Sixteen very large "tar" deposits are estimated to contain about 2,100 billion bbl of oil in place. This is nearly as much heavy oil as the world's total discovered recoverable oil reserves. The seven largest "tar" deposits of the world contain 98% of the world's heavy oil; that is, these seven heavy-oil deposits contain about as much oil in place as the world's 264 giant oil fields. These accumulations are remarkable not only for their size, but also for their unconventional geologic settings--in traps that feature various degrees of stratigraphic control in combination with structure; in marginal marine or nonmarine sediments; and commonly in thermally immature sediments without associated local source beds.

For very large tar-sand accumulations to be formed and localized, the following are geologic prerequisites: (1) a paleodelta system comprising organic-rich source beds and very far-reaching, efficiently interfingered carrier sandstones; (2) a widespread regional cap restraining vertical fluid escape from the underlying paleodelta and channeling fluid movements laterally to the edges of the basin; (3) a homocline with updip stratigraphic convergence and, preferably, low-amplitude arching plunging into the basin; and (4) degradation of oil to heavy sour "tars" by water washing and bacterial action.

Most of the settings reviewed indicate moderately rich source beds widespread over large areas, excellent gathering and focused drainage systems in paleodeltas, very long migration distance, and the predominance of regional structural and stratigraphic factors over local structural factors in determining both size and sites of accumulations.

Some authors already have suggested the possibility that supergiant accumulations of medium-gravity producible oil may lie undiscovered in settings similar to the very large "tar" sands.

The writer concludes that foreland basins are the most favorable areas for the possible existence of very large "tar" sand-like accumulations, but are the areas that have the highest risk for their degradation. The central parts of large rift basins are better protected against oil degradation, but gathering areas and migration distances are considerably more restricted than in foreland basins. The possibilities of passive, Atlantic-type, margin settings are unpredictable. Small intermountain basins can offer possibilities, if thick and exceptionally rich source beds are present.