A large area of overpressured Upper Cretaceous and Tertiary rocks has been identified in the Green River basin of Wyoming. Source-rock, pressure, and temperature data from the El Paso Wagon Wheel No. 1 and Belco 3-28 Merna wells (originally proposed as nuclear stimulation sites) in the northern part of the basin reinforce previously reported conclusions regarding the cause of overpressuring: the generation of gas in low-permeability rocks. Pressure gradients in these wells exceed 0.8 psi/ft (18.1 kPa/m) and a maximum gradient of more than 0.9 psi/ft (20.4 kPa/m) may be present in the Merna well. If true, this would be the highest subsurface pressure gradient ever reported in the Rocky Mountain region.

Observations relevant to overpressuring in these wells include: (1) the coincidence of the onset of overpressuring and the top of the gas-saturated interval; (2) the source rocks of the gas are the interbedded coal and other carbonaceous lithologies; (3) the organic matter in the source rocks is predominantly a humic-type, capable of generating mainly gas; (4) the average total organic carbon content is about 2.0%; (5) the vitrinite reflectance at the top of overpressuring is 0.75 to 0.84 R_o, values that are consistent with the beginning of thermal gas generation; and (6) the low permeability (< 0.1 md), and stratigraphic and sedimentologic heterogeneity of the reservoirs provide an effective pressure seal.

Previously reported overpressuring mechanisms such as aquathermal pressuring, clay transformations, undercompaction, and dewatering of shales do not appear to be significant factors that contribute to overpressuring in this area.

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