This is gas that is stuck in a very tight formation underground, trapped in unusually impermeable, hard rock, or in a sandstone or limestone formation that is unusually impermeable and non-porous (tight sand).

Tight gas sands is one type of unconventional reservoirs. Typical unconventional reservoirs include also coal bed methane, heavy oil and gas shales. Tight reservoirs contain no natural fractures and cannot be produced economically without hydraulic fracturing. Tight reservoirs are in essence reservoirs with low permeability. It has been coined by some as a reservoir of natural gas with an average permeability of less than 0.1 millidarcies or by others as a reservoir with an average effective gas permeability of less than 0.6 mD. A tight gas reservoir is often defined also as a gas bearing sandstone or carbonate matrix, which may or may not contain natural fractures which exhibits an in situ permeability to gas of less than 0.10 mD. Many ultra tight gas reservoirs may have in situ permeability down to 0.001 mD.

A number of reasons can contribute to making a reservoir tight. Basically, the permeability that determines the ease at which a fluid can flow is a function governed by Darcy’s Law of fluid flow in a porous medium. Effective porosity, viscosity, fluid saturation and the capillary pressure are some of the important parameters that affect the permeability of a reservoir.