During the past 4 years, a previously unexplored 3,000-sq mi (7,800 sq km) overpressured area in the eastern Green River basin has developed into a major gas province which should ultimately produce more than 20 Tcf. Production is from lenticular sandstones in the Upper Cretaceous Lewis Shale and Mesaverde Group. Abnormally high pressure gradients of 0.5 to 0.86 psi/ft are caused by the generation of natural gas from coals and carbonaceous shales in the Mesaverde Group and perhaps from other source rocks such as the marine Lewis and Cody Shales. Because cumulative gas generation from coals increases approximately exponentially with increases in temperature and depth, the largest volumes of gas and the highest pressures should have been generated in the deepest parts of th basin. The deepest rocks (15,000 to 20,000 ft; 4,600 to 6,100 m) are sparsely explored but may prove to be the most productive parts of the overpressured area for the following reasons. (1) Higher pressures result in more gas in the available pore space. (2) Sufficient gas should have been generated at these depths to fill all available pore space in Mesaverde and Lewis sandstones, and to reduce water saturation to an immobile minimum. More total pay should thus be expected than in shallower areas where water production is a common problem. (3) Higher pore-fluid pressures increase the ease with which natural fracturing of rock units can occur and more fracturing should enhance reservoir performance. (4) Younger sandstones in the Upper Cretaceous Lance and Paleocene Fort Union Formations are also overpressured in the deepest basin areas because of gas generation from associated coals and carbonaceous shales. These formations should contain significant gas accumulations.