Gases are the lightest products and products of an crude oil  refinery or gas condensate.
Types of gases:
 1- Natural gas: which mainly consumption is the same CH4 and a bit of C2H6, and there is little contamination and compact (CNG) for use in cars designed for CNG.
  Its density: Specifications (0.7 kg / m3-0.18kg / m3)
Liquid gas: LPG: Liqufide Petroleum Gas is composed of two main factors of:( iso butane +normal butane  and propane) and compressed to come in liquid and is very cold due to compression. 
Specific Gravity @ 15˚C = (0.5250-0.5800)
 1- Specification: NG  
 2- Specification: LPG