Thermal controls on biodegradation around the Peace River tar sands: Paleo-pasteurization to the west

Bitumens of the Alberta tar sands share a similar primary source rock, but exhibit varying levels of degradation. In the Peace River tar sands area, the wide range of API gravity and sulphur content in Lower Cretaceous reservoirs is investigated in terms of biodegradation processes, using detailed geochemical analyses, burial history modelling and charge/degrade modelling. Decreasing tar sand degradation to the west can be attributed to increasing reservoir temperatures which retard biodegradation. Gething oils west of the Peace River tar sands are derived from the same source, but are minimally degraded due to reservoir pasteurization at 80 °C shortly after charging. Long charge times, reservoir filling and possibly compositional or very low temperature degradation retardation have limited biodegradation of the Alberta tar sands despite long oil residence times and low reservoir temperatures.