Controls on minibasin infill in the Nordkapp Basin: Evidence of complex Triassic synsedimentary deposition influenced by salt tectonics

Abstract

The Nordkapp Basin is a northeast-trending upper Paleozoic rift basin located in the western Barents Sea. The presence of salt structures makes this basin of interest for hydrocarbon exploration. Nevertheless, no commercial discoveries have been made so far, and the basin remains poorly understood in terms of distribution of reservoirs and source rocks within its salt minibasins. In this study, a three-dimensional data set together with well data in a small area located in the southwestern subbasin of the Nordkapp Basin has been studied, with the main purpose of obtaining a better understanding of salt kinematics and minibasin infill, with special focus on the Triassic. The minibasin infill of the Nordkapp Basin has been subdivided into five second-order sequences, which mark the main regional events recorded in the Barents Sea. Further subdivision into third-order sequences has been made in the Triassic in response to the local halokinetic movements of the basin. In terms of salt kinematics, the Nordkapp minibasins have been affected by the following: (1) late Permian–Early Triassic minor salt mobilization, (2) latest Early Triassic reactive diapirism, (3) Early to Middle Triassic transition between reactive–passive diapirism, (4) Middle Triassic–Jurassic passive diapirism, (5) Late Jurassic–Early Cretaceous gravity gliding, and (6) Cenozoic regional contraction. The rapid changes in depositional environment observed in well 7228/7-1A, together with the large variability of seismic responses and reflection terminations, provide evidence of a complex Triassic paleogeography controlled by salt tectonics. Early to Middle Triassic differential loading of minibasins into the underlying salt formed broad diapir-induced highs, which might have acted as local sources of sediment for submarine debris flows and fan deltas developed adjacent to salt structures. During the Middle Triassic–Jurassic last stage of passive growth, salt diapirs formed local topographic highs covered by ephemeral roofs and caused narrow areas of deformation widths less than 1 km (3281 ft), developing composite halokinetic sequences. In minibasins, salt evacuations and associated subsidence controlled the spatial transgressive–regressive trends of fluviodeltaic systems coming from the southeast of the Barents Shelf. Rapidly subsiding minibasins were flooded by marine conditions, and intervals of organic-rich source rocks might have been deposited in salt withdrawal-related depressions. As subsidence decreased, newly developed regressive sequences of northwest-trending fluviodeltaic systems sourced from the southeast covered previous marine environments, possibly depositing potential fluviodeltaic reservoirs within the salt minibasins.