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Hydrocarbons occluded by asphaltenes
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ScienceDirect Russian Geology and Geophysics 59 (2018) 975–982 www.elsevier.com/locate/rgg
Hydrocarbons occluded by asphaltenes V.A. Kashirtsev * A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia Institute of Oil and Gas Problems, Siberian Branch of the Russian Academy of Sciences, ul. Oktyabrskaya 1, Yakutsk, 677891, Russia Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia Received 25 April 2018; accepted 25 April 2018
Abstract Homologous series of n-alkenes and dimethylalkanes with the odd or even number of carbon atoms in the molecule have been identified in chloroform extracts from the organic matter of Upper Paleozoic deposits of the Vilyui syneclise penetrated by the superdeep well SV-27 at depths below 5 km. It is presumed that these unusual hydrocarbons resulted from the destruction of asphaltene occlusions under severe P–T conditions at great depths and that the hydrocarbon generation began in the zone of postdiagenetic transformations of sediments. This hypothesis was tested in the sections of deposits whose organic matter underwent catagenesis of different grades. On the basis of these results, zones of emergence, transition, and destruction of occlusions have been recognized. © 2018, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: bitumen; hydrocarbons; asphaltenes; occlusion
Introduction The superdeep well SV-27 was drilled at the top of the Khapchagai uplift, Vilyui syneclise. It penetrated Mesozoic and Upper Paleozoic deposits to the depth 6519 m. At present, the temperature at the well bottom is 173 °C. The earliest geochemical data on the SV-27 core were reported in (Kontorovich et al., 1988). Later studies provided more information on the geochemistry of both aliphatic and aromatic hydrocarbon fractions (Kashirtsev et al., 2016, 2017). They showed that the distribution of n-alkanes in the bitumens changed at depths below 5 km. The distribution maximum shifts toward lower molecular weight compounds (C18 and C19), and the pristane : phytane ratio decreases below unity, although the facies features of the coal-bearing rock mass remain practically unchanged. Studies by chromatography– mass spectrometry revealed a series of formerly unknown hydrocarbons, first as traces and then at concentrations increasing with sampling depth. Two homologous series of n-alkenes (m/z 55, 69) were recognized in the “new” hydrocarbons: one with only odd and the other with only even numbers of carbon atoms. Two homologous series of 2,4- and 2,7-dimethylalkanes were found (m/z 85 and m/z 127), also
* Corresponding author. E-mail address: [email protected] (V.A. Kashirtsev)
with only odd and only even numbers of carbon atoms, respectively. Alkylcyclohexanes with predominance of odd carbon numbers were detected. Among aromatic hydrocarbons, four new diastereomeres of 17-desmethyl, 23-methyl monoaromatic steroids C27 were identified. It is hypothesized that the emergence of premature and new hydrocarbons at great depths in the apocatagenesis zone of the Vilyui syneclise is related to asphaltene degradation and that it reflects the composition of components occluded and adsorbed by asphaltenes at their nascence. Many papers have been dedicated to the structure of asphaltenes and their aggregates, as well as to hydrocarbons occluded and adsorbed by asphaltenes. Most of these works involve experiments on asphaltene degradation by mild thermolysis, pyrolysis, and chemical agents (Aref’ev et al., 1980; Borisova, 2009, 2016; Derakhshesh et al., 2013; Ekweozor, 1985; Gordadze et al., 2015; Kontorovich et al., 1987; Liao and Geng, 2002; Melenevskii et al., 2009; Tian et al., 2012; Yang et al., 2009; Zhao and Liao, 2012). According to current notions, macromolecular bodies of asphaltenes and their aggregates contain chemical fragments that can be extracted from the host molecule by various procedures of pyrolysis and chemical degradation, disrupting covalent links. Experiments indicate that these fragments may provide valuable information on the hydrocarbons, which reflect the properties of the maternal organic matter at early stages of its thermal evolution.
1068-7971/$ - see front matter D 201 8, V.S. So bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.rgg.201 + 8.07.017
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They can also protect occlusions from secondary modifications, including biodegradation (Snowdon et al., 2016). This paper considers natural asphaltene degradation under severe P–T conditions at great depths and compares hydrocarbons from the occlusions with the corresponding components at early hydrocarbons generation stages.
