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 The data in this Fact Sheet are compiled from commercial databases (NRG Associates, I.H.S. Petroconsultants), unpublished government data, and the published literature.

Natural bitumen (often called tar sands or oil sands) and heavy oil differ from light oils by their high viscosity (resistance to flow) at reservoir temperatures, high density (low API gravity), and significant contents of nitrogen, oxygen, and sulfur compounds and heavy-metal contaminants. They resemble the residuum from the refining of light oil. Most heavy oil is found at the margins of geologic basins and is thought to be the