Heavy oil and natural bitumen-strategic petroleum resources

Because conventional light oil can typically be produced at a high rate and a low cost, it has been used before other types of oil. Thus, conventional oil accounts for a declining share of the Earth's remaining oil endowment. In addition to assessing conventional oil resources, scientists of the U.S. Geological Survey's Energy Resources Program collect data on the abundant energy resources available as heavy oil (including extra-heavy oil) and natural bitumen; see definitions in sidebar. The data in this Fact Sheet are compiled from commercial databases (NRG Associates, I.H.S. Petroconsultants), unpublished government data, and the published literature.