Gas and oil are commonly, though not always, found together. Most oil wells produce some gas. Many gas wells produce condensate, a natural gas liquid made up of lighter liquid hydrocarbons. Some gas wells, like a lot of the shale wells I've seen, produce just dry gas.

 

Gas is measured in standard cubic feet (scf @ 1 atmosphere & 60F). Multiples are mcf (1,000), mmcf (millions), BCF (billions), TCF (trillions). One mcf contains roughly 1 million BTUs of heating value; on average, 5.8 mcf of gas is the energy equivalent of 1 barrel of crude oil.