may production data, page-42

There are various different levels of "proven" oil reserves (P1, P2, P3) for crude oil reserves. They can be summarized as follows:

Proved 1P - at least 90% probability
Proved + Probable 2P - at least 50% probability
Proved + Probable +Possible 3P - at least 10 % probability

However, to make things more confusing, some people do not use the definition of geological probability for the existence of an oil reserve, but vague assertions like:

Proved P1 - reasonable certainty
Probable P2 - more likely than not
Possible P3 - less likely than probable