ON THE LITHOLOGY AND MINERALOGY OF DEPOSITS OF THE TYUMEN FORMATION IN TOMSK OBLAST

L. V. Peshekhonov, E. V. Vladimirova

(Presented by the scientific seminar of the departments of general geology, historical geology, and combustible minerals)

At every stage of the comprehensive geological study of the West Siberian Lowland, much attention has been devoted to questions of the lithology and mineralogy of Mesozoic rocks in connection with their oil-and-gas potential. This is explained by the fact that lithological-mineralogical investigations make it possible to establish mineralogical assemblages that are one of the principal criteria for subdividing and correlating the Mesozoic sections of this region. The results of such investigations have been published in a considerable number of works that describe, with varying degrees of completeness, the lithological-mineralogical features of deposits of the Tyumen Formation in Tomsk Oblast [1, 6, 7]. It should be noted that the conclusions in these works are based on analysis of material from the same reference wells. Nevertheless, on a number of questions the conclusions are contradictory. This is confirmed by the discrepancy in the composition of the correlative mineralogical assemblages distinguished by different authors (Table 1).

In addition to the reference wells, by the present time a considerable number of exploratory wells have been drilled in Tomsk Oblast. The results of the lithological-mineralogical study of core material from such wells in a number of areas (Fig. 1), carried out by us from 1969 to 1972, form the basis of the present work. They made it possible to refine and supplement the characterization of the deposits of the Tyumen Formation.

In Tomsk Oblast, as over most of the West Siberian Lowland, the deposits of the Tyumen Formation, distinguished by N. N. Rostovtsev in 1954, occur at the base of the Mesozoic cover. The age of the formation has been determined from spore-pollen assemblages and remains of leaf flora as Early–Middle Jurassic [3, 4, 5]. The rocks of the Tyumen Formation rest unconformably either directly on the basement, whose structure includes Paleozoic formations of different composition, or on a kaolinite-type weathering crust. The deposits of the Tyumen Formation include grusites, conglomerates, sandstones, aleurolites, argillites, siderites, and coals.

Grusites occur at the base of the Tyumen Formation in the Uralovskaya, Peredovaya, Yuzhno-Cheremshanskaya, and other areas. They are firmly cemented rocks, from light gray to black in color, composed of 60–70% angular fragments of quartzites, carbonaceous-siliceous shales, quartz, and feldspars, with a minor content of fragments of micaceous and clayey shales and effusive rocks. The sizes of the fragments reach 3–4 mm; fragments of size [[unclear: continuation cut off at bottom of page]]

Fig. 1. Areas within which deposits of the Tyumen Formation were studied from cores of exploratory wells: 1 — Strezhevaya (well 13), 2 — Emtorskaya (well 300), 3 — Nikolskaya (well 2), 4 — Kiev-Eganskaya (wells 350, 351), 5 — Kedrovskaya (well 35), 6 — Matyushkinskaya (well 34), 7 — Poludennaya (well 223), 8 — Peredovaya (well 190), 9 — Mezhozernaya (well 196), 10 — Uralovskaya (well 301), 11 — Olenya (well 127), 12 — Ozernaya (well 71), 13 — Pervomaiskaya (well 262), 14 — Lomovaya (well 200), 15 — Cheremshanskaya (well 2), 16 — Yuzhno-Cheremshanskaya (wells 335, 337), 17 — Kolotushnaya (well 260).

with a size of 1.5–0.8 mm. In a number of areas (Cheremshanskaya), the grussites consist exclusively of fragments of siliceous rocks. Owing to the intensive development of regeneration cement of siliceous composition, the grussites become massive and difficult to distinguish from primary quartzites. In the Peredovaya area, kaolinite clots 1–3 mm in size play a significant role in the composition of the clastic fraction of the grussites. The cement of the grussites is basal, of mixed composition. Along with kaolinite, siderite of fine-grained structure is usually present in the cement. In such varieties of grussites, the fragments are overgrown by siderite grains 0.05–0.1 mm in size, thereby producing features of rim cement. Dolomite and calcite are considerably less abundant in the cement and occur irregularly.

