Editing Hubbert peak theory (section)

===Energy return on energy investment===
The ratio of energy extracted to the energy expended in the process is often referred to as the Energy Return on Energy Investment (EROI or [[EROEI]]). Should the EROEI drops to one, or equivalently the [[Net energy gain]] falls to zero, the oil production is no longer a net energy source.

There is a difference between a barrel of oil, which is a measure of oil, and a [[barrel of oil equivalent]] (BOE), which is a measure of energy. Many sources of energy, such as fission, solar, wind, and coal, are not subject to the same near-term supply restrictions that oil is.{{citation needed|date = May 2017}} Accordingly, even an oil source with an EROEI of 0.5 can be usefully exploited if the energy required to produce that oil comes from a cheap and plentiful energy source. Availability of cheap, but hard to transport, natural gas in some oil fields has led to using [[natural gas]] to fuel [[enhanced oil recovery]]. Similarly, natural gas in huge amounts is used to power most [[Athabasca tar sands]] plants. Cheap natural gas has also led{{citation needed|date = May 2017}} to [[ethanol fuel]] produced with a net EROEI of less than 1, although figures in this area are controversial because methods to measure EROEI are in debate.

The assumption of inevitable declining volumes of oil and gas produced per unit of effort is contrary to recent experience in the US. In the United States, as of 2017, there has been an ongoing decade-long increase in the productivity of oil and gas drilling in all the major tight oil and gas plays. The US Energy Information Administration reports, for instance, that in the Bakken Shale production area of North Dakota, the volume of oil produced per day of drilling rig time in January 2017 was 4 times the oil volume per day of drilling five years previous, in January 2012, and nearly 10 times the oil volume per day of ten years previous, in January 2007. In the Marcellus gas region of the northeast, The volume of gas produced per day of drilling time in January 2017 was 3 times the gas volume per day of drilling five years previous, in January 2012, and 28 times the gas volume per day of drilling ten years previous, in January 2007.<ref>US Energy Information Administration,  [https://www.eia.gov/petroleum/drilling/ Drilling productivity report], 15 May 2017, (see “Report data” spreadsheet).</ref>