Calcium Isotopic Composition of Oil-Bearing Rocks and Waters of the West Siberian Lowland
Abstract Generated abstract
This study examines variations in the calcium isotopic composition of oil-bearing sedimentary rocks and formation waters of the West Siberian Lowland, with attention to calcium mobility, lithology, depth, water chemistry, and possible biological influences. The authors used deuteron-activation analysis on calcium extracted as calcium fluoride to determine Ca48/Ca40 ratios in 39 rock and water samples, reporting a measurement precision of about 0.3 percent. The results indicate substantial variability, with generally lighter calcium at greater rock depths, heavier values in organic limestones and siderites, contrasts between argillites and sandstones, and differences associated with sodium-chloride and calcium-chloride waters. Formation waters from the Sovetskoe field show heavier calcium isotopic composition with increasing depth and calcium concentration, and the authors suggest a possible relation between heavier Cretaceous rock calcium and zones of oil-field concentration.
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CALCIUM ISOTOPE COMPOSITION OF OIL-BEARING ROCKS AND WATERS OF THE WEST SIBERIAN LOWLAND
V. L. KOKUNOV, A. D. NAZAROV, Yu. M. STOLBOV, R. P. MESHCHERYAKOV,
A. E. KONTOROVICH, Yu. F. TARABYKIN
(Presented by the scientific seminar of the Geological Prospecting Faculty)
Interest in the study of the behavior of calcium isotopes in nature has repeatedly been emphasized by leading researchers in this field, for example by Rankama [1].
Among metals, in terms of abundance in the Earth’s crust, calcium occupies third place after aluminum and iron and belongs to the group of principal rock-forming elements of rocks of the most varied genesis.
Meanwhile, calcium has 6 stable isotopes: Ca^40, Ca^42, Ca^43, Ca^44, Ca^46, and Ca^48. The mass ratio of the extreme calcium isotopes is 1.20 and in magnitude is second only to the isotope constants of the lightest elements—hydrogen and helium. Consequently, calcium belongs to the group of elements of high geochemical mobility. At the same time, the geochemical migration of calcium is extremely diverse both under hypogene and hypergene conditions. The absence of insoluble salts leads to very complex pathways of calcium migration, especially in the hypergene environment. In substitution series calcium occupies an intermediate position; that is, some elements are capable of replacing calcium in compounds, while it replaces others itself.
Finally, changes in the isotopic composition of calcium may occur not only in physicochemical processes, but also as a result of the decay K^40 → Ca^40.
All this indicates the possibility of significant separation of calcium isotopes in natural processes. Among the processes in the study of which the isotopic method of investigating calcium may probably prove fruitful, the following may be included:
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Processes occurring during the formation of a magmatic body (crystallization, isotope-exchange chemical reactions, diffusion, ionic substitution, gravitational settling).
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Processes occurring at the contacts of an intrusive body with enclosing rocks (diffusion, thermodiffusion, ionic substitutions).
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Processes occurring during the formation of postmagmatic solutions (isotope-exchange chemical reactions, diffusion, ionic substitution).
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Processes leading to the destruction of primary rocks and the migration of calcium into water with the formation of secondary deposits (chemical dissolution of rocks, ion exchange, isotope-exchange chemical reactions, biological processes).
- Biological processes.
In setting up the work, we were primarily interested in the behavior of calcium in sedimentary rocks and its participation in biological processes.
The authors are clearly aware that, when discussing the results of isotopic analysis of calcium from sedimentary formations of any sizable territory, it is necessary to take into account the nature of the isotopic composition of the calcium of the initial material that went into the formation of the sedimentary rocks. Thus, it is quite obvious that when limestones are destroyed in the source area, in the course of their “melting,” the calcium entering the sedimentation area will undoubtedly be light in isotopic composition. As for “silicate” calcium, i.e., calcium entering the mineral lattice of plagioclases, which are an indispensable component of clastic material, it will necessarily be heavier in its isotopic composition. In view of this, analyses of different signs may be obtained for different areas. However, for such an element as Ca, it proves impossible to obtain a gaseous compound necessary for carrying out accurate mass-spectrometric measurements; moreover, the accuracy of the best mass-spectrometric measurements of the ratio of the most abundant calcium isotopes, \(Ca^{40}/Ca^{44}\), whose variation is approximately two times smaller than the variation of the ratio \(Ca^{40}/Ca^{48}\), at present amounts to \(\pm 0.5\%\), which is clearly insufficient for solving most geological problems.
