If you check oil & gas company press releases you might find that some natural gas wells also produce a number of barrels of condensate per day, these natural gas wells were in gasfields (not oilfields). This natural gas was called non-associated gas. Associated gas was in solution with oil in oilfields and seperated out by GOSP's during the production process. Gas caps are the top portions of oil fields below the cap rock that contain natural gas. They also contained condensates (lighter petroleum molecules). Natural gas liquids or NGL's were stripped out of natural gas at surface facilities, these included butane and ethane. After the oil from an oilfield was pumped out, the gas caps were tapped for natural gas, condensates, and NGL's.

EIA definition of NGL's:

Natural gas liquids (NGL): Those hydrocarbons in natural gas that are separated from the gas as liquids through the process of absorption, condensation, adsorption, or other methods in gas processing or cycling plants. Generally such liquids consist of propane and heavier hydrocarbons and are commonly referred to as lease condensate, natural gasoline, and liquefied petroleum gases. Natural gas liquids include natural gas plant liquids (primarily ethane, propane, butane, and isobutane; see Natural Gas Plant Liquids) and lease condensate (primarily pentanes produced from natural gas at lease separators and field facilities; see Lease Condensate).

http://www.eia.doe.gov/glossary/glossary_n.htm