PRECAMBRIAN BASEMENT FORMATIONS OF THE NORTHEASTERN PART OF THE WEST SIBERIAN PLATE

(ERMAKOV EXPLORATORY BOREHOLES)

S. N. Gulyaev, A. F. Senakolis

(Presented by Professor A. V. Aksarin)

During oil-exploration work in the northeastern part of the West Siberian Plate, at the latitude of the Arctic Circle, four deep boreholes in the Ermakov and Shchuchya areas penetrated formations of the pre-Jurassic basement, which are conventionally assigned to the Sinian [9]. A detailed study of the sections penetrated by the boreholes, and a petrographic investigation of the constituent rocks, will make it possible to determine more correctly the degree of metamorphism of the ancient strata and the features of the geological structure of the junction zone of the West Siberian and Siberian platforms.

In the Ermakov area, located 10–15 km west of the Yenisei River (Fig. 1), the Precambrian is penetrated by boreholes 1-RE, 2-RE, and 3-RE and lies directly beneath Jurassic deposits, respectively in the intervals 1420–2000 m, 1437–1666 m, and 1565–1705 m. In the Shchuchya area, situated somewhat farther to the west, basement rocks were penetrated by borehole 1-RSh in the interval 1668–1893 m. The Precambrian section is most completely characterized by core material from borehole 1-RE, and only slightly in the Shchuchya area. However, the clear differentiation of the constituent rocks on electrical logging diagrams makes it possible to characterize the penetrated formations in all boreholes (Fig. 2).

In borehole 1-RE, where the greater part of the Precambrian section, with a total thickness of up to 680 m, was penetrated, three members may be distinguished according to the composition of the constituent rocks: the upper—shaly—member, occurring in the interval 1420–1486 m; the middle—carbonate—member, the base of which was penetrated at a depth of 1886 m; and the lower—carbonate-shaly—member, forming the base of the section.

The rocks composing the upper member have a greenish-gray, green, more rarely dark-green color, and a clearly expressed schistosity. Thin calcite and quartz veinlets are often observed in them.

Fig. 1. Overview map of the work area.
1 — work area

On the electrical logging diagrams, the formations of the upper member are characterized by comparatively low resistivities, varying from 20 to 200 mm. Microscopic study of the rocks of this part of the section showed that they are dominated by rocks with a low degree of metamorphism and clearly expressed relicts of primary formations. Phyllitized aleuritic–clayey shales with fragments of sericite schists and chlorite rocks were encountered here.

Phyllitized aleuritic–clayey shales with fragments of clayey shales, as seen under the microscope (thin section 22), consist for 30% of small fragments and for 50% of a grayish-brown clayey substance in transmitted light. The size of the fragments ranges from 0.04 to 0.08 mm; rare grains reach 0.4–0.7 mm. Both small and large fragments have an irregular angular shape and are represented mainly by quartz; altered feldspars are noted more rarely. The terrigenous material is distributed uniformly in the clayey mass; accumulations of fragments in the form of lenses measuring 0.15 × 2 mm are sometimes encountered. It is interesting to note that the degree of compaction in these lenses is low; the cement envelops the grains in the form of films, and incorporation and conformal structures are not observed. In the thin section a lamina enriched in fragments of sericite schists was found; the length of the fragments reaches 6 mm, with a thickness not exceeding 1 mm. The composition of the schists is generally uniform; the differences consist in the sizes of the sericite flakes and in the amount of admixture of quartz grains.

The cementing mass is colored grayish-brown and brown and is composed of a fine-scaly clayey substance. The flakes of the latter have an ordered orientation, expressed in simultaneous darkening of the thin section when the microscope stage is rotated. It should be emphasized that areas of the thin section where the clayey substance is more intensely colored are distinguished by a higher degree of recrystallization. As is evident from the description, these formations are characterized by the presence of features typical of sedimentary formations (bedding, expressed in the alternation of layers of different coloration, an admixture of ter-

Fig. 2. Primary Precambrian sections.
1—carbonates; 2—shales; 3—thin-section interval; 4—depth, in m.

Visible well labels: Well 1-RB, Well 3-RB, Well 2-RB, Well 1-GRShch.
Visible logging labels include: GK, NGK, PS.

…terrigenous material, aleuritic-pelitic structure) and a weak manifestation of metamorphism, which consists in the appearance in the deposits of an ordered orientation during recrystallization of the clayey mass.

In addition to those just described, green chloritic rocks occur in the unit. Study under the microscope showed that they formed through chloritization of the enclosing formations, whose primary nature is sometimes rather difficult to establish. Thus, in thin section 21 it is seen that the rock consists 70% of a scaly aggregate, the size of whose constituent grains does not exceed 0.2 mm. The orientation of the chlorite is ordered only to a slight degree; there are many areas where the chlorite forms small rosettes and radial-fibrous aggregates. Along with chlorite, accumulations of quartz grains play a noticeable role in the composition of the rock, the crystal size of which ranges from 0.05 to 0.1 mm. Quartz is distributed throughout the thin section; usually the lenticular accumulations are arranged as a chain parallel to, or at a small angle to, the schistosity, which suggests a superimposed character of silicification.

