NATURE AND CAUSES OF OVERPRESSURING IN MUDROCKS OF THE GULLFAKS AREA,NORTH-SEA

Pore-fluid pressure gradients in Lower Teritary to Upper Jurassic mudr ocks in the Gullfaks area show significant variations, both between an d within individual structural compartments. Detection and quantificat ion of abnormal pore-pressures, based on drilling parameters and wire line logs, are significantly influenced by petrological variations in the mudrocks and to not provide adequate direct pressure indicators. E mpirical models show that the degree of overpressuring in mudrocks abo ve the Gullfaks reservoirs cannot be fully explained by processes such as shale dehydration, compaction disequilibrium, aquathermal effects or in situ hydrocarbon generation, and other processes, such as caproc k failure and subsequent migration of hydrocarbons (mainly gas) into t he overlying mudrocks, appear to be influential in generating the obse rved pattern of overpressure.