Abstract The West Siberia basin is the largest petroleum province in Russia, with 80% of the country's gas resources in the Cenomanian Pokur Formation. Significant undiscovered gas resources have been assessed as on trend with the giant gas fields. However, the origin of the large amounts of dry, isotopically light gas is still an enigma, albeit extensively addressed in the literature. This study aims at quantifying the gas contribution from all relevant thermal sources. The West Siberia Basin is the world's largest intracratonic basin, comprising up to 12 km of Mesozoic and Cenozoic clastic rocks. The Basement is composed of Palaeozoic accretionary crust. Northward-trending Permian–Triassic rifts were filled by fluvial–deltaic sediments from the south and east, punctuated by marine transgressions from the north. Cenozoic basin inversion formed traps for petroleum. A regional high-resolution 3D basin simulation was used to model the thermal evolution of the northern West Siberia basin. Geostatistical modelling was applied to assess source rock richness and quality. Basal heat flow was modelled by calibration to bottom-hole temperature and vitrinite measurements. Hydrocarbon generation kinetic parameters were derived from measurements performed on West Siberia rock samples. Thermal gas charge expelled from the hydrocarbon kitchen drainage areas of key fields were compared with the gas volumes accumulated in these fields. The study found that Cretaceous terrestrial sources can generate sufficient early thermal gas to charge accumulations in the South Kara Sea area, and additional Jurassic sources can charge the remaining accumulations of the study area if favourable conditions apply. Biogenic gas is likely to have contributed to the gas accumulations. Mixing of thermal and biogenic gas could explain the observed isotopic composition. Sensitivity analyses show that the timing of structuring and uplift is the most critical factor of the assessment. Variations in glaciation, heat flow and source kinetics show less effect on the hydrocarbon accumulation.