Petrographic Types of Tertiary-Age Coals in the Western Part of Tomsk Oblast

PETROGRAPHIC TYPES OF TERTIARY-AGE COALS IN THE WESTERN PART OF TOMSK OBLAST

O. A. DULZON, V. L. KOKUNOV

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

Brown coals of the lignite type, associated with deposits of Tertiary age on the territory of Tomsk Oblast, have been known in the vicinity of the city of Tomsk since the 1930s of the present century (A. V. Aksarin, 1959). They occur at comparatively shallow depths below the ground surface, possess fairly good technological properties, and undoubtedly in the near future will be of practical interest.

Recently, on the territory of the Kozhevnikovsky and Bakcharsky districts of Tomsk Oblast, during the drilling of wells by the Tomsk Complex Expedition, a whole series of new occurrences of brown-coal deposits associated with the Novomikhailovskaya suite of Tertiary age was discovered. The depth of occurrence of the coals ranges from 20 to 148 meters. Coal seams were penetrated by a number of wells (3k, 4, 5, 6k, 7, 9k, 10k, 11, 12, 13, 14, and 16), traced along the Iksha River for a very considerable distance, and, most likely, represent two coal seams of highly variable thickness, with a layered structure and rather high ash content.

A petrographic description of Tertiary-age coals of Tomsk Oblast is not found in the literature, and this brief article gives, of course, far from a comprehensive description of the petrographic types of Tertiary coals of Tomsk Oblast, but only of those that we have had occasion to study.

Macroscopically, the coals belong to earthy and lignitic varieties. Usually the first variety consists of loose earthy coals with an uneven, rough fracture. Bedding is not at all expressed in them; the structure is lumpy. Their color is grayish-brown, and the luster is dull.

The second variety of coals is composed of small or large pieces of lignitized wood and has a lighter yellowish-brown color and a dull luster. At the same time, the woody structure of individual fragments is very conspicuous.

The coals are light, with a specific gravity on the order of 0.80–0.90 g/cm³.

Study of the coals under the microscope showed that these coals are typical humic coals and are composed of remains of terrestrial vegetation. Remains of algae in them are rare.

Among the earthy coals, the following petrogenetic types are distinguished under the microscope:

1. Clarain type.
2. Durain-clarain type.
3. Clarain-durain type.
4. Durain type.

The remaining 9 samples belong to the lignite type of coal. Thus, earthy varieties of coal are predominant in structure.

The clarain type of coal under the microscope is characterized by a dark-brown and yellowish-brown coloration.

Among the plant fragments there are observed: bands of semitellinite of dark-brown color with unfilled or partly filled cell cavities (the size of the bands ranges from 0.40 × 0.80 mm to 0.40 × 4.00 mm), lenses of homogeneous tellinite of dark-brown color measuring 0.20 × 4.00 mm with the cracking characteristic of it.

Among the liptinite components there are: yellow cutinite with indistinct denticles, yellow and light-orange resinite measuring 0.053 mm, and algotellinite of greenish-golden color measuring 0.053 mm.

According to the predominant kind of liptinite components in coals of this type, three petrographic varieties may be distinguished:

1) sporinite clarain,
2) cutinite clarain,
3) mixtinite clarain.

The durain-clarain type of coal under the microscope is characterized by light-brown and dark-brown coloration.

Plant fragments are observed in the form of lens-shaped bands of semitellinite of dark-brown and yellowish-brown color with partly filled cell cavities, often bordered by cutinite. The size of the lenses is 0.10 × 4.00 mm and 0.08 × 2.00 mm. In addition, bands of homogeneous tellinite of dark-brown color, measuring 0.30 × 1.40 mm, occur. Among the liptinite elements there are: isolated inclusions of yellowish resinite, sporinite (macrospores) of brownish-yellow color measuring 0.08 × 4.00 mm, and yellow cutinite with distinct and indistinct denticles. The accumulation of cutinite indicates the flow of water in the peat bog.

