Abstract: Pressure compartments and their interdependence on overpressure mechanisms: Examples from the North Sea

SWARBRICK, RICHARD

Pressure compartments are recognized by combining the mapped areal distribution of excess pressure above hydrostatic (overpressure) with a structural interpretation. Each region with similar amount of overpressure, and where geology justifies lateral connectivity and pressure equilibration, forms a single pressure compartment. Amount of overpressure in each compartment depends on sealing capacity of lateral and vertical barriers, and the overpressure mechanism. The most likely overpressure mechanisms are disequilibrium compaction, and gas generation. Overpressure generated by disequilibrium compaction varies with sedimentation rate and permeability. To achieve very high pressures requires early sealing, and continuous burial. Overpressure created by gas generation is largely restricted to deep, high temperature regions. Lateral transfer of overpressure within reservoirs is very effective at redistributing overpressure, increasing crestal and diminishing basal pressure. Locally this mechanism is responsible for seal breaching of pressure compartments.

Data from Jurassic North Sea clastic reservoirs show an increase in the amount of overpressure from basin margin to axis. In the Central North Sea pressures at the structural crest within most pressure compartments are close to fracture pressure, inferred from the minimum leak-off pressures in the overlying seal, suggesting vertical seal control on pressure distribution. The Northern North Sea has a similar pattern of highest overpressure in the deep, axial regions, diminishing towards the basin, but high variability in the amounts of overpressure at any depth. The throw of the faults does not always exceed the reservoir thickness, and overpressure distribution suggests lateral seals are less effective than in the Central North Sea.

AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria