Large gas resources occur in low-permeability Upper Cretaceous and lower Tertiary reservoirs in the Greater Green River basin of Wyoming, Colorado, and Utah. Most of the gas-bearing reservoirs are overpressured, beginning at depths of 8,000 to 11,500 ft (2,440 to 3,500 m). The reservoirs are typically lenticular nonmarine and marginal marine sandstones. In situ permeabilities to gas are generally less than 0.1 millidarcy (md) and porosity ranges from 3 to 12%. Secondary porosity, after dissolution of framework grains and cements, is the dominant type of porosity. Gas accumulations are characterized by the presence of updip water-bearing reservoirs and downdip gas-bearing reservoirs. The top of these overpressured gas-bearing reservoirs cuts across structural and stratigra hic boundaries and is not associated with any particular lithologic unit. These overpressured accumulations are the result of gas accumulating at rates greater than it is depleted.

Data from reference wells indicate that in the deeper parts of the basin the relatively closed nature of this system imposes severe restrictions on the ability of gas to migrate appreciable distances from the interbedded source rocks. Consequently, the temporal relationships of hydrocarbon generation and migration with respect to the development of structural and stratigraphic traps is not as important in these unconventional reservoirs as in more conventional reservoirs. The more important factors related to gas generation and occurrence are source rock (quantity and quality), organic maturation, thermal history, formation pressure, and porosity and permeability variations.