PETROGRAPHIC CHARACTERIZATION OF COALS FROM THE BEREZOVSKOYE DEPOSIT OF THE KANSK–ACHINSK BROWN COAL BASIN

O. A. Dulzon, V. L. Kokunov

(Presented by Prof. A. V. Aksarin)

The Berezovskoye brown-coal deposit is located within Krasnoyarsk Krai and, in tectonic terms, is confined to the northwestern marginal part of the Minusinsk trough of the Sayano-Altai folded region.

The macroscopic description of the coals was carried out on core material. Samples were taken directly during the drilling of boreholes located along two intersecting profiles in the western periclinal part of the Berezovskaya trough.

As a result of the macroscopic study, by luster the coals were divided by us into dull coals, constituting 79%, and semidull with semilustrous coals, constituting 20.8% of the total coal mass.

In terms of macrostructure, the coals of the Berezovskoye deposit are subdivided into striated—67.8%, striated-banded—27.1%, banded—2.6%, and homogeneous—2.5%. Thus, the greater part of the studied coals is either striated or striated-banded.

In the coals under study, two simple lithotypes are clearly distinguished macroscopically—vitren and fusen—and a third, of complex composition: semidull or semilustrous coal.

Microcomponent Composition of the Coals

The microscopic description of the coals was carried out in transparent polished sections in transmitted light, but with obligatory checking of the behavior of the microcomponents under crossed nicols.

In describing the polished sections, the nomenclature of microcomponents according to GOST 9416-60 was adopted. The percentage composition of the microcomponents was calculated in the polished sections using an integration stage of the ISA type, and conversion to the percentage composition of microcomponents in the seams was carried out according to the method of Z. V. Ergolskaya [4].

Before proceeding to the characterization of the microcomponents of the Berezovskoye coals, it should be noted that they are composed mainly of the remains of higher plants that underwent processes of gelification, and to a lesser extent fusenization and eluvation. A sharp predominance is observed of gelified lignin-cellulose plant tissues in the form of microcomponents of the vitrinite group over the microcomponents of the other groups.

A very characteristic feature of the Berezovskoye coals is

the structural character of almost all stem remains. This feature indicates that the process of gelification did not proceed to completion, to the formation of a completely structureless, homogeneous gelified mass, and consequently the burial of plant remains took place rather rapidly.

Below is a description of the groups of microcomponents characteristic of the Berezovka coals.

Vitrinite Group

Within this group, components are distinguished that are recognizable both with the naked eye and only under the microscope.

Vitrene. This simple ingredient (lithotype) is readily distinguished in the Berezovka coals by the naked eye. It stands out sharply from the main coal mass by its darker, uniform coloration, as well as by its stronger luster. On a vertical fracture, vitrene commonly occurs as very small lenses measuring \(0.5 \times 3.0\ \mathrm{cm}\) and as bands up to \(1.0\ \mathrm{cm}\) wide. It is most often observed in the form of long, thin lenses corresponding to small branches, stems, and sometimes comparatively large tree trunks. On the bedding surface, vitrene also appears as bands oriented in various directions.

The presence of vitrene lenses gives the coals a streaky structure, and streaky coals occupy a dominant position in the seams of the Berezovka deposit.

The lenses and bands of vitrene are highly characterized by endogenous fracturing perpendicular to the bedding.

Under the microscope, this group includes collinite “A,” collinite “B,” and several varieties of telinite.

Collinite “A” is represented by lenses and bands with sharp outlines and an absence of cellular structure. In transmitted light, with one nicol, it has a reddish-brown, more rarely dark-brown color. In crossed nicols it does not produce pseudostructures. Small lenses and bands have only vertical fracturing, while larger ones have both vertical and oblique fracturing.

Collinite “B” has a lighter color in transmitted light than collinite “A” and a weakly expressed xylovitrene structure. Collinite “B” consists of small rounded lumps, representing small fragments of tissues that have already lost the sharpness of their outlines. This component is the most widely distributed in the Berezovka coals, making up the principal mass of the coals.

