Hydrogen sulfide
Small amounts of hydrogen sulfide can be found in crude petroleum. Sour natural gas can contain up to 28%. But, sour natural gas must be cleaned before it can enter a long distance pipeline. Pipelines limit hydrogen sulfide to 3 grains per thousand cubic feet of natural gas. Volcanoes and hot springs give off some H2S, where it probably is made by the hydrolysis of sulfide minerals, i.e. MS + H2O to give MO + H2S.