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
Shale gas in the United States in Illinois, Indiana, and Kentucky. The New Albany has been a gas producer in this area for more than 100 years, but recent higher gas prices and improved well completion technology have increased drilling activity. Wells are 250 to deep. The gas is described as having a mixed biogenic and thermogenic origin. As of 2007, operators had completed approximately 50 wells in the Pearsall Shale in the Maverick Basin of south Texas. The most active company in the play has been TXCO Resources, although EnCana and Anadarko Petroleum have also acquired large land positions in the basin. The gas wells