Influence of basement faults on the development of salt structures in the Danish Basin

Abstract

Scaled centrifuged models based on interpreted seismic profiles were used to simulate the salt structures in the Danish Basin. The models consisted of a viscous layer simulating rock salt, overlain by layers of cohesive sand that mimic brittle failure in natural overburdens. Model results support the hypothesis that basement faults trigger the growth of many salt diapirs in the Danish Basin. In plan view, model diapirs are localized as buoyant walls along faults in the overburden which collapsed due to basement fault movement. With further burial, single diapirs rose from these buoyant walls. Some of the diapirs surfaced through the faulted overburden, a few were trapped beneath the competent ‘Cretaceous’ units and others were starved due to lack of supply from below. In section, model diapirs were asymmetric and were rooted on the tip of the faults or located on the rotated footwall blocks. Model diapirs not located over faults were triggered by differential loading and collapsing of overburden layers due to basement block rotation. Models show that the presence of fewer salt structures in the northern than the southern part of the basin is due to differential subsidence and flow of buoyant material from north to south, where they rise diapirically. In the model, the pressure due to loading by overburden layers changed from 1.3 × 10 2 Pa in the south to 2.3 × 10 2 Pa in the northern part of the basin. In general, the model results illustrate that basement faults influence diapirs by accumulating buoyant material in half-grabens and introducing a slope and steps at the base of the buoyant layer, weakening its overburden and causing differential loading and compaction. The spreading overhangs of salt diapirs beneath the Cretaceous chalk in the Danish Basin would seal any potential pre-Cretaceous reservoir rocks.