Considering the finite aspect of the world, there is a fixed quantity of resources, often referred to as total resources. Based upon what has been discovered and its cost of recovery, only a segment of the total resources can be considered as reserves. Thus, reserves are resources that are available under current market conditions (cost and technology). A significant amount of resources could be known to be present. Still, if the costs of their extraction and transformation are prohibitive, they will not be used, as they are considered sub-economic.
Growth in the price of a resource (mainly due to scarcity and growing demand) can increase the quantity of resources that are recoverable in addition to technological improvements enabling resources that were not previously recoverable to become recoverable. Exploration can also identify resources that can either be immediately recoverable or could potentially be if investments were made. Some resources can be considered potentially unrecoverable since they exist in concentrations and/or locations that are unlikely to ever be available.
For petroleum, most of the exploration cycle is completed, implying that it is unlikely that additional reserves of significance are going to be found. The question of the cost of recovery remains as scarcity (peak oil) has become a strong force in growing prices. Under such circumstances, new resources, such as tar sands, can become available and thus be considered as reserves.