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[References: 33 tit.]

The Vankor Field, located in the West Siberia Basin, Russia, contains oils at various levels of biodegradation. A study of reservoir oils and rock extracts from the Lower Cretaceous sandstones in the Nkh 3-4 interval reveals that increasing biodegradation when moving from the gas-oil contact (GOC) towards the oil-water contact (OWC) is strongly controlled by rock properties. Biodegradation is most advanced where the OWC is located in the high permeability reservoir interval. High permeability rocks also allow for more extensive upward mixing of biodegraded oil. Changes in the oil and gas fractions due to biodegradation also affect the gas-oil ratio as dry gas is less soluble in heavy oil. This explains the presence of reservoir oil saturated with gas near the OWC. The latter also can be partly due to the admixture of methane through methanogenic processes at the OWC. Upward movement of microbial gas bubbles enhances the mixing of biodegraded oil in the high permeability reservoir intervals.