Origin of overpressures on the Halten Terrace, offshore mid-Norway: the potential role of mechanical compaction, pressure transfer and stress

The Halten Terrace, a structural element on the mid-Norwegian margin, is characterized by distinct spatial variations in subsurface fluid pressure. Jurassic formations on the western part of the Halten Terrace are highly overpressured (>30 MPa), whereas the same formations have fluid pressures close to hydrostatic in the central and eastern parts of the terrace. The Cretaceous formations are moderately overpressured in the whole area. The porosities in the overpressured middle Jurassic formations and lower Cretaceous shales are only slightly influenced by the present-day overpressures, suggesting that most of the overpressure development was recent. Overpressure generation as a result of mechanical compaction, induced by the weight of the Plio-Pleistocene sediments, is not sufficient to explain the overpressure magnitudes in the Jurassic and Cretaceous formations. Results from forward modelling of mechanical compaction suggest that the deep Ras Basin adjacent to the Halten Terrace has been highly overpressured since Cretaceous time. Pressure transfer from this basin to the western part of the Halten Terrace is therefore a possible explanation for the observed overpressure. Migration of the overpressured fluids may have occurred through the Klakk Fault Complex separating the basin from the terrace, during an event related to a basinal stress reorientation in early Pliocene time.