Individual varieties of the chloritic formations make it possible to draw a conclusion about the primary rocks from which they formed. Thus, study of thin section 21a showed that it consists of 50–60% chlorite, 20% plagioclase, and 20% quartz. The remaining minerals account for no more than 10%. The fine-scaly chlorite has a bright-green color with clearly expressed anomalous interference colors. The length of the flakes reaches 0.3 mm, while their thickness does not exceed 0.02 mm. The flakes of the chloritic aggregate have an ordered orientation, which is responsible for the crystallization schistosity. Fine and large crystals of plagioclase with clearly expressed polysynthetic twins are uniformly distributed in the chloritic mass. The size of the fine grains does not exceed 0.1–0.2 mm; the large ones reach 1–1.2 mm in length and are cut along cleavage by extremely thin chloritic veinlets. The fine grains are distributed everywhere; the large ones are observed in separate areas of the thin section.

In addition to chlorite and plagioclase, quartz is a noticeable impurity; it is developed as rare large veinlets 3–4 mm thick and in the form of small accumulations of lenticular shape. The size of the quartz crystals in the veinlets reaches 0.8 mm, and in the lenticular accumulations, 0.1 mm. Another impurity, not exceeding 5% in content, consists of accumulations of an opaque mineral, evidently opacitized dark-colored minerals.

In the microscopic study of this rock we found no relics of layering. The presence of microlites and porphyritic segregations of plagioclase, and the disordered arrangement of their crystals, allow the conclusion that we are dealing with a chloritized and silicified plagioclase porphyrite.

The middle unit is composed of dark-gray and black fine- and medium-grained carbonate rocks with a banded structure, which is revealed by the fine alternation of black and dark-gray bands. Calcite and quartz-calcite veinlets usually run parallel to the banding. On electrical logging diagrams, the carbonate rocks are distinguished by high resistivities varying from 500 to 7000 ohm·m. Microscopic study showed that the carbonate rocks of this unit are characterized by noticeable metamorphism. Schistose limestones, tremolite calcite schists, and medium-grained silicified marbles were encountered here.

The schistose limestones are the least metamorphosed varieties, characterized, together with crystallization…

...by schistosity and by the presence of relics of the primary structure (thin section 20). Under the microscope it is seen that the rocks consist 80% of calcite, the grain size of which varies mainly from 0.06 to 0.1 mm. The texture is determined by the alternation of coarse- and fine-grained bands. The transition from coarse-crystalline to fine-grained bands is gradual, whereas the boundary between the fine-crystalline bands and the overlying coarse-grained bands is sharp. The thickness of the layers does not exceed 0.5 mm. The relatively fine-grained interlayers are enriched in black carbonaceous matter, which is concentrated in the form of the thinnest laminae. The character of the alternation of carbonate grains of different sizes and the confinement of accumulations of organic matter mainly to the finest-grained laminae make it possible to speak with confidence of relict bedding in this rock.

Another distinctive feature of the formations described is that the composing calcite crystals have a clearly expressed elongated form. The ratio of width to length in the grains ranges from 1 : 3 to 1 : 10. The elongated crystals, being oriented parallel to the bedding noted above, emphasize the crystallization-schistose texture. Thus, the presence of primary-sedimentary and metamorphic textures is the principal distinguishing feature of the schistose limestones described. (Analogous rocks of Precambrian age are widely distributed on the eastern slopes of the Kuznetsk Alatau [10].) The limestone that gave rise to the metamorphic rock described had a fine-grained structure, which can be observed in small lenses of carbonate composition that did not undergo recrystallization. Their size is 0.6 mm; their shape is rounded, more rarely lenticular. Recrystallized carbonates envelop the lenses of the primary rock. In calcite and quartz veinlets that cut the limestones along the schistosity, grains of pyrite and flakes of biotite are noted.

Tremolite calcite crystalline schists do not show relics of the original sedimentary deposits. Under the microscope it is seen (thin section 17) that they consist 70% of calcite, while 30% is tremolite. The structure of the rock is nematoblastic. Calcite crystals reach 0.5–0.8 mm in length, not exceeding 0.2 mm in cross section. The elongated grains, as in the case described earlier, have an ordered orientation, owing to which crystallization schistosity is revealed; it is emphasized by the uniform arrangement of tremolite prisms. Perpendicular to the schistosity there is an alternation of bands composed of small and large calcite grains, which gives the rock a banded character.

