ON THE CONDITIONS FOR THE FORMATION OF ACCUMULATIONS AT THE BOBROVSKOYE FIELD IN ORENBURG OBLAST BASED ON CARBON ISOTOPE DATA

V. L. Kokunov, Yu. N. Komarov, N. K. Grigoriev

(Presented by Professor A. V. Aksarin)

In recent years the question of isotope geochemistry has received much attention from both Soviet and foreign scientists. This is because the study of the isotopic composition of oil, natural gas, and rocks is of very great interest in elucidating the features of the formation of their accumulations.

In the present work, a study was made of the isotopic composition of carbon in oil; therefore, the state of knowledge concerning the isotopy of precisely this element is presented here. The isotopic composition of carbon in oil interested researchers from the very beginning almost exclusively in connection with its origin.

From the works of C. West (1945), A. V. Trofimov (1952), G. Craig (1954), N. S. Filippova (1955), F. A. Alekseev and V. S. Lebedev (1964), it is evident that the carbon of oils is characterized by the same order of magnitude of $\delta C^{13}$ as the carbon of the biosphere as a whole.

The mean value of $\delta C^{13}$ of the carbon of oils, calculated from these data, is $-3.1\%$. This value differs noticeably from the value characterizing the average isotopic composition of terrestrial plants ($\delta C^{13} = -2.5\%$) and especially marine plants ($\delta C^{13} = -1.4\%$). Hence it follows that both continental and marine oil are enriched in the isotope $C^{12}$ in comparison with the parent material. Such isotope fractionation may be explained by the difference between the lipid and hydrocarbon fractions of modern plants, by the selective preservation and concentration of the lipid fraction of organic matter.

According to C. Silverman, Epstein (1958), and Vyshemirsky (1968), oils attributed by their conditions of occurrence to marine deposits have $\delta C^{13}$ from $-2.3$ to $-2.96\%$, whereas oils of continental sediments have from $-2.99$ to $-3.25\%$, i.e., are enriched in the isotope $C^{12}$.

C. Silverman (1964) believes that the isotopic composition of oil can serve as a criterion for assigning it to one or another type, which is also supported by V. S. Vyshemirsky (1971). Nevertheless, the same C. Silverman believes that in individual cases migration may also affect the isotopic composition of oil. For example, the nature of the change in the isotopic composition of oil (as a whole by $0.04\%$) within the Kirkuk field (Venezuela) is best explained by Silverman (1965), taking geological data into account, precisely by migration.

In this connection, the interpretation by R. G. Pankina et al. (1971) of the results of studying variations in the isotopic composition of the sulfur of hydrogen sulfide at the Orenburg gas-condensate field deserves attention; there the authors judge the direction in which the field trap was filled from the lightening of the isotopic composition of sulfur.

Thus, on the basis of previous studies of the geochemistry of carbon isotopes in oils, the following conclusions may be drawn:

1. Fluctuation of the isotopic composition in oils of different ages, compositions, and areas indicates the identity of the source material from which the oil was formed, and points to its facies affiliation.

2. A difference in the isotopic composition of the carbon of oil within one bed in a trap of 0.4–0.8% indicates lateral migration of the oil.

In studying the isotopic composition of oils in the TPI laboratory, calcium carbonate ($\mathrm{CaCO_3}$), kindly supplied to us by V. S. Lebedev, having $\delta C^{13} = -0.85\%$ and $C^{12}/C^{13} = 89.65$, measured relative to Craig’s standard [6], was taken as the working standard. All measurements were made relative to the working standard and then converted to PDB according to the formula of H. Craig [6].

In the regional tectonic setting, the Bobrovskoye field is located on the southeastern slope of the Russian Platform and is confined to the so-called Bobrovsk–Pokrovka swell, which on the north and southwest is bounded by a zone of development of a thick sequence of terrigenous Lower Carboniferous sediments filling the floor of the Kama–Kinel depression. The structural plan of the Bobrovskoye field at the top of the Bashkirian stage consists of three brachyanticlinal uplifts of almost latitudinal extent: Spiridonovskoye, Proskurinskoye, and Savelyevskoye. They are separated from one another by shallow troughs, with the Proskurinskoye and Savelyevskoye uplifts arranged en echelon relative to each other.

The geological section of the Bobrovskoye field is represented by Archean formations; Devonian deposits comprising the Eifelian and Givetian stages; Upper Devonian deposits comprising the Frasnian and Famennian stages; three divisions of the Carboniferous system; two divisions of the Permian system; Triassic deposits comprising the Buzuluk and Tananlyk formations of the Vetluga stage; Jurassic and Neogene deposits; and Quaternary formations. The exposed thickness of the section is 3988 m.

Productive here are dolomites at the base of the Artinskian stage, limestones in the upper part of the Bashkirian stage, dolomites of the upper half of the Oka superhorizon, sandstones of the Bobrikov horizon, and limestones in the upper part of the Tournaisian stage.

