Some Results of Determining the Isotopic Composition of Carbon in Oils, Gases, Condensates, and Dispersed Organic Matter of Western Siberia
Abstract Generated abstract
This paper reports preliminary determinations of carbon isotope composition in oils, condensates, hydrocarbon gases, and kerogen from the West Siberian Lowland, with examples mainly from fields in Tomsk Oblast. Using MI-1305 mass spectrometers, the authors compare C12/C13 ratios across productive horizons and dispersed sapropelic organic matter, noting variations with stratigraphic position, burial depth, and hydrocarbon type. The results indicate enrichment of West Siberian oils, condensates, and gases in the light carbon isotope, supporting a biogenic interpretation, while observed differences among oils and condensates are tentatively attributed to isotope fractionation during migration and to bacterial oxidation. These interpretations are presented as preliminary and requiring confirmation from a larger data set.
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SOME RESULTS OF DETERMINING THE ISOTOPIC COMPOSITION OF CARBON IN OILS, GASES, CONDENSATES, AND DISPERSED ORGANIC MATTER OF WESTERN SIBERIA
N. K. GRIGOR’EV, V. M. EFREMOV, V. L. KOKUNOV, A. E. KONTOROVICH,
V. Kh. KUMYKOV
(Presented by Professor A. V. Aksarin)
In the deposits of the Meso-Cenozoic cover and the basement of the West Siberian Lowland, carbon has been recorded in the following forms.
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Free carbon in the form of graphite, contained in clastic rocks of Jurassic and Cretaceous formations as fine inclusions and in a dispersed state. The presence of graphite was noted by a number of authors as early as 1957 (F. S. Buzulukov et al., 1957), and recently has been confirmed by X-ray structural analysis by P. A. Trushkov, a staff member of SNIIGGiMS (oral communication). This fact is of considerable interest and deserves close attention.
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Carbon in the form of dioxide, constantly recorded in dissolved state in formation waters, as well as carbon dioxide gas included in natural gases, sometimes in significant quantities. Thus, at the Mezhovskoye and Veselovskoye fields, individual wells yield inflows of carbon dioxide gas from fractured basement granites equal, respectively, to 1.5–2 thousand m³/day and even 200–250 thousand m³/day.
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Carbon bound in carbonates. This form of occurrence of carbon should be subdivided into the following varieties:
a) Carbon in the carbonate cement of clastic rocks. Here, as indicated (F. A. Alekseev et al., 1967), at least two generations of calcites and siderites are possible.
b) Carbon of calcite and siderite in carbonate interbeds.
c) Carbon included in calcium and magnesium bicarbonate dissolved in formation waters.
- Carbon of natural hydrocarbons of the petroleum series, contained in the Meso-Cenozoic section of the West Siberian Lowland. Here carbon is contained:
a) in oils, condensates, natural and associated gases;
b) in bitumens;
c) in dispersed organic matter.
- Carbon of hydrocarbons of the coal series, contained in the Meso-Cenozoic section of the lowland, included in:
a) coals;
b) dispersed organic matter of the sapropelic series;
c) detritus and dispersed organic matter of the humic series.
- Carbon included in compounds of soil and bog formations.
The ultimate task of the present investigation is a comparative study of the isotopic composition of oils, condensates, gases, and dispersed organic matter of the West Siberian Lowland.
In carrying out such a study, however, a number of difficulties arise. The ratio of carbon isotopes \(C^{12}/C^{13}\) in organic matter, other conditions being equal, is smaller the greater the molecular weight of the compounds studied. Therefore a simple comparison of the isotopic composition of carbon in bitumoids and oils is methodologically incorrect: in the overwhelming majority of cases bitumoids are richer than oils in high-molecular asphalt-resin compounds, condensed aromatic hydrocarbons, and high-boiling fractions, which in itself already accounts for the predominance in them of the isotope \(C^{13}\) as compared with oils.
In this connection, our study provides for:
a) study of the isotopic composition of unfractionated samples of oils, condensates, and gases;
b) comparative study of the isotopic composition of asphaltenes, resins, normal, isoaliphatic, naphthenic, and naphthene-aromatic hydrocarbons of bitumoids and oils. Comparison of the isotopic composition of hydrocarbon compounds of oils and bitumoids is, naturally, carried out on fractions boiling within the same temperature intervals;
c) comparative study of the isotopic composition of carbon in the kerogen of dispersed organic matter of various classes and in coals.
