Pomerantz, A. E.; Hammond, M. R.; Morrow, A. L.; Mullins, O. C.; Zare, R. N. Asphaltene Molecular-Mass Distribution Determined by Two-Step Laser Mass Spectrometry. Energy Fuels 2009, 23, 1162− 1168. (15) Sabbah, H.; Morrow, A. L.; Pomerantz, A. E.; Mullins, O. C.; Tan, X.; Gray, M. R.; Azyat, K.; Tykwinski, R. R.; Zare, R. N. Comparing Laser Desorption/Laser Ionization Mass Spectra of Asphaltenes and Model Compounds. Energy Fuels 2010, 24, 3589− 3594. (16) Sabbah, H.; Morrow, A. L.; Pomerantz, A. E.; Zare, R. N. Evidence for Island Structures as the Dominant Architecture of Asphaltenes. Energy Fuels 2011, 25 (4), 1597−1604. (17) Sabbah, H.; Pomerantz, A. E.; Wagner, M.; Muellen, K.; Zare, R. N. Laser Desorption Single-Photon Ionization of Asphaltenes: Mass Range, Compound Sensitivity, and Matrix Effects. Energy Fuels 2012, 26 (6), 3521−3526. (18) Groenzin, H.; Mullins, O. C. Molecular Size and Structure of Asphaltenes from Various Sources. Energy Fuels 2000, 14 (3), 677− 684. (19) Abdallah, W. A.; Yang, Y. Raman Spectrum of Asphaltene. Energy Fuels 2012, 26 (11), 6888−6896. (20) George, G. N.; Gorbaty, M. L. Sulfur K-Edge X-Ray AbsorptionSpectroscopy of Petroleum Asphaltenes and Model Compounds. J. Am. Chem. Soc. 1989, 111 (9), 3182−3186. (21) Mitra-Kirtley, S.; Mullins, O. C.; Vanelp, J.; George, S. J.; Chen, J.; Cramer, S. P. Determination of the Nitrogen Chemical Structures in Petroleum Asphaltenes Using XANES Spectroscopy. J. Am. Chem. Soc. 1993, 115 (1), 252−258. (22) Bergmann, U.; Groenzin, H.; Mullins, O. C.; Glatzel, P.; Fetzer, J.; Cramer, S. P. Carbon K-Edge X-Ray Raman Spectroscopy Supports Simple, Yet Powerful Description of Aromatic Hydrocarbons and Asphaltenes. Chem. Phys. Lett. 2003, 369 (1−2), 184−191. (23) Andrews, A. B.; Edwards, J. C.; Pomerantz, A. E.; Mullins, O. C.; Nordlund, D.; Norinaga, K. Comparison of Coal-Derived and Petroleum Asphaltenes by 13C Nuclear Magnetic Resonance, DEPT, and XRS. Energy Fuels 2011, 25 (7), 3068−3076. (24) Netzel, D. A. Quantitation of Carbon Types Using DEPT QUAT NMR Pulse SequencesApplication to Fossil-Fuel-Derived Oils. Anal. Chem. 1987, 59 (14), 1775−1779. (25) Voelkel, R. High-Resolution Solid-State 13C-NMR Spectroscopy of Polymers. Angew. Chem. Int. Ed. Engl. 1988, 27 (11), 1468−1483. (26) Solum, M. S.; Pugmire, R. J.; Grant, D. M. 13C Solid-State NMR of Argonne Premium Coals. Energy Fuels 1989, 3 (2), 187−193. (27) Burnham, A. K.; Happe, J. A. On the Mechanism of Kerogen Pyrolysis. Fuel 1984, 63 (10), 1353−1356. (28) Ward, R. L.; Burnham, A. K. Identification by 13C NMR of Carbon Types in Shale Oil and Their Relation to Pyrolysis Conditions. Fuel 1984, 63 (7), 909−914. (29) Birdwell, J. E.; Ruble, T. E.; Laughrey, C. D.; Roper, D. R.; Walker, G. Differentiating Organic Carbon Residues in Spent Oil Shale. In 30th Oil Shale Symposium; Colorado School of Mines: Golden, CO, 2010. (30) Doddrell, D. M.; Pegg, D. T.; Bendall, M. R. Distortionless Enhancement of NMR Signals by Polarization Transfer. J. Magn. Reson. 1982, 48 (2), 323−327. (31) Michon, L.; Martin, D.; Planche, J. P.; Hanquet, B. Estimation of Average Structural Parameters of Bitumens by 13C Nuclear Magnetic Resonance Spectroscopy. Fuel 1997, 76 (1), 9−15. 7323

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