Experimental study of kerogen maturation by solid-state 13C NMR spectroscopy

Experimental study of kerogen maturation by solid-state 13C NMR spectroscopy

Fuel 118 (2014) 308–315 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel Experimental study of kerogen...

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Fuel 118 (2014) 308–315

Contents lists available at ScienceDirect

Fuel journal homepage: www.elsevier.com/locate/fuel

Experimental study of kerogen maturation by solid-state spectroscopy

13

C NMR

N. Burdelnaya a,⇑, D. Bushnev a, M. Mokeev b, A. Dobrodumov b a b

Institute of Geology of the Komi Science Center of the Ural Branch RAS, 54 Pervomaiskaya St., Syktyvkar 167982, Russian Federation Institute of Macromolecular Compounds, RAS, 31 Bolshoy pr., Saint Petersburg 199004, Russian Federation

h i g h l i g h t s  Cobs decreases during the artificial maturation experiments.  Branched aromatic carbons transform into bridgehead aromatic carbons.  –(CH2)–O–C structures in kerogen break up at the early stages of catagenesis.  The spatial remoteness of alkyl chains from aromatic cores is fixed up to the oil window.

a r t i c l e

i n f o

Article history: Received 5 April 2013 Received in revised form 1 November 2013 Accepted 4 November 2013 Available online 15 November 2013 Keywords: Kerogen Solid-state 13C NMR Hydrous pyrolysis Artificial maturation

a b s t r a c t Chemical changes in the structure of kerogen during simulated thermal maturation were investigated by solid-state 13C NMR spectroscopy. The spin counting technique was used to evaluate the share of ‘‘observable’’ carbon atoms with increasing hydrous pyrolysis temperature. The obtained one-dimensional CP– MAS spectra showed that up to 50% of ether bonds were destroyed during the initial stages of sample heating. In spite of the loss of a significant part of alkyl chains, kerogen remained aliphatic. The aromatic structure of kerogen underwent considerable changes: two-dimensional 1H–13C heteronuclear solidstate NMR (HETCOR) spectra showed that in the process of hydrous pyrolysis, the redistribution of carbon atoms took place – branched aromatic carbons transform into bridgehead aromatic carbons. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Kerogen is a polymer whose nature depends on the initial organic matter that is decomposed and the conditions of that deposition [1,2]. Studies on the structure of kerogen are very important for petroleum geology with respect to the prognosis of the generated fluid content and also the stages of oil and gas generation [3]. Moreover, to successfully solve the problem regarding the search of alternative fuel types, it is necessary to study oil shales – a complicated but prospective raw material for the fuel and chemical industries – in detail. It is also important to know the structure of kerogen to optimize the processing of raw oil shale [4–8]. Recently, the structure of kerogen has been widely investigated by high-resolution solid-state NMR spectroscopy using the techniques of cross-polarization and magic angle spinning (CP–MAS) [8–16]. The combination of these techniques allows for not only for an increase in sensitivity but also for the determination of the ⇑ Corresponding author. Tel.: +7 8212249548; fax: +7 8212240970. E-mail address: [email protected] (N. Burdelnaya). 0016-2361/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2013.11.003

average dipole–dipole interaction and chemical shift anisotropy, which considerably narrows the lines in solid-state spectra. The use of one-dimensional NMR spectroscopy to study the structure of kerogen and similar insoluble natural substances is based on its high selectivity: individual functional groups can be distinguished in spectra because of their different chemical shifts. However, this does not provide the opportunity to characterize detailed differences between the atoms included in such groups and in many cases does not allow for the signals to be clearly distinguished as a result of their considerable overlap due to broad lines and the diversity of structures of such functional groups. Twodimensional NMR spectroscopy, based on the registration of dipolar interactions between magnetic nuclei, helps to solve these problems and reduce peak overlap [10]. As dipolar interactions act through space, the correlation of unprotonated carbons with unbound protons is possible, which provides additional structural information [10]. There are rather few paper about the study of kerogen structure during maturation based on NMR spectroscopy [10,14,16]. This work presents the results of a study on the structural changes undergone by kerogen during the thermal maturity of organic

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N. Burdelnaya et al. / Fuel 118 (2014) 308–315 Table 1 Geochemical characteristics of the samples before and after hydrous pyrolysis.