Telinite. The microcomponents of this group have a characteristic reddish-brown color in transmitted light, with one or another manifestation of plant structure. Among them are plant fragments with cellular cavities completely filled in, but preserving the pattern of the plant tissue. Among them, fragments with a xylovitrene structure are distinguished, as well as fragments with a xylenic structure. All these microcomponents are observed in polished sections in the form of fragments of one shape or another, densely or sparsely saturating the collinitic mass of the coal. This group includes:

1. Telinite “A” is reddish-brown in color and has the form of bands with traces of conducting tissues of plants. The cell walls are clearly visible. In crossed nicols, telinite “A” does not produce pseudostructures.

2. Telinite “B” is represented by convex lenses with sinuous boundaries, bordered by cutinite, measuring \(0.16 \times 2.0\ \mathrm{mm}\). The lenses are filled with loose parenchymatous gelified tissue of reddish-

brown in color. Probably the original material of telinite “B” consists of leaves of ginkgo and coniferous plants. This is clearly indicated by both their shape and their dimensions.

1. Telinite “B” consists of fragments of every possible shape and has two characteristic features: a reddish-brown color and a tile-like cellular structure. These are fragments of the bark tissue of woody plants. The fragments of bark tissue evidently represent transverse sections of the cork of stems and twigs of woody forms of plants.

Bark tissue is often observed whose cell walls are impregnated with a yellow or dark-orange substance. This suberinite-like substance, filling the cavities of plant cells in a coarse network, probably contributes to the preservation of fragments during decomposition processes. In any case, when cork cells decompose, cavities filled with a yellow substance remain intact, and then fragments of a “vesicular structure” of yellow color with threadlike, streaky spots of reddish-brown color are observed.

Yu. A. Zhemchuzhnikov and A. I. Ginzburg [5] call similar yellow formations suberinic substance. They believe that bark tissue produces suberinic substance. Botanists [2] suppose that suberin impregnates the walls of cork cells and makes them impermeable to water and gas. Suberinite is formed together with friable cork cambium.

In our study of the material composition of the Berezovsk coals, fragments of bark tissue with the characteristic tabular structure of cells inherent in cork, without yellow substance, were assigned to telinite “B” and counted as microcomponents of the vitrinite group. Bands with a yellow “vesicular structure,” where suberinic substance predominates over the reddish-brown gelified material, were assigned by us to suberinite and counted as representatives of the liptinite group.

Semivitrinite Group

This group includes structureless semicolinite and structured semitelinite, which in transmitted light have a light-brown or brown color. The structure of the plant fragments is the same as in the vitrinite group; the main distinction of the components of the semivitrinite group is their brown color.

Semicolinite has a weakly expressed fine-lumpy (xylovitrainous) structure. Bands and lenses are brown and dark brown in color. For fragments with dark-brown coloration, as representatives of this group, the superposition of the fusenization process on the gelification process is characteristic.

Semitelinite possesses a cellular (xylene) structure and is observed as strips measuring \(0.6 \times 2.0\) mm and small plates measuring \(0.2 \times 0.4\) mm. It occurs in all coal varieties. The main features are the xylene structure and brown color.

Mixtinite is a fine mixture of collinite with micrinite or mineral impurities. Usually mixtinite has a dark-brown color owing to small black plates of fusenoattrite. In the durain and fusinite-semitelinite types of coal, mixtinite plays the role of the main mass cementing the plant fragments.

Fusinite Group

This group of microcomponents, which sometimes constitutes entire lithotypes of the Berezovskoye coals, includes fragments with clear relics of the anatomical structure of the coal-forming plants and, depending on the degree to which gelification is superimposed on the process of fusinization, is subdivided into micrinite, semifusinite, and fusinite.

Micrinite occurs rarely in the Berezovskoye coals and is a structureless microcomponent with smooth outlines, saturating the collinitic mass to a greater or lesser degree. The fragments are black in color and not less than 0.02 mm in size.

Semifusinite is also rarely found in the coals and consists of fragments of plants having a dark-brown, almost black color. In color it occupies an intermediate position between semitellinite and fusinite. Depending on structure, however, we subdivide semifusinite into two varieties. Fragments with a xylovitrain structure, having a dark-brown, almost black color, are assigned by us to semifusinite “A,” while fragments with a xylene structure are assigned to semifusinite “B.” Mutual transitions of gelified tissues into fusinized ones are observed within a single fragment.

