Abstract: Tar Sands Give Clues to Search for Supergiant Oil Fields

G. J. Demaison

Sixteen very large "tar" deposits are estimated to contain 2,165 billion bbl of oil in place. This is about as much heavy oil as the world's total discovered medium and light-gravity oil in place. These accumulations are remarkable not only for their size, but also for their unconventional geologic settings: mainly stratigraphically controlled traps in deltaic or nonmarine sedimentary rocks; often in thermally immature sedimentary rocks without associated local source beds.

For very large tar sands the following are prerequisites: (1) a paleodelta system comprising organic-rich source beds and very far reaching, efficiently interfingered carrier sands; (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; (4) degradation of oil to heavy sour "tars" by water washing and bacterial action.

In summary, most of the reviewed settings point to moderately rich source beds widespread over large areas, excellent gathering and focused drainage systems in paleodeltas, very distant migration, and predominance of regional 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.

This inquiry concludes that foreland basins carry the lowest risk for the possible presence of tar sands but 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 totally unpredictable. Small intermontane basins can offer possibilities, provided thick and exceptionally rich source beds are present.

About half of the oil found so far on our planet is in a few very large, stratigraphically controlled accumulations. Understanding of the factors that led to the genesis of the very large tar sands brings an added dimension to the search for new reserves in old and not-so-old provinces. Awareness of these factors will help explorationists to recognize the potential for supergiant oil in geologic settings that often may appear unattractive by conventional standards.

AAPG Search and Discovery Article #90976©1976 AAPG-SEPM-SEG Pacific Section Meeting, San Francisco, California