a

Sample (temperature, °C)

Corg, %

H/C

a

K-2 (initial) K-2 (250 °C) K-2 (275 °C) K-2 (300 °C) K-2 (325 °C) Chut (initial) Chut (250 °C) Chut (275 °C) Chut (300 °C) Chut (325 °C)

28.4 30.8 26.5 25.4 11.6 19.5 19.1 18.7 13.9 10.4

1.37 1.20 1.14 1.06 0.79 1.23 1.14 1.14 1.03 0.88

2 16 40 62 214 11 4 9 67 122

b, %

Tmax, °C

PI

OI

HI

TOC, %

406 413 – 424 427 413 415 417 421 427

0.01 0.05 – 0.15 0.18 0.08 0.08 0.12 0.15 0.23

46 11 – 7 7 10 9 6 5 7

600 582 – 513 476 614 591 591 535 448

28.5 30.0 – 27.2 31.3 19.4 19.7 20.8 19.8 19.6

b – the bitumen ratio (the ratio of the chloroform soluble bitumen to the organic carbon content in the rock).

2.3. Solid-state

Table 2 Results of calculation of the observed C (spin counting). Sample

C, %

m, mg

I, abs

T1q(1H), ms

Cobs, %

a-gly

32.0 64.2 69.6 66.0 70.8 69.6 67.5

117.85 65.00 63.45 62.80 57.50 61.20 63.30

100 83.6 67.4 40.2 70.2 55.8 47.8

47 5.5 4.7 2 5.5 4.4 3.1

100 88.71 69.70 59.03 76.36 60.70 57.02

K-2 (initial) K-2 (300 °C) K-2 (325 °C) Chut (initial) Chut (300 °C) Chut (325 °C)

matter (OM). Solid-state 13C NMR was used for qualitative and quantitative analyses of the studied kerogen. To observe the changes in the kerogen NMR spectra, we carried out experiments on the artificial maturation of organic matter.

13

C NMR

High-resolution solid-state magic angle spinning NMR spectra were obtained on an AVANCE II-500WB (Bruker) spectrometer at room temperature. The operating frequency for 13C was 125.8 MHz. Samples were packed in 4 mm zirconia rotors and spun at 10–13 kHz. In experiments with cross-polarization, a repetition time of 3 s and 100 kHz proton decoupling were used. Two-dimensional C–H correlation spectra were obtained using the standard HETCOR technique [23] with frequency switched Lee–Goldburg (FSLG) homonuclear dipolar decoupling at 110 kHz. All chemical shifts are presented in ppm relative to TMS (tetramethylsilane). The deconvolution of the spectra obtained was performed using the DMFIT software program [24]. 2.4. Hydrous pyrolysis

2. Materials and methods 2.1. Material The kerogen samples used in the study originated from Upper Jurassic and Upper Devonian deposits of the East European Platform (Russia). Upper Jurassic oil shales (Tithonian) were collected from an outcrop located not far from the settlement of Koygorodok (in the south of the Republic of Komi). The organic carbon content was 28.4%. Shale OM is characterized by a high content of organically bound sulfur and belongs to the II-S type [17]. According to the data on polycyclic biomarkers, shale OM is immature [18]. A Domanik (Middle Frasnian) sample presented by thin-layer oil shale was obtained from the Chut River outcrop (Ukhta District, Republic of Komi). This shale is a bituminous siliceous shale [19], Corg content of the sample was 19.5%. The level of organic matter thermal maturity was also not high [20,21]. The elemental composition of the kerogen corresponds to type II kerogen [20,22].

Seventy-eight ml autoclaves were used for hydrous pyrolysis. Oil shale samples were divided into several parts, and each (approximately 25 g of rock chips) was heated in an autoclave at a fixed temperature for 24 h. In each autoclave, 20 ml of distilled water was added. The temperatures used for hydrous pyrolysis were 250 °C, 275 °C, 300 °C and 325 °C. The temperature was measured by an electronic thermoregulator integrated into the furnace and was additionally controlled by a chromel–aluminum thermocouple (type R). Before autoclaving, the rock had been preliminarily extracted. After autoclaving, the rock was taken out and extracted, and kerogen was then isolated from the residue by sequential acid treatment [25,26]. Kerogen was prepared from the shales by sequential treatment with concentrated hydrochloric and hydrofluoric acids in accordance with the technique described in [27]. Inorganic component removal was controlled by the combustion of the obtained residue in a muffle furnace for 1 h (ash content measurement). The temperature in the furnace was 1100 °C. Soluble organic components were removed by kerogen extraction with chloroform.

