Towards a better understanding of the highly overpressured Lower Triassic Bunter reservoir rocks in the Terschelling Basin

Abstract

Abstract Large lateral variations in pore-fluid pressures and associated overpressures occur in the compartmentalized Lower Triassic Bunter reservoir rocks in the Terschelling Basin (northern Dutch offshore). This study describes the main controlling factors on the distribution of these overpressures by combining seismically derived structural interpretations with pressure data. The most important controlling factor is considered to be the presence and type of both lateral and vertical permeability barriers. Lateral permeability barriers are formed by Zechstein salt and faults. The most effective vertical permeability barrier for maintaining high overpressures is the Upper Triassic Röt evaporite. Overpressures of more than 30 MPa are observed in the Bunter compartments where the Röt is present and laterally continuous. Overpressures are lower than 15 MPa in compartments where the Röt was affected by Mid-Cimmerian erosion. Based on structural observations, it is concluded that the highly overpressured compartments became fully hydraulically restricted during the Late Jurassic and Early Cretaceous. Calculations show that subsequent sedimentary loading, during the Cretaceous and Cenozoic, was able to generate the observed overpressures.