In this connection, experimental studies were carried out on the possibilities of activation methods for measuring the isotopic composition of calcium on the accelerators of the Tomsk Polytechnic Institute: a betatron and a cyclotron. An assessment of the possibilities of neutron-activation and deuteron-activation analyses of the isotopic composition of calcium showed that the most acceptable activation method is deuteron-activation analysis, since in essence it is precise, does not require a special determination of the total amount of calcium submitted for analysis, and does not require a very high degree of purity of the analyzed Ca compound.
With the aid of deuteron-activation analysis on the cyclotron, it is possible to determine the isotopic composition \(Ca^{40}/Ca^{48}\) on samples with a calcium content of about \(0.25\) g.
For analysis of the isotopic composition of calcium, the latter is extracted from the rock chemically and converted into calcium fluoride, i.e., the compound \(CaF_{2}\) is used for the analysis. As the standard, especially pure \(CaF_{2}\) produced by domestic industry is taken.
The accuracy achieved (20 measurements on one sample) is characterized by a root-mean-square error of \(\pm 0.3\%\).
In all, 39 calcium samples from rocks and formation waters of the West Siberian Lowland, listed in Tables 1 and 2, were analyzed. It is rather difficult to discuss the results presented in the tables, because at present the geochemistry of calcium isotopes has practically hardly been studied.
Nevertheless, comparing the results of the determination, the following may be noted:
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For the calcium of sedimentary rocks and formation waters, where it occurs in the form of calcium carbonate and “silicate” calcium, a fluctuation of the values of \(\beta Ca^{48}\) from \(+1.13\) to \(-0.56\) is characteristic.
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The constructed plot of the dependence of the content of the isotope \(Ca^{48}\) in rocks on depth (Fig. 1), despite the large scatter of points, shows a general lightening of the isotopic composition of calcium in the rocks with depth. A similar dependence in the lightening with depth of the isotopic composition
Isotopic composition of calcium
Table 1
| No. | Field name | Well no. | Sampling interval (depth, m) | Rock name | Age | Ca^48/40 |
|---|---|---|---|---|---|---|
| 1 | Dyundinskoye | 2-r | 328 | Sandstone | Upper Jurassic | 0 |
| 2 | Rassokhinskoye | 1-r | 377-f | 1391—1405.4 | Sandstone | Upper Jurassic |
| 3 | » | » | 371-f | 1501.2—1528.3 | Argillite | Upper Jurassic |
| 4 | Sukhodudinskoye | 2-r | 267-f | — | Sandstone | Lower Cretaceous |
| 5 | » | » | 275-f | 1309—1312.3 | Sandstone | Lower Cretaceous |
| 6 | Messoyakhskoye | 1-r | 115-f | 1835—1839.2 | Sandstone | Lower Cretaceous |
| 7 | Baklanskoye | 2-r | 227 | 2517—2523 | Sandstone | Lower Cretaceous |
| 8 | Tavolinskoye | 1-r | 426 | 2228.38—2228.83 | Sandstone | Lower Cretaceous |
| 9 | » | » | 440 | 2452.01—2458.36 | Sandstone | Upper Jurassic |
| 10 | Tarasskoye | 1-r | 450 | 2030.7—2035.56 | Sandstone | Lower Cretaceous |
| 11 | » | » | 468 | 2253.5—2256.5 | Sandstone | Lower Cretaceous |
| 12 | » | » | 468a | 2256.5—2262.0 | Sandstone | Lower Cretaceous |
| 13 | » | » | 483 | 2416.2—2421.33 | Sandstone | Lower + Middle Jurassic |
| 14 | Kazanskoye | 1-r | 523 | 2443.02—2448.59 | Aleurolite | Upper Jurassic |
| 15 | Pokamaiskoye | 10-r | 44 | 2174.7—2178.7 | Shell limestone | Upper Jurassic |
| 16 | » | » | 45 | » | Sandstone | Upper Jurassic |
| 17 | » | » | 77 | 2224.7—2227.7 | Sandstone | Lower + Middle Jurassic |
