The quantities of oil in a deposit are listed under two categories: resources and reserves.
Resources are deposits that could be converted to reserves if economic factors, prices and costs, were favorable.
Reserves are deposits that can be extracted for a profit.
What makes a deposit profitable involves many things:
- The size of the deposit
- The quality of the deposit - the type of hydrocarbons in the deposit,
is it lighter fractions of oil or heavier or a mixture of the two. It
also depends on the amount of undesirable contaminants in the oil
such as sulfur.
- The depth of the deposit
- Ease or difficulty in accessing the deposit
- The flow rate achievable from the deposit: the number of barrels of
oil per day that can be extracted.
- How much gas is associated with the deposit and the type of gas
- Ease of getting the oil to a refinery
- How much of the total oil in the ground can eventually be extracted.
The amount of oil that can be extracted is heavily reliant on the technology used to extract it. With the introduction and use of technologies not previously available more oil might be able to be extracted from the deposit, or oil from deposits not previous considered for extraction could be converted to reserve status and become available to be extracted. When this happens the hydrocarbon resources have become extended.
Similarly, new technology might make oil in different types of deposits capable of being extracted. This is what happened with oil shale deposits when fracking was developing. Until then oil shale deposit were largely untouched.
By being able to extract oil from such new sources also extends the hydrocarbon resource.