RESULTS OF STUDYING THE CARBON ISOTOPIC COMPOSITION OF ROCKS AND OIL FROM THE MEGION FIELD

V. L. Kokunov, N. K. Grigor’ev, R. S. Sakhibgareev,
V. A. Marchenko, N. A. Brylina, N. A. Arkhipova

(Presented by Prof. A. V. Aksarin)

In studying the isotopic composition of any element of the Earth’s crust, knowledge of the forms in which it occurs there acquires considerable importance, since this makes it possible to judge the nature of the isotopic-exchange reactions in which the given element took part and thereby to understand its evolution in the course of the history of geological development.

When summarizing the first results of studying the carbon isotopic composition of natural objects of Western Siberia [5], we indicated that in the deposits of the Meso-Cenozoic cover and the basement of the West Siberian Lowland, carbon has been recorded in the following forms:

1. Free carbon in the form of graphite, contained in clastic rocks of Jurassic and Cretaceous formations as small inclusions and in a dispersed state. The presence of graphite was noted by a number of authors as early as 1957 [1], and recently it has been confirmed by X-ray structural analysis by P. A. Trushkov, a staff member of SNIIGGiMS [8]. This fact is very interesting and deserves close attention.

2. Carbon in the form of dioxide, constantly recorded in dissolved form in formation waters, as well as carbon dioxide gas, which is part of natural gases, sometimes in significant quantities. Thus, at the Mezhovsk and Veselovsk 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 [9].

3. Carbon contained in carbonates. This form of occurrence of carbon must be subdivided into the following varieties:
   a) carbon in the carbonate cement of clastic rocks; here at least two generations of calcites and siderites are possible [1]; b) carbon of calcite and siderite in carbonate interbeds; c) carbon entering into the composition of calcium and magnesium bicarbonate dissolved in formation waters.

4. Carbon of natural hydrocarbons of the petroleum series contained in the section of the Mesozoic and Paleozoic of the West Siberian Lowland. Here carbon is contained:

a) in oils and condensates (liquid hydrocarbons);
b) in hydrocarbon gases of oil, oil-gas, oil-gas-condensate, gas-condensate, and gas fields, as well as in hydrocarbon gases dissolved in formation waters (gaseous hydrocarbons);

c) in free and dispersed bitumens A;
d) in bound bitumens C.

1. Carbon of hydrocarbons of the coal series contained in the section of the Meso-Cenozoic and Paleozoic lowland, comprising: a) coals; b) dispersed organic matter of the sapropelic series; c) detritus and dispersed organic matter of the humic series; d) mixed sapropelic-humic organic matter.

2. Carbon entering into compounds of soil and bog formations.

By prior agreement, our field of attention included only the oil of beds Yu-1 and B-VIII, as well as limestone sandstone reservoirs from beds Yu-1, B-VIII, and A-1 of the Megion field, i.e., objects of the third and fourth groups of the above classification.

The results of studying the isotopic composition of carbon in the oil of the Megion field are summarized in Table 1. Let us consider them.

Table 1

Isotopic composition of carbon in the oil of the Megion field

In bed Yu-1 of the Megion field, the carbon isotope ratio $C^{12}/C^{13}$ is 91.71. If the isotopic composition of the carbon of oil from bed Yu-1 of the Megion field is compared with the isotopic composition of the carbon of oils from bed Yu-1 of some other field, it turns out that in the oil of bed Yu-1 of the Malo-Rechenskoye field, also located on the Nizhnevartovsk arch, the carbon isotope ratio $C^{12}/C^{13}$ is 92.13 (our determinations, 1969), i.e., it differs only slightly.

