Hydrogen sulfide
Like sour oil, there is some natural gas naturally high in sulfur, called sour gas (natural gas with little or no sulfur is called sweet gas). The sulfur is present in the form of hydrogen sulfide, a toxic gas that must be removed for safety reasons. Most natural gas has less than 1 percent hydrogen sulfide, but deposits have been discovered with more than 80 percent hydrogen sulfide. Sour gas is most often associated with carbonate rocks, and especially with anhydrite. Hydrogen sulfide is also corrosive, so that gas high in hydrogen sulfide requires special corrosion-resistant production equipment,as well as extra safety precautions. The requirement that gas processors remove the sulfur causes sour gas to receive a lower producer price than sweet gas.