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The Lower Cretaceous Mannville Group oil sands of northern Alberta have an estimated 270.3 billion m3 (BCM) (1700 billion bbl) of in-place heavy oil and tar. Our study area includes oil sand accumulations and downdip areas that partially extend into the deformation zone in western Alberta. The oil sands are composed of highly biodegraded oil and tar, collectively referred to as bitumen, whose source remains controversial. This is addressed in our study with a four-dimensional (4-D) petroleum system model. The modeled primary trap for generated and migrated oil is subtle structures. A probable seal for the oil sands was a gradual updip removal of the lighter hydrocarbon fractions as migrated oil was progressively biodegraded. This is hypothetical because the modeling software did not include seals resulting from the biodegradation of oil.
Although the 4-D model shows that source rocks ranging from the Devonian–Mississippian Exshaw Formation to the Lower Cretaceous Mannville Group coals and Ostracode-zone-contributed oil to Mannville Group reservoirs, source rocks in the Jurassic Fernie Group (Gordondale Member and Poker Chip A shale) were the initial and major contributors. Kinetics associated with the type IIS kerogen in Fernie Group source rocks resulted in the early generation and expulsion of oil, as early as 85 Ma and prior to the generation from the type II kerogen of deeper and older source rocks. The modeled 50% peak transformation to oil was reached about 75 Ma for the Gordondale Member and Poker Chip A shale near the west margin of the study area, and prior to onset about 65 Ma from other source rocks. This early petroleum generation from the Fernie Group source rocks resulted in large volumes of generated oil, and prior to the Laramide uplift and onset of erosion (∼58 Ma), which curtailed oil generation from all source rocks. Oil generation from all source rocks ended by 40 Ma. Although the modeled study area did not include possible western contributions of generated oil to the oil sands, the amount generated by the Jurassic source rocks within the study area was 475 BCM (2990 billion bbl).
Debra Higley has 5 years experience in uranium exploration and 26 years as a research geologist with the U.S. Geological Survey. Her research interests include reservoir characterization, petroleum system modeling, and petroleum resource assessment in basins in North and South America. She received her M.S. degree in geochemistry and Ph.D. in geology from the Colorado School of Mines, and her B.S. degree in geology from Mesa State College, Colorado.
Mike Lewan is an organic geochemist and petroleum geologist for the Central Energy Resources Team of the U. S. Geological Survey (Denver, Colorado). Prior to his 17 years with the U.S. Geological Survey, he worked for 13 years at the Amoco Production Co. Research Center (Tulsa, Oklahoma) and 3 years with Shell Oil Co. Offshore E&P Office (New Orleans, Louisiana). He received his Ph.D. from the University of Cincinnati, M.S. degree from Michigan Technology University, and B.S. degree from Northern Illinois University.
Laura Roberts graduated with a degree in geology from Colorado College. She is a recently retired emeritus scientist who had worked for the U.S. Geological Survey since 1977. Her research includes coal geology and coal resource assessment of the Fort Union Formation in the northern Powder River Basin, Montana, and the Cretaceous coals of the Colorado Plateau, and the burial and thermal history of petroleum source rocks in the Uinta-Piceance and Wind River basins.
Mitchell Henry retired from the U.S. Geological Survey after 30 years of research in direct geochemical detection and remote sensing of hydrocarbons, and domestic and international petroleum resource assessments. He received his B.S. degree from Midwestern University and his M.S. degree and a Ph.D. from Texas A&M University.