The West Siberia Basin is the world’s largest intra-cratonic basin, filled with up to 10 km of Mesozoic and Cenozoic clastic sedimentary rocks (Figs. 1 and 2). Basement is composed of Paleozoic accretionary crust. Permian-Triassic rifting is accommodated along northward striking extensional grabens. The rifted basin was filled by fluvio-deltaic sediments prograding from the south and east, punctuated by periods of marine transgression from the north. Cenozoic basin inversion formed traps for petroleum. The combination of rifting, basin fill, abundant source and reserovir rocks, Cenozoic structuring and recent glaciation provide a natural laboratory to study how these factors combine to control thermal evolution of the basin and timing of hydrocarbon generation. A regional high-resolution 3D basin simulation using structural grids from 2D seismic and well data were used to model the basin thermal evolution. The basin model was calibrated against bottom hole temperature and vitrinite measurements. Kinetic activation energies were derived from measurements performed on West Siberia rock samples. Many giant gas fields have been discovered in the Cenomanian Pokur Fm reservoir in the northern West Siberia basin. The origin of the large amounts of very dry, isotopically light gas is still an enigma, albeit extensively adressed in literature. The 3D basin simulation was used to quantify the thermal charge from key source intervals. These are 1) Middle Jurassic lacustrine coals and shales; 2) Late Jurassic marine bituminous shales; 3) Cretaceous delta plain humic and coaly shales. In addition, the contribution from biogenic gas is discussed. The modelling results show that the volumes of early mature gas generated from Cretaceous sources are not sufficient to charge all Cenomanian gas fields. Additional contributions from Jurassic sources are required to charge the fields. Biogenic gas is likely to also contribute. Trap timing and Cenozoic uplift and erosion are key factors controlling the volumes of gas accumulated in the Cenomanian fields.


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