oil gas, any of a group of fuel gases produced from oil by exposing it to high temperatures. High-Btu oil gas is so called because of its high heating value; it is often used to supplement natural gas during periods of high demand. Refinery oil gases are produced as byproducts during normal heat treatment in oil refining. Their chief use is in the heating of refinery equipment. Typically, oil gas consists of methane, ethane, propane, butane, and some of their derivatives.