Investigations of Upper Cretaceous and lower Tertiary rocks in the Pacific Creek area of the northeastern Green River basin show that studies of organic matter content, type, maturity, subsurface temperature, and reservoir pressure will help define prospective gas-saturated intervals and delineate areas of maximum gas-resource potential not included in previous U.S. Geological Survey resource estimates.

The onset of overpressuring occurs at about 11,600 ft (3,500 m), near the base of the Upper Cretaceous Lance Formation. Drill-stem test data indicate that at about 12,800 ft (3,900 m) the pressure gradient is as high as 0.84 psi/ft (19.0 kPa/m). Current data indicate that the active generation of large amounts of wet gas is important to the development of this overpressuring. A reversal of the spontaneous potential curve is nearly coincident with the top of overpressuring and is probably caused by a reduction of formation-water salinity. Very small amounts of water produced during thermochemical decomposition of organic matter and the dehydration of clays may provide enough low-salinity water to dilute effectively the original formation water, so that the resistivity of the formation ater (Rw) is greater than that of the mud filtrate (Rmf).

Humic-type kerogen dominates the organic matter. Total organic carbon contents range from 0.25 to 7.84 wt. %, averaging 1%. The top of overpressuring and beginning of important wet-gas generation occur at vitrinite reflectance values of 0.76 to 0.84 and color alteration values of about 2.8 on a scale of 1 to 5. The present minimum temperature at the top of overpressuring is 190°F (88°C; determined from uncorrected bottomhole temperatures). The preservation of abnormally high pressures is probably due to presently active generation of gas in a thick interval of discontinuous, very low-permeability shales, siltstones, and sandstones.

The U.S. Geological Survey is studying gas-bearing intervals in the very few wildcat penetrations of deep tight sandstone intervals in Rocky Mountain basins to better define a possible major gas resource.

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