Gravelites are of limited distribution. We have described them in the lower parts of the section in the Cheremshanskaya, Poludennaya, and Ozernaya areas. In the Poludennaya area, the gravelites have much in common with grussites in the composition of the clastic material and cement, differing from them only in the noticeable roundness of the fragments. In the Cheremshanskaya area, the gravelites, as in the Peredovaya area, consist predominantly of fragments of microquartzites, cherts, quartz, micaceous siliceous-micaceous and clayey shales 0.5–1.5 mm in size, but their cement is represented by coarse- to medium-grained sandstones. Previously, gravelites were described in the lower part of the Tyumen Formation in the Kolpashev-

well. According to some data, in gravelites fragments of siliceous rocks predominate [1]; according to other data, pebbles of light-gray clays predominate [7].

Coarse- to medium-grained sandstones are rather widely distributed in the lower part of the Tyumen Formation in the Matyushkinskaya, Cheremshanskaya, South Cheremshanskaya, and other areas. Earlier, similar sandstones were described in the section of the Kolpashevo well [1, 7] and in the Teguldet district [6]. These sandstones are usually gray and light gray, indistinctly bedded. The clastic material in them amounts to 95–97%; it is poorly rounded and comparatively poorly sorted. Fragments 1–0.5 mm in size make up 15–38%. Fragments 0.5–0.25 mm in size predominate. Fragments 0.25–0.1 mm in size amount to as much as 10–18%. Fragments smaller than 0.1 mm occur in insignificant amounts. Sometimes the sandstones contain rare pebbles of argillites 15–10 mm in size, which makes it possible to distinguish intraformational conglomerates. Quartz fragments in the sandstones make up 35–50% of the clastic material and are characterized by an irregular and slightly elongate form. Feldspars constitute 5–20% of the detrital fraction. They are represented by fragments of irregular and tabular form and mainly by potassium varieties. Rock fragments make up 45–55% and are characterized by irregular and rounded forms. They are represented chiefly by cherts, quartzites, carbonaceous-siliceous and siliceous-micaceous shales. In insignificant amounts there are fragments of clayey and chloritic shales and micropegmatites. Biotite and muscovite occur as individual flakes. Siderite, dolomite, and calcite, filling pores in the sandstones, are present constantly but in insignificant amounts (up to 0.5%). Only the dolomite content sometimes reaches 5%. Coarse- to medium-grained sandstones are characterized by pressure-solution cementation combined with pore cement of kaolinite-hydromica composition, less commonly of dolomitic and calcitic composition.

Fine- to medium-grained sandstones are confined to the lower parts of the Tyumen Formation in the Kolotushnaya, Emtorskaya, South Cheremshanskaya, and Cheremshanskaya areas. In the Kiev-Eganskaya, Ozernaya, and Matyushkinskaya areas they compose thin interbeds in the upper part of the formation as well. Sandstones in which the clastic material amounts to 93–95% predominate. In well-sorted sandstones, fragments 0.5–0.25 mm in size amount to as much as 73%. In poorly sorted sandstones the 1–0.5 mm fraction is 0–12%, the 0.5–0.25 mm fraction is 25–43%, the 0.25–0.1 mm fraction is 12–48%, and silty material is up to 8–15%. In these sandstones, from bottom to top along the section, the amount of quartz fragments increases from 30 to 40%, feldspars from 10 to 25%, while the content of rock fragments decreases from 45–50 to 30%. In general, in textural and structural features and in the composition of the clastic material, authigenic minerals, and cement, the fine- to medium-grained sandstones are similar to the coarse- to medium-grained sandstones. Along with such sandstones, in the lower half of the formation in the Kiev-Eganskaya and other areas, sandstones with basal, poorly sorted dolomitic cement sometimes occur; in the upper part, sandstones with basal cement of calcitic overgrowth composition occur.

