Petrographic analysis under microscope served to determine the degree of diagenetic changes in the Carboniferous sandstones sampled from the borehole core Drogomyśl IG-1. The degree of diagenetic engagement in the sandstones was found to be growing with the depth below the ground level. The transformation is revealed by disappearance of quartz grain edges, partial or entire dissolution of quartz grains with re-crystallization of chalcedony and also by: Si-enrichment of feldspars and mica, kaolinization of feldspars and biotite, carbonization of quartz, feldspars and mica as well as chlorization of biotite. On the basis of their mineral composition the sandstones were classified to the group of minerals which are both mechanically and chemically stable. Results of performed X-ray diffraction analysis of clay minerals in the sandstones indicated to the temperature range of diagenetic transformation from 120oC to above 160oC. It should be emphasized that the appearance of secondary porosity in the sandstones, as a result of diagenetic transformation, may have positive impact on the gas accumulation. Therefore, tha described method can be useful for evaluation of diagenetic potential of sandstones.