The Marcellus Formation in the Northeast United States has been known for centuries for being rich in hydrocarbons. But it is a mudstone, with the result that it has very low permeability. The advent of fracking at the beginning of the 21st century raised the possibility that this extensive hydrocarbon resource could be exploited. It has been, but the fracking technology still results in rapid decline curves with significant gas being left in place; hence the initial economic promise is only partially achieved. The details of geology are much less important for this type of play than for traditional oil and gas operations; more important are the economic parameters, specifically the hydrocarbon price and the political environment. Significant production of gas from northeast Pennsylvania using fracking technology started in 2009; calculations of production to date indicate that approximately 50% of the exploitable gas has already been produced given current technology and economic framework, meaning that by 2040 this resource will no longer be a significant energy source for the US economy.