Systematics and controls of in-reservoir oil biodegradation

Petroleum biodegradation causes systematic variation in oil molecular composition. Most biodegradation in the reservoir takes place in a basal degradation zone, near the OWC or in transition zones where free water is available for the transport of nutrients. The reservoir temperature history and oil mixing are the primary controls on oil biodegradation. The shape of the molecular composition gradients and final oil quality are controlled by the relationship between biodegradation rate, fresh oil charge rate, diffusion of oil components towards the biodegradation zone, and mixing of fresh and degraded oil. In this work, we review the systematic variations in molecular composition resulting from in-reservoir oil biodegradation and its main controls. We also present some case histories to demonstrate how these processes impact compositional and fluid property gradients (API gravity and viscosity) in heavy and extra-heavy oil accumulations.