公定価格
Price controls grew even more complex with the Energy Act of 1980, which exempted Devonian gas shales (shales deposited during the Devonian geologic period) from price controls (but not gas shales deposited during other geologic periods), as well as low-permeability formations and coalbed methane. In addition, production from these sources earned tax credits for the producers for qualified wells drilled before January 1, 1992; the tax credits expired at the end of 2002.