Exergy is the energy content of a unit of petroleum. It is calculated from the combustion equation of a fraction of petroleum, and equations that come from thermodynamics. It is the value used by the EIA when they report energy production. API is the standard used by the petroleum industry to define crude density realive to water. Higher API means a lower density. As can be seen by the graph as a petroleum fractions' density increases (API decreases) so does its energy content. 35.7° is used in the study as the average conventional crude API value (140,000 BTU/gal).