Abstract

Reservoir pressures within the lithologic column in the Andarko Basin are a tiered system. The overpressured zone termed the Megacompartment Complex (MCC) is overlain and underlain by normally pressured intervals. Compartments are classified in three different groups or levels. The basinwide MCC is termed Level 1 compartment. Within the MCC, reservoirs that form fields or districts usually have similar pressure values. These fields or districts are termed Level 2 compartments. Detailed analyses of initial pressure, fluid types, and decline curves indicate that Level 1 and 2 compartments contain many smaller and isolated (sealed) compartments termed Level 3 type. The seals occur mainly in clay- or sand-rich rocks and may exhibit diagenetic banding patterns.

Banding patterns in clay-rich rocks seem to form independently of sedimentary textures or result from the enhancement or modification of sedimentary features. Diagenetic bands in sandstones typically consist of silica- and carbonate-cemented layers that are separated by clay-coated porous layers. Styloites and other pressure-solution features such as penetrating grain boundaries suggest a mechanism for the source of silica cements. The integration of tectonic history, stratigraphic relationships, facies distribution, thermal history, and diagenetic patterns of seal zones suggests that seals and compartments evoloved primarily during the Pennsylvanian orogenic episode. This occurred during the rapid subsidence phase of the orogeny during a period of approximately 30 million years.