The oils of bed B-VIII of the Megion field differ little in isotopic composition from the oil of bed Yu-1; in any case, the value of the carbon isotope ratio $C^{12}/C^{13}$ for the oils of bed B-VIII ranges within 91.58—91.89, with an average of 91.70. This is very alarming, since the isotopic composition of the carbon of oils studied by us from the Sovetskoye oil field from analogous beds behaves somewhat differently. Thus, the average value of the carbon isotope ratio of oil in bed B-VIII of the Sovetskoye field is 92.13, i.e., the oil here is “lighter” in isotopic composition than in the bed

B-VIII of the Megion field. But most importantly, at the Sovetskoe field, with increasing depth of occurrence of the deposits, the isotopic composition of the carbon in the oils gradually becomes lighter; this does not contradict the general character of the change in the carbon isotopic composition of oils in the overwhelming majority of the studied fields of the Soviet Union and foreign fields. The equality of the \(C^{12}/C^{13}\) ratios in the Yu-1 and B-VIII formations of the Megion field indicates a connection between the formations, apparently due to the presence of disjunctive disturbances, as we noted for the Myldzhinskoe gas-condensate field of Tomsk Oblast [5]. The foregoing is illustrated more clearly by Table 2.

Table 2

With an analytical precision of \(\pm 0.1\), such a distribution of the isotopic composition of carbon indicates a connection between formations B-VIII and Yu-1 at the Megion and Myldzhinskoe fields. This assumption agrees well with the available geological materials [4].

Analyzing the results of studying the isotopic composition of the carbon of oil in formation B-VIII of the Megion field, we have already noted that the difference in the value of the \(C^{12}/C^{13}\) ratio in different wells is 0.41, or in \(\delta C^{13}\%\) is \(-2.94 - (-3.20) = 0.34\%\). An analogous difference in the content of the heavy isotope within one and the same formation at the Quiriquire field in Venezuela led S. Silverman [11] to conclude that the deposit in the Pliocene sediments at this field owes its origin to the migration of oil from the underlying Miocene sediments and the subsequent upward movement of the oil along an uplift in reservoirs of Pliocene age.

For a visual assessment of the character of the change in the isotopic composition of carbon in a deposit of any field, we [5] proposed constructing maps of isoconcentrates of the isotopic composition of oil carbon. The first isoconcentrate maps were constructed by us for deposits in formations A-1 and B-VIII of the Sovetskoe oil field. It turned out that the concentration of the heavy isotope \(C^{13}\) in the deposits of the Sovetskoe field varies in a very peculiar way, not following the general character of the change in other physical properties of the oil. If one takes into account the usual general picture of changes in the physical properties of oils in deposits, namely that in the marginal parts of deposits near the OWC the oil density, its viscosity, etc. increase [3], then one would also expect a weighting near the OWC of the isotopic composition of the oil carbon. But the distribution of stable carbon isotopes in the oil of formation A-1 of the Sovetskoe field does not follow the above-mentioned general pattern. The lines of isoconcentrates of the carbon isotopic composition intersect the OWC; moreover, a lightening of the carbon isotopic composition is observed from the northeast to the southwest.

An entirely analogous picture is also obtained for formation B-VIII of the Sovetskoe field, in which the ratio of carbon isotopes in the oil also becomes regularly lighter in the direction from the northeast to the southwest.

A schematic map of the variation in the isotopic composition of oil carbon in horizon B-VIII was also constructed by us for the Megion field. Here, too, the concentration of the heavy isotope gradually decreases in the direction from northeast to southwest (Fig. 1). Two questions quite naturally arise: what is the reason for this phenomenon, and why at

Fig. 1. Schematic map of the variation in the isotopic composition of oil carbon in horizon B-VIII of the Megion field: 1 — outer contour; 2 — inner contour; 3 — numerator — well number, denominator — value of the heavy carbon-isotope content relative to the standard (PDB), in %; 4 — lines of equal concentrations of the isotopic composition of oil carbon

the Sosninsko-Sovetskoe field the concentration of the isotope C¹³ changes smoothly, whereas at the Megion field it changes abruptly. Most likely, the reason for the change in the concentration of the isotope C¹³ in the reservoir is the process of lateral migration of hydrocarbons. Any porous bed through which a complex gas—oil—water system moves may be likened to a chromatographic column, in which, according to the currently prevailing theory of nonstationary processes of heat and mass transfer, isotope separation must necessarily occur. In this case the heavy isotopes lag behind the more migration-capable light carbon isotopes.

Thus, from the change in the isotopic composition of oil carbon it proved possible to determine the direction in which the traps of the Sosninsko-Sovetskoe and Megion oil fields were filled. In the present case, as the constructed schematic map shows, filling of the trap of the Megion field took place from the northeast.

