Source: adapted from M. Simmons (2008).
Oil reserves come in two forms:
- Conventional. Oil that comes in liquid form which is transportable and possible to refine. It represents the form that is the most desirable to extract since it is of high quality.
- Unconventional. Oil that comes in other forms that are much more difficult to extract let it be in deep waters or as oil shale, and of lower quality.
The availability of oil resources is correlated with market price and technology. Like many resources, quality (often measured in the level of concentration) is inversely proportional to quantity. High-quality oil and gas resources have mostly been tapped, leaving more marginal resources such as tar sands and gas shale to be considered. Still, extracting low-quality resources is an expensive and technically intensive endeavor.