[en] The paper presents brief discussion of the present state of knowledge of the processes of isotope fractionation of coal, proceeding in the nature, with a special attention paid to geochemical cycle of origin of oil. Moreover, the results of isotope studies on 54 samples of Polish oils, and especially those concerning the samples of oils from the Lublin region are briefly discussed. The isotope studies have shown that 8 types of oil derived from Devonian and Carboniferous deposits of that region may be characterized as heavily metamorphosed and well-advanced in maturity processes. A group of 3 oils from Deblin, Abramow and Kock 2 (first horizon) drillings differs from the former in their features, which seems to indicate origin under conditions of lower temperature and lower degree of matamorphism. (author)