Graph of the certainty level of estimates for oil and gas reserves
Million Dollar Way pointed me to a great visual illustration of this idea, explained by the US EIA:
For many purposes, oil and natural gas resources are usefully classified into four categories:
- Remaining oil and gas in-place (original oil and gas in-place minus cumulative production at a specific date)
- Technically recoverable resources
- Economically recoverable resources
- Proved reserves
Check out this illustration, which is obviously not to scale:
Illustration tells the story better than my words could.
Source: U.S. Energy Information Administration – Oil and natural gas resource categories reflect varying degrees of certainty.