Zonation of deposits of hard coals of different porosity in the Upper Silesian Coal Basin

Abstract

The article presents the results of tests of porosity of the Upper-Silesian Coal Basin (USCB) hard coals. The porosity was determined for various lithotypes of hard coal, collected in different areas of the Upper-Silesian Coal Basin. Samples of hard coal were collected in 60 seams of 16 coal mines, from the depth of approximately between 350 and 1200 m. There are also presented differences in values of open porosity of coal depending on the depth of occurrence, as well as chronostratigraphic and tectonic correlation of a seam. Uniaxial compression strength of the tested coals falls in the strength class from very low to very high (from 8.1 to 51.5 MPa), open porosity is 0.68–12.5% and total porosity is 3.29–17.45%. With an increase in depth, in general, open porosity of coals decreases. There is an apparent decrease in open porosity correlated with the age of hard coals. It was observed that the lower and upper limits of open porosity ranges of variability shift towards lower values the older the coals are, from the Łaziskie Beds to the Jaklowieckie Beds. The older the coals the greater the average drop in the share of open porosity in total porosity from 60% for the youngest coals to merely several per cent for the oldest ones. The highest values of total porosity (over 10%) were observed in the youngest coals (Cracow sandstone series and siltstone series). Older coals reached more diversified values of total porosity (3.29–17.45%). Based on the conducted tests, zones correlated with tectonics of the basin and with deposition of lithostratigraphic beds of specific age, where values of porosity of hard coals differ, were determined in the area of the USCB. However, in spite of their correlation to a structural unit, the coals demonstrated a decrease in open porosity with an increase in their geologic age. Thus, there is an apparent influence of the deposition depth of coal within a given structural unit on open porosity following the age of coal, i.e. correlation to an older and older chronostratigraphic unit. Porosity tests of hard coal are published in scientific journals yet, most often, there is no information on the type of porosity. Such detailed characteristics of open porosity for coal of the Upper Silesian Coal Basin, as in the article, have not been prepared yet, making our research novel. Such broad tests of porosity of hard coals, correlated with their strength and referring to the deposition depth of seams as well as a correlation to lithostratigraphic units within individual structural units in the USCB, had not been conducted yet. We believe it is a sufficient argument to start detailed research into the open porosity of hard coals. The results of the tests are important from both the research and practical points of view. Knowledge of coal porosity is indispensable in solving many engineering problems of geological engineering, mining geomechanics and mining e.g.: assessing behaviour of coal when it is mined and processed, assessing the possibility of capturing methane, which makes up over 90% of natural gas, from coal in the areas of methane-rich deposits; the possibility of storing various substances in abandoned hard coal mining areas (e.g. CO2 storage within the structure of coal), assessing risk of occurrence of certain natural hazards resulting from mining deposits (e.g. coal and rocks outburst hazard), as well as assessing the possibility of applying unconventional methods of mining hard coal deposits (e.g. underground coal gasification).

Chemostratigraphic Approach to the Study of Resources’ Deposit in the Upper Silesian Coal Basin (Poland)

“…In a stratigraphic context, differences in mineralogy and/or chemistry can lead to the formation of a geochemical system, which provides valuable information about geological settings, climate, and the processes of sediment production and preservation. The stratigraphic record exhibited changes in the concentration of certain elements over time as a result of geological conditions, such as tectonic, climatic, redox, oceanographic, biotic, and other processes [15][16][17]26,46].…”

“…The Al vs. Si binary diagram showed a negative trend between the two elements, with relatively high concentrations of Al and low concentrations of Si (Figure 7a). This can be explained by the proportion of Al-bearing clay minerals decreasing as Si-bearing minerals (e.g., quartz, zircon, and titanite) increased and vice versa [9,26,50]. The Al2O3 vs. K2O graph illustrates a positive trend between the two elements (R 0.70 and 0.66, in Unit 1 and Unit 3, respectively) (Figure 8a, Table 3), suggesting that K, along with Al, was at least partly concentrated in clay minerals.…”

“…The Al vs. Si binary diagram showed a negative trend between the two elements, with relatively high concentrations of Al and low concentrations of Si (Figure 7a). This can be explained by the proportion of Al-bearing clay minerals decreasing as Si-bearing minerals (e.g., quartz, zircon, and titanite) increased and vice versa [9,26,50].…”

“…The older Paralic series contains marine and brackish fauna horizons, while the younger Limnic series is deposited in a continental paleoenvironment [25]. Most of the coal seams are placed in the Mudstone Series and the Upper Silesian Sandstone Series (Westphalian), where over 3 m thick coal seams are deposited [26]. These series belong to limnic deposits.…”

Influence of Grain Size and Gas Pressure on Diffusion Kinetics and CH4 Sorption Isotherm on Coal

“…larger mesopores and macropores function in coal as pores that transport adsorbate [3]. a crucial element influencing pore structure in coal and porosity is the maceral structure [4]. according to the literature [5], macerals of the inertinite group have the highest porosity, while those of the liptinite group have the lowest.…”