What is the difference between reserves, resources and the R/P ratio?

These are three progressively narrower categories. The resource base is all the fuel estimated to exist underground. Technically recoverable resources are what existing machinery could physically produce, ignoring cost. Proved reserves are only what can be produced profitably at today's prices with today's technology. The reserves-to-production (R/P) ratio divides proved reserves by annual output: about 54 years for oil, 49 for gas, 139 for coal and 26 for uranium.

Energy accounting draws the same hard line company accounts draw between potential revenue and money in the bank. The broadest layer is the resource base: the total estimated volume of a fuel in the ground, regardless of whether it can be extracted. One step closer to reality are technically recoverable resources, the share existing machinery could physically produce if cost were no object. The layer that matters to investors, governments and planners is proved reserves, the resource that geological and engineering data show can be produced profitably at current prices. This distinction sits at the heart of the peak oil debate.

The shale revolution is the clearest illustration. In the mid-2000s US oil and gas output was in what looked like irreversible decline. Hydraulic fracturing combined with directional drilling supplied the key to deep, tight formations such as the Marcellus in Pennsylvania and the Permian Basin in Texas. Those formations had always been there; without the key they simply did not count as reserves. Once the key was found, the United States flipped almost overnight from a leading energy importer to a leading exporter. That inventory growth was not the discovery of more oil but a redefinition of what counts as recoverable.

The reserves-to-production ratio divides total proved reserves by current annual production to estimate the remaining life of a carbon asset: roughly 54 years for oil, 49 for natural gas, 139 for coal and 26 for uranium. It is tempting to read oil's 54 years as a wall arriving in the mid-2070s, but in 1980 oil's R/P ratio was also around 30 to 40 years. New supply has been found at roughly the rate old supply is consumed, pushing the peak steadily further out.

The same distinction explains strategic petroleum reserves. The US SPR, created after the 1973 oil embargo and the world's largest, consists of vast engineered salt caverns in Texas and Louisiana with capacity for over 700 million barrels of crude; China and India have since built similar networks. But the real constraint is not the size of the tank: there is a hard, finite ceiling on how much carbon the atmosphere can absorb. As renewable economics erode the profitability of the most expensive fossil resources, what we may be approaching is peak demand rather than peak supply.

The real question of the twenty-first century is no longer 'when does the tank run dry?' but 'when do we decide to stop pumping?'

This question is covered in depth in The Full Spectrum Every Energy Source Explained — A Singapore Perspective，第二章 2.3