The Powder River Basin of northeastern Wyoming contains numerous subsurface pressure anomalies, both above and below normal hydrostatic. A study of approximately 5000 drill-stem-tests (DST’s) from the Powder River Basin reveals the geographic distribution of pressure compartments. The abnormally high pressure compartments within the Muddy Sandstone are selected initially for detailed analysis. Over-pressure in the Muddy pressure compartments is approximately 2000 psi over normal hydrostatic. Lithologie units surrounding a pressure compartment are sampled and measured for threshold displacement pressure with Mercury injection techniques. The seals surrounding the pressure compartment are not impermeable rocks; rather, they are capillary seals due to anomalously high displacement pressure in isolated portions of the Muddy Sandstone. These zones of high displacement pressure appear to correlate with unconformities within the Muddy Sandstone. This model of pressure compartmentalization is in marked contrast to Gulf Coast type models where overpressured aquifers are a result of disequilibrium compaction. Rocky Mountain pressure compartments require more effective sealing, as provided by capillarity and low-permeability rocks.