The study of oil and bitumoid fractions of the same type selected in this way will make it possible to reveal the influence on the isotopic composition of dispersed organic matter and oils of similar composition of the processes of primary and secondary migration, of the properties (if they exist) of the isotopic composition of hydrocarbons of the dispersed organic matter of source strata and daughter oils, and of the dependence of the isotopic composition of oils and dispersed organic matter on the geological conditions of occurrence (temperature, depth, etc.).
At present, implementation of the planned program is only beginning. The isotopic composition of oils, condensates, and gases has been studied using a number of fields in Tomsk Oblast as examples, as has the isotopic composition of kerogen using sapropelic organic matter of the Maryanovskaya Formation and its analogues as examples.
The isotopic ratios were measured on MI-1305 mass spectrometers by both single-beam and double-beam methods (Tables 1–5). The arithmetic mean error of a single determination by the single-beam method was \(\pm 0.4\%\) (Table 14). Each sample was analyzed from 2 to 4 times.
The results of the study of the isotopic composition of carbon in oils, condensates, gases, and dispersed organic matter of the West Siberian Lowland are summarized in Tables Nos. 1, 2, 3, 4, 5, 6, 7. Let us consider them.
Oils. The isotopic composition of carbon in oils was studied using the Sovetskoye, Malorechenskoye, and Sredne-Vasyuganskoye fields as examples.
In bed \(A_1\) of the Sovetskoye field, the ratio of carbon isotopes \(C^{12}/C^{13}\) varies (6 samples analyzed) from 91.32 to 91.93, with an average of 91.62. In bed \(A_{10}\) of the same field, the concentration of the light carbon isotope in the oil is still greater; the ratio \(C^{12}/C^{13}\) in it is 92.67. The “lightest” is the oil from bed \(B_8\) of the same field. The ratio of carbon isotopes in it (2 samples) is 93.84.
The Upper Jurassic oils of bed \(Yu_1\) were studied using the Malorechenskoye and Sredne-Vasyuganskoye fields as examples. In terms of the ratio of carbon isotopes they are very close to one another. The ratio \(C^{12}/C^{13}\) in them is
92.59 and 92.50, respectively. In comparison with the oil of bed \(A_{10}\) and especially \(B_8\), the Upper Jurassic oils are richer in the isotope \(C^{13}\). Only in bed \(A_1\) does the oil contain more of the heavy carbon isotope \(C^{13}\).
The oils of the Tyumen suite were studied using the example of a paraffinic methane oil from bed M of the Sovetskoe field (Medvedevo uplift).
Table 1
Isotopic composition of carbon in oils and gases, determined by the precision method
| Name of sample, field, and well nos. | \(\delta C^{13}\) relative to the standard \((\mathrm{CaCO}_3)\) | \(C^{12}/C^{13}\) relative to the standard | \(\delta C^{13}\) relative to the standard (oil 38) | \(C^{12}/C^{13}\) relative to the standard |
|---|---|---|---|---|
| Oil | ||||
| Sovetskaya area, well 38/1675—1680 | —2,416 | 91,86 | — | — |
| Sovetskaya area, well 38/1675—1680 | —2,457 | 91,90 | — | — |
| Sovetskaya area, well 38/1675—1680 | —2,551 | 91,98 | — | — |
| Malorechenskaya area, well 117 (2458—2474) | —2,8668 | 92,28 | —0,1904 | 92,07 |
| Okha area, well no. 800 (Sakhalin) (32—68) | —2,131 | 91,60 | +0,5171 | 91,89 |
| Mukhto area, bed III (Sakhalin) | —2,089 | 91,55 | +0,5347 | 91,55 |
| Sredne-Vasyuganskaya area, well no. 5 (2308—2322) | —2,830 | 92,29 | —0,1013 | 91,98 |
| Sosninskaya area, well no. 39 (1679—1690) | — | — | —0,5975 | 92,43 |
| Coal tar | —2,959 | 91,97 | +0,1774 | 91,72 |
| Associated petroleum gases | ||||
| Sosninskaya area, well no. 29 (1665—1673) | —3,1504 | 93,04 | —1,2441 | 93,04 |
| Sosninskaya area, well no. 36 (1661—1689) | —3,9483 | 93,36 | —1,477 | 93,26 |
| Sovetskaya area, well no. 29 (1667—1676) | — | — | —1,298 | 93,08 |
| Sovetskaya area, well no. 39 (1678—1690) | — | — | —1,619 | 93,38 |
| Chebachya area, well 117 (1932—1970) | —2,554 | 92,07 | —0,2929 | 92,15 |
uplift). The ratio of carbon isotopes \(C^{12}/C^{13}\) in it is 93.23. This value is somewhat lower than in the oil of bed \(B_8\), but higher than in the oils of all the other productive horizons studied.
