The truth about Triassic salt tectonic models for the Northern North Sea

The evaporite-dominated Zechstein Supergroup (Upper Permian) is well-developed in the northern North Sea sector of the Norwegian continental shelf. Here, syn- and predominantly post-depositional flow of Zechstein salt resulted in the formation of large salt-tectonic structures, controlling the tectono-stratigraphic evolution and hydrocarbon potential of this prolific petroleum province. The timing of initial salt flow, at least on marginal terraces and intra-basin platforms, is relatively well-constrained, with most salt-tectonic studies suggesting diapir formation mainly initiated in the Triassic, with welding occurring in the Late Triassic to Early Jurassic. Much more poorly understood is the mechanism driving this earliest stage of salt flow, with two models being proposed: (i) extension of Lower Triassic overburden (i.e. reactive diapirism); or (ii) Early Triassic differential sedimentary loading (i.e. passive diapirism). Choosing between these two models has ‘kinematic consequences’, with reactive diapirism, which is driven by thin- (i.e. basement-detached) or thick-skinned (i.e. basement-involved) extension, requiring broadly age-equivalent shortening elsewhere within the basin, in the form of squeezing of pre-existing diapirs or the formation of salt-detached buckle folds. In contrast, passive diapirism, in which salt-tectonic motions are primarily vertical, does not require complementary shortening. Furthermore, establishing which model is most applicable to a particular geological setting is important, with both making significantly different predictions regarding Triassic reservoir distribution and trap geometry. In this talk I present published and unpublished data from the northern North Sea and other salt-tectonic basins to examine diagnostic criteria that may help us differentiate between these two models. I show that, due to poor seismic imaging of lowermost Triassic strata, and similarities in finite structural style related to reactive and passive diapirism, differentiating between these two models is extremely difficult, arguing it is therefore important to consider both when exploring for salt-related Triassic traps.