Propane is primarily a byproduct of domestic natural gas processing, though some propane is produced from crude oil refinement. Propane is a three-carbon alkane gas (C3H8) that is stored under pressure inside a tank and is a colorless, odorless liquid. As pressure is released, the liquid propane vaporizes and turns into gas that is used in combustion. An odorant, ethyl mercaptan, is added for leak detection.