Fusinite forms whole interbeds in the Berezovskoye coals and is then regarded by us as a lithotype. In external appearance it resembles ordinary charcoal, with a fibrous structure and a silky luster.

The number of clearly visible fusain interbeds in boreholes varies from 1–2 to 17. Their frequent alternation can in all probability be explained by the fact that in the coal-accumulation basin there was a periodic rise and fall in the groundwater level.

The fusinite fragments encountered in the Berezovskoye coals have the form of thin interlayers, from barely visible to 0.1–1.5 cm and more in width. Characteristic of fusinite is its tabular structure, which is clearly observed on the bedding planes. The fusinite plates have the form of regular quadrangles or parallelepipeds, variously oriented. In transverse coal hand specimens, fusinite is observed in the form of strokes and also as narrow bands. This is clearly an autochthonous formation.

The walls of fusinite cells are completely opaque, black; the shape of the cell cavities is varied, and their dimensions are usually 0.02–0.04 mm. Depending on the direction of the section of the plant, it may range from square, with slightly rounded corners, to strongly elongated in oblique or radial section, and with figures of bordered pores in tangential section. Sometimes a gradual transition of fusinite into semitellinite “B” is observed within a single fragment.

The shape of the cell openings in fusinized fragments depends on which part of the stem, little stem, or twig is observed in the polished section. When we are dealing with the pith part of the stem, we see rather large cell cavities arranged in disorder. Such a picture, in the opinion of V. V. Drushchits [3], is produced by transverse sections of ginkgo plants.

Fragments of the marginal parts of the remains of stems, little stems, and twigs show cell cavities of regular subquadrangular form, which are arranged strictly concentrically; very often the annual growth rings of the plants are preserved.

It is necessary to point out the following regularity with respect to the preservation of the cellular structure of fusain fragments in the Berezovskoye coals—

...coals. If fusinite in the seam is contained in more or less thin interlayers or inclusions, then its cellular structure is well preserved. If, however, fusain forms thick interlayers, then a stellate structure of the fusain, or one even more comminuted, is often observed here.

In some cases the amount of fusain is so considerable that the coal becomes matte in the hand-specimen fracture. The thickness of such beds rapidly pinches out, reaching half a meter and more. In Berezovskoye coals, fusainite also occurs in the form of cubic pieces and strips with frayed ends, more often embedded in a semi-matte than in a semi-bright principal mass. In all probability, this modification of fusinite inclusions formed from allochthonous source material, in connection with the swamping of new areas as the water level rose.

The phenomenon of introduction of allochthonous material, previously oxidized and yielding fusinite in the form of plates and strips with frayed ends (which is clearly observed under the microscope), is encountered very often in Berezovskoye coals.

Leiptinite Group

In the coals of this deposit the leiptinite group is represented by sporinite, cutinite, resinite, and suberinite. In transmitted light with one Nicol prism, leiptinite elements have a golden-yellow, yellow to dark-orange color. In reflected light they have a pale-gray color and high relief.

Sporinite in the coals is represented by coats of macro- and microspores.

Macrospores were observed by us only in two polished sections, in the form of fragments of exine measuring 0.02 × 0.25 mm. The exines of macrospores have a yellow color and sharp relief; in crossed Nicols they have a weak gray color and relief. The surface of the macrospores has a knobby structure.

Microspores are observed in the coal in the form of small yellow streaks. The sizes of microspore exines range from 0.015 to 0.075 mm. The form of the microspores is simple—thin, short streaks, occasionally bent in the form of hooks. Microspores are usually evenly dispersed in the collinitic mass.

Cutinite. In Berezovskoye coals there are two varieties of cutinite: thin and thick. Thin cutinite is observed with clearly expressed serrations, sometimes without them (the thickness does not exceed 0.02 mm). With one Nicol prism, cutinite has a bright-yellow color, and in crossed Nicols it becomes gray. Cutinite usually rims the lenticles of tellinite “B.” Cutinite is encountered rarely and does not form large accumulations. Often cutinite is observed in the form of fragments up to 0.30 mm in size. Thick cutinite is encountered very rarely and is not characteristic of the coals of the Berezovskoye deposit.

Resinite occurs in the coals in the form of angular and oval bodies. Resinite has yellow, light-orange, and dark-orange colors, and with crossed Nicols a darker color.

