Biodegradation of high-viscosity oil of the Ashal'chinskoe field by indigenous microflora of soil on the basis of fen peat and humus with a high concentration of nutrients and microorganisms has been studied under laboratory conditions on model soil systems. It has been shown that after the adaptation period of ten days, the total population of heterotrophic bacteria increases by two orders of magnitude and the activity of soil enzymes increases by a factor of 2-3.5. Oil utilization was 84% over 180 days of the experiment. During this period of time, all petroleum hydrocarbons: alkanes, naphthenes, and arenes, are oxidized by 68-99%; the total biodegradation of resins and asphaltenes making 85.6 and 55.2%, respectively; i.e., the resins are oxidized easier that the asphaltenes. It has been found that the biotransformation of the high-molecular-weight hetero compounds resins and asphaltenes results in partial degradation of saturated structures and accumulation of oxygen in the composition of average molecules, with the average size of the block decreasing and naphthene structures partially degrading.