Fine-grained sandstones are widely developed in all the sections studied. Silty sandstones predominate, with horizontal lenticular and cross bedding. More rarely, the sandstones are evenly grained, homogeneous, and indistinctly bedded. From bottom to top along the section in the sandstones, the content of clastic material decreases from 80–95 to 60–70%; fragments of rocks of siliceous composition decrease from 50–55 to 10–15%; the quartz content increases from 30 to 50%, and feldspars from 10–15 to 25–30%. The predominant constituent in the fine-grained sandstones is—

the fraction 0.25–0.1 mm. Fragments 0.5–0.25 mm in size are present in amounts of 5–15%. Aleuritic material constitutes 10–25%. Siderite is present in the form of microlaminae and microlenses in amounts of 1–3%, more rarely 10–15%. Dolomite and calcite occur in small quantities, acting as pore and contact cement. In the sandstones the cement is predominantly pore, contact, locally basal, of hydromica–kaolinite composition.

Siltstones are very widely developed. They are gray and dark gray; more rarely light gray, horizontally, lenticularly, and cross-bedded. According to the grain size of the clastic material, coarse-grained, fine-grained, and mixed-grained siltstones are distinguished. In coarse-grained siltstones the 0.25–0.1 mm fraction constitutes 5–15%, the 0.1–0.05 mm fraction 65–85%, and the 0.05–0.01 mm fraction 15–20%. In fine-grained siltstones, fragments 0.05–0.01 mm in size dominate; sandy and coarse-aleuritic material does not exceed 15–25%. In mixed-grained siltstones, sandy, coarse-aleuritic, and fine-aleuritic material occur in approximately equal amounts. The clastic fraction contains quartz—45–70%. Feldspars make up 15–25%; among them plagioclases predominate. Rock fragments usually constitute 10–15%; they are represented by siliceous rocks. In the upper part of the formation there are interbeds of siltstones enriched in biotite and muscovite up to 3–5%, sometimes up to 13%. Usually, however, mica does not exceed 0.5–1%. The siltstones constantly contain clayey material of kaolinite–hydromica composition in amounts of 5–15%, more rarely up to 35%; siderite in the siltstones is 1–5%, dolomite 1–3%, sometimes up to 17%, and calcite usually not more than 0.5–1%. The siltstones are characterized by contact, locally basal, cement of clayey, more rarely dolomitic composition, in combination with pore cement of calcitic composition.

Argillites are characterized by a gray, dark-gray to black coloration. They contain carbonized plant detritus in varying amounts and therefore are sometimes coaly. Horizontal- and wavy-bedded varieties predominate. Where sandy and aleuritic material is present, the argillites are lenticular- and cross-bedded. They constantly contain aleuritic quartz–feldspar material in amounts of 5–25%. The bulk of the argillites is characterized by a scaled structure, oriented parallel to the bedding, and by a kaolinite–hydromica composition. It sometimes contains small, irregularly shaped aggregates of pelitomorphic siderite in amounts of 6–8%. More rarely, the argillites contain calcite and dolomite, which fill rare small pores.

Coal occurs as interbeds 0.1–0.4 mm thick in the lower part of the formation. They are a semimatte, brittle, black, structureless coaly mass with inclusions of plant detritus.

Siderites are confined to the lower part of the formation. They are characterized by a brownish-gray color and a spherulitic structure. These are dense, indistinctly bedded rocks that sometimes contain up to 10–20% quartz–feldspar material of aleuritic size. The sizes of the spherulites vary within 0.1–0.9 mm. Isometric, round, concentrically zoned forms predominate; elliptical forms are encountered more rarely. They are cemented by a yellowish-brown pelitomorphic or fine-grained sideritic mass. In the cores of the spherulites there are either quartz or feldspar fragments, or the largest crystals of calcite or dolomite. Sometimes fragments of plants with cellular structure are noticeable in the cores of the spherulites.

of the Tyumen Formation, we note that everywhere its deposits formed in continental facies.