Among the leiptinite elements, resinite is distinguished by its even contours and by the presence of fissuring in the form of a fine network, while the integrity of the resinite bodies is preserved. The size of the resinite bodies ranges from 0.05 to 0.30 mm.

Suberinite. Suberinite material, impregnating the bark tissues in a sparse network, is fairly widely distributed in the coals of the Berezovskoye, as well as the Irsha-Borodinskoye and Nazarovskoye deposits [1, 8], which we studied earlier.

The color of suberinite ranges from amber-yellow to orange-yellow; the structure is fibrous, determined by the structure of the walls of cork tissues. Pure suberinitic matter is not found in the coals and only surrounds the cork cells with a thin network. In crossed nicols, suberinite does not produce pseudostructures, and its color does not change.

Fragments of cork tissue are abundantly impregnated with suberinitic matter and form a vesicular tissue in which the cells of the cork proper appear against a yellow background as bright-brown spots.

Alginite Group

This group is represented by algotelenite, which occurs in the coals rather often, but in small amounts, approximately 0.33–0.57%. Algotelenite has a semiellipsoidal form and oval outlines. The color of algotelenite is golden-greenish in one nicol, and light gray in crossed nicols. In some specimens, mottling is observed—probably a reflection of the internal structure. This phenomenon is especially clearly observed in colonies of large size (0.5 mm). The sizes of algotelenite colonies range from 0.04 × 0.40 mm to 0.05 × 0.10 mm. More often, algotelenite colonies of 0.040 × 0.05 mm are observed. Algotelenite of this appearance is assigned by all researchers to the genus Pila.

Mineral Inclusions in Coals

Mineral inclusions in coals belong to the terrigenous, syngenetic type. Macroscopically they are not detected.

The terrigenous minerals include siliceous minerals (chalcedony, quartz) and clay minerals (kaolinite, hydromicas).

Quartz is the most widespread mineral. It occurs in the form of angular, almost round grains measuring from 0.02 × 0.02 to 0.05 × 0.08 mm, immersed in the collinitic mass.

Chalcedony is observed very rarely in the collinitic mass of the coal, in the form of small angular inclusions measuring 0.021 × 0.042 mm.

Kaolinite is represented by small flakes dispersed in the collinitic mass.

Hydromicas occur in the coal as a dispersed, disseminated clayey substance. In the presence of this impurity, the collinitic mass acquires a dirty-brown color.

Petrogenetic Types of Coals

Study of the coals of the Berezovskoye deposit under the microscope showed that their composition includes representatives of the class of humites and sapropelite-humites, which are subdivided into a number of petrogenetic types of coals, representing the corresponding facies of coal accumulation.

The class of humites in the Berezovskoye coals includes the following petrogenetic types.

Claren Type — KT

This is a representative of stagnant bogs with a high degree of waterlogging. Claren coals constitute 81.1% of the thickness of the Berezovskoye seam. The claren type of coal is subdivided into two petrogenetic subtypes with their own varieties.

I. Dull, streaked clarain coal with vitrain and fusain—KT-1.

1. Telinite clarain.
2. Fusinite-semitelinite clarain.

II. Semi-dull, streaked clarain coal with vitrain—KT-2.

1. Cutinite clarain.
2. Sporinite clarain.
3. Sporinite-cutinite clarain.

Dureno-clarain type—DKT

The dureno-clarain type of coal is representative of the facies of a fen bog with very weak flow-through. In all, this coal type accounts for 13.0%. This coal type forms persistent interbeds with a thickness from 1.0 to 6.0 m. This coal type is subdivided into two petrographic varieties:

1. Telinite dureno-clarain,
2. Fusinite-semitelinite dureno-clarain.

Clarain-durene type—KDT

The clarain-durene type, as representative of the facies of fen bogs with weak water flow-through, also forms beds with thicknesses from 1.0 to 1.5 m and constitutes 2.4% of the seam. Coal of this type is subdivided into three petrographic varieties:

1. Telinite clarain-durene,
2. Cutinite clarain-durene,
3. Sporinite-cutinite clarain-durene.

Durene type—DT

The durene type of coal is representative of the facies of a flowing fen bog. This coal type forms weakly expressed interbeds with thicknesses from 1.0 m to 1.5 m and constitutes 2.5% of the seam mass.

