Seismic interpretation and characterization of anhydrite caprocks in the Tromsø Basin, SW Barents Sea

Abstract

The origin of anhydrite caprocks found in many evaporite basins is currently under debate. Existing models in the literature for anhydrite formation include rotated stratigraphic layering, rotated crestal caprock and diagenetic anhydrite. In this work, we interpret high-quality 2D seismic reflection data to understand the processes associated with anhydrite caprock formation. Our study focuses on anhydrite caprock formation in the Tromsø Basin, which represents one of several underexplored salt basins in the Arctic. We use well log data and seismic stratigraphic as well as seismic attribute analyses to characterize the ‘anhydrite’ layers associated with salt diapirs. Our results reveal that the anhydrite layers are high-amplitude reflections at the crests, which are often found close to the seabed and lateral caprocks on the flanks of salt diapirs. A third class of anhydrite layers is folded, faulted and boudinaged intrasalt stringers encased by the salt diapirs. Lateral caprocks in the Tromsø Basin are formed in response to rotated crestal caprock failure. The crestal caprocks are secondary in origin, occur at shallow depths, and are formed as insoluble residues from the dissolution of halite by the ingress of basinal fluids into the tops of diapirs. Anhydrite stringers, or floaters, originate from tectonic-related multi-dimensional salt flow. Salt diapirs in the Tromsø Basin are associated with varied anhydrite caprocks; their formation cannot be fully explained using a single model. Our work demonstrates from seismic reflection data the occurrence of caprock events that formed progressively on the flanks of salt diapirs due to local folding.