Factors to watch in evaluating oil and gas reserves.
- Price changes can increase or decrease reserves
- Improved technologies can increase the reserves
- New discoveries can increase the reserves. Elsewhere there are studies that predict what is left to be discovered.
- Political realities can increase but more frequently decrease reserves.
So reserves is an economic concept unlike the size of the total resource which is a geological concept and the technically recoverable portion of the total resource which is an engineering concept. These three concepts and the quantification that corresponds to each concept has a relationship which is not always as clear as it should be and ofttimes controversial especially when things are changing rapidly.
BP-statistical-review-of-world-energy-2014-full-report