| 18 | Teterevskoye | 726-r | 231 | 1558.2—1561.7 | Limestone | Upper Jurassic |
| 19 | Trekh-ozyornoye | 66-r | 286 | 1477.8—1474.3 | Siderite | Lower Cretaceous |
| 20 | » | 67-r | 290 | 1538.3—1541.8 | Sandstone | Upper Jurassic |
| 21 | Omskoye | exploratory | 744 | 2300 | Argillite | Upper Jurassic |
| 22 | » | exploratory | — | 2758 | Argillite | Lower + Middle Jurassic |
| 23 | » | » | 741 | 2107—2111.1 | Sandstone | Lower Cretaceous |
| 24 | » | » | 745 | 2210 | Argillite | Lower Cretaceous |
| 25 | » | » | — | 2759 | Sandstone | Lower + Middle Jurassic |
| 26 | N.-Vartovskoye | 1-r | 1 | 2468—2478 | Argillite | Lower + Middle Jurassic |
| 27 | B. Rechenskoye | 2-r | — | 2762 | Limestone, marl | Lower + Middle Jurassic |
| 28 | Cheryomshanskoye | — | 5 | 2549—2556 | Argillite | Lower + Middle Jurassic |
| 29 | » | 1-r | — | 2645—2657 | Argillite | — |
| 30 | Mulym’inskoye | » | — | 1450—1500 | Argillite | Upper Jurassic |
| 31 | Sarataskoye | — | — | 2765 | Argillite | Paleozoic |
| 32 | Fyodorovskoye | 33-r | 59 | 1674.55—1680.55 | Argillite | Lower Cretaceous |
| 33 | Senkinskoye | 7-r | 20 | 2064.7—2153.9 | Argillite | Upper Jurassic |
| 34 | Sovetskoye | 78 | 1k | 2216—2222 | Aleurolite | Lower Cretaceous |
| 35 | Sosninskoye | 54 | 3k | 1737 | Argillite | Lower Cretaceous |
Chemical composition of waters
Table 2
| No. | Area, name, and well No. | Age and depth interval | pH | Sample form | Ca | Mg | Na+K | Fe²⁺³ | NH₄ | Cl | HCO₃ | SO₄ | I | Br | Mineralization, mg/l | Calcium isotope composition δCa⁴⁸/Ca⁴⁰ |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1 | Sosninskaya, 318 bis | Pokur Formation, 1342—1524 m | 7,75 | mg/l | 439,4 | 69,02 | 6371,0 | 3,0 | 18,0 | 10693,3 | 195,2 | traces | 9,0 | 53,0 | 18880,92 | −0,10 |
| 1 | Sosninskaya, 318 bis | Pokur Formation, 1342—1524 m | 7,75 | meq | 21,97 | 5,18 | 277,0 | — | 1,0 | 301,2 | 3,2 | — | 0,08 | 0,66 | 18880,92 | −0,10 |
| 1 | Sosninskaya, 318 bis | Pokur Formation, 1342—1524 m | 7,75 | % meq | 7,2 | 1,67 | 90,8 | — | 0,33 | 98,5 | 1,28 | — | 0,2 | 18880,92 | −0,10 | |
| 2 | Sovetskaya, 70-r | Vartovskaya Formation, 1724—1691 m | 7,2 | mg/l | 1401,1 | 65,8 | 6359,5 | 0,15 | 20,0 | 12409,0 | 166,3 | 3,43 | 15,0 | 57,0 | 20414,1 | +0,30 |
| 2 | Sovetskaya, 70-r | Vartovskaya Formation, 1724—1691 m | 7,2 | meq | 70,06 | 5,48 | 276,5 | — | 1,11 | 349,55 | 2,72 | 0,07 | 0,12 | 0,71 | 20414,1 | +0,30 |
| 2 | Sovetskaya, 70-r | Vartovskaya Formation, 1724—1691 m | 7,2 | % meq | 19,6 | 1,55 | 78,5 | — | 0,32 | 99,06 | 0,72 | — | 0,2 | 20414,1 | +0,30 | |
| 3 | Sosninskaya, 511 | Kulomzinskaya Formation, 2174—2189 m | 6,7 | mg/l | 2254,5 | — | 7896,0 | 0,15 | 30,0 | 16129,8 | 146,5 | 6,0 | — | — | 26390,0 | +0,50 |
| 3 | Sosninskaya, 511 | Kulomzinskaya Formation, 2174—2189 m | 6,7 | meq | 112,5 | — | 343,3 | — | 1,67 | 455,0 | 24,0 | 0,12 | — | — | 26390,0 | +0,50 |
| 3 | Sosninskaya, 511 | Kulomzinskaya Formation, 2174—2189 m | 6,7 | % meq | 24,6 | — | 75,04 | — | 0,34 | 99,45 | 0,52 | 0,03 | — | — | 26390,0 | +0,50 |
| 4 | Sosninskaya, 3-r | Kulomzinskaya Formation, 2216—2222 m | 7,7 | mg/l | 2500,0 | — | 7894,93 | 2,0 | 23,0 | 16300 | 300,9 | — | 10,9 | 85,0 | 26966,00 | +0,50 |
| 4 | Sosninskaya, 3-r | Kulomzinskaya Formation, 2216—2222 m | 7,7 | meq | 125,0 | — | 338,91 | 0,04 | 1,28 | 459,15 | 4,93 | — | 0,09 | 1,06 | 26966,00 | +0,50 |
| 4 | Sosninskaya, 3-r | Kulomzinskaya Formation, 2216—2222 m | 7,7 | % meq | 26,8 | — | 72,8 | — | 0,3 | 98,7 | 1,05 | — | 0,02 | 0,23 | 26966,00 | +0,50 |
Fig. 1. Plot of the dependence of the isotopic composition of calcium \((\mathrm{Ca}^{48}/\mathrm{Ca}^{40})\) on depth, age, and lithological type of rocks.
