When oil production first began in the mid-nineteenth century, the largest oil fields recovered fifty barrels of oil for every barrel used in the extraction, transportation and refining. This ratio is often referred to as the Energy Return on Investment (EROI or EROEI). This ratio becomes increasingly inefficient over time: currently, between one and five barrels of oil are recovered for each barrel used in the recovery process. The reason for this efficiency decrease is that oil becomes harder to extract as an oil field is depleted.

Certain types of energy are more convenient than others—because of the energy density and relative safety of gasoline at room temperature and atmospheric pressure, it is uniquely suitable for transportation. Oil is also usable as a chemical feedstock (i.e., can be used as a base building material for chemicals and materials), whereas sources such as wind, nuclear and solar are not. Therefore, it is possible that oil would continue to be extracted and refined even after it consumes net energy to do so.

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