ALBERT

Description: Understanding the factors controlling the development of accommodation above collapsing salt diapirs and their influence on reservoir distribution is critical in reducing exploration risk in salt-influenced sedimentary basins. In this study, we use an integrated subsurface data set (three-dimensional and two-dimensional seismic reflection, wire-line-log, core, and biostratigraphic data) from the Upper Jurassic of the Cod terrace, Norwegian North Sea, to understand the influence of rifting on accommodation creation and shallow-marine deposition during the initial-stage collapse of salt diapirs. We demonstrate that rifting resulted in the rise and fall of salt diapirs, and the formation of supra-diapir minibasin-style depocenters that became sites for deposition and preservation of up to 500 m (1640 ft) thick net-transgressive shallow-marine sandstone reservoirs. Maximum thickness is recorded in the axis of minibasins with a reduction in thickness of up to 65% noted on their flanks. The stratigraphic architecture of individual minibasins is variable. Proximal-to-distal facies variations from shoreface to offshore shelf and commensurate changes in reservoir quality occur over scales larger than individual minibasins. These deposits contain large sand volumes, and are not confined to areas of localized sandstone subcrop. In combination, these features suggest that the minibasins formed a linked network supplied by regional sediment-routing systems. The results of this study provide a new tectono-stratigraphic model for prediction of reservoir presence, thickness, and continuity in diapir-collapse minibasins along salt walls in the Central North Sea, and in other less mature, data-poor basins where reservoirs have been identified in depocenters above salt walls.

Description: Syndepositional deformation in salt-influenced rift basins is complex, being driven by a combination of normal faulting and the growth of salt structures such as diapirs. Owing to a lack of data with which to simultaneously constrain basin structure and synrift stratigraphic architecture, we have a poor understanding of how these processes control shallow-marine deposition in such settings. To improve our understanding we here use seismic reflection and borehole data from the Norwegian Central North Sea to investigate the role that syndepositional fault growth and salt movement played in controlling the sub-regional stratigraphic architecture of a net-transgressive shallow-marine synrift succession (Middle to Late Jurassic). The rift-related structural framework, which is usually dominated by normal fault-bound horst and graben, is strongly modified where an Upper Permian salt layer (Zechstein Supergroup) is sufficiently thick and mobile to act as an intra-stratal detachment, giving rise to decoupled rift-related basement and cover structural styles. Furthermore, cover extension allows the salt to rise diapirically, resulting in the formation of large salt diapirs and supra-salt normal faults formed owing to late-stage salt withdrawal and diapir collapse. Rift-related normal faulting and the growth of salt structures had a dual control on the depositional thickness and facies distribution within the net-transgressive, predominantly shallow-marine, Middle to Upper Jurassic synrift succession. The resulting facies architecture reflects a delicate balance between fault- and salt flow-driven accommodation creation and intra- and extra-basinal sediment supply. Where sediment supply and accumulation rate exceeded accommodation, little or no change in facies is observed across syndepositional structures. In contrast, where accommodation outpaced sediment supply and accumulation rate, footwall-attached shorelines locally developed adjacent to large, thick-skinned normal faults, with deeper water conditions persisting in the adjacent hanging wall. Flooding of structural elements was strongly diachronous and influenced by the underlying rift-related topography, which was characterized by intra-basinal horst and graben. This paper highlights the key role that salt plays in modifying the tectonostratigraphic evolution of rift basins, suggesting that existing models, based on salt-free structural templates, need to be modified.