Calcite marbles occurring within the middle member opened by borehole 1-PE do not show a schistose texture (thin sections 18b). They consist of large calcite grains, the sizes of which range from 0.4 to 0.8 mm. The structure of the marbles is granoblastic. The presence of a large number of parallel veinlets of calcite composition emphasizes the banded texture [3]. As in the carbonate formations described earlier, quartz veinlets are developed in the marbles.

Among the carbonate rocks of the middle member in borehole 1-PE, isolated beds of dark-gray and black crystalline schists are noted, characterized by a lepidogranoblastic structure and by mineral associations: quartz—biotite—chlorite—calcite (thin section 19) and quartz—muscovite—biotite—zoisite—calcite (thin section 16).

The lowermost member of the section of borehole 1-PE is represented by an alternation of dark-gray schistose limestones (thin section 15), black marbles (thin section 14), and quartz-biotite-calcite schists analogous to the rocks described above.

In boreholes 2-RE, 3-RE, and 12RSh the upper member has been penetrated; in the composition of the constituent rocks it is similar to the upper parts of the section in borehole 1-RE. Here microscopic study has established sericite and sericite-biotite schists with relicts of the primary structure of sedimentary rocks, expressed in the presence of bedding, emphasized by changes in the content of silty admixture by layers (thin sections 2, 4, 5, 6, 9, 13), and gravelites consisting of fragments of quartz and sericite schists, cemented by a weakly crystallized clayey substance (thin sections 7, 11). Sericite and sericite-biotite greenish-gray and dark-gray schists predominate in the composition of the upper member, while gravelites occur only very rarely as thin (up to 10 cm) interbeds. Sometimes, among the deposits of the member, medium-grained crystalline schists with an association of the following minerals are noted: quartz—muscovite—chlorite—calcite (thin sections 3, 10).

The carbonate member is penetrated only in part and is represented, as in borehole 1-RE, by dark-gray and black banded marbles and schistose limestones (thin section 12).

In degree of metamorphism the rocks studied belong to a transitional stage from initial to regional metamorphism (the muscovite-chlorite zone of initial metamorphism and the biotite zone of regional metamorphism [5]).

The section penetrated by the boreholes in the area of the Ermakovo settlement is not comparable with any Paleozoic formations in the basement of the northeast of the West Siberian Plate [2, 4, 8, 9] and differs substantially, in the degree of metamorphism of the rocks composing it, from the Late Precambrian of the Turukhansk [7] and Igarka [1, 6, 7] districts of the Siberian Platform. And only in the area of the town of Igarka, in the Igarka suite, the age of which, according to absolute-age data, is dated at 887 million years [1, 6], are some fragments in tuff breccias represented by chlorite-sericite schists. This makes it possible to consider that in the Ermakovo area a more ancient sequence of the Upper Precambrian has been penetrated, which underlies the Igarka suite and is not exposed within the northwestern margin of the Siberian Platform.

LITERATURE

1. O. A. Votakh, N. F. Ivlev, S. P. Mikutskii. The Precambrian of the Igarka district. Doklady AN SSSR, vol. 154, 1964.

2. V. I. Dragunov, A. A. Smirnov, I. E. Chernysh. Lower Paleozoic deposits in the basement of the eastern part of the West Siberian Lowland (Elogui reference borehole). Doklady AN SSSR, vol. 172, No. 2, 1966.

3. N. A. Eliseev. Metamorphism. Publishing House of Leningrad State University, 1959.

4. A. S. Zaporozhtseva. Lower Paleozoic deposits of the Ust-Port area. Tr. NIIGA, vol. 78, 1954.

5. A. G. Kossovskaya, N. V. Logvinenko, N. D. Shutov. On the stages of formation and alteration of terrigenous rocks. Doklady AN SSSR, vol. 116, No. 2, 1957.

6. V. M. Krymskii. Stratigraphic relationship of the Igarka suite with the Strel’nogorsk and Lower Sukharihino effusives. Doklady AN SSSR, vol. 164, No. 5, 1965.

7. S. P. Mikutskii et al. Geological structure and prospects for oil-and-gas potential of the northwest of the Siberian Platform. Tr. SNIIGGiMS, issue 28, 1963.

8. A. D. Miroshnikov. On the stratigraphic subdivision of marine Paleozoic deposits in the northeastern part of the West Siberian Lowland. Geologiya i razvedka, 1963, No. 6.

9. V. D. Nakaryakov. Structure of the pre-Mesozoic basement of the Yenisei zone of the West Siberian Platform. Geologiya i geofizika, 1961, No. 3.

10. A. F. Senokolis, V. A. Shipitsyn, I. I. Koptsev. New data on the stratigraphy and lithology of Upper Precambrian and Lower Cambrian deposits of the Batenev Ridge. Izv. Tomskogo politekhn. in-ta, vol. 151, 1966.