An investigation has been made of the compositions of two characteristic fossil fuels encountered in Permian deposits in Tatarstan: heavy crude oil with a density of 0.9413 g/cm(3), concentrated in reservoir rock, and semiviscous bitumen with a density of 0.9945 g/cm(3), dispersed in bituminous sandstone outcrops. These are classified as type B-2 and B-1 oils respectively. It has been established that oils of these types have a common origin but differ substantially in their component and fractional compositions. Here a distinguishing feature of the bitumen, besides the absence of normal and isoprenoidal alkanes, and also the low content of vanadyl porphyrins, is the considerable predominance of asphaltenes and resins extracted with an alcohol-benzene mixture and rich in hydroxyl and ester groups. It has been shown that thermal degradation of the bitumen is more intense than that of the crude oil, with the formation of considerable amounts of hydrogen sulphide and mercaptans owing to the breakdown, above all, of resins extracted from silica gel with an alcohol-benzene mixture and formed with the participation of sulphur and oxygen atoms. As a result of comparing the natural variations observed in the composition of the Permian specimens investigated with experimental data obtained during the action of atmospheric and microbiological factors on Devonian type Al paraffinic crude oil, it is concluded that, by comparison with heavy crude oil, bitumen in rock is the product not only of more extensive biochemical oxidation, but also of subaerial weathering of, it appears, fairly light crude oil that has migrated from lower-lying layers.