Grussandstones and gravelites are proluvial-deluvial sediments that formed through the destruction of local uplifts of the basement. Sandstones and siltstones are mainly deposits of alluvial plains. Argillites, siderites, and coals formed chiefly under swamp and lake conditions. In general, the deposits of the Tyumen Formation are characterized by the constant presence of plant material, weak rounding and poor sorting of clastic material, the widespread development of siderite and dolomite, and a considerable degree of diagenetic alteration. The latter was expressed in compaction of clastic material with the formation of pressure-solution cementation, in dissolution of the outlines of quartz and feldspar grains. These changes were also manifested in deformation of biotite and its chloritization, in recrystallization of calcite and dolomite, and in the neoformation of quartz, albite, anatase, and leucoxene in the pore space. All this worsened the reservoir properties of the terrigenous rocks of the Tyumen Formation. However, coarse-clastic rocks in which the cement does not exceed 5–10% may serve as oil and gas reservoirs [6].

Table 1

Terrigenous-mineralogical assemblages of the Tyumen Formation according to different authors

In connection with the particular conditions of formation, the deposits of the Tyumen Formation are characterized by a specific assemblage of heavy-fraction minerals. The mineral that constantly and everywhere predominates is siderite, making up 30–97% of the heavy fraction. In all studied sections, zircon (10–30%) and tourmaline (5–10%), which are products of redeposition of the weathering crust, occur in increased amounts.

In the area of the South-Cheremshanskaya and Kolotushnaya areas, the heavy fraction contains significant amounts of epidote (5–8%) and spinel (20–25%), which indicates the influence of local source areas. Among other minerals, in the form of isolated and rare grains, garnet, rutile, ilmenite, apatite, hornblende, magnetite, sphene, anatase, leucoxene, pyrite, and galena are found irregularly. In individual samples, staurolite, kyanite, and andalusite are present. Analysis of the distribution and composition of heavy-fraction minerals within the studied areas establishes a single correlative mineralogical assemblage of siderite-zircon-tourmaline composition.

References

1. F. S. Buzulukov, T. I. Gurova, L. I. Korobeinikova, V. A. Pluman, A. G. Poda, E. G. Sorokina, K. V. Yaskina. Lithology of the Mesozoic and Cenozoic of the West Siberian Lowland. Moscow, Gostoptekhizdat, 1957.

2. A. V. Golbert, L. G. Markova, I. D. Polyakova, V. N. Saks, Yu. V. Teslenko. Paleolandscapes of Western Siberia in the Jurassic, Cretaceous, and Paleogene. Moscow, “Nauka,” 1968.

3. T. I. Gurova, V. P. Kazarinov, V. N. Sokolov. Geology and oil-bearing capacity of the West Siberian Lowland. Proceedings of VNIGRI, issue 114. Leningrad, Gostoptekhizdat, 1958.

4. V. P. Kazarinov. Mesozoic and Cenozoic Deposits of Western Siberia. Moscow, Gostoptekhizdat, 1958.

5. T. I. Osyko. Lower and Middle Jurassic (Zavodoukovskaya Series). In: Geological Structure and Prospects for Oil-and-Gas Potential of the West Siberian Lowland. Moscow, Gostoptekhizdat, 1958.

6. S. G. Sarkisyan, T. I. Gurova, E. F. Sorokina, M. V. Korzh. Lithology of the Mesozoic and Paleogene of the Oil-and-Gas-Bearing Regions of the West Siberian Lowland. Moscow, “Nauka,” 1967.

7. E. V. Shumilova. Terrigenous Components of Mesozoic and Cenozoic Deposits of the West Siberian Lowland. Novosibirsk, 1963.