From the geological point of view, there are two types of oil reservoirs. These types influence also the extraction methods, which have a great impact over the price of the crude.
Oil in place is that type which contains the naturally massed oil. However, companies can't extract the full content of crude, also sometimes it is hard and very expensive to dig for it. Not every reservoir is placed near the crust of the earth and the finding of productive oil fields is even today a matter of luck and coincidence. Nowadays, the technical progress, the better and cheaper exploitation methods, can convert every deposit to a useful one. Still, there is a difference between the costs of the losing and the winning methods.
Extracting conventional oil is the easiest and it also goes cheaper and easier throughout the refining process. This oil is found near the crust of the earth, has a better density and viscosity gravity. To this type of oil counts the Brent crude or the WTI.
The unconventional oils, while having a larger amount in deposit than conventional crude, are more complex to extract. The geological circumstances require specialized extraction technology and machines. Such unconventional oils you may find in oil sands, shales, coal-based and bitumen-based liquid supplies, under the oceans and at the Polar regions.
Although there are several statistics and charts about the oil reserves worldwide, it is a matter of fact, that many details do not fit the truth. For instance, there are a lot of unproven reserves. The proof of an oil reserve is based on the productivity, which has to be minimal of 90%. In cooperation with them, officially, the unproven reserves are not seen as useful in the present. Of course, under the principle of recoverable fields, they might be useful in the future.
On the other hand, many states do not want to show their whole crude possessions, because of some more or less justified economical or political perspectives. Officially it is said that Saudi Arabia would have the greatest amount of oil, followed by Russia and Iran. But there is also Norway or the Unites States, which show a great number of refineries. Just look at the pictures below. You may see some differences, for instance the great amount in Canada, which sparely appears on the “Who has oil” chart. This is just one of the proofs, that the geological mentioned oil reserves often do not match the political and economical based statistics.
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