Petroleum in the form of tar sands may occur either as disseminated deposits or as veins or dike-like deposits. Disseminated occurrences may result from inspissation (drying up) of a preexisting pool of liquid and gaseous hydrocarbons into heavier residues with the loss of the lighter fractions leaving behind a fossil oil field. They also may originate as primary mixtures of rock and bitumen. However, such distinctions may seem farfetched compared to some of the largest known deposits in the world like the Athabasca tar sands of Alberta, Canada, which are of Cretaceous or Devonian origin and contain 100-300 billion bbl of oil.

Exploration and production of synthetic oil from tar sands were reported in many parts of the world, ranging from the Santa Maria field in California to sand asphalt deposits in Utah and around the Oklahoma Panhandle, to other fields in the United States, Canada, USSR, Venezuela, Trinidad, Cuba, and Germany. Experimentation, research, and mechanical design are the keys to commercial recovery of crude oil from tar sands (which are also a matter of supply and demand).

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