Any electricity future dependent on significant coal or gas resources brings with it the added risk of fuel availability. The McKelvey diagram is a useful visualization for classifying resources by their degrees of geologic assurance and economic recoverability.
In a McKelvey diagram, the large rectangle represents the entire existing quantity of the given mineral, coal for example. This entire rectangle is known as the resource base. Resources in the lower right corner of the grid have highly speculative existence and would be very expensive to mine even if they did exist. Resources in the upper left corner are proven to exist (for example, an as-yet-unmined area in an existing mine) and can be mined at lower cost than the price they can be sold for, under existing market prices and technologies. These resources are known as reserves. As the price of coal increases, some of the demonstrated subeconomic resources will become economic, and shift into the reserves box. As more resources are proven or hypothesized, they will shift to the left in the grid. The entire resource base has the potential to become reserves, but only if we can find everything (and know conclusively that it is everything) and it can all be extracted profitably. The outline of this diagram can be used to understand the risk of future coal and gas availability.
America’s domestic coal supply, traditionally taken for granted, is not guaranteed. The current U.S. coal reserves are estimated at 260 billion short tons; in 2009, the U.S. consumed 1 billion short tons of coal. Adding in demonstrated subeconomic resources increases the estimate to almost 500 billion short tons, but this may prove optimistic. For example, the Gillette coalfield in Wyoming’s Powder River Basin, today’s most productive U.S. coalfield, was once thought to contain over 200 billion tons of coal. But in 2009, the U.S. Geological Survey reassessed the economically recoverable resource at a little over 10 billion tons. A similar downgrade has been predicted for other coalfields, meaning the U.S. may have just a 60-year supply of economically recoverable coal at current rates of consumption.
The story for natural gas is very similar. At the 2009 consumption rate of 22.7 trillion cubic feet of gas per year, the current recoverable resource estimate of domestic natural gas will last 77 years. And using high-end estimates for shale gas will extend the supply only to 100 years.
McKelvey, V.E. 1972. “Mineral Resource Estimates and Public Policy.” American Scientist 60 (1): 32-40.