Objects and materials studied Given that within the Siberian Platform or adjacent marginal depressions there are no other wells penetrating the sedimentary cover to a depth lower that 5 km, sections of folds of the Kharaulakh anticlinorium (Fig. 1) were chosen as reference for a comparative study because the organic matter (OM) of Carboniferous and Permian marine deposits experienced profound transformations in apocatagenesis stages in those segments. A total of 45 OM extracts, mainly from mudstones, have been examined. Other objects expected to illustrate the degree of “unusual” hydrocarbon generation and occlusion by asphaltenes were Jurassic–Lower Cretaceous sections exposed in natural outcrops near the Anabar Bay and Olenek River. Judging from vitrinite reflectance (Ro = 0.4), OM in those sections is in the protocatagenesis–early mesocatagenesis stages. Extracts from 63 samples were examined. The biogeochemical characterization of the sections is provided in (Kashirtsev et al., 2018).
Methods Rock samples from well cores and natural outcrops were extracted with chloroform. The maltene fraction of the bitu-
mens obtained after asphaltene precipitation with an excess of petroleum ether was resolved in columns with ASK silica gel and aluminum oxide into fractions of aliphatic and aromatic hydrocarbons, benzene tars, and alcohol–benzene tars. The hydrocarbon fractions were studied on a gas chromatograph 6890 combined with a high-performance mass-selective detector Agilent 5973N. Mass chromatograms were obtained by total ion current (TIC) record and single ion monitoring (SIM) of characteristic fragment ions. Individual hydrocarbons were identified by computerized search of the library of the National Institute of Standards NIST-08, according to data from the literature, and, mainly, by the reconstruction of structures from electron impact ion fragmentation patterns (Lebedev, 2003; Petrov et al., 1986).
Discussion Nearly all hydrocarbon spectra deduced from TIC masschromatograms of samples from the Kharaulakh anticlinorium (apocatagenesis stage) were similar to each other and to deep-seated samples from the Vilyui syneclise (Kashirtsev et al., 2017). The same range of unusual hydrocarbons was identified by scanning over fragment ions: homologous series of 2,4-dimethylalkanes (m/z 85) with only odd numbers of carbon atoms and 2,7-dimethylalkanes (m/z 127) with even carbon numbers (Fig. 2). Homologous series of alkenes with double bonds at the first (alkenes-1) and third (alkenes-3) carbon atoms were identified according to the m/z 69 ion, also with odd and even carbon numbers (Fig. 3). Note that n-alkenes with only even carbon numbers were identified among occlusion hydrocarbons in Tarim oils (China) (Tian et al., 2012). Alkylcyclohexanes (m/z 83) of Kharaulakh bitu-
Fig. 1. Locations of well sections and natural outcrops studied.
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Fig. 2. Composite m/z 85 and 127 mass-chromatograms of aliphatic hydrocarbon fractions of bitumens from Devonian, Carboniferous, and Permian mudstones of the Kharaulakh anticlinorium. 15–33, n-alkanes; Pr, pristane; Ph, phytane; 2,4-Dma, 2,4-dimethylalkanes; 2,7-Dma, 2,7-dimethylalkanes.
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Fig. 3. Total ion current mass-chromatograms of the aliphatic hydrocarbon fraction of bitumen from Carboniferous deposits of the Kharaulakh anticlinorium. Mass-fragmentograms m/z 69, 83 and mass spectra of alkene-3 and alkylcyclohexane C21. Bottom: the mass-spectrum and structure of squalene (peak S) in the TIC chromatogram.
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Fig. 4. Mass-chromatograms of aromatic bitumen fractions from (A) Carboniferous deposits of the Kharaulakh anticlinorium and (B) Cretaceous deposits of the Lena–Anabar basin. Mass-spectra of 17-desmethyl, 23-methylaromatic steroid C27. MPh, Methylphenanthrenes; DMPh, dimethylphenanthrenes; P, pyrene; R, retene; TTHPh, 1,1,7,8-tetramethyl-, 2,3,4-tetrahydrophenanthrene.
Fig. 5. Mass-chromatogram of the aliphatic bitumen fraction from Cretaceous clays of the Lena–Anabar basin, fragmentogram m/z 69 and mass-spectra of octadecene-3 and nonadecene-1.
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Fig. 6. Total ion current mass-chromatogram and fragmentograms of the aliphatic (m/z 69,85,127) and aromatic (m/z 366) fractions from Lower Jurassic mudstones of the Govorovskaya well.
