"Instead of remaining constant, discoveries per foot have fallen drastically during the last 35 years from a maximum value of 276 bbls/ft during the period 1928-1937 to a present figure of about 35 bbls/ft. From this, it is evident why the estimate derived deductively from the Zapp hypothesis is about 3.5 times the highest figure that can be justified by the discovery data — an over-estimate of about 425 billion barrels."
[Zapp put out the estimates for the USGS and they were wrong, in a big way.]
—Resources and Man: a Study and Recommendations; National Academy of Sciences-National Research Council. W.H. Freeman & Company, 1969; pg. 186.
Zapp, A.D. 1962. Future petroleum producing capacity of the United States. U.S. Geological Survey Bulletin. 1141-H