"liquid natural gas[1] (mainly propane and butane)

That doesn't make sense to me. Either it's natural gas, or propane-butane. Where I live at least, there are requirements for what natural gas is supposed to be. There can be some amout of impurities in it but it's still supposed to be mostly methane.

Maybe they meant that the propane and butane impurities stayed low while methane escaped?