Reserves are an uncertain quantity. The primary reason for calculating the reserves in a field is to assist with a set of investment decisions in regard to that field. If the estimated  ("Proved" and "Probable" reserves are simply estimates with differing margins of error) reserves appear to justify and support the economic decision to develop then we have a producing field. If they do not, then  we have nothing except a lease committment. We can think of reserves as "latent oil" which will only become a tangible commercial reality when a certain economic thresholds are met.

The size of the reserve is therefore the product of a complex economic calculus which assigns relative values to the geologic structure, the prevailing technology, the socio-political environment, and present and anticipated market demand, and a host of other factors. Change any single variable in this complex calculation and you can justify any position you like. Or, you can do as the OPEC nations have done, and simply pick an arbitrary number. A discussion of Peak Oil based on reserve estimates is an endless debate akin to arguing over the number of CERA employees that can stand on a wellhead.

Flows, on the other hand, are tangible and real. They can be counted and verified. Flows can also be obstructed by changes in the technological envelope, socio-political upheavals, and by simple economics. If oil flows are being diverted to domestic internal uses then less will come to market and the market price will be higher.

For some nations, Peak Oil may have already arrived. They cannot afford liquid fuels at the current price level; the flow is interrupted and they learn to live without. If there is a conflict with Iran then it is likely flows will be interrupted and we will learn to live with less.

Key to this approach is the recognition that the world may have unlimited reserves but constrained flows. In such an environment the tangible effects we associate with Peak Oil may be experienced well before we have exhausted world reserves.

Cheers!