As for the nature of the change in the concentration of the heavy isotope C¹³ in the area of wells 512 and 510, where the concentration of C¹³ proved to be reduced, the possibility is not excluded that this zone of bed BVIII is connected with a deeper oil-bearing horizon (bed Yu₁), in which the oil is enriched in the light isotope C¹². The characteristic bend of the isoconcentrates indicates the linear nature of this zone, which may be caused by a disjunctive disturbance connecting the above-mentioned beds.

It is known that one of the principal methods for establishing faults when there is a significant displacement amplitude of a given block consists of geological constructions (maps, profiles, sections). Meanwhile, in buried deposits, tectonic disturbances are often recorded with a small displacement amplitude or are manifested only as zones of fracturing. In this case, all geological constructions for identifying them essentially prove powerless or, at any rate, unconvincing. Contradictory judgments therefore often arise as to the presence or absence of faults. In particular, in productive bed B-VIII of the Megion field, V. P. Markevich [6] distinguished three faults with a small displacement amplitude, whereas other investigators deny their existence. Therefore the data obtained at “Giprotyumenneftegaz” by R. S. Sakhibgareev on the quantitative distribution of authigenic feldspars within this pool are of definite theoretical and methodological interest. As has already been described, the regeneration cement is represented by quartz and feldspars (albite). The latter, as a rule, forms in pore spaces as discontinuous regeneration rims. The thickness of the newly formed discontinuous albite rims, the number of regenerated plagioclase fragments, and at the same time the total content of feldspathic (albitic) cement in oil-saturated sandstones located near the oil-bearing contour are considerably smaller than in water-saturated rocks of the same type. Thus, in oil-saturated sandstones of near-contour areas the amount of feldspathic cement does not exceed 0.16% (wells 547, 548). In similar water-saturated sandstones (similar values of granulometric coefficients, amount and quality of clay cement), the role of feldspathic neomineral formations increases sharply, and their amount often reaches 2 percent or more (wells 525, 25, 540). It follows from this that with the entry of oil into the beds, the formation of authigenic feldspars, as well as of a number of other minerals [7], ceased or, at any rate, slowed sharply. In this connection it would be logical to assume that, in the crest and near-crest parts of the pool described, the difference in the degree of authigenic mineral formation should have manifested itself even more sharply [7], since oil first fills the more uplifted parts of the bed. In reality, however, this is not observed. Moreover, within the oil-saturated part of the bed, three zones with an increased amount of feldspathic cement are clearly distinguished; in essence, these are not controlled by the structure (Fig. 2).

However, even in these zones there is less authigenic feldspar than in rocks of the same type saturated with water.

The position and direction of these zones, with the exception of the upper one, generally correspond to the faults identified from geological constructions by V. P. Markevich (Fig. 3). He allows, however, that with the appearance of additional factual material the position of these faults will be refined.

Such a connection of the zones described with faults is probably explained by the influx along them of waters of greater mineralization. In...

Fig. 2. Map of the distribution of feldspar cement in bed B-VIII of the Megion field: 1 — well; in the numerator on the right is its number, in the denominator is the amount of feldspar cement; 2, 3 — inner and outer oil-bearing contours; 4 — zones of anomalous values; 5 — faults according to V. P. Markevich.

Fig. 3. Dependence of the concentration of \(C^{13}\) in the cement of sandstones on the value of the \(C_{\mathrm{org}}\) content.

At the same time, it is not excluded that oil also entered from the Jurassic deposits along these weakened zones. This is confirmed by the fact that in zones of increased feldspar cement content the concentration of the heavy isotope \(C^{13}\) decreases (Fig. 2).

The results of the study of the isotopic composition of carbon contained in the rocks of the productive sequence of the Megion field are given in Table 3. Rocks of beds Ю₁ (5 samples), Б₈ₙ (11 samples), and А₁—₂ (7 samples) were studied. In lithological composition these are calcareous sandstones, aleuritic clays, and one sample of siderite. The isotopic composition of carbon in the aleuritic clays is lighter in comparison with the isotopic composition of carbon from the carbonate cement of the sandstones. Their average values are, in the clays, \((-2.35\%)\), and in the sandstones, \((-1.44\%)\). Only two samples of sandy rocks gave extreme values of the isotopic composition of carbon, significantly heavier than in the majority of samples \((-0.56\%)\).

