(from the article `natural gas`) ...with crude oil. Natural gas is often found dissolved in oil at the high pressures existing in a reservoir, and it also can be present as a gas cap ...
Natural gas associated with oil accumulations, either dissolved in oil or found as a cap of free gas above oil in reservoir.
Natural gas found as part of or in conjunction with other constituents of oil, this is as opposed to gas found on its own. The expression has come to mean natural gas produced along with crude oil.
natural gas, commonly known as gas-cap gas, which overlies and is in contact with crude oil in the reservoir. 
Natural gas associated with oil accumulations, which may be dissolved in the oil at reservoir conditions or may form a cap of free gas above the oil.
natural gas produced with crude oil from the same reservoir