2.2. Bulk analyses

3. Results and discussion

The organic carbon (Corg) content in the rock was defined using the ‘‘express carbon analyzer AN-7529’’ with a preliminary rock constituent that was insoluble in concentrated hydrochloric acid. Glucose was used as a standard. The Corg data obtained during the analysis were recalculated with respect to the initial rock. Rock-Eval pyrolysis was conducted in the crude oil geochemistry laboratory at OJSC TomskNIPIneft using Rock-Eval 6 Turbo analyzer. Kerogen elemental analysis was carried out at the Institute of Biology Komi SC UB RAS on an elemental EA 1110 (CHNS-O) CE Instruments analyzer.

3.1. Rock-Eval analysis According to Rock-Eval pyrolysis data, with increasing hydrous pyrolysis temperature, the hydrogen index decreases, and the Tmax value increases from 406 (R-2, source) – 412 °C (Chut, source) up to 427 °C for both samples (Table 1), which clearly indicate structural changes in kerogen but nevertheless do not correspond to organic matter with sufficiently high maturity. The HI of rocks heated to 325 °C was greater than 400 mg HC/g TOC; thus, kerogen did not lose its status as type II and still showed a considerable generation potential. The Upper Jurassic oil shales were

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Fig. 1. CP/MAS spectra of the Tithonian (K-2) and Domanik (Chut) kerogens before and after hydrous pyrolysis.

Fig. 2.

13

C NMR subspectra of all C (Call) protonated (Cp) and nonprotonated (Cq + CH3) carbons (after dipolar dephasing).

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311

characterized by a considerable decrease in their OI values compared with those of a non-heated sample and a rock heated at 250 °C. Bitumen quantitative content expressed through the bitumen ratio (b) grows during artificial maturation. Maximum conversion is achieved in Tithonian oil shales, where at 325 °C b is 2 times higher than in the Chut sample at the same temperature (Table 1).

tures of these acids mostly contain not polycondensed aromatic rings but isolated benzene rings replaced by oxygen-containing functional groups and alkyl chains [32,33]. Thus, with increasing autoclave temperature, the informativeness of kerogen spectra decreases.

3.2. Spin counting

The 13C NMR spectra of kerogens obtained using the crosspolarization technique (CP) are presented in Fig. 1. The spectra correspond to numerous 13C NMR spectra of the kerogen of different fossil fuels and have two characteristic bands attributed to aromatic and aliphatic carbon [13]. The initial kerogen spectra R-2 and Chut are noticeably different from each other. While R-2 is characterized by 5 signals, at 35 ppm (alkyl structures), 75 ppm (alkyl structures bonded to oxygen), 135 ppm (aromatic carbon), 175 ppm (carboxyl carbon) and 210 ppm (carbonyls composing approximately 1% of the total amount of observable carbons), in the Chut sample spectrum, only 2 signals are clearly observed – at 35 ppm and 135 ppm. The absence of signals of aliphatic ether bonds can be explained either by the higher maturity of Domanik OM, in which functional groups containing oxygen disappear at the beginning of maturation [4], or an original structure of kerogen that is different from the Jurassic structure [34,35]. During hydrous pyrolysis, with the increase in temperature, there are changes in the structure of less mature kerogen (sample R-2), reflected in the CP–MAS spectra. During the initial stage of the hydrothermal experiment, the signal intensity decreased in the 60–90 ppm range typical for O-alkyl carbon atoms. Meanwhile at 250 °C, up to 50% of ether bonds were lost at 300 °C, the signal in this region vanished completely (Fig. 1). The same changes took place for the carboxyl and carbonyl groups (signals in the 175– 210 ppm range). With the increase in the hydrous pyrolysis temperature, the Tithonian and Domanik kerogen spectra became similar (Fig. 1).

