Salt Tectonics Early Movement (STEM): Impact on Triassic and Jurassic stratigraphy and on petroleum systems in the Dutch offshore

Summary

The STEM project investigated the Triassic structural evolution of the Central and Northern Dutch offshore. The research focused on early salt tectonic events and their expression at different locations in the basin. The research was carried out at two scales of investigation: 1) at a regional scale; and 2) at a block scale through three case studies. The project used a multidisciplinary approach by combining seismic-based structural analysis, 2D structural restoration, palynological analysis and geochemical analysis. The first two techniques were used to understand the salt tectonics of individual salt systems and of basin, and the latter two techniques were used to distinguishing between in-situ Triassic salt and remobilised Permian salt at selected locations. The STEM Project provided a new structural framework for the Triassic in the Dutch offshore and achieved the aims set.   

Background

This research topic was previously understudied in the Dutch sector, with more attention paid to Jurassic and Cretaceous salt tectonic events.

Project objective

The main aim of this project was to provide insights into the location, geometry and kinematics of individuals salt systems, such as salt welds, allochthonous salt systems, growth fault/raft systems and collapse salt bodies, and to build a comprehensive structural evolution of the study area during the Triassic.

Project results

The geochemical study revealed that Permian and Triassic salts can be dated with S-isotope stratigraphy and so can be geochemically be distinguished from each other. This result opens a new avenue for future research, with the aim being to develop a new tool for Permian and Triassic salt dating.

The structural analysis carried out using seismic and structural restoration techniques revealed that salt tectonics started during the Early Triassic, but only as isolated mini basins in the northern part of the Dutch sector (Block A). The detailed tectonostratigraphic analysis carried in the three case studies were also successful.

The regional structural analysis showed that the basin was composed of three distinctive zones during the Middle and Late Triassic, and the study proposed a new conceptual regional kinematic model of the Triassic that details successive events such as 1) the salt diapirism at the onset of the Middle Triassic; 2) shortening of these salt diapirs due to concentric gravitational gliding around the Triassic basin margins; and 3) local extrusion of allochthonous salt sheets associated with this shortening of salt diapirs.

As a result, the STEM Project provided a new structural framework for the Triassic in the Dutch offshore and achieved the aims set. The results indicate that salt tectonics played a strong role in shaping the basin and specific structures as early as the Middle Triassic. The implications of this research are far reaching in regards of the petroleum geology of the study area and specifically the implication for Triassic/Jurassic reservoir characterisation and hydrocarbon maturity and migration modelling in the vicinity of salt structures.