Microhardness and Coalification Parameters as Sensitive Indicators of Tectonic Deformation in Coal Seams: A Case Study

The formation of hard coal seams is the outcome of multi-stage, complex transformations of organic matter that lead to an increase in carbon content, a decrease in volatile components, and a progressive evolution of the rock’s structure and texture. Diagenetic and metamorphic processes, which underpin coal formation, largely determine its petrographic and geochemical characteristics, but they are not the only factors controlling the final properties of coal. An equally important role is played by the tectonic history of the region in which the coal seams occur. In this study, we carried out an integrated analysis of coal rank, based on vitrinite reflectance measurements (R0), and mechanical properties, using Vickers microhardness tests (Hv). Coal samples were collected from both sides of a fault plane within a single seam. The results show that the presence of the fault is clearly reflected in the measured parameters. Vitrinite reflectance generally increases towards the fault zone, but in the immediate vicinity of the fault, it exhibits a slight decrease. Subtle yet systematic changes are also observed in microhardness, particularly in the Hv values. The results show that vitrinite reflectance (R0) and microhardness (Hv) vary in a very similar manner—both parameters decrease as the degree of structural degradation of coal increases within the fault zone. This consistent response of R0 and Hv to local structural damage suggests that they may serve as sensitive indicators of the presence and extent of influence of small-scale tectonic dislocations. Their combined application provides additional information on the potential occurrence of a fault and on the degree of structural disturbance of coal in its vicinity.