To determine the structural features of the samples, 13C NMR spectra with cross-polarization were obtained, allowing for the observation of the signals of practically all of the carbon atoms with high sensitivity due to polarization transfer from 1H nuclei (100% of abundance vs. 1% for 13C). However, it is worth noting that these spectra do not allow us to make qualitative evaluations because signal enhancement implies that the signal intensities are influenced by the kinetics of this transfer of magnetization (signal intensity depends on transfer efficiency for each group of atoms). Moreover, not all of the carbon nuclei contribute to the spectrum signal, and the part of such ‘‘non-observable’’ carbon nuclei in the spectrum sometimes reaches 50% in some samples [28,29]. This is behavior is associated with the fact that the intensity of the signals of a number of 13C nuclei in such objects decreases or totally disappears due to the influence of several factors, which are important when conducting NMR experiments, especially when using the cross-polarization technique. The main problems are the presence of paramagnetic centers, the spatial remoteness of proton nuclei and the high molecular mobility of structural fragments. To estimate the fraction of carbon nuclei invisible in NMR spectra, several experiments were carried out using the spin counting technique described in [30,31], allowing for the determination of part of the carbon nuclei actually observed, Cobs. The values of Cobs obtained for the initial kerogens and samples heated in an autoclave at 300 °C and 325 °C are presented in Table 2. The analysis shows that the Cobs values for initial kerogens reach up to 76.4–88.7% and decrease to as low as 57% with increasing heating temperature; therefore, the contribution of ‘‘invisible’’ carbon nuclei in the original samples is much lower than that in the heated samples. The authors [14] calculated Cobs for kerogens with different levels of maturity. The obtained Cobs values indicate a decrease in the share of carbon atom nuclei observed in kerogen of higher maturity. This decrease may be due to the presence of a more condensed aromatic structure of thermally mature kerogen and, as a result, the remoteness of protons. On the other hand, while studying the solid-state NMR spectra of humic and fulvic acids, quite low values of Cobs were observed [28–31], though the chemical struc-

3 K-2, initial

2,5

Caliph /Carom

2

Chut, 250 o C Chut, 275 o C

1,5

K-2, 250 o C Chut, initial

K-2, 300 o C

1 K-2, 325 oC

0,5

K-2, 275 o C

Chut, 300 o C Chut, 325 oC

0 0,5

0,7

0,9

1,1

1,3

H/C Fig. 3. Plot of Caliph/Carom versus the atomic ratio H/C.

1,5

3.3. 13C CP NMR spectra of kerogen before and after hydrous pyrolysis

3.4. CP edited spectra In the acquired spectra, there is an overlap of protonated and quaternary aromatic carbon signals (110–150 ppm range). To identify these signals, spectra with a short contact time (up to 100 ls) were additionally obtained. In these spectra, carbons with strong dipolar coupling with protons (protonated carbons) are mostly observed. Also, the spectra of nonprotonated carbons were recorded (for a period lasting approximately 50 ls, dipolar decoupling was turned off, which resulted in the destruction of magnetization in the carbons bonded to protons; CH3 and CH2 group signals could be observed due to weakened C–H dipolar interactions due to the fast rotation and mobility of the latter groups). The corresponding subspectra are presented in (Fig. 2). The signal corresponding to quaternary carbons and CH3 mobile groups is much lower than the signal corresponding to protonated carbon in the ‘‘aliphatic’’ region. Therefore, it is possible that a considerable portion of linear alkyl chains with the most intense signal is concentrated in kerogen. Branched carbon structures (quaternary carbon) are observed to a lesser degree. With the increase in the hydrous pyrolysis temperature, a considerable amount of alkyl chains disappeared, as indicated by the signal reduction at 30 ppm. The concentration of the terminal methyl groups (CH3) remained practically constant as the heating temperature increased. The given conclusion well correlates with the results from the work [16]. The authors [16] studied changes of kerogen structure near two igneous dike intrusions in the Illinois Basin. As far as coal is away from the dike intrusion, the removal of CH2 groups in kerogen structure occurs faster than the removal of methyl groups.

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Fig. 4. 2D 1H–13C HETCOR spectra of Domanik kerogen at different contact times before and after hydrous pyrolysis.

