Computerized chromato-mass spectrometry has been used to study higher cyclic biomarkers (steranes, tri-, tetra- and pentacyclic terpanes) of crude oils from the main oil-bearing complexes in East Siberia. On the basis of the results obtained, the conditions of generation of crude oils have been determined, and the composition of their initial organic matter has been established. It was noted that early crude oils of Riphean, Vendian and Lower Cambrian age were generated from marine organic matter considerably refined by micro-organisms at the sedimentation stage. Crude oils of Upper Permian, Lower Triassic and Lower Jurassic deposits from the Vilyuisk syneclise were generated from typical continental organic matter. All the crude oils studied are catagenetically mature.