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Interplay Between Depth-Dependent Leakage, Fault Sealing and Pore Pressure Buildup on Selected Areas of the Norwegian Continental Shelf


Basin subsidence results in porosity reduction and fluid expulsion. The expelled fluids either move laterally in carrier beds, along faults, or through imperfect caprocks. Likewise, hydrocarbons move largely due to buoyancy, and so reach the surface unless they are trapped. Subsurface fluid pressures hold information on the connectivity of fluid migration routes. Hence, overpressure differences testify to poor connectivity and ineffective or lacking migration paths.

We observe that fluid pressures in reservoirs on the NCS increase with depth: from close to hydrostatic in most open carrier beds above 2500m to close to lithostatic at ca 5000m burial depth. We also observe that fault-sealed hydrocarbon columns in areas with little clay smear are generally small above 2500m, but that they are larger at greater depths. Furthermore, faults that in some places need to be open to allow hydrocarbons to migrate can at other places serve as sealing elements for fault-dependent accumulations.

These observations can to some extent be explained by depth-dependent diagenesis in fault planes. Such diagenesis may lead to cementation of faults that are open to across-fault fluid flow at the time when the fault movement ceased. Non-homogeneous fault planes can have sealing properties and trapping capabilities locally before they become completely sealed and set up fluid overpressure differences. We therefore envisage the cementation of low-clay-smear faults as a three-step process through geologic time. Initially, the faults are open to water flow and hydrocarbon migration. Later, the faults become sufficiently sealing along parts of the fault planes to allow accumulation of hydrocarbons, whereas other parts of the fault planes are open to hydrocarbon movement. In the last stage, the entire fault planes become cemented, and pressure buildup results because pore fluids that are released during compaction cannot escape easily.

Hydrocarbon column heights will be influenced by such fault behavior. The accumulations that are trapped by partly cemented fault planes can be charged by migration from the basinward side of faults until the fault planes become completely sealing. The fault-sealed columns can at this stage be influenced by local as well as by long-distance charge. Migration from source rocks within the pressure compartment and leakage at the weakest point of the same compartment will exert the main controls on hydrocarbon columns after sealing of the entire fault planes.