Global Petroleum Systems Analyses as Related to Heavy Oil and Bitumen Occurrence
ExxonMobil Exploration Company, Houston, Texas, USA
Heavy oils and bitumen are the near end products in a geologic continuum from petroleum generation within sedimentary basins to its complete destruction at or near the Earth’s surface. Since this migration is driven by the inherent buoyancy of oil and gas and the partial destruction occurs in the near surface then heavy oils and bitumen occur to varying degree in most oil generative sedimentary basins. The generation – destruction process can occur at varying rates but seems to generally occur over a time ‘window’ of 100my+/-. The economics associated with heavy oil development are favored where the resource has relatively close proximity to market and infrastructure. Western Canada and Venezuela contain the world’s largest accumulations in mature petroleum provinces with additional, more remote accumulations at Olenek in East Siberia, Russia, and Madagascar. Examples do exist of accumulations that have not been thoroughly degraded.
In general foreland basins present an extremely good setting to generate large volumes of petroleum. These basins are often characterized by large drainage areas, multiple (or extremely potent) source rocks and large basin-margin trapping configurations. Most importantly the evolution of a foreland basin, as its foldbelt encroaches onto a craton, drives a number of required play parameters. The initial foreland sedimentation provides a sand prone basal unit which provides a drainage conduit and reservoir for immense accumulations. As the foldbelt encroaches further toward the craton the eroding sediment loads, matures and expels hydrocarbons from underlying sources. The evolving peripheral bulge can provide the locus for broad structural trapping. Continued deformation will begin to ‘consume’ the early foreland sediments and can destroy much of the original ‘fetch’ area for sourcing.
The Western Canada Sedimentary Basin is the classic example of a high potency foreland basin. The basin contains approximately 42 billion bbls+/- of conventional oil and 2 trillion bbls+/- of Heavy oil and bitumen. Source rocks range in age from Middle Devonian to Late Cretaceous and all are associated with oils and gases generated from them. Facies stacking patterns, source maturity and the relationship of these sources to the basal Cretaceous unconformity and overlying Mannville sand controls which sources can contribute to the heavy oil accumulations on the basin margin. Although a large volume of the heavy oil is actually reservoired in the Devonian Grosmont Formation, it is genetically unrelated to prolific Devonian source rocks. There is only evidence of relatively minor oil leakage from the Devonian to the Mannville. The Triassic has similar difficulty accessing the Mannville. The Mississippian has a broad erosional subcrop to the Mannville and the Exshaw source is a major contributor to the Mannville. The Jurassic Nordegg source has similar accessibility to the Mannville but a much reduced subcrop and could contribute but in lesser volumes. Post-Mannville sources of the Colorado Group are also significantly oil prone but lack conduits to the underlying Mannville. Oils from the Colorado have migrated from the deep foreland, across the top of underlying Mannville heavy oils and come to rest in conventional traps as far east as Central Saskatchewan. The Mannville itself is a prolific source of gas (biogenic to thermogenic) and a minor source of oil which has negligible impact on resource endowment. Mannville gas perhaps increases the efficiency of the Mannville as conductor bed by occupying (and removing) conventional trap space on the way to the heavy oil accumulations.
Trapping and degradation of crude oils in Western Canada occurred sometime at the end Cretaceous to Early Tertiary. At this time the final loading of the basin occurred in response to cessation of foldbelt deformation combined with a sea-level fall. The final basin fill comprised alluvial and fluvial coarse clastics. A number of theories have been proposed for trapping the Western Canada heavy oils and most of them probably apply to varying degrees at different locations. A marginal bulge emphasized by salt withdrawal from the Middle Devonian likely created the closure and early biodegradation probably impaired oil viscosities to the point of stagnation. Biodegradation and viscosity increases in a general way as the regional oil – water contact is approached. As the continent began to drain, sediment removal occurred and the supergiant oil accumulations on the eastern flank of the basin began to exhume. Approximately 1.5 Km of sediment was removed from the basin in the foredeep with lesser amounts on the eastern flank. Heavy oils at outcrop are testimony to the near complete destruction in the Athabasca area with progressively less degradation to the south.
AAPG Search and Discovery Article #90075©2008 AAPG Hedberg Conference, Banff, Alberta, Canada