Legend:
1 — sandstones, 2 — argillites
organic carbon (the carbon of oils, gases, and dispersed organic matter) compels one to assume an influence of biological processes on the isotopic composition of calcium in the rocks. The scatter of points on the graph is undoubtedly a reflection of the complex nature of the transformations experienced by calcium over the course of geological history. Thus, rocks of organic origin (coquinas, limestones) give a calcium isotopic composition that is considerably heavier relative to the standard \((+0.58 \div +1.12)\). The isotopic composition of calcium is also heavier in siderites \((+1.08)\). If, however, one considers the isotopic composition of calcium contained in sandstones, siltstones, and argillites, that is, in different lithological types of clastic rocks, it turns out that in argillites it is lighter relative to the standard, whereas in sandstones it is heavier (Fig. 2).
We attempted to analyze with which forms of calcium in argillites and sandstones particular isotopes are predominantly associated.
Fig. 2. Frequency distribution of values of the calcium isotope composition in sandstones and argillites.
Legend:
1 — sandstones, 2 — argillites
Testing argillite samples for carbonate content, i.e., for the presence of calcium carbonate in them, showed its absence in a number of samples. Nevertheless, “silicate” calcium was extracted from these samples and analyzed. It has an elevated content of \(Ca^{48}\) relative to the standard. In the same argillite samples that showed the presence of carbo-
Fig. 3. Scheme of the location of rock-sampling points for the calcium isotope composition.
Legend:
I — oil-bearing areas, II — water-bearing areas, III — internal boundary of the West Siberian Plate; 1–16 — names of the areas: 1 — Nizhne-Vartovskaya, 2 — Sovetskaya, 3 — Kedrovskaya, 4 — Narymskaya, 5 — Silginskaya, 6 — Kazanskaya, 7 — Tavolginskaya, 8 — Tartasskaya, 9 — Sargatskaya, 10 — Bolsherechenskaya, 11 — Baklyanskaya, 12 — Cheremshanskaya, 13 — Trekhozernaya, 14 — Mulmyinskaya, 15 — Teterevskaya, 16 — Potanayskaya.
calcium carbonate, its isotopic composition is light relative to the standard. Sandy rocks containing calcium carbonate also yield a light isotopic composition relative to the standard. The absence of a direct relationship between the amount of calcium carbonate in rocks and its isotopic composition indicates the need for mandatory separate determination of the isotopic composition of carbonate and “silicate” calcium, which provides much new material for determining the geochemical role of calcium in the section of sedimentary formations of Western Siberia.
- The attempt we have made to note the features of the isotopic composition of calcium as a function of the type of waters saturating them makes it possible preliminarily to state the following:
a) rocks of Cretaceous age saturated with mineralized water of sodium-chloride composition contain calcium with a heavy isotopic composition relative to the standard (from \(+0.02\) to \(+0.33\) in \(\beta\ \mathrm{Ca}^{48}\));
b) rocks of Jurassic age saturated with water of the same composition contain calcium of an even heavier isotopic composition (up to \(+0.61\) \(\delta\ \mathrm{Ca}^{48}\));
c) zones where the rocks are saturated with mineralized water of calcium-chloride composition have a sharply light isotopic composition of calcium (\(-0.37\)).
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The studied isotopic composition of calcium contained in the formation waters of the Sovetskoe oil field shows that, with increasing depth of occurrence of the aquifer, the isotopic composition also becomes heavier along with the increasing calcium content (from \(-0.1\) to \(+0.5\) \(\mathrm{Ca}^{48}\)).
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Consideration of the plan of the location of sampling points reveals that in zones of concentration of oil fields the isotopic composition of calcium in rocks of Cretaceous age is heavy, whereas in zones where oil fields are absent the isotopic composition of calcium is light (Fig. 3).
REFERENCES
- K. Rankama. Isotopes in Geology. I. L. M., 1956.