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mens are dominated by odd-carbon molecules, whereas evencarbon ones are much less abundant. Nearly all extracts contain squalene, and its concentrations in some samples are so large that they are visible in TIC mass-chromatograms (Fig. 3). As in Vilyui extracts, four diastereomeres of 17-desmethyl, 23-methylmonoaromatic steroid C27 (17-DSM,23MMAS) were identified in aromatic fractions from Kharaulakh bitumens (Fig. 4a). In bitumens from sections near the Anabar Bay and Olenek River, where fine-grained Upper Jurassic–Lower Cretaceous rocks consist mainly of soft clays and the maturity of organic matter ranges between the protocatagenesis and mesocatagenesis stages, tentative search for compounds not yet occluded was performed with regard to m/z 366, not to miss the “new” steroid. The search was successful, and a series of samples with 17-DSM,23MMAS was detected at the Jurassic/Cretaceous boundary (Fig. 4b). The aliphatic fractions contained homologous series of alkenes-1 and alkenes-3 with the same properties and features as formerly described. They also contained 2,4- and 2,7-dimethylalkanes and alkylcyclohexanes dominated by odd-carbon compounds (Fig. 5). The question arose of at what depths (catagenesis stages) the occlusion of unusual hydrocarbons detected at the levels of proto- and mesocatagenesis was completed. In the Govorovskaya well (Figs. 1, 6), alkenes, dimethylalkanes, and 17-DSM,23MMAS not captured by asphaltenes were detected at the depth 1380 m (stage MC11). Minor amounts of these components occur at depths to 1800 m (MC11). At greater depths, no occluded matter was found in organic matter extracts, whereas asphaltenes are degraded and the specific hydrocarbons emerge again at depths below 5 km under severe P–T conditions (SV-27, AC). Thus, three zones can be outlined: occlusion nascence (PC–MC11), transition (MC11– AC1), and destruction (>AC1). Conclusions The entire range of unusual hydrocarbons detected in the section of superdeep well SV-27 at depths above 5 km and in Paleozoic deposits of the Verkhoyansk fold belt was formed by the degradation of asphaltene macromolecules under severe P–T conditions and by the release of hydrocarbons occluded at early stages of hydrocarbon generation. Judging from the catagenetic zonation of the nascence, transition, and degradation of occluded hydrocarbons, their contribution to the formation of oil fields was small.
References Aref’yev, O.A., Makushina, V.M., Petrov, A.A., 1980. Asphaltenes as indicators of the geochemical history of oil. Int. Geol. Rev. 24, 723–728. Borisova, L.S., 2009. Geochemistry of oil asphaltenes of West Siberia. Geologiya Nefti i Gaza, No. 1, 76–80. Borisova, L.S., 2016. Asphaltenes inherit the kerogen genetic code. Geologiya Nefti i Gaza, No. 6, 75–78. Derakhshesh, M., Gray, M.R., Dechaine, G.P., 2013. Dispersion of asphaltene nanoaggregates and the role of Rayleigh scattering in the absorption of visible electromagnetic radiation by these nanoaggregates. Energy and Fuels 27, 680–693. Ekweozor, C.M., 1985. Characterisation of the non-asphaltene products of mild chemical degradation of asphaltenes. Organic Geochem. 8 (10), 1053–1058. Gordadze, G.N., Giruts, M.V., Koshelev, V.N., Yusupova, T.N., 2015. Distribution features of biomarker hydrocarbons in asphaltene thermolysis products of different fractional compositions (using as an example oils from carbonate deposits of Tatarstan oilfields). Petrol. Chem. 55, 22–31. Kashirtsev, V.A., Fomin, A.N., Shevchenko, N.P., Dolzhenko, K.V., 2016. New monoaromatic steroids in organic matter of the apocatagenesis zone. Dokl. Earth Sci. 469, 815–818. Kashirtsev, V.A., Dolzhenko, K.V., Fomin, A.N., Kontorovich, A.E., Shevchenko, N.P., 2017. Hydrocarbon composition of bitumen from deeply buried terrestrial organic matter (zone of apocatagenesis). Russian Geology and Geophysics (Geologiya i Geofizika) 58 (6), 702–710 (869–879). Kashirtsev, V.A., Nikitenko, B.L., Peshchevitskaya, E.B., Fursenko, E.A., 2018. Biogeochemistry and microfossils of the Upper Jurassic and Lower Cretaceous, Anabar Bay, Laptev Sea. Russian Geology and Geophysics (Geologiya i Geofizika) 59 (4), 386–404 (481–501). Kontorovich, A.E., Borisova, L.S., Melenevskii, V.N., 1987. Some essential geochemical features of oil asphaltenes. Geokhimiya, No. 10, 1423–1432. Kontorovich, A.E., Polyakova, I.D., Kolganova, M.M., Soboleva, E.I., 1988. Organic matter transformations in meso- and apocatagenesis. Sovetskaya Geologiya, No. 7, 26–36. Lebedev, A.T., 2003. Mass-Spectrometry in Organic Chemistry [in Russian]. BINOM, Moscow. Liao, Z., Geng, A., 2002. Characterization of nC7-soluble fractions of the products from mild oxidation of asphaltenes. Organic Geochem. 