The influence of faults and intraplate stresses on the overpressure evolution of the Halten Terrace, mid-Norwegian margin

The Halten Terrace is a structural element of the Meso-Cenozoic mid-Norwegi an margin. The pore fluid pressure distribution in the faulted Jurassic for mations on the Halten Terrace is characterized by significant lateral varia tions. In general, the fluid overpressure increases stepwise across faults from east to west, from zero (hydrostatic fluid pressure) to about 30 MPa. Fault-bounded pressure cells can therefore best explain the fluid pressure distribution. The results of analyses of log-derived porosities indicate th at the high overpressure in the westernmost pressure cell was built up rece ntly. However, despite the high sedimentation rates during Plio-Pleistocene , the high overpressure cannot be explained by local mechanical compaction. Alternative explanations for the high overpressure proposed by other autho rs are based on pore fluid volume increase (e.g. hydrocarbon generation). W e propose that the high overpressure is caused by fluid flow from the deep Ras Basin to the western part of the Halten Terrace, through fractures in t he Mesozoic, deep seated Klakk Fault Complex. Opening of fractures in this fault zone by seismic and static mechanisms is possible in the present-day intraplate stress field, which is characterized by a NW-SE-oriented maximum horizontal stress direction. During Miocene, the maximum horizontal stress was E-W oriented, which implies a stress rotation during Pliocene. The E-W orientation of the maximum horizontal stress has impeded the initiation an d opening of fractures in the N-S-striking Klakk Fault Complex during Mioce ne. Fluid flow from the Ras Basin through faults of the Klakk Fault Complex can therefore have occured since Pliocene. Thus, the rotation of the intra plate stress directions can explain why the build-up of overpressure on the western part of the Halten Terrace occured recently, as indicated by the r esults of porosity analyses. Understanding the overpressure evolution of th e Halten Terrace is important for exploration in that area, as hydrocarbons have been found in the hydrostatic pressure cells, whereas they are absent in the high overpressure cells. (C) 2000 Elsevier Science B.V. All rights reserved.