Deep Burial Microbial Sterilization Limits the Canadian Tar Sand Reservoirs

Jennifer Adams¹, Steve Larter¹, Cindy Riediger¹, and Martin Fowler^2
1 University of Calgary, Calgary, AB
² Geological Survey of Canada, Calgary, AB

Biodegraded oils dominate the world oil inventory; the highly biodegraded oils of the Lower Cretaceous tar sands in the Alberta basin contain over 270 billion m³ of oil (33% of total world petroleum reserves). Despite the supergiant nature of this accumulation, the sharp transition between non-degraded or only mildly degraded oils basinward (west) of the Peace River tar sands and the much more heavily degraded oils at Peace River and Athabasca has long puzzled geologists.

Detailed geochemical analyses of bitumen, oil and gas from across the Peace River oil sands region were used to map biodegradation levels across the area. Biomarkers show a common source (Exshaw Formation) for all Lower Cretaceous reservoired oil in the area. The differing thermal histories of these reservoirs can account for the sharp transition in biodegradation level west of the Peace River deposits. Geochemical proxy-calibrated reconstructions of the burial and thermal history of the Lower Cretaceous reservoirs demonstrate that the well preserved oils west of the Peace River accumulations resulted from deep burial and sterilization of reservoirs at temperatures above 80-90°C shortly after charging. The eastern reservoirs remained biologically active degrading the oils to bitumen. Biodegradation models show that oil charge history and associated water displacement from the reservoirs, as well as thermal history and oil composition explain the varying degradation levels in the tar sand reservoirs east of the sterilization boundary. The western edge of the Canadian tar sands represents a deep biosphere thermal boundary frozen through early Tertiary uplift of the Alberta basin.