Propane is lighter than air
Propane is a liquefied petroleum gas that comes out of both oil and gas wells. Propane does not occur naturally though. Raw crude oil or raw natural gas is refined to make different types of petroleum products, one of which is propane. Following it's refinement, propane is stored as a liquid under pressure until utilized, at which point it becomes a gas. Propane is heavier than air and will seek the lowest space available.