The aleuritic clays were combusted in the apparatus in which oil is usually prepared for analysis; that is, the samples were calcined in a stream of oxygen at a temperature of \(800—900^\circ C\). As a result of this operation, both organic matter, \(C_nH_{2n+2}+O_2 = mCO_2 + mH_2O\), and all calcareous material of the clays, \(CaCO_3 + O_2 \to CaO + CO_2\), should pass into \(CO_2\). As a result, we should obtain \(CO_2\) with a total isotopic composition, which greatly complicates interpretation of the data.

Table 3

Carbon isotopic composition of rocks from the Megion oil field

In three clay samples the carbonate content was determined with a Clark apparatus. The analysis showed the absence of carbonate material in the clays; thus, the clays gave us the isotopic composition of the organic material contained in them, which proved to be “heavier” than oil, but “lighter” than rocks containing carbonates.

Of greatest interest, in our view, are the results of determining the isotopic composition of carbon in the cement of sandy rocks of the main productive beds of the Megion field Yu₁, B VII, A₁–₂. A comparison of the mean values for the content of the heavy isotope C¹³ shows an unquestionable increase in its concentration with depth, i.e., from A₁–₂ to Yu₁.

It is still too early to draw final conclusions about the distribution of the heavy isotope C¹³ over the area of the field. The point is that the concentration of the isotope C¹³ in sandstone cement depends on a whole series of both natural and technical factors.

Indeed, checking the dependence of the concentration of the heavy isotope C¹³ on carbonate content showed (Fig. 4) that with an increase in the carbonate content of the reservoir, the isotopic composition of the carbon entering the carbonate becomes “heavier.” And although, as is evident from the graph, no direct dependence is observed, nevertheless the question arises that study of the nature of the distribution of concentrations of the heavy isotope C¹³ should be carried out on samples with equal, or at least known, carbonate content.

An attempt to establish a dependence between the concentration of the heavy isotope C¹³ and the content in the sample under study of organic substances that can be extracted from them by hot extraction with organic solvents (benzene and chloroform) showed that, with an increase in the content of C_org. in sandstones, the isotopic composition of carbonate carbon becomes “heavier” (Fig. 4). The isotopic composition of carbon in the carbonate cement of most sandstone samples becomes significantly heavier after extraction (Table 4).

Fig. 4. Dependence of the concentration of the heavy isotope \(C^{13}\) in the cement of sandstones on the carbonate content.

Table 4

Isotopic composition of sandstones with calcite cement

The latter circumstance compelled us once again to reconsider the established procedure for preparing carbonate samples for mass-spectrometric analysis. The point is that any sedimentary rock practically always contains dispersed organic matter. But it had been accepted that, during acid treatment, the organic matter does not take part in the reaction:

\[ 2\mathrm{CaCO}_3 + 4\mathrm{HCl}_2 \longrightarrow 2\mathrm{CaCl}_2 + 2\mathrm{CO}_2 + 2\mathrm{H}_2\mathrm{O}. \]

We collect and analyze the carbon dioxide gas, while the organic matter remains in the calcium chloride solution. The graph we constructed (Fig. 5)

showed that this is not quite so. If the results of determining carbon isotopes in the rock before extraction are plotted on the ordinate axis, and after extraction on the abscissa axis, then in the case of neutral behavior of the organic matter during the reaction of carbonate with acid, the graph should yield a straight line cutting off equal segments on the axes. In reality, as can be seen from the graph, the isotopic composition of carbon before extraction is clearly lighter. This indicates, in our view, that during the etching reaction the extracted bitumens “make heavier” the isotopic composition of the carbon of the rocks through an isotope-exchange reaction. The latter circumstance still requires support by a sufficiently large number of analyses, but this fact must be taken into account when studying the isotopic composition of carbon in the carbonate cement of clastic rocks.

Fig. 5. Comparison of the values of the concentrations of $C^{13}$ in the cement of sandstones before and after extraction.

