The Tunguska basin, eastern Siberia, contains 3.5-8 km of Late Precambrian to Triassic sedimentary and igneous rocks. Source-reservoir-seal systems are present throughout the Upper Precambrian to Permo-Carboniferous interval. Hydrocarbon generation and accumulation largely preceded the formation of the Siberian traps, a Late Permian to Middle Triassic association of effusive and explosive extrusives and intrusive dolerites. The intrusives occur mainly in Palaeozoic strata and have profoundly affected hydrocarbon accumulation. The major process is of destruction of hydrocarbon accumulations, owing to the fact that substantial volumes of the Palaeozoic basin fill has been heated to 150 degrees C plus. At lower temperatures experienced further from the contacts between the intrusions and the country rocks, organic matter thermal maturation levels may significantly exceed those related to burial alone. Water-mineral-hydrocarbon interactions in association with magmatic heating have produced a range of effects, including the generation of hydrocarbons rich in sulphur compounds such as mercaptans.