33, 1477–1486. Melenevskii, V.N., Kontorovich, A.E., Kashirtsev, V.A., Kim, N.S., 2009. Biomarkers in products of the pyrolysis of asphaltenes from old oils of East Siberia as indicators of the settings of source rock formation. Neftekhimiya 49 (4), 1–8. Petrov, A.A., Golovkina, L.S., Rusinova, G.V., 1986. Album of Mass-Spectra of Oil Hydrocarbons [in Russian]. Nedra, Moscow. Snowdon, L.R., Volkman, J.K., Zhang, Z., Tao, T.G., Peng, Liu, 2016. The organic geochemistry of asphaltenes and occluded biomarkers. Organic Geochem. 91, 3–15. Tian, Y., Yang, C., Liao, Z., Zhang, H., 2012. Geochemical quantification of mixed marine oils from Tazhong area of Tarim Basin, NW China. J. Petrol. Sci. Engin. 90-91, 96–106. Yang, C., Liao, Z., Zhang, L., Creux, P., 2009. Some biogenic-related compounds occluded inside asphaltene aggregates. Energy and Fuels 23, 820–827. Zhao, J., Liao, Z., Chrostowska, A., Liu, Q., Zhang, L., Graciaa, A., Creux, P., 2012. Experimental studies on the adsorption/occlusion phenomena inside the macromolecular structures of asphaltenes. Energy and Fuels 26, 1746–1755.
Editorial responsibility: A.E. Kontorovich
ScienceDirect Russian Geology and Geophysics 59 (2018) 975–982 www.elsevier.com/locate/rgg
Hydrocarbons occluded by asphaltenes V.A. Kashirtsev * A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia Institute of Oil and Gas Problems, Siberian Branch of the Russian Academy of Sciences, ul. Oktyabrskaya 1, Yakutsk, 677891, Russia Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia Received 25 April 2018; accepted 25 April 2018
Abstract Homologous series of n-alkenes and dimethylalkanes with the odd or even number of carbon atoms in the molecule have been identified in chloroform extracts from the organic matter of Upper Paleozoic deposits of the Vilyui syneclise penetrated by the superdeep well SV-27 at depths below 5 km. It is presumed that these unusual hydrocarbons resulted from the destruction of asphaltene occlusions under severe P–T conditions at great depths and that the hydrocarbon generation began in the zone of postdiagenetic transformations of sediments. This hypothesis was tested in the sections of deposits whose organic matter underwent catagenesis of different grades. On the basis of these results, zones of emergence, transition, and destruction of occlusions have been recognized. © 2018, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: bitumen; hydrocarbons; asphaltenes; occlusion
Introduction The superdeep well SV-27 was drilled at the top of the Khapchagai uplift, Vilyui syneclise. It penetrated Mesozoic and Upper Paleozoic deposits to the depth 6519 m. At present, the temperature at the well bottom is 173 °C. The earliest geochemical data on the SV-27 core were reported in (Kontorovich et al., 1988). Later studies provided more information on the geochemistry of both aliphatic and aromatic hydrocarbon fractions (Kashirtsev et al., 2016, 2017). They showed that the distribution of n-alkanes in the bitumens changed at depths below 5 km. The distribution maximum shifts toward lower molecular weight compounds (C18 and C19), and the pristane : phytane ratio decreases below unity, although the facies features of the coal-bearing rock mass remain practically unchanged. Studies by chromatography– mass spectrometry revealed a series of formerly unknown hydrocarbons, first as traces and then at concentrations increasing with sampling depth. Two homologous series of n-alkenes (m/z 55, 69) were recognized in the “new” hydrocarbons: one with only odd and the other with only even numbers of carbon atoms. Two homologous series of 2,4- and 2,7-dimethylalkanes were found (m/z 85 and m/z 127), also
* Corresponding author. E-mail address: [email protected] (V.A. Kashirtsev)