In view of the above, we interpret the results of isotope analysis of the carbonate cement of sandy rocks from the productive beds of the Megion field only on extracted samples.

The value of the concentration of the heavy isotope $C^{13}$ in beds $A_{1-2}$, $B_V$, and $Yu_1$ averages $-0.4\%$.

The small number of (extracted) samples does not make it possible to interpret the analytical results with confidence; nevertheless, as preliminary conclusions we consider it necessary to state the following.

In bed $B_{VIII}$, the isotopic composition of carbon in the cement of the sandstones becomes “lighter” in the direction from the outer contour of the pool to the inner one. In beds $A_{1-2}$ and $Yu_1$, the isotopic composition of carbon in the cement of the sandstones becomes lighter from northwest to southeast. Apparently, the direction

the lightening of the isotopic composition of carbon in the carbonate cement of the sandstones should be regarded as indicating the direction of the first movement of pore and silt waters after their expulsion into reservoirs. These must be directions toward better reservoir permeability and toward hypsometrically elevated parts of the structure. This is also indicated by the nature of the distribution of reservoir types, carbonate content, the thickness of carbonate interbeds in the strata, etc.

Conclusion

The average isotopic composition of carbon in siderites in Western Siberia, according to Galimov’s data, is \(-0.23\%\), i.e., close to the isotopic composition of marine limestones. According to our data, the isotopic composition of siderite carbon is \(\delta C^{13} = -1.33\%\), which is twice as light as the lightest value reported by Galimov.

The average isotopic composition of the carbonate cement of freshwater sandstones, according to R. Clayton and E. Degens [10], is characterized by \(\delta C^{13} = -0.99\%\).

According to our data, it is \(-1.27\%\) before extraction and \(\delta C^{13} = -1.06\%\) after extraction. Consequently, if one judges by the isotopic composition of the carbon of calcite cement from the sandstones of bed B-VIII of the Megion field, they must be assigned to freshwater sandstones. In our view, no categorical conclusions should be drawn in this case, since during oil migration through the bed, as is attested by the isoconcentration map, isotopic exchange between oil carbon and carbonate carbon is possible in the direction of lightening the isotopic composition of the carbonate carbon.

LITERATURE

1. F. S. Buzulukov, G. I. Gurova et al. Lithology of the Mesozoic and Cenozoic of the West Siberian Lowland. Gostoptekhizdat, 1957.

2. E. M. Galimov. Geochemistry of Stable Carbon Isotopes. “Nedra,” 1968.

3. Sh. K. Gimatudinov. Physics of the Oil Reservoir. Moscow, Gostoptekhizdat, 1963.

4. F. G. Gurari et al. Changes in Composition and Forecast of the Quality of Mesozoic Oils of the West Siberian Lowland. Proceedings of Zap.-Sib. NIGNI, 1965.

5. V. L. Kokunov et al. On the isotopic composition of \(Ca^{48}/Ca^{40}\) in oil-bearing rocks and waters of the West Siberian Lowland. Proceedings of SNIGGIMS, 1969.

6. V. P. Markevich. The History of Geological Development and Oil-and-Gas Potential of the West Siberian Lowland. “Nauka,” 1966.

7. G. E. Prozorovich, Z. L. Valyuzhenich. Regeneration of quartz and pelitization of feldspars in oil-bearing and water-bearing sandstones of the Ust-Balyk oil field. DAN SSSR, vol. 168, No. 4, 1966.

8. P. A. Trushkov. Graphite in Mesozoic deposits of the West Siberian Lowland. 1969.

9. K. A. Shpilman. Prospects for Oil-and-Gas Potential of the Ob–Irtysh Interfluve. Dissertation for the degree of Candidate of Geological-Mineralogical Sciences. Novosibirsk, 1967.

10. R. N. Clayton, E. F. Degens. Use of carbon isotope analysis of carbonates for differentiating fresh-water and marine sediments. Bull. Amer. Ass. Petr. Geol. V. 43, 1959.

11. S. Silverman. Migration and segregation oil and gas. Tluids is subsurface enbironments. Sympos. Am. Ass. Petr. Geol., memoir 4, 1965.