Tight gas reservoirs generally are defined as gas-bearing rocks with an in-situ permeability to gas of less than 0.1 md. In 1980, the National Petroleum Council estimated that undiscovered recoverable gas resources in low-permeability (tight) rht) reservoirs in the United States, excluding eastern gas shales, are about 574 tcf (16 × 10¹² m³). The possible recovery of this volume of gas was based on $9/mcf (1979 dollars) and a level of recovery technology presently unknown (advanced technology). Most of this resource is in the western United States.

The two main types of tight rocks are shallow high-porosity reservoirs and deep low-porosity reservoirs. The high-porosity rocks are tight because of their fine grain size. The low-porosity rocks are tight because diagenetic alteration has resulted in them having small microvugs (porosity) poorly connected by sheet or cracklike, very thin capillaries. The low-porosity rocks are very stress sensitive and may be overpressured, and all tight rocks are susceptible to formation damage to varying degrees during drilling and treatment.

The western tight gas reservoirs can be placed in two major geologic and engineering categories: blanket and lenticular. Blanket reservoirs generally respond to artificial stimulation in a predictable manner, whereas the response of lenticular reservoirs is mostly unpredictable.

Many obstacles complicate the identification and recovery of gas from these unconventional reservoirs. Accurate borehole-geophysical log analysis is generally difficult, and economic stimulation (commonly artificial fracturing) of the tightest rocks is difficult or impossible with present technology. Geologists and engineers must better understand the characteristics of these reservoirs, work together to improve present recovery technology, and develop new, innovative drilling and stimulation techniques.