Source: Worldwatch Institute. Data updated with the BP Statistical Review of World Energy.
- New reserves. No large scale new oil reserves have been found since the 1970s, which may lessen the oil reserves that could be added to the 1,000 billion barrels of proven reserves. There has been serious issues concerning the real availability of oil reserves, as some figures have been inflated to uphold the confidence of markets and investors. Since reserves in many countries, mainly OPEC countries, are not audited by external sources, reporting agencies are likely to have overestimated potential oil reserves. Still, with technological development and investments new reserves can be brought online, as the example of tar sands and shale oil underline, but their economic recoverability tend to involve much higher prices.
- Demand. The consumption of oil is far from being a constant growth process. Between 1990 and 2000, annual oil consumption increased by 14% with expectations that this process may go on up to 2020, especially in Pacific Asia with countries such as China importing more oil, which explains the surge in global oil production that took place after 2000. Consequently, if demand goes up, the time remaining before the exhaustion of global oil supplies could get shorter. Still, demand can also decline for several reasons, namely with technological improvements, shifts to alternative sources of energy, as well as with economic downturns where global and sectorial demand can face setbacks.
- Recoverability. An historical perspective on the exploitation of resources reveals that resources that are the easiest to access are exploited first, while resources that are more difficult to access are left for later times (if not overlooked). Oil extraction has followed the same principle as most of the easy access oil has now been extracted and what remains is located in more remote areas (subarctic; offshore), is much deeper of is much more complex to extract (e.g. tar sands and shale oil). This implies that the oil that can be extracted is much more difficult to recover than the oil that has been extracted so far. The last few hundred billion barrels of oil may be economically unrecoverable.