Estimating the amounts of oil and gas that will be produced from known fields beyond proved reserves was an important part of the 1987 assessment of U.S. oil and gas potential. These resources, inferred reserves, are comparable in magnitude to proved reserves. Their estimation was based upon the only nationwide series of successive estimates of proved reserves of fields grouped by year of discovery. Although that data series ended in 1979, it documented a pattern of field growth that could be extrapolated for known fields. The estimated inferred reserves for the lower 48 states were 16 billion bbl of crude oil and 96 tcf of natural gas.