Figure 4. Pressure-depth profile from Keyes field, Cimarron County, Oklahoma. The freshwater (0.43 psi/ft) and brine (0.465 psi/ft) gradients indicate the underpressured nature of compartments below the shallow Ogallala aquifer. Seal zones separate clusters of p-d values calculated from reservoir pressures. In the Keyes area, the upper Morrow Purdy sandstone has the highest p-d values (0.25-0.29 psi/ft), whereas values in the Chase and Council Groups are lowest (0.15 psi/ft). Note that Purdy values are higher than those in deeper lower Morrow ‘Keyes’ and Chester compartments. Hydrostatic value (0.43 psi/ft) for the Ogallala is an historic (pre-irrigation) value taken from the literature.