This coal type makes up an insignificant part of the seam and does not have a broad distribution in space or in time.

This coal type is subdivided into two petrographic varieties:

1. Telinite durene.
2. Semifusinite durene.

Fusinite-semifusinite type—FST

The fusinite-semifusinite type of coal is representative of the facies of a weakly waterlogged forest bog. It forms individual lenses 0.5–2.0 m thick and constitutes 0.2% of the Berezovskoye seam. Coal of this type is subdivided into two varieties:

1. Homogeneous fusain coal.
2. Fusinite-semitelinite coal.

Sapropelite-humite class—SG

Representatives of sapropelite-humite coals are characterized by the presence of algotelinites in the humic collinitic mass. The percentage composition of algotelinite in these coals is low, but from the genetic point of view these coals are representatives of the facies of waterlogged

lakes. Sapropelite–humic coal is represented by an algotelinite clarene. Sapropelite–humite coals occur rarely. This type of coal forms small interbeds and constitutes 0.8% of the seam thickness.

In the present work the petrographic and material composition of Jurassic brown coals from the Berezovskoye deposit of the Kansk-Achinsk coal-bearing basin is considered.

In studying the geological structure of the Berezovskoye deposit, many common features were found that are characteristic of a number of other deposits of the Kansk-Achinsk basin (Abanskoye, Irsha-Borodinskoye, Nazarovskoye), which makes it possible to expect much in common also in the material composition of the coals [1–8].

Most significant is the uniform and very close time of formation of the coal-bearing series, as well as of the coal seams enclosed in them. The age of the deposits studied is dated by impressions of leaf flora identified by A. V. Aksarin, and also by spore-pollen assemblages identified by N. S. Sakhanova-Grigorieva and L. G. Markova.

Let us note the following important factor that has affected the commonality of features in the material composition of the coals of the deposits studied. The above-mentioned deposits are included in one of the coal-accumulation belts of P. I. Stepanov [6], which according to N. M. Strakhov [7] is an ancient zone of humid climate. A study of the composition of the coal-forming plants, as well as the study of the composition of the vegetation cover of this zone carried out by us, showed that the composition of the vegetation cover, as well as the composition of the coal-forming plants, has very much in common. Naturally, with such a great similarity in the composition of the coal-forming plants, it is difficult to expect a large difference in the material composition of the coals of the seams studied. The source material of the coals of the Berezovskoye deposit consists mainly of woody forms of plants of conifer–ginkgo composition and, only to a lesser extent, sphagnum moss material.

According to data on petrographic composition and ash content, the coals of the Berezovskoye deposit apparently were formed mainly in raised-type bogs.

In conclusion it should be noted that the microcomponent composition of the coals of the Berezovskoye deposit, especially of the Berezovsky seam, makes it possible to expect good technological qualities of coals suitable not only for energy purposes but also for chemical processing.

REFERENCES

1. A. V. Aksarin, O. A. Dulzon, V. L. Kokunov, L. A. Pukhlyakov. Material composition of the Jurassic coals of the Berezovskoye and Abanskoye deposits of the Kansk-Achinsk basin. Report on contractual subject No. 76/62 with the Krasnoyarsk Administration. Manuscript, funds of KGU, 1964.

2. P. A. Bogdanov. Botany. Goslesbumizdat, 1961.

3. V. V. Drushchits, T. A. Yakubovskaya. Paleobotanical Atlas. Moscow State University Press, 1961.

4. Z. V. Ergolskaya. Methodology of petrographic investigation of a coal seam for determining coal quality. Funds of ZSGU, 1947.

5. Yu. A. Zhemchuzhnikov, A. I. Ginzburg. Fundamentals of Coal Petrology. Publishing House of the Academy of Sciences of the USSR, 1960.

6. P. I. Stepanov, S. I. Mironov. Geology of Caustobiolith Deposits. ONTI, 1967.

7. N. M. Strakhov. Fundamentals of the Theory of Lithogenesis. Publishing House of the Academy of Sciences of the USSR, 1962.

8. V. L. Kokunov. Petrographic characterization of the Borodinsky seam of the Irsha-Borodinskoye deposit of the Kansk brown-coal basin. Izv. TPI, vol. 122, issue 1, 1964.