In the ‘‘aromatic’’ region, two overlapped signals at 126 and 137 ppm are clearly observed. The signal in the 125–126 ppm range refers to protonated aromatic carbon, and the signal at 137 ppm is associated with branched aromatic carbon Carom [11]. In accordance with [11], in the studied samples with different levels of thermal maturity, two types of quaternary aromatic carbons were detected – quaternary carbon (bridgehead carbon) atoms at the junction between aromatic rings close to protons and aromatic carbon connected to alkyl groups. The signal for protonated Carom corresponding to the quaternary aromatic carbon response region (at 125 ppm) was also registered. Wei et al. [13] and Tong et al. [36] registered the signal characteristic of Cbrigehead at 129.6 and 132 ppm, respectively. Protonated aromatic carbon resonates at 126 ppm [36,37]. In the table presented by [13], chemical shifts in different model polyaromatic compounds (from naphthalene up to coronene) are presented. Their average values range from 122 up to 140 ppm, and there seems to be no direct correlation between the chemical shift value and the number of carbon atoms present in the condensed system. In general, according to Mao et al. [11], the bridgehead carbon band is shifted to the weaker

regions (d is more than 135 ppm) with the increase in the concentration of aromatic clusters. However, it is not possible to categorize the signal in the 125–129 ppm range as belonging to protonated aromatic carbon. As a result of the effect of hydrothermal treatment on the kerogen of both Tithonian and Domanik shales, the intensity of the bands for Cproton and Cquart increases. Thus, the amount of quaternary aromatic carbons bonded to alkyl groups increases. As a result, due to the loss of alkyl chains, the rearrangement of the remaining carbon structures transferring to aromatic rings takes place. The geopolymer aromatic system itself remains relatively unchanged during the process of maturity, at least at these stages of catagenesis. This conclusion is true only for ‘‘observable’’ carbon atoms. If the decrease in the Cobs values when the samples were heated is considered, in the case of kerogen undergoing a stronger thermal treatment (300 °C, 325 °C), additional structure aromatization, reflected by the appearance of polycondensed aromatic systems, takes place. As indicated by Wilson et al. [9], for many carbons and macerals, the NMR signal of nonprotonated aromatic carbon increases with thermal maturity [9]. Therefore, it is possible to conclude that the kerogen samples

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313

Fig. 5. 2D 1H–13C HETCOR spectra of Tithonian kerogen at different contact times before and after hydrous pyrolysis.

considered in this study did not reach ‘‘sufficient’’ maturity to achieve structural order. Based on a mature row of kerogens (Ro from 0.52% up to 1.81%), Wei et al. [13] showed that there exists a direct correlation between the increase in aromatic cluster size and the increase in kerogen thermal maturity. The authors of this work introduced the parameter vb, which denotes the size of aromatic clusters determined as the ratio of the integral spectral intensity of ‘‘condensed’’ carbons to the total number of carbons [13]. This parameter is directly correlated to vitrinite reflectance and increases with increasing kerogen maturity. 3.5. Caliph/Carom ratio The Caliph/Carom ratio underwent considerable changes – with the increase in the hydrothermal treatment temperature (Fig. 3), it decreased from 2.53 (for R-2) and 1.35 (for Chut) to 0.47 (for R-2) and 0.65 (for Chut) at 325 °C. The calculation of this ratio is based on the integral intensities of signals referring to primary, secondary, tertiary, and quaternary carbon atoms composing

acyclic hydrocarbon structures in the 10–40 ppm range for Caliph and 100–150 ppm for Carom. This ratio correlates with the atomic ratio H/C well – when heating takes place, H/C values and the Caliph/ Carom ratio decrease. For Tithonian kerogen, a drastic transition in values from source rock to kerogen takes place at 250 °C, after which there is a smooth reduction in Caliph/Carom associated with changes in this ratio for Domanik shales. 3.6. 2D 1H –

13

C HETCOR NMR spectra

To obtain additional information regarding the spatial bonds between different functional groups, two-dimensional correlation spectra C–H (2D HETCOR) with different contact times (0.2– 1 ms) were obtained. The spectra were obtained for original kerogens and for ones heated at 300 and 325 °C. Depending on the contact time, in these spectra, cross-peaks were observed. If either there is a direct C–H bond or the contact time is considerable, it is possible to observe the cross-peaks of the carbons at a distance of up to 15 Å from protons (Figs. 4 and 5). If a comparison is drawn

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between the correlation peaks in the spectra aromatic region and the lower contact time (0.2–0.35 ms), it is possible to notice the higher intensity of the cross-peak between the aromatic carbons and aromatic protons in the case of hydrothermally treated kerogen (Figs. 4 and 5). It is also necessary to mention that, as a result of sample thermal treatment, due to the loss of a portion of the nalkyl chains in kerogen indicated by the 2D HETCOR spectra, for the same contact time, the cross-peak in the aliphatic region is less intense. For a contact time of 1 ms, the initial and heated samples have a cross-peak between the aliphatic carbons at 30 ppm and the protons at 2 ppm corresponding to methyl and methylene groups in alkyl chains. The spectrum with a 1 ms contact time contains a cross-peak between the aromatic carbons at 133 ppm and aliphatic protons at 2 ppm (as has already been mentioned, at shorter contact times, the cross-peak cannot be seen), which testifies to a certain spatial remoteness of alkyl chains from aromatic cores. The intense cross-peak between the aromatic carbons at 128 ppm and aromatic protons at 7 ppm reflects the spatial proximity of the aromatic core to its own protons. Specifically, after the hydrous pyrolysis of the rocks, the signal at 133 ppm, characteristic of non-heated kerogen, correlated with aliphatic protons (2 ppm) and indicating the presence of alkyl radicals relative to the aromatic ring at a distance of 1.5 nm (approximately 9–10 single carbon–carbon bonds), is shifted to a higher field (128 ppm). Thus, the substituted aromatics (133 ppm) transform into a condensed polyaromatic structure (128 ppm). With short contact times of 0.25 ms and 0.35 ms, in the 2D HETCOR spectra obtained for the Chut initial sample and the samples thermally heated at 300 and 325 °C, as it has already been mentioned, the cross-peak between the aromatic carbons at 133 ppm and aliphatic protons at 2 ppm is missing, and the cross-peak between the aromatic carbons at 128 ppm and the protons attached directly to the aromatic cores becomes more intense (Fig. 4). Therefore, the missing cross-peak of the methyl group at a short contact time indicates that the group is not attached directly to the aromatic core. The two-dimensional C–H correlation spectra of humic acids presented in [10], which were also obtained at different contact times (0.1 ms, 1 ms and 10 ms), demonstrate a wide range of different structures present in humic acids. A weak cross-peak between the aliphatic carbons at 30 ppm and protons at 2 ppm indicates a low degree of structural aliphaticity. With the increase in contact time, the cross-peak becomes even less clear and more elongated. Thus, the kerogen of Upper Jurassic and Domanik shales contains more methylene units. Still, at 325 °C, the intensity of the cross-peak attributed to methyl and methylene groups in the structure of kerogen weakens. The signal in the 65–85 ppm range observed in the 13C NMR CP–MAS spectra of initial Upper Jurassic kerogen, corresponding to the groups –CH(OH)– or CH2–O–C, is as intense as that in humic acid spectra [10]. In the spectra of mature kerogen, this signal is missing [11]. As mentioned earlier, with the increase in the hydrothermal experiment temperature, in the one-dimensional spectra of heated kerogen, this signal disappears; in the twodimensional spectrum of kerogen heated at 300 °C, there is no signal (Fig. 5). 4. Conclusions The structure of II and II-S type kerogen, isolated from Domanik and Tithonian oil shales, was studied in the process of artificial maturing at temperatures 250 °C, 275 °C, 300 °C and 325 °C by MAS and 2D 13C NMR spectroscopy. It was found that with the increase of hydrous pyrolysis temperature, the share of ‘‘visible’’ carbon nuclei and the general informativeness of kerogen spectra somehow decreases. Considerable changes connected with ether

decomposition at the initial stages of artificial maturing take place in the structure of less mature kerogen. In the aromatic region, three types of aromatic carbon were determined–protonated aromatic carbon at 125 ppm and two types of unprotonated Carom (Cbridgehead and carbon substituted with alkyl chains). The signal of the first one coincides with the signal of protonated carbon atoms (i.d. 125 ppm), and the signal of the last one is observed at 137 ppm. Upon heating kerogen, the redistribution of carbon atoms takes place. Branched aromatic carbons transform into bridgehead carbons; at the same time, most protonated aromatics remains. Kerogen 2D 13C NMR spectra analysis proved the idea of remoteness of alkyl chains from aromatic cores in kerogen structure up to the beginning of the stage oil window. Taking into consideration high kerogen conversion into soluble organic substance under hydrothermal influence on the rock, it is seen that by the beginning of oil generation stage, the structure of kerogen of Tithonian and Domanik shales (II-S and II type kerogen) becomes similar losing most aliphatic structures and functional groups. Acknowledgments The authors would like to express their deepest gratitude to Dr. Goncharov for making the Rock-Eval pyrolysis possible at the TomskNIPIneft and Dr. Kondratenok for making the elemental analysis at IB Komi SC UB RAS possible. This research was supported by the Grant of the Russian Foundation for Basic Research No. 11-05-00699-a and the program of fundamental research of UB RAS No. 12-M-57-2047. References [1] Behar F, Vandenbroucke M. Chemical modeling of kerogens. Org Geochem 1987;11:15–24. [2] Vandenbroucke M, Largeau C. Kerogen origin, evolution and structure. Org Geochem 2007;38:719–833. Review. [3] Tissot BP, Welte DH. Petroleum formation and occurrence. 2nd ed. Berlin: Springer Verlag; 1984. p. 151. [4] Oil shale Ed, Yena TPh, Chilingarayna JV. Leningrad: Nedra; 1980. p. 262. in Russian. [5] Zelenin NI, Ozerov IM. Reference oil shales. Leningrad: Nedra; 1983. p. 248. in Russian. [6] Burnham AK. Slow radio-frequency processing of large oil shale volumes to produce petroleum-like shale oil, LLNL, Report UCRL-ID-155045; 2003. [7] Burnham AK, McConaghy JR. Comparison of the acceptability of various oil shale processes 26th oil shale symposium golden CO., United States October 16, 2006 through October 18; 2006. [8] Cao X, Birdwell JE, Chappell MA, Pignatello JJ, Mao J-D. Characterization of oil shale, isolated kerogen, and post-pyrolysis residues using advanced 13C solidstate nuclear magnetic resonance spectroscopy. AAPG Bull 2013;97:421–36. [9] Wilson MA, Pugmire RJ, Karas J, Alemany LB, Woolfenden WR, Grant DM, et al. Carbon distribution in coals and coal macerals by cross polarization magic angle spinning C-13 nuclear magnetic-resonance spectrometry. Anal Chem 1984;56(6):933–43. [10] Mao J-D, Xing B, Schmidt-Rohr K. New Structural information on a humic acid from two-dimensional 1H–13C correlation solid-state nuclear magnetic resonance. Environ Sci Technol 2001;35:1928–34. [11] Mao J, Fang X, Lan Y, Schimmelmann A, Mastalerz M, Xu L, et al. Chemical and nanometer-scale structure of kerogen and its change during thermal maturation investigated by advanced solid-state 13C NMR spectroscopy. Geochem Cosmoch Acta 2010;74:2110–27. [12] Werne-Zwanziger U, Lis G, Mastalerz M, Schimmelmann A. Thermal maturity of type II kerogen from the New Albany Shale assessed by C-13 CP–MAS NMR. Solid State Nucl Magn Reson 2005;27:140–8. [13] Wei Z, Gao X, Zhang D, Da J. Assessment of thermal evolution of kerogen geopolymers with their structural parameters measured by solid-state 13C NMR spectroscopy. Energy Fuels 2005;19:240–50. [14] Smernik RJ, Schwark L, Schmidt MWI. Assessing the quantitative reliability of solid-state 13C NMR spectra of kerogens across a gradient of thermal maturity. Solid State Nucl Magn Reson 2006;29:312–21. [15] Bushnev DA, Burdelnaya NS, Mokeev MV, Gribanov AV. Chemical structure and 13C NMR spectra of the kerogen of carbonaceous rock masses. Dokl Earth Sci 2010;430(5):210–3. [16] Cao X, Chappell A, Schimmelmann A, Mastalerz M, Li Y, Hu W, et al. Chemical structure changes in kerogen from bituminous coal in response to dike

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