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Anomalously low fluid pressures are found in the Lower Cretaceous, Mesozoic, and Paleozoic rocks of the Denver basin. Drill-stem test data and published hydrogeologic information are used to construct a potentiometric map for the Lower Cretaceous sandstones in the area. Normally, one expects the potential surface to be at or near the land surface (0.43 psi/ft). However, the potential surface for the Lower Cretaceous sandstones and underlying Paleozoic rocks is up to 2,500 ft (762 m) beneath the land surface (0.35 psi/ft) in parts of the Denver basin in Colorado and the Nebraska panhandle. The low pressures seem especially anomalous considering the elevation of the outcrops along the Rocky Mountain Front and the Black Hills.
The hydrostratigraphy is defined based on the known regional geology. Structure, isopach, and lithofacies maps are used to estimate the hydraulic characteristics of the rocks in the basin. A numerical model is constructed, based on the hydrostratigraphy, which simulates the regional flow system. Both transient and steady-state flow regimes are simulated. The interaction of the Lower Cretaceous sandstones with overlying and underlying hydrostratigraphic units is investigated. The significance of recharge in the outcrop areas is evaluated. The model is used to define the conditions under which subnormal fluid pressures may occur. The subnormal fluid pressures are reasonably explained as a consequence of regional ground-water flow.
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