The group hydrocarbon composition of natural bitumens of different levels of the Mortuk field (West Kazakhstan) and the group-structural parameters of the oils, resins and asphaltenes contained in the bitumens have been studied. It has been established that the bitumens investigated are of the aromatic-naphthenic hydrocarbon type. The main relationships governing change in the elemental, group and group-structural composition of Mortuk natural bitumens as a function of the depth of occurrence of the bitumen-containing rock have been revealed. The absence of any compositional or structural criteria distinguishing the high-molecular components of natural bitumens and heavy crude oils of the same type has been shown.