The controls on the composition of biodegraded oils in the deep subsurface - Part 3. The impact of microorganism distribution on petroleum geochemical gradients in biodegraded petroleum reservoirs

► We report geochemistry and microbiology through a biodegraded oil reservoir. ► Biodegradation of oil is focussed in a bioreactor zone at the base of the oil column. ► Hydrocarbon biodegradation alters many compound classes at different rates. ► The numbers of organisms coincide with a zone of intense biodegradation. ► Enhanced bacterial numbers in the upper part of the oil water transition zone.