A comparison of the isotopic composition of carbon in West Siberian oils and in oils from other regions of the USSR and the USA (see Petrenko, 1962) shows the closeness, with respect to this parameter, of oils from different oil-and-gas-bearing basins of the globe. Thus, in oils of the USA the \(C^{12}/C^{13}\) ratio ranges from 90.98 to 94.16, and in oils of the Soviet Union from 91.20 to 92.40. We have carried out a series of determinations of the isotopic composition in oils of the Sabo field (Sakhalin). The \(C^{12}/C^{13}\) ratio in them ranges from 91.75 to 92.75.
For the Neocomian oils (beds of groups A and B), there is a tendency toward an increase in the concentration of the light isotope \(C^{12}\) downward through the section.
Condensates and gases. The isotopic composition of carbon in condensates was studied using the example of the Myldzhinskoye, Severo-Vasyuganskoye, and Ust-Silginskoye fields.
Table 2
Isotopic composition of carbon in oils of fields of Tomsk Oblast
| Name of field (area, well No.) | No. of analyses | Interval, m | $C^{13}$, % meas. | $C^{12}/C^{13}$ |
|---|---|---|---|---|
| Sosninskaya area, well 29 | 2 | 1665—1673 | 1,176 | 91,32 |
| Sosninskaya area, well 36 | 3 | 1661—1689 | 1,171 | 91,72 |
| Sovetskaya area, well R-21 | 4 | 1667—1694 | 1,174 | 91,46 |
| Sovetskaya area, well 38 | 8 | 1675—1690 | 1,173 | 91,56 |
| Sovetskaya area, well 17 | 3 | 1686—1690 | 1,170 | 91,82 |
| Sosninskaya area, well 51 | 3 | 1661—1689 | 1,164 | 92,32 |
| Sosninskaya area, well 35 | 2 | 1937—2004 | 1,160 | 92,67 |
| Sosninskaya area, well 1—R | 4 | 2130—2140 | 1,147 | 93,80 |
| Sosninskaya area, well 4 | 7 | 2147—2155 | 1,146 | 93,88 |
| Sredne-Vasyuganskaya, well 5 | 3 | 2308—2322 | 1,162 | 92,53 |
| Malorechenskaya area, well 117 | 6 | 2458—2474 | 1,161 | 92,59 |
| Medvedevskaya area, well R—7 | 2 | 2528—2537 | 1,154 | 93,18 |
| Sovetskaya area, well 58 | 2 | 1,154 | 93,18 | |
| Sosninskaya area No. R—19 | 1 | 2147—2153 | 1,168 | 91,93 |
| Sovetskaya, well 27 | 7 | 2141—2153 | 1,168 | 91,99 |
Table 3
Isotopic composition of kerogen of dispersed organic matter
| Name of area and well No. | No. of analyses | Interval, m | $C^{13}$, % meas. | $C^{12}/C^{13}$ |
|---|---|---|---|---|
| Ermakovskaya, 1—R sample No. 1 | 3 | 1190—1141 | 1,174 | 91,48 |
| Pokrovskaya, No. 16 | 2 | 1593—1610 | 1,172 | 91,65 |
| Vikulovskaya | 1 | 1919—1925 | 1,183 | 90,73 |
| Barabinskaya, No. 1—R | 2 | 2074—2080 | 1,167 | 92,07 |
| Omskaya, well 1—R, sample 141 | 2 | 2329,7 | 1,165 | 92,24 |
| Sargat, well 4—R, No. 13 | 2 | 2367—2403 | 1,157 | 92,84 |
| Sargat, well No. 6, sample 240 | 2 | 2425—2431 | 1,176 | 91,31 |
In all productive horizons of the Myldzhinskoye gas-condensate field, the ratio of the concentrations of carbon isotopes $C^{12}/C^{13}$ is very close and ranges from 93,88 to 94,77. However, a clearly marked tendency is observed for this ratio to increase from bed Ю₁ to bed Б₈. It is equal to 93,88; 94,42 and 94,77 in beds Ю₁, Б₂₃ (Achimov unit), and Б₈, respectively. It is interesting to note that, in this same direction, the content of condensate in the gas decreases.
