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Pressure Regime in the Upper Cretaceous Shales and Sandstones in the Washakie Basin, Wyoming
The Upper Cretaceous shales in the Washakie Basin below a depth of 9000 ± 1000 feet typically are overpressured. The change of pressure regime from normally pressured to overpressured coincides with marked changes in the geochemical and geophysical properties of the Upper Cretaceous rock/fluid system, as reflected in increases in acoustic transit time, hydrocarbon production index, vitrinite reflectance, aromaticity of kerogen, progressive clay diagenesis, displacement pressure, and sealing capacity of the fine-grained rocks.
The Upper Cretaceous shale section in the Washakie Basin below 9000 ± 1000 feet is overpressured on a basinwide scale. The driving mechanism of the pressure compartmentalization is the generation and storage of liquid hydrocarbons that subsequently react to gas, converting the fluid-flow system to a multiphase regime in which capillarity controls permeability. Sandstone bodies within the overpressured shale section are subdivided stratigraphically and diagenetically into relatively small, isolated, gas saturated, anomalously pressured compartments. The major difference between pressure compartmentalization in the Upper Cretaceous shale and the sandstone bodies is one of scale. In both cases, the major driving mechanism of compartmentalization is hydrocarbon generation, storage, and subsequent reaction. In the Washakie Basin, the abnormally pressured sandstones are gas saturated. The most significant aspect of developing prospects in the gas saturated portion of the basin is the delineation of gas storage capacity and deliverability in the potential reservoir target.
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