Permo-Triassic basin development on the Horda Platform and Stord Basin – Pre-rift architecture and rift phase 1 evolution

Abstract The northern Horda Platform and Stord Basin is a part of the multi-phase North Sea rift system. Crustal extension of the area have taken place since the Devonian, in several pulses of rift activity. This study focuses on the development of the Permo-Triassic rift structures and the implications for sediment accumulation in relation to the large-scale structures. To achieve this, the region is studied via the interpretation of a large network of 2D and 3D seismic surveys and correlated to a limited number of wellbores. A selection of seismic sections, time-structure maps and thickness maps are produced to elucidate the underlying structures as well as how and where Permo-Triassic faults created accommodation, providing further insight into the temporal history of the Permo-Triassic northern North Sea rift phase (Rift Phase 1 – RP1). Devonian orogenic collapse shear zones are mapped to investigate relationships between inherited structures and fault zones being formed during RP1. The study area is divided into three distinct structural domains (northern, central and southern) based on characteristics inherited from Devonian structures and onshore-offshore correlation. Furthermore, relationships between RP1 fault zones and Devonian shear zones are established and described as exploitative, cross-cutting, detachment-style and non-interaction modes. The inherited structures are shown to have had a profound effect on the basin development during RP1 by controlling fault positions, fault displacement, fault plane polarity and large-scale strain transfer between the domains of the study area. A rigid structural element, the Utsira High, is shown to have influenced the development of the Stord Basin by strain localization east and west of the rigid block. Zechstein evaporites, found in basins to the south of the study area, is shown to have been blocked by footwall uplift on the Hardangerfjord Shear Zone and hinterland uplift resulting from continuous uplift from the late Carboniferous. Sediment accommodation was created in the RP1 rotated half-grabens. The sedimentary units are divided into tectonostratigraphical units and tied to well-log interpretations, providing a spatial and temporal framework for the RP1 deposits. RP1 initiation is proposed to be Permian, with cessation in the early Late Triassic Carnian age. Post-rift deposits revealed a reactivation of major faults in the early Norian age. Sediment dispersal systems are discussed and major drainage systems are revealed by applying fault growth theory and palaeotopography. The Hardangerfjord and Sognefjord drainage systems are proposed to be the major systems of drainage into the Stord Basin and the Horda Platform, respectively. This study documents improved understanding of multi-phase rift basins and the influence of inherited structures that can be applied to future work. More specifically, the temporal development of the Permo-Triassic rift phase in the northern North Sea is discussed. This study contributes to constrain the temporal aspects of RP1, and provides several large-scale sediment supply routes to the basins of the study area. The Horda Platform and Stord Basin are currently being evaluated for CO2 sequestration (CCS), and the >5 km thick Permo-Triassic strata are one of the target groups for reservoir rocks. Understanding the structures and deposits in these systems are therefore vital for identifying and assessing new reservoirs