The isotopic composition of condensates in bed Yu\(_1\) of the North Vasyugan and Ust-Silga pools differs from that at the Myldzhin field by a higher concentration of the isotope C\(^ {13}\). The C\(^ {12}\)/C\(^ {13}\) ratio in the condensates of these pools is, respectively, 92.84 (average of two samples) and 92.24.
Table 4
Isotopic composition of condensates
| Name of fields (area, well Nos.) | No. of analyses | Interval, m | C\(^ {13}\), % meas. | C\(^ {12}\)/C\(^ {13}\) |
|---|---|---|---|---|
| North Vasyugan, well No. 1 | 2 | 2402—2412 | 1.156 | 93.01 |
| North Vasyugan, well No. 6 | 3 | 2313—2329 | 1.162 | 92.50 |
| Ust-Silga, well No. 4 | 8 | 2333—2320 | 1.161 | 92.58 |
| Myldzhin area, well 1 | 6 | 2088—2093 | 1.163 | 92.41 |
| Myldzhin area, well 5 | 6 | 2400—2369 | 1.155 | 93.10 |
| Myldzhin area, well 4 | 3 | 2261—2274 | 1.140 | 94.59 |
| Myldzhin area, well 3, sample 2 | 2 | 2174—2146 | 1.136 | 94.77 |
| Myldzhin area, well 3, sample 1 | 2 | 2396—2355 | 1.129 | 95.41 |
Table 5
Isotopic composition of carbon in gases from fields of Tomsk Oblast
| Name of fields (areas, well Nos.) | No. of analyses | Interval, m | C\(^ {13}\), % meas. | C\(^ {12}\)/C\(^ {13}\) |
|---|---|---|---|---|
| Leyushinskaya reference well | 4 | 1.140 | 94.42 | |
| Myldzhin area, well 3, sample 1 | 2 | 2355—3396 | 1.142 | 94.24 |
| Myldzhin area, well 3, sample 2 | 2 | 2164—2174 | 1.143 | 94.15 |
| Severnaya area, well 262 | 2 | 1708—1715 | 1.148 | 93.71 |
| Chebachya area, well 217 | 2 | 1932—1971 | 1.150 | 93.53 |
The isotopic composition of hydrocarbon gases has been studied mainly using the Myldzhin field as an example. The C\(^ {12}\)/C\(^ {13}\) ratio in gases of beds Yu\(_1\) and B\(_8\) is, respectively, 94.24 and 94.15. These gases are richer in the light carbon isotope than the gases of the Anastasyevo-Troitskoye and Chelbasskoye fields in the Fore-Caucasus and of the Korobkovskoye field in the Lower Volga region (Vdovykin et al., 1964). I. G. Petrenko (1962) cites data of E. T. Allen and A. L. Day on the isotopic composition of carbon in methane from hot springs and geysers of Yellowstone National Park. The average C\(^ {12}\)/C\(^ {13}\) ratio in methane from these springs is 91.05; i.e., methane from hot springs is sharply enriched in the heavy carbon isotope C\(^ {13}\) in comparison with gases from fields of the West Siberian Lowland, the Fore-Caucasus, and the Lower Volga region.
Kerogen of dispersed organic matter. The isotopic composition of carbon in the kerogen of dispersed organic matter was studied using as an example organic matter of the sapropelite class from Upper Jurassic bituminous argillites. According to the data obtained, the ratio of carbon isotope concentrations \(C^{12}/C^{13}\) in the kerogen of sapropelic organic matter ranges from 91.48 to 92.84, with an average of 92.16. In comparison with coals (Petrenko, 1962), sapropelic organic matter is richer in the light isotope \(C^{12}\). The \(C^{12}/C^{13}\) ratio in coals does not exceed 91.39. It is interesting that in bituminous coals and shales the value of this ratio is higher (91.15—92.00 in bituminous coals and 90.52—92.70 in shales) and is close to the values characteristic of the organic matter of the Upper Jurassic bituminous sequence of the West Siberian Lowland.
