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The “Tar sands” of the Athabaska or “McMurray bituminous sands” of Lower Cretaceous age in northeastern Alberta, Canada, are believed to have derived their bituminous content from the underlying coral-reef reservoirs of Upper Devonian age which come in contact and near-contact with the overlying sands at the unconformity between the Lower Cretaceous and the Devonian. The white quartz sands were supplied from the pre-Cambrian “Athabaska sandstone” lying at the northeast on the Canadian shield, and it is suggested that the escape of oil and gas through fissures and fractures from the Devonian reservoirs into the “Tar sands” occurred primarily during, but also possibly subsequent to, the deposition of the Cretaceous “Tar sands.” It is also believed that the Lower Cretaceous heavy oil of Lloydminster, Alberta, has had a similar history.

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