Regional pressure compartments

Definition

Most deep sedimentary basins of the world contain a shallow, normally pressured hydraulic system overlying one or more hydraulic compartments whose pressures are above or below the normal static water pressure.^[1] In this discussion these hydraulic compartments are called pressure compartments. They are defined within a basin from pressure measurements.

Seals

Seals isolate fluids within compartments from fluids that are in pressure communication with the surface (normally pressured hydraulic system). The top seal is more or less horizontal and usually cuts across stratigraphic units. It forms where authigenic minerals (usually calcite) fill the pore spaces. The bottom seal of regional compartments is usually a well defined lithostratigraphic unit. Commonly, the side seals are thin, mineralized zones paralleling vertical faults.

Pressure compartment recognition

Pressure compartments develop after the compartment has been sealed, isolating the fluids inside the compartment. Pressure in the compartment can be over, under, or normal. We can map a compartment's top, bottom, and sides from pressure measurements. Since pressure measurements are the main identifying criteria, the normally pressured compartments are usually not recognized.

Schematic of pressure compartment

Figure 1 See text for explanation.

The schematic cross section and pressure-depth plot in Figure 1 shows how changes in the fluid pressure gradient correlate with the top and bottom seals of a regional pressure compartment in a foreland basin.

Multiple pressure compartments

Figure 2 .

Many sedimentary basins contain two or more pressure compartments. The cross section in Figure 2 illustrates the multiple pressure compartments of the Anadarko basin.

Regional pressure compartment

The cross section below illustrates a regional pressure compartment from the Anadarko basin of Oklahoma.

Figure 5-24 .

Example: gradient profile

Following is a pressure-depth plot of a well through the Anadarko regional pressure cornpartment. Fluid pressure goes from normal above the top seal to overpressured within the pressure compartment, then back to normal below the bottom seal. Many regional pressure compartments contain smaller subregional and local pressure compartments. Al-Shaieb et al.^[2] use the following terms:

Regional pressure compartments—first-order or mega-compartments
Subregional compartments—second-order compartments
Local compartments—third-order compartments

The fluid pressure gradient variations within the pressure compartment on the plot below represent the smaller second- and third-order compartments.

Figure 5-25 Modified.

References

↑ Powley, D., E., 1990, Pressures and hydrogeology in petroleum basins: Earth Science Reviews, vol. 29, p. 215–226.
↑ Al-Shaieb, Z., Puckette, J., O., Abdalla, A., A., Ely, P., B., 1995b, Three levels of compartmentation within the overpressured interval of the Anadarko basin, in Ortoleva, P., J., ed., Basin Compartments and Seals: AAPG Memoir 61, p. 69–83.

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Regional pressure compartments

[ch05r14-1] Powley, D., E., 1990, Pressures and hydrogeology in petroleum basins: Earth Science Reviews, vol. 29, p. 215–226.

[ch05r2-2] Al-Shaieb, Z., Puckette, J., O., Abdalla, A., A., Ely, P., B., 1995b, Three levels of compartmentation within the overpressured interval of the Anadarko basin, in Ortoleva, P., J., ed., Basin Compartments and Seals: AAPG Memoir 61, p. 69–83.

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