There is no consensus on the question of how the \(C^{12}/C^{13}\) ratio changes during metamorphism of organic matter. F. E. Wickman (1956) found no dependence of the isotopic composition of coals on their metamorphism. K. Rankama (1956) believed that, with increasing metamorphism of organic matter, the concentration of the light isotope in it should decrease.
From the materials obtained on the isotopic composition of carbon in the kerogen of sapropelic organic matter it is evident that there is a correlation between the value of the \(C^{12}/C^{13}\) ratio and the thickness of the overlying deposits. The deeper the sedimentary sequence is buried and, consequently, the higher the metamorphism of the dispersed organic matter, the greater the \(C^{12}/C^{13}\) ratio. The emerging regularity is opposite to that proposed by K. Rankama.
The enrichment of the oils, condensates, and hydrocarbon gases of the West Siberian Lowland in the light carbon isotope \(C^{12}\), characteristic of organic matter, is a weighty argument in favor of their biogenic origin. In this respect, the example cited above for methane is characteristic: for methane of hot springs and geysers the \(C^{12}/C^{13}\) ratio is 91.05, for swamp-gas methane 94.40. In the gases of the West Siberian Lowland this ratio is 94.20, i.e., very close to that in methane of undoubtedly biogenic origin. It is considerably more difficult to resolve the question of the causes of variation in the carbon isotope ratio in different oils. The changes in the isotopic composition of carbon in Upper Jurassic and Neocomian oils cannot be explained by differences in the composition of the initial organic matter. Indeed, the sandstones of beds \(Ю_1\) and \(Б_8\) accumulated in a marine basin, and the organic matter dispersed in them and in the underlying and overlying argillite sequences has a predominantly sapropelic composition. The Hauterivian–Barremian sequence, including beds \(А_1\) and \(А_{10}\), accumulated under nearshore-marine and continental conditions. The organic matter dispersed in it is enriched in humic material and belongs to the classes of sapropelite-humites and humites. Terrestrial plants are richer in the light carbon isotope than marine plants. In this connection one would expect that the ratio of isotope concentrations \(C^{12}/C^{13}\) should be higher in beds \(А_1\) and \(А_{10}\). In reality, such a relationship does not occur.
If one assumes, as V. V. Ivantsova, A. E. Kontorovich, G. P. Sverchkov, and some other researchers believe, that the oils of the Upper Jurassic and Neocomian owe their origin mainly to the organic matter of the Upper Jurassic and Valanginian, then the following hypothesis suggests itself. As is known, partial fractionation of isotopes may occur during the migration of hydrocarbons. During the migration of hydrocarbons into bed \(Ю_1\), which directly underlies the Upper Jurassic bituminous argillites, such fractionation
undoubtedly had less influence on the isotopic composition of the oils than during their migration into bed \(B_8\) and the overlying beds through a thick sequence of argillites. This explains the enrichment of the oil of bed \(B_8\) in the isotope \(C^{12}\) in comparison with the oil from bed \(Ю_1\).
Subsequently, the oils of the Neocomian, especially those of its upper horizons, underwent bacterial cryptohypergenic oxidation (Gurari, 1962; Kontorovich, Stasova, 1964; Rylkov, 1967). In this process, bacteria assimilated predominantly the light isotope of carbon (Vinogradov, 1954), which led to the relative enrichment in the isotope \(C^{13}\) of the oils occurring in the beds of group A.
The fractionation of carbon isotopes in the process of vertical migration of hydrocarbons can probably explain the increase in the concentrations of the light isotope \(C^{12}\) in the condensates of the Myldzhino field from the older productive horizons to the younger ones. Such an assumption is in good agreement with the available geological materials (Gurari et al., 1965).
The assumptions put forward do not contradict modern concepts of the mechanism of isotope fractionation and agree well with the available geological and geochemical materials; however, they require confirmation on a larger body of factual material.
LITERATURE
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