with only odd and only even numbers of carbon atoms, respectively. Alkylcyclohexanes with predominance of odd carbon numbers were detected. Among aromatic hydrocarbons, four new diastereomeres of 17-desmethyl, 23-methyl monoaromatic steroids C27 were identified. It is hypothesized that the emergence of premature and new hydrocarbons at great depths in the apocatagenesis zone of the Vilyui syneclise is related to asphaltene degradation and that it reflects the composition of components occluded and adsorbed by asphaltenes at their nascence. Many papers have been dedicated to the structure of asphaltenes and their aggregates, as well as to hydrocarbons occluded and adsorbed by asphaltenes. Most of these works involve experiments on asphaltene degradation by mild thermolysis, pyrolysis, and chemical agents (Aref’ev et al., 1980; Borisova, 2009, 2016; Derakhshesh et al., 2013; Ekweozor, 1985; Gordadze et al., 2015; Kontorovich et al., 1987; Liao and Geng, 2002; Melenevskii et al., 2009; Tian et al., 2012; Yang et al., 2009; Zhao and Liao, 2012). According to current notions, macromolecular bodies of asphaltenes and their aggregates contain chemical fragments that can be extracted from the host molecule by various procedures of pyrolysis and chemical degradation, disrupting covalent links. Experiments indicate that these fragments may provide valuable information on the hydrocarbons, which reflect the properties of the maternal organic matter at early stages of its thermal evolution.
1068-7971/$ - see front matter D 201 8, V.S. So bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.rgg.201 + 8.07.017
976
V.A. Kashirtsev / Russian Geology and Geophysics 59 (2018) 975–982
They can also protect occlusions from secondary modifications, including biodegradation (Snowdon et al., 2016). This paper considers natural asphaltene degradation under severe P–T conditions at great depths and compares hydrocarbons from the occlusions with the corresponding components at early hydrocarbons generation stages.
Objects and materials studied Given that within the Siberian Platform or adjacent marginal depressions there are no other wells penetrating the sedimentary cover to a depth lower that 5 km, sections of folds of the Kharaulakh anticlinorium (Fig. 1) were chosen as reference for a comparative study because the organic matter (OM) of Carboniferous and Permian marine deposits experienced profound transformations in apocatagenesis stages in those segments. A total of 45 OM extracts, mainly from mudstones, have been examined. Other objects expected to illustrate the degree of “unusual” hydrocarbon generation and occlusion by asphaltenes were Jurassic–Lower Cretaceous sections exposed in natural outcrops near the Anabar Bay and Olenek River. Judging from vitrinite reflectance (Ro = 0.4), OM in those sections is in the protocatagenesis–early mesocatagenesis stages. Extracts from 63 samples were examined. The biogeochemical characterization of the sections is provided in (Kashirtsev et al., 2018).
Methods Rock samples from well cores and natural outcrops were extracted with chloroform. The maltene fraction of the bitu-
mens obtained after asphaltene precipitation with an excess of petroleum ether was resolved in columns with ASK silica gel and aluminum oxide into fractions of aliphatic and aromatic hydrocarbons, benzene tars, and alcohol–benzene tars. The hydrocarbon fractions were studied on a gas chromatograph 6890 combined with a high-performance mass-selective detector Agilent 5973N. Mass chromatograms were obtained by total ion current (TIC) record and single ion monitoring (SIM) of characteristic fragment ions. Individual hydrocarbons were identified by computerized search of the library of the National Institute of Standards NIST-08, according to data from the literature, and, mainly, by the reconstruction of structures from electron impact ion fragmentation patterns (Lebedev, 2003; Petrov et al., 1986).
Discussion Nearly all hydrocarbon spectra deduced from TIC masschromatograms of samples from the Kharaulakh anticlinorium (apocatagenesis stage) were similar to each other and to deep-seated samples from the Vilyui syneclise (Kashirtsev et al., 2017). The same range of unusual hydrocarbons was identified by scanning over fragment ions: homologous series of 2,4-dimethylalkanes (m/z 85) with only odd numbers of carbon atoms and 2,7-dimethylalkanes (m/z 127) with even carbon numbers (Fig. 2). Homologous series of alkenes with double bonds at the first (alkenes-1) and third (alkenes-3) carbon atoms were identified according to the m/z 69 ion, also with odd and even carbon numbers (Fig. 3). Note that n-alkenes with only even carbon numbers were identified among occlusion hydrocarbons in Tarim oils (China) (Tian et al., 2012). Alkylcyclohexanes (m/z 83) of Kharaulakh bitu-
Fig. 1. Locations of well sections and natural outcrops studied.
