Eons ago alternate layers of mud and organic matter were deposited at the bottom of a sea. Thousands of feet of overburden formed over the layers. If an impermeable capstone had formed over the petroleum, an oil pool would be available for tapping today. Either a cap did not form or one formed and was subsequently eroded away. In either case the absence of a cap has allowed the light ends of the petroleum to escape to the atmosphere, thereby, leaving the asphalt remainder.
In the recent past attempts have been made to extract hydrocarbons from oil shale by cooking it at high temperature in an enclosed pressure vessel. This “cracks” the molecular chain of the asphalt and releases some lighter compounds which when cooled form liquid petroleum. The petroleum thus formed is heavy crude of less than average value. The yield of crude is low and the cost of extraction is high. Therefore, there are no American oil shale processing facilities in operation.
Other technologies have been suggested, such as removal of the oil by using a solvent which then needs to be distilled from the oil-solvent mixture for reuse. However, none of the suggestions have produced an economical process.