Degree of Advancement of Petroleum Exploration in United States
Authors: King Hubbert M. 
Published: May 2021

Abstract
Because gas and oil are exhaustible resources, the discovery and production history of these fuels in any particular area must be characterized by a beginning, a period of increase, a period of decline, and ultimately, an end. In this sequence, the most significant dates are neither those of the beginning nor of the end, but that of the transition between the period of increase and the period of decline. The petroleum industry in the conterminous United States is by now sufficiently advanced in its evolution that several lines of evidence are consistent in their indication of the approximate degree of that advancement.
One suite of diagnostic data comprises the cumulative production, cumulative proved discoveries, and proved reserves. For more than 40 years the curve of cumulative production of crude oil has closely followed that of cumulative discoveries, with a time lag of about 10—12 years. The peak in the rate of proved discovery occurred about 1956. The peak in proved reserves, which should occur about midway between the peaks in the rates of discovery and of production, actually occurred at about 1961. The peak in production rate should occur in the late 1960's.
Another suite of data is provided by successive studies made by the Petroleum Administration for War and by the National Petroleum Council, in which the oil discovered has been allocated to the years of discovery of the producing fields. These, when corrected to an estimated ultimate growth, indicate that by the end of 1966 about 136 billion bbl of producible crude oil had been discovered. The rate of discovery per year, averaged for successive 5-year periods, reached a peak of 3.57×109 bbl/yr during the period 1935-1940, and has declined subsequently to a present rate of less than 2×10&lt;sub&gt;9&lt;/sub&gt; bbl/yr.
Another method of analysis is that proposed by A. D. Zapp (1961,1962), of relating discoveries per foot of exploratory drilling to cumulative footage. By the end of 1966 it is here estimated that about 1.52×109 ft of exploratory drilling had been done. The discoveries per foot, averaged for each successive 108 ft, reached a peak of 276 bbl/ft during the third 108-ft interval, which included the discovery of the East Texas field. Subsequently, the rate of discovery per foot has declined exponentially to a present average rate of about 35 bbl/ft.
These several lines of evidence are consistent indicating that:
1. The peak in time-rate of crude-oil production will probably occur near the end of the 1960 decade, and the ultimate amount of oil production will probably be less than 200 billion bbl.
2. The peak in the rate of natural-gas production will probably occur in the late 1970's, with ultimate production between 900 and 1,200 trillion cu ft.
3. The ultimate cumulative crude-oil production from fields already discovered in the conterminous United States probably represents between 68 and 85 per cent of the total crude oil ultimately to be discovered.
4. The evidence available offers scant hope that recently postulated future discovery rates in excess of 80 bbl/ft, with attendant discoveries of additional hundreds of billions of barrels of crude oil, during the next 2 or more billion ft of exploratory drilling, will ever, in fact, be realized.