Free (non-associated) gas – aggregate state of gas when its particles (molecules, atoms) are moving freely evenly filling all the available volume in porous and fissured rock in case of no external forces.

Free gas may be found under oil in an oil reservoir or above other fluids in equilibrium state with the same diluted gas.

Free gas may be produced from coal beds in mines and may become very hazardous factor in case of violation of safety rules. It may become associated (occluded) and vice versa, or may be coming out as currents from natural exposures on the Earth surface or blown out under high pressure from volcanic crater during an eruption.

In nature there may be atmospheric free gas and free gas in lithosphere. From the point of view of showing types there may be – gas-geneous, gas-accumulating (accumulation of gas), circulating (air-driven), combined. From the point of view of its chemical composition it may be: hydrocarbon, carbon-dioxide, nitrogen, sulfurous, helium-rich, etc. Each of these types is found in nature – both in pure form and in various mixes. Big free gas accumulations of commercial value are found in lithosphere.