Sustainability 2011 , 3 1867
Keywords: EROI; oil; gas; depletion; energy cost- Introduction
Petroleum, including crude oil, natural gas, and natural gas liquids, is industrialized society’s most
important fuel. Since its discovery in the United States in 1859, the use of petroleum has increased
rapidly in both absolute terms and relative to other fuels. It accounted for about two thirds of total fuel
use in the 1970s [1]. Since the oil crises of the 1970s, many entities within the United States have
attempted to devise alternatives to oil. Nevertheless we consume today about the same proportion of
petroleum as in the 1970s. As the easier-to-find and exploit resources are increasingly depleted, we
have to turn to other, more difficult and expensive resources. The deep water Gulf of Mexico
exploration and exploitation efforts are but one example. Getting oil from these more difficult
environments is more expensive, and any oil company will tell you that the easy oil is gone.
It takes energy as well as money to produce energy. One important issue pertaining to petroleum
availability in the United States is Energy Return on Investment (EROI), the ratio of energy returned
compared to the energy used to get it. A more energy-intensive process of production, other things
being equal, results in a lower energy return on energy (and dollar) investment. In theory, EROI takes
into consideration all energies produced and all energies consumed to get that production. In practice,
EROI is usually calculated from the direct and indirect energy used to produce a given amount of
energy Murphy et al. in press [2].
The U.S. oil and gas industry is traditionally the most energy-using industry in the United States,
and the energy intensity of getting energy did not escape the notice of M. King Hubbert, the most
important analyst of oil production patterns in the United States, who mentioned it in his notes for his
deposition before the 93rd U.S. Congress. However, few or no analysts attempted to quantify that
relation until Hall and Cleveland undertook this analysis in 1981 [3]. They concluded that the energy
found per foot of all types of drilling while seeking and producing oil and gas declined from about 50
barrels of oil (including gas on an energy basis) in 1946 to about 15 in 1978. They also found that the
energy cost increased from about 0.1 to 2 barrels equivalent per foot. EROI was not calculated
explicitly in that paper, but one can infer that the EROI implied by these data declined during that
period from at least 50:1 to about 8:1. They also found that while the (inferred) EROI declined over
time it was greatly influenced by the amount of drilling, and that a large amount of drilling effort in
any given year was associated with a low EROI relative to the secular trend and the converse.
Previously Davis had reported on a similar relation for return per drilling effort [4]. An update to the
Hall and Cleveland study was published by Cleveland in 2005 [5] that estimated that the EROI for oil
and gas for the United States had declined from a peak of about 30:1 in 1972 to about 13:1 in 1982,
during a period of very intense drilling, but that the ratio had recovered to about 18:1 in 1997. He also
found that if corrections were made for the quality of the different fuels the ratio had declined from
20:1 to about 11:1 from 1954–1997. Since the data that have been analyzed previously covered only a
short time span (1946–1977 or 1954–2002 at best) our objective is to analyze the data, including