Compressed Natural Gas or CNG is stored on the vehicle in high-pressure tanks - 20 to 25 MPa (200 to 250 bar, or 3,000 to 3,600 psi). Natural gas consists mostly of methane and is drawn from gas wells or in conjunction with crude oil production. As delivered through the pipeline system, it also contains hydrocarbons such as ethane and propane as well as other gases such as nitrogen, helium, carbon dioxide, sulphur compounds, and water vapour. A sulphur-based odourant is normally added to CNG to facilitate leak detection. Natural gas is lighter than air and thus will normally dissipate in the case of a leak, giving it a significant safety advantage over gasoline or LPG.