In the Bashkirian stage, the oil-bearing rocks are limestones, which are distinguished as productive bed A-4. The limestones are organogenic, porous, and cavernous; the average porosity is 16%, and permeability ranges from 0.01 to 927 md. The pool is massive; the accepted oil–water contact mark is −1919 m. The reservoir pressure is below hydrostatic and is 190–105 atm; the yields are small—44.3 t/day through a 10-mm choke at an excess buffer pressure of 18 atm (well 78).

The main reserves of the Bobrovskoye field are confined to the Oka superhorizon; they occur among dense anhydrites in the upper sulfate–carbonate member. Within the field, four productive beds are traced (O-II, O-III, O-IV, and O-IVa). Common to all beds is the irregularity of the thicknesses due to the replacement of dolomites by anhydrites and the sharp heterogeneity of the reservoirs.

properties. The porosity of the productive beds is 12%. Permeability is from 1 to 20 millidarcies. Flow rates vary over wide limits, from 9.8 m³/day to 161 m³/day through a 10 mm choke.

Samples were taken from 8 wells: 4 samples from wells Nos. 78, 92, 254, 71 of the Bashkirian stage, and 5 samples from wells of the Oka subhorizon, Nos. 98, 318, 91, 203, 117. The results of the determinations are given in Table 1. As can be seen from the table, the carbon isotope ratios $C^{12}/C^{13}$

Table 1

of bed A4 (Bashkirian stage) range from 86.56 to 86.29, and of bed O2 (Oka subhorizon) from 86.37 to 86.29.

If the latest achievements of the theory of heat and mass exchange are taken into account, then the change in the isotopic composition of the carbon of oil, this certain integral physical characteristic, can be explained only as a consequence of lateral migration of oil through the beds. In our opinion, the best representation of this phenomenon is provided by maps of isoconcentrations of the isotopic composition of the carbon of oil in the pool. From these positions, the direction of lightening of the isotopic composition of carbon should be documented

Fig. 1. Diagram of the location of oil sampling points from the Bashkirian stage of the Bobrovskoye field: 1 — isoconcentrations of the isotopic composition of carbon in $C^{13}$ PDB, %; 2 — outer oil-bearing contour; 3 — inner oil-bearing contour; 4 — well numbers.

...the direction from which the trap was filled with oil, as is shown in Fig. 1.

The accuracy of the analyses ranges within $\pm 0.05\%$. This gives grounds to hope that the variation in the isotopic composition of carbon in the oils of the Bobrovskoye field is free from the influence of measurement error; however, for compiling a more reliable map of isoconcentrates for the $A_4$ and $O_2$ beds, there are still insufficient data.

Thus, from the results of studying the isotopic composition of the oils of the Bobrovskoye field, it proved possible to determine the probable direction of oil migration along which the traps of the Bobrovskoye field were filled. According to the data obtained, filling of the traps occurred from the side of the Buzuluk depression, which is consistent with the general views of a number of authors on the role of the Buzuluk depression in the formation of fields of the Bobrovsko-Pokrovsky swell.

LITERATURE

1. F. A. Alekseev, V. S. Lebedev. Isotopic composition of the carbon of oil and gas. “Geology of Oil and Gas,” 1964, No. 7.

2. V. S. Vyshemirsky, E. F. Doilnitsyn. Isotopic composition of the carbon of oils of the Bobrikovsky horizon of the Saratov Volga region. DAN SSSR, vol. 178, No. 1, 1968.

3. V. S. Vyshemirsky. On the possible oil-and-gas potential of the Paleozoic of the West Siberian Lowland. In the book: “Problems of the Oil-and-Gas Potential of Siberia.” “Nauka,” Siberian Branch of the USSR Academy of Sciences, 1971.

4. R. G. Pankina, S. P. Maksimov, I. A. Shpilman, G. Ya. Shutov. On the genesis of hydrogen sulfide of the Orenburg gas-condensate field. “Geology of Oil and Gas,” 1971, No. 1.

5. A. V. Trofimov. Isotopic composition of carbon of magmatic rocks. DAN SSSR, vol. 85, No. 1, 1952.

6. H. Craig. Isotope standards carbon and oxigen and correction factors for mass-spectrometric analyses of carbon dicside. Geoch. et Cosmoch. Acta, vol. 12, 1957.

7. S. R. Silverman, S. Epstein. Carbon isotopic composition of petroleum and other sedimentary organic materials. Bull. Am. Ass. Petr. Geol., vol. 42, 1958.

8. S. R. Silverman. Investigations of petroleum origin and evolution mechanisms by carbon isotope studies. New York, 1964.

9. S. R. Silverman. Migration and segregation oil and gas. Fluids is subsurface environments. Sympos. Am. Ass. Petr. Geol. memoir, No. 4, 1965.

10. S. West. The relative abundance of the carbon isotopes in petroleum. Geoph., vol. 10, No. 3, 1945.