V.A. Kashirtsev / Russian Geology and Geophysics 59 (2018) 975–982
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Fig. 2. Composite m/z 85 and 127 mass-chromatograms of aliphatic hydrocarbon fractions of bitumens from Devonian, Carboniferous, and Permian mudstones of the Kharaulakh anticlinorium. 15–33, n-alkanes; Pr, pristane; Ph, phytane; 2,4-Dma, 2,4-dimethylalkanes; 2,7-Dma, 2,7-dimethylalkanes.
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Fig. 3. Total ion current mass-chromatograms of the aliphatic hydrocarbon fraction of bitumen from Carboniferous deposits of the Kharaulakh anticlinorium. Mass-fragmentograms m/z 69, 83 and mass spectra of alkene-3 and alkylcyclohexane C21. Bottom: the mass-spectrum and structure of squalene (peak S) in the TIC chromatogram.
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Fig. 4. Mass-chromatograms of aromatic bitumen fractions from (A) Carboniferous deposits of the Kharaulakh anticlinorium and (B) Cretaceous deposits of the Lena–Anabar basin. Mass-spectra of 17-desmethyl, 23-methylaromatic steroid C27. MPh, Methylphenanthrenes; DMPh, dimethylphenanthrenes; P, pyrene; R, retene; TTHPh, 1,1,7,8-tetramethyl-, 2,3,4-tetrahydrophenanthrene.
Fig. 5. Mass-chromatogram of the aliphatic bitumen fraction from Cretaceous clays of the Lena–Anabar basin, fragmentogram m/z 69 and mass-spectra of octadecene-3 and nonadecene-1.
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Fig. 6. Total ion current mass-chromatogram and fragmentograms of the aliphatic (m/z 69,85,127) and aromatic (m/z 366) fractions from Lower Jurassic mudstones of the Govorovskaya well.
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mens are dominated by odd-carbon molecules, whereas evencarbon ones are much less abundant. Nearly all extracts contain squalene, and its concentrations in some samples are so large that they are visible in TIC mass-chromatograms (Fig. 3). As in Vilyui extracts, four diastereomeres of 17-desmethyl, 23-methylmonoaromatic steroid C27 (17-DSM,23MMAS) were identified in aromatic fractions from Kharaulakh bitumens (Fig. 4a). In bitumens from sections near the Anabar Bay and Olenek River, where fine-grained Upper Jurassic–Lower Cretaceous rocks consist mainly of soft clays and the maturity of organic matter ranges between the protocatagenesis and mesocatagenesis stages, tentative search for compounds not yet occluded was performed with regard to m/z 366, not to miss the “new” steroid. The search was successful, and a series of samples with 17-DSM,23MMAS was detected at the Jurassic/Cretaceous boundary (Fig. 4b). The aliphatic fractions contained homologous series of alkenes-1 and alkenes-3 with the same properties and features as formerly described. They also contained 2,4- and 2,7-dimethylalkanes and alkylcyclohexanes dominated by odd-carbon compounds (Fig. 5). The question arose of at what depths (catagenesis stages) the occlusion of unusual hydrocarbons detected at the levels of proto- and mesocatagenesis was completed. In the Govorovskaya well (Figs. 1, 6), alkenes, dimethylalkanes, and 17-DSM,23MMAS not captured by asphaltenes were detected at the depth 1380 m (stage MC11). Minor amounts of these components occur at depths to 1800 m (MC11). At greater depths, no occluded matter was found in organic matter extracts, whereas asphaltenes are degraded and the specific hydrocarbons emerge again at depths below 5 km under severe P–T conditions (SV-27, AC). Thus, three zones can be outlined: occlusion nascence (PC–MC11), transition (MC11– AC1), and destruction (>AC1). Conclusions The entire range of unusual hydrocarbons detected in the section of superdeep well SV-27 at depths above 5 km and in Paleozoic deposits of the Verkhoyansk fold belt was formed by the degradation of asphaltene macromolecules under severe P–T conditions and by the release of hydrocarbons occluded at early stages of hydrocarbon generation. Judging from the catagenetic zonation of the nascence, transition, and degradation of occluded hydrocarbons, their contribution to the formation of oil fields was small.
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Editorial responsibility: A.E. Kontorovich