In (vanilla) Factorio all fluids are mechanically identical, with the only cosmetical difference that gasses fill pipe & tank viewports from top to bottom.
Petroleum Gas seems to roughly correspond to Butane & Propane (and maybe Ethane and some traces of Methane ?) IRL.
(The icon is for ethylene, a precursor of polyethylene plastic, not methane.)For example, light hydrocarbons are steam-cracked in an ethylene plant, and the produced ethylene is polymerized to produce polyethene.