Asphyxiant gas can smell the odorants, and assigning a different smell to each gas may be impractical. Another difficulty is that most odorants (e.g., the thiols) are chemically reactive. This is not a problem with natural gas intended to be burned as fuel, which is routinely odorized, but a major use of asphyxiants such as nitrogen, helium, argon and krypton is to protect reactive materials from the atmosphere. The dangers of excess concentrations of nontoxic gases has been recognized for centuries within the mining industry. The concept of black damp (or "stythe") reflects an understanding that certain gaseous mixtures could lead to
Natural gas in the United States 25.7 TCF in 2014, an increase of 21 percent. Remaining proved reserves increased overall, from 301 TCF in 2013 to 338 TCF in 2013 (the last year for which reserves are available), an increase of 11 percent. The rise in gas production despite fewer rigs drilling has been explained by the greater efficiency in drilling, and the greater productivity of shale gas wells. Top producing gas fields in the United States, 2013 Most oil fields produce some gas, and vice versa, but the ratio of oil and gas varies considerably. Some gas fields, such as coalbed methane wells, and some