The clarain-durain type of coal is composed of collinitе, cellular fragments, and liptinite elements.

In the light-brown collinitе there are observed: plates of semifusinite of black-brown color with a destroyed cellular structure (plate size 0.10 × 0.20 mm); strips and small lenses of semitellinite of light-brown and dark-brown color with partly filled cell cavities, bordered by cutinite and without it (strip size 0.20 × 1.00 mm and small-lens size 0.10 × 0.50 mm); small fragments of small lenses of homogeneous tellinite of dark-brown color (small-lens size 0.053 × 0.60 mm). Transverse shrinkage cracks are clearly visible.

Among the liptinite elements there are: yellow cutinite with distinct and indistinct denticles, isolated inclusions of yellow resinite.

The durain type of coal is characterized by a predominance of cellular fusinized components over collinitе.

In the collinitic light-brown mass there are observed: fragments of transverse sections of wood with clearly expressed annual rings. The color of the wood is black, the size of the fragments is 1.20 × 4.60 mm. Bands of light-orange tissue with clearly expressed rounded openings—traces of resinite—occur, as do bands of semitellinite of light-brown color with partly filled cell cavities. Isolated lenses of tellinite have a light-orange color, and the cell cavities in them are filled with suberinite material.

Of the liptinite elements there are: yellow cutinite with indistinct denticles, isolated resinites of yellow and light-orange color, measuring 0.053 × 0.14 mm, 0.053 × 0.053 mm, and isolated yellow sporinite.

It should be noted that for the durenic type of coal a sharp predominance of cutinitic material over the other liptinite elements is characteristic.

The lignitic type of coal usually consists of large fragments of lignitized woody tissues of brown color with bordered pits, characteristic of coniferous plants of the Pinus, Picea type, or else of tissues with scalariform perforation of conducting vessels, characteristic of deciduous woody plants of the Betula type. The woody tissues are most often deformed and compressed, but, when compared with the photographs of V. E. Moskaleva (1960), they show almost complete similarity. Usually pieces of lignitized wood are cemented by a certain amount of attrital collinitic mass, consisting of semi-decomposed woody tissue, sometimes of parenchymatous leaf tissue. The collinite usually contains fungal sclerotia. Coals of this type usually do not contain liptinite elements.

In conclusion it is necessary to note that, as the petrographic study of the coals has shown, the coal-forming plants of these coals were mainly plants of ancient appearance, belonging both to coniferous and to deciduous species, as is evidenced by transverse, tangential, and also longitudinal sections of the wood of the above-mentioned plant types recorded in polished sections.

Very interesting is the discovery in polished sections of the described coals of fungal sclerotia characteristic of Mesozoic coals, namely sclerotia possessing a compacted structureless peripheral ring resembling a shell. These fungi, until now considered characteristic of Mesozoic and even Paleozoic coals (Yu. A. Zhemchuzhnikov, A. I. Ginzburg, 1960), were found by us in samples from borehole 9 k.

The rather high ash content and variable petrographic composition characterize the coals as representatives of the facies of flowing, silted-up lowland-floodplain bogs, which were often subjected to processes of eluviation (washing out of humic substances).

We did not carry out a technological study of the coals, but, evidently, they will give a fairly high percentage yield of primary tars, since analogous coals from the Yarskoe, Kazanskoe, Rezhenskoe, and Shegarskoe deposits give on average 12–17.5% yield of primary tars (Aksarin A. V. 1959).

Thus, the coals studied are earthy and lignitic coals typical of the Tertiary period, of very low coalification, at the stage of brown coals.

LITERATURE

1. A. V. Aksarin, Nonmetallic Minerals of Tomsk Oblast, Tomsk, 1959.
2. Yu. A. Zhemchuzhnikov, A. I. Ginzburg. Fundamentals of Coal Petrology. Publishing House of the Academy of Sciences of the USSR, 1960.
3. V. E. Moskaleva. The Structure of Wood and Its Changes under Physical and Mechanical Actions. Publishing House of the Academy of Sciences of the USSR, 1960.