Salt Tectonics in the Northern Dutch Offshore (2015)

This MSc thesis has been written by former intern Matthijs van Winden.

Until recently, the northern Dutch offshore was a relatively under-explored area. Newly acquired 3D seismic data provides new opportunities for exploration and allows re-evaluation of existing play concepts. The presence and movement of Upper Permian Zechstein evaporates in this area has had a major effect on the geological development of this area. The timing of this halokinesis affects depositional patterns, structural development, hydrocarbon migration, trap formation and other aspects of plays in the DEFAB area of the Dutch offshore. This is the first study to look specifically into salt tectonics in this area of the Dutch offshore.

This study analyses a range of salt structures in the Step Graben, Dutch Central Graben and adjacent platform areas. Salt structures are described and characterized in a salt structure inventory. Assessment of timing of salt structure growth and its effect on depositional patterns shows initiation of salt movement in the Triassic, salt tectonic climax in the Jurassic and renewed salt movement in the Cretaceous. However, salt movement is not always consistent throughout the study area. A correlation can be seen between the location of salt structures within the structural elements, the way salt structures developed and the timing of salt movement. For example, isolated salt diapirs almost exclusively occur within the Dutch Central Graben, while in the Step Graben and towards platform areas, salt walls and elongated salt pillows occur above major basement faults.

To be able to assess salt tectonic evolution within a structural context, a 2D structural restoration of the Step Graben and Dutch Central Graben was performed. Results provide a conceptual model, constraining periods tectonism, deposition, erosion and salt movement in these graben systems. With this model, salt tectonic development of the study area can be viewed in a structural framework. It becomes evident that observations in depositional patterns, distributions of salt structure types and timing of salt movement have to be regarded as the consequence of an interplay of salt  ovement and structural development of the Step Graben and Dutch Central Graben.

Keywords: Salt tectonics, northern Dutch offshore, Dutch Central Graben, DEFAB, Structural restoration, Exploration