On the Geochemistry of Carbon Isotopes in Oils from Fields of Tomsk Oblast
Abstract Generated abstract
This paper examines stable carbon isotope variation in oils from the Sovetskoe oil field in Tomsk Oblast, focusing on productive beds A-I and B-VIII. Using isotopic analyses of oil samples from multiple wells, the authors compare carbon isotope ratios across reservoir positions and find that the observed spatial pattern does not follow the expected enrichment in heavier carbon near the oil water contact. Instead, both beds show a regular lightening of carbon isotopic composition from northeast to southwest, which the authors interpret as evidence of lateral hydrocarbon migration and isotope separation during movement through porous strata. They suggest that the Sovetskoe traps were filled from the Koltogor trough and propose that carbon isotope data may help infer hydrocarbon pool zonality in adjacent structurally favorable areas, although this remains conjectural pending further analyses and drilling.
Full Text
On the Geochemistry of Carbon Isotopes in Oils from Fields of the Tomsk Region
V. L. Kokunov, N. K. Grigor'ev, V. M. Efremov,
A. E. Kontorovich
(Presented by the scientific seminar of the Geological Exploration Faculty)
The first results of determining the isotopic composition of carbon in oils, condensates, and gases from a number of fields in the Tomsk Region enabled us [2] to draw a conclusion regarding the organic origin of the carbon of the hydrocarbons studied and to note a tendency toward a lighter isotopic composition of oil carbon with depth. An analogous correlation between the value of the ratio \(C^{12}/C^{13}\) and the thickness of the overlying deposits was also obtained for the kerogen of sapropelic organic matter of the Upper Jurassic bituminous sequence, as we have already noted [2]. Apparently, the pattern obtained can be explained by partial fractionation of carbon isotopes in the course of their vertical migration; this is especially clearly seen in the example
Table
Isotopic composition of the carbon of oil from beds A-1 of the Sovetskoe field
| Location of wells | No. of wells | \(C^{13}\) PDB | \(C^{12}/C^{13}\) |
|---|---|---|---|
| In the water-floating part of the pool | 38 | −3.45 | 92.07 |
| In the water-floating part of the pool | 22 | −3.49 | 92.11 |
| In the crest, fully saturated part of the bed | 29 | −2.93 | 91.49 |
| In the crest, fully saturated part of the bed | 36 | −3.18 | 91.72 |
| In the crest, fully saturated part of the bed | 17 | −3.04 | 91.60 |
| In the crest, fully saturated part of the bed | 21 | −3.17 | 91.71 |
| Directly at the OWC | 51 | −3.39 | 92.01 |
| Directly at the OWC | 39 | −3.44 | 92.06 |
| Directly at the OWC | 42 | −3.34 | 91.97 |
of the Myldzhinskoe gas-condensate field, where the concentration of the light isotope \(C^{12}\) in condensates increases from the more ancient productive horizons to the younger ones.
Further accumulation of the results of isotopic analyses of the carbon of oils made it possible to discern traces of lateral migration of hydrocarbons.
In studying the isotopic composition of carbon in oils from the main productive beds of the Sovetskoe oil field, A-I and B-VIII, it was found that the concentration of the heavy isotope \(C^{13}\) in the pool of bed A-I varies from well to well (Table 1). If one takes into account the usual general pattern of changes in the physical properties of oils in pools, which consists in the fact that in the marginal parts of pools, near the OWC,
Fig. 1. Distribution of stable carbon isotopes in oils of beds A-I and B-VIII of the Sovetskoe oil field.
Legend: 1 — outer oil-bearing contour of bed A-I; 2 — isolines of equal ratio of stable carbon isotopes \(C^{12}/C^{13}\) in bed A-I; 3 — isolines of equal ratio of stable carbon isotopes \(C^{12}/C^{13}\) in bed B-VIII; 4 — inner oil-bearing contour; 5 — wells from which samples were taken for isotopic analysis, and their numbers; 6 — direction of lateral migration.
the density of the oil, its viscosity, etc. increase [1], one would also expect the isotopic composition of carbon to become heavier near the OWC. However, the distribution of stable carbon isotopes in the oil of bed A-I does not obey the above-mentioned general regularity. As is shown in Fig. 1, a lightening of the isotopic composition of carbon is observed from the northeast to the southwest.
A completely analogous picture is also obtained for bed B-VIII, in which the ratio of carbon isotopes in the oil also becomes regularly lighter in the direction from northeast to southwest.
Quite naturally, the question arises as to the causes of such a phenomenon. Most likely, the cause is the process of lateral migration of hydrocarbons. Any porous bed through which trans-
the motion of a complex gas—oil—water system can be likened to a chromatographic column in which, according to the theory of nonstationary heat- and mass-transfer processes [3], isotope separation must necessarily take place.
The quite sufficient precision (±0.05%) makes it possible to hope that the change in the isotopic composition of carbon in the oils of the main beds of the Sovetskoe oil field is free from the influence of measurement error.
Thus, on the basis of the results of studying the isotopic composition of the oils, it proved possible to determine the direction in which the trap of the Sovetskoe field was filled. According to the data we obtained, the traps of the Sovetskoe field were filled from the direction of the Koltogor trough, which is also consistent with the general views of very many authors on the role of the Koltogor trough in the formation of the Sovetskoe oil field.
A second conclusion from the results of studying the carbon isotopic composition of the oils of the Sovetskoe field is the possibility of drawing conclusions about the character of possible pools in areas adjacent to the Sovetskoe field. This, of course, concerns areas adjoining the field that have suitable structural conditions. This consideration is based on the fact that, in the process of lateral migration, the most migration-capable components will be those that possess a lighter isotopic composition, i.e., gases and light oil fractions. Thus, in the direction of migration from depressions toward uplifts, there should first be small “residual” oil pools, then the main oil pools, further on—oil-and-gas pools, and, finally, purely gas pools.
The indicated zonality, which can be recorded by isotopic-analysis data, is for the time being conjectural and must be confirmed by additional analyses and, possibly, by drilling.
LITERATURE
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Sh. K. Gimatudinov. Physics of the Oil Reservoir. Gostoptekhizdat, 1963.
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N. K. Grigor’ev, V. L. Kokunov, A. E. Kontorovich. First information on the isotopic composition of carbon in oils, gases, and dispersed organic matter of the West Siberian Lowland. In: Geology and Oil-and-Gas Potential of the Southeast of the West Siberian Plate. Proceedings of SNIIGGiMS, issue 69, 1967.
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Investigation of a Nonstationary Process of Heat and Mass Transfer. Minsk, “Nauka i tekhnika,” 1966.