History of Filling of the Northern Alberta Oil Sands Based on 4-D Petroleum System Models
Debra K. Higley, Michael D. Lewan, and Laura N. R. Roberts
US Geological Survey, Denver, CO
An estimated 1,600 billion barrels (BB) of bitumen are present in the oil sands of northern Alberta. The primary reservoir is the McMurray Formation of the Lower Cretaceous Mannville Group. The source of this vast resource remains controversial, with some researchers advocating mostly Paleozoic source rocks, and others advocating mostly Mesozoic source rocks.
Two four-dimensional petroleum system models of northern Alberta were constructed to determine the contributions to these oil sands of oil and gas from Devonian through Cretaceous source rocks. The models were identical except for petroleum generation kinetics, with one based on hydrous pyrolysis (HP) and the other based on Rock-Eval pyrolysis (REV) kinetic parameters. Both models used the same kinetic parameters for secondary generation from petroleum. These kinetic parameters, in conjunction with measured hydrogen indices and total organic carbon data, were used in the models to evaluate timing and extent of petroleum generation and expulsion from Devonian through Cretaceous source rocks. Integrated flow-path and hybrid Darcy algorithms were used to model migration of the expelled petroleum and the relative percent contributions through time of each source rock to petroleum accumulations.
Structural relief on the top of each reservoir layer is the primary control on migration pathways for oil and gas that (1) is not trapped in the reservoir layer, (2) does not breach bounding seal lithologies, or (3) escape along open faults. Also critical are location of reservoir strata relative to vertical and lateral seals, and to thermally mature source rocks. Biodegradation traps such as the oil sands are not modeled; instead the generated and expelled petroleum accumulates in associated subtle structures and migrates northeast to the Precambrian shield.
Truncation of Jurassic through Devonian rocks against the angular unconformity at the base of the Mannville Group increased oil and gas contributions through time from Mississippian through Lower Cretaceous source rocks. Petroleum generation from all source rocks began near the present-day Rocky Mountains, and migrated to the northeast as much as 350 km. The HP and REV models show that the oil sands are located at focal points of the petroleum migration pathways and the initial and major source interval for the oil sands is the Gordondale Member of the Jurassic Fernie Formation. The northeast migration was the main reason why the Gordondale did not contribute petroleum to accumulations south of the oil sands. This early contribution was important in preventing lithification of petroleum impregnated Mannville strata.
Petroleum source rocks that contribute to the Mannville Group layers range from the Mississippian Exshaw to the Ostracode zone and coals of the Mannville Group. Using a transformation ratio (TR) of 0.1% for onset of petroleum generation, the Gordondale started generating about 109 Ma (HP) and 80 Ma (REV). Devonian through Lower Cretaceous source rocks peak generation (TR=50%) near the deformation belt, proximal and east of the Canadian Rocky Mountains, began 70 Ma (HP) and 65 Ma (REV). Mannville Group conventional accumulations that are sourced from the Gordondale peaked at 70 Ma (HP) and 55 Ma (REV). Onset of petroleum generation and expulsion is 70 to 65 Ma (HP and REV) from the Mississippian Exshaw Formation, Triassic Doig Member, and coals and Ostracode zone of the Mannville Group. Source rocks of the Duvernay Formation of the Devonian Woodbend Group expelled petroleum starting 70 (HP) and 75 Ma (REV); contributions were mainly to the Woodbend Group. Shales of the Upper Cretaceous Colorado Group did not contribute to Mannville and older accumulations. Oil and gas generation ended about 40 Ma for the HP and 45 Ma for the REV model.
The volume of generated petroleum from the Gordondale in our study area is 380 x 109 m3 (2,400 BB) based on HP kinetics, and 58 x 109 m3 (370 BB) using REV kinetics. This volume disparity results from differences in the timing and extent of petroleum generation as determined by the HP and REV kinetics, with the former being more representative of early petroleum generation from high-sulfur organic matter than the latter.
AAPG Search and Discovery Article #90075©2008 AAPG Hedberg Conference, Banff, Alberta, Canada