Print this pageAnomalous Pressure Regimes In the Cretaceous Sandstones of the Denver Basin, Colorado, Wyoming, and Nebraska
ZHAO, HAN Q.
Decompacted sonic velocity panels for the central Denver Basin were constructed from digital sonic logs after filtering for lithology. It is clear from the panels that Cretaceous shales in the central Denver Basin are characterized by low sonic velocity and high rock density and resistivity, indicating that the rock volume is anomalously pressured and gas saturated on a regional scale.
Within the anomalously pressured shale volume, there are many isolated, gas- saturated depletion drive sandstone reservoir compartments. Individual reservoir compartments can be overpressured, underpressured, or normally pressured. The traps for these reservoirs initially involved a combination of stratigraphic, and structural elements combined with other low-permeability barriers. Intensive generation of hydrocarbons from the shale volume and the subsequent filling of the sandstone compartments with hydrocarbons led to the expulsion of most of the free water, and conversion of the fluid-flow system from a single-phase to a multiphase regime. This resulted in the evolution of the low-permeability barriers into capillary seals, which enclosed the compartments in three dimensions. Water chemistry studies indicates that the individual sandstone compartments are still relatively closed fluid-flow systems.
Sandstone reservoirs outside the anomalously pressured shale volume are normally pressured. These sandstones follow a normal hydrostatic gradient (relative to the outcrop on the eastern margin of the Denver Basin) and are under water drive, with an open-fluid-flow system. A permeability barrier between these normally pressured Cretaceous sandstones and the anomalously pressured shale volume and the sandstone compartments within it inhibits present-day communication of fluids.
This study was funded by the Gas Research Institute under Contracts Number 5089-26O-1894 and 5091-221-2146.