Severočeské doly a.s.



The North Bohemian Brown Coal Basin is a relict of a Tertiary sedimentary basin, filled with sedimentary material mostly during the Miocene era. At that period, 22 to 17 million years ago, more than 500 metres of clays, sands and organic material was gathered in the basin. A brown coal seam developed in most parts of the basin, formed from layers of peat deposited in a Tertiary swamp. Near river mouths feeding the swamp with water, the peat sedimentation was compressed by deposited sands and clays. At these locations the seam is either completely replaced by alluvial or delta sediments, or split into several benches. According to the manifestation of these impacts on the seam cross-section, the Basin can be divided into several parts. Most affected by sand and clay deposits was the Žatec delta area. In the remaining part, a more or less integral, 25 to 45 metres thick brown coal seam developed. The outcrop of the coal seam or its equivalent represents the present basin border. Today's remnants of the Basin are spread over an area of about 870 km2. The deepest part of the Basin is the so called Central Region between the towns of Litvínov, Osek and Duchcov and villages Lom and Mariánské Radčice. At present, the coal seam is mined by four open cast mine sites. Two of these - Bílina and Tušimice, are run by Severočeské doly a.s. Chomutov.

The deepest bedrock of the basin is formed mainly by Proterozoic gneisses of the Krušnohorský/Ore Mountains Crystalline Massif, in the Teplice area by a paleorhyolite body of the Carboniferous age and in the Žatec area a considerable part of the bedrock is formed by sandy-clayey sediments of the Permian-Carboniferous age. Large areas of these rocks are additionally covered by up to 200 metre thick maritime Cretaceous sediments - marlstones and sandstones.
The stratigraphic section through the sedimentary fill of the Basin shows that among the oldest basin fill material are Oligocene volcanic rocks of the Střezovské series, forming the coal seam's bedrock, but also the current mountain ranges of České středohoří and Doupovské hory. The weathered horizons originating from these volcanic rocks gradually turn to clays and sands of the Duchcovské series, which already represent the early stages of the Basin's sedimentation history in Lower Miocene. The gradual transformation of the river system which created the Duchcovské series in a huge peat-forming swamp, is in the section indicated by the presence of thin coal sub-seams. Higher up in the section we find a coal seam mainly in the Žatec delta, split into several bands. In the sandy part of the Žatec delta, the coal seam section has been completely replaced by alluvial sandy and clayey sediments. The coal seam and its time equivalents (including the body of the Bílina delta in the coal seam's overburden) belong to the Holešické series. Thickness of the Holešické series varies between 20 and 300m, depending on whether it is represented by the coal seam itself or by its sandy and clayey equivalents. The Libkovické series above them consist of clayey sediments which were formed in a large lake and which represent a major part of the overburden sediments of the Basin. In the deepest part of the Basin, its thickness represents 300 metres. In the central part of the Basin, there developed, at the top of the section, the lakustrine sediments of the Lomské series with only a few metres thick coal seam (with high ash content) at the base.

Bílina mine site
The Bílina open cast mine site with a depth of over 200 m (the deepest point is practically the same level as the Baltic sea and the top level of the highest overburden bench has an elevation of between 220 and 280 m a.m.s.l.) is the deepest mine site in the North Bohemian Brown Coal Basin. The bottom of Bílina mine site is practically the lowest open point which can be reached in the Czech Republic. The coal seam extracted at the deposit is between 25 and 35 m thick and has an average ash content in anhydrous state of 26.9%, content of sulphur 1.03% and a calorific value in its original state of 13.929 MJ/kg. From the degree of carbonisation point of view, it is brown coal in the stage of orthophase bordering with metaphase. Total mineable reserves were as at 31.12.2012, 155 million tons. The best quality seam is at some parts devalued by historical underground mining operations. The overburden of up to 200 m thickness is to a large degree formed by sediments of the so called Bílina delta (Holešické series) and to a lesser degree by lakustrine clayey sediments of the Libkovické Series. The body of the Bílina delta has been opened by the Bílina open cast mine in a transversal direction. Available documentation shows that the body is formed by a very tricky complex of sandy delta corpuses, layers of lakustrine clays and an entanglement of sandy fills of river beds bordered by sandy plain sediments. The complex geological structure complicates the mining operations in the form of water-saturated sandy bodies, difficult-to-mine bodies of sandstones and layers of claystones with the presence of hard, pelocarbonated interbeds. The deposit is being mined in the east to west direction. On the northern side the deposit is limited by a series of zones with the seam absent, on the south by a seam outcrop in the vicinity of the Bílinský fault, and on the western side the deposit is limited by an administratively drawn line - the mining claim border.

Tušimice mine site
The Tušimice open cast mine site, even after extraction of so-called down-timed working face, in the western part of the Tušimice mining area, with its more than 4 km long working face is the one of largest open cast mine in the Basin. The coal seam mined at the Tušimice-Libouš deposit is between 25 and 35 m thick and has an average ash content in anhydrous state of 36.8%, content of sulphur 2.7% and calorific value in its original state of 10.40 MJ/kg. From the degree of carbonisation point of view, it is brown coal in the stage of orthophase. However, the degree of carbonisation is lower than in the Bílina mine site. Total mineable reserves were as at 31.12.2012, 230 million tons. The overall geological reserves are however 461 million tons and the use of homogenising technology enables production of more fuel than responses to the amount of mineable reserves. The extracted coal is used to produce fuel mixes for power generation. The seam is split by large parting layers (Žatecká delta deposits) into three bands. The coal seam overburden up to 120 m thick is formed mainly by lakustrine clays of the Libkovické Series. The part of deposit is being mined from the south to the north and, the second part is extracted in the east direction. In the north, the deposit is limited by the coal seam outcrop at the foothills of the Krušné Hory/Ore Mountains, in the west by the mined out areas of the former Merkur open cast mine and on the eastern and north-eastern side by an administratively drawn line - the mining claim border. Geological structure of the overburden is relatively simple. The overlaying clays can be divided into a lower layer of kaolin to illite like clays of the Holešické Series, and a top layer with an increased content of montmorilonite, belonging to the Libkovické Series. Mining operations are complicated mainly by difficult geo-technical conditions in the vicinity of the coal seam outcrop at the Krušné Hory /Ore Mountains foothills and by local presence of hardened, thin pelocarbonate layers.

Severočeské doly a.s.

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