A team of researchers has recently examined the data (of which there is precious little) relating to energy returned on energy invested (EROEI), or net energy.

Their study begins by stating, “Few issues, maybe none, are as important to industrial societies and their economies as the future of oil and gas supplies…. We no longer find large, cheap and easy to exploit reserves…” (p. 2).

The authors then point to a greatly overlooked aspect in the energy debate: “A critical issue missing from this debate is not how much oil is in the ground, or even how much we may be able to extract, but rather how much we can extract with a significant energy surplus. In other words, what we need to know is the net, not gross, energy available” (p. 2, emphasis added).

This research is an attempt to quantify what common sense tells us must be true: if we are drilling more, finding less, and the fields which we do find are either smaller, in more difficult locations, or of a lesser quality, then it stands to reason that it will cost not only more money but also more energy to find, develop and produce that energy.

They conclude that "global supplies of petroleum available to do economic work are considerably less than estimates of gross reserves and that EROI is declining over time..." (p. 1).

Their study is modestly entitled “A Preliminary Investigation of Energy Return on Energy Invested for Global Oil and Gas Production.”
Their pioneering attempt to quantify the decline in EROEI is a welcome addition to the literature.
This link provides the abstract and the link to the complete study (14 pgs):
http://www.mdpi.com/1996-1073/2/3/490

More general information on EROEI may be found here:
http://en.wikipedia.org/wiki/EROEI