During the past 4 years, a previously unexplored 3,000 sq-mi (4,828 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 of gas. Production is from lenticular sandstones in the Upper Cretaceous Lewis Shale and Mesaverde Group. Abnormally high fluid pressure gradients of .5 to .86 psi/ft are caused by the generation of natural gas from coals in the Mesaverde Group and perhaps from other source rocks. Gas generation from coals is believed to increase exponentially with increases in temperature and depth. Therefore, the largest volumes of gas and the highest pressures have been generated in the deepest parts (15,000 to 20,000 ft, 4,572 to 6,096 m) of the basin. The deepest areas ar 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. 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

Figure

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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. These formations should contain significant gas accumulations.

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