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 a Haynesville Shale gas well. Haynesville shale wells have also been drilled in northeast Texas, where it is also known as the Bossier Shale. From 2008 through 2010 Encana accumulated a "large land position" (250,000 net acres) at an "average $150/acre" in the Collingwood Utica shale gas play in Michigan's Middle Ordovician Collingwood formation. Natural gas is produced from the Collingwood shale and the overlying Utica shale. The Michigan public land auction took place in early May 2010 in one of "America's most promising oil and gas plays". The Devonian-Mississippian New Albany Shale produces gas in the southeast Illinois Basin