Geological characterization of sand-prone subaqueous delta systems: a case study of the Upper Jurassic Sognefjord Formation (Troll Field, Northern North Sea, offshore Norway) and global examples

Abstract

Core sedimentology, stratigraphic architecture and 3D seismic geomorphology are integrated in order to: (a) demonstrate the criteria for recognition of coarse-grained subaqueous deltas in the stratigraphic record; (b) compare them with modern examples; (c) develop a new method to extract progradation rates from ancient shallow-marine clinoforms; and (d) refine the depositional model of the Upper Jurassic Sognefjord Formation, which forms the main reservoir in the super-giant Troll Field (Norwegian North Sea). The Sognefjord Formation is a 10-200 m thick clastic wedge, deposited in ca. 6 Myr, by a fully marine deltaic system that was sourced from the Norwegian mainland. A series of 10-60 m thick, westerly-dipping subaqueous clinoform sets are developed within this unit and can be mapped for several tens of kilometres along strike. Within each clinoform set, clinothems are formed by regressively stacked sandstone-rich bedsets, devoid of subaerial facies and separated by thin mudstone intervals. Near-horizontal trajectories are observed in each clinoform set, and the sets are stacked vertically. In the eastern half of the field, individual clinoforms are relatively gently dipping (1-6°) and bound thin (10-30 m) clinothems dominated by fine-grained, hummocky cross-stratified sandstones. Towards the west, clinoforms gradually become steeper (5-14°) and bound thicker (15-60 m) clinothems that comprise medium-grained sandstones in their upper parts. Topsets are usually well developed. Quantification of clinoform age and progradation rates is constrained by regionally correlatable bioevents, and relies on exponential age-depth interpolations. The facies break that mirrors the foreset-to-bottomset transition, which represents storm wave base, is subsequently dated, and progradation rates are measured along transects tied to well correlations and seismic interpretations. The results indicate falls in progradation rate (from 500 to 30 km/Myr) and net sediment flux (from 90 to 10 km2/Myr), and a simultaneous rise in vertical sedimentation rate (from 15 to 70 m/Myr) towards the basin; these variations are attributed to the progradation of the subaqueous delta into progressively deeper waters associated with along-shore currents that provide net sediment transport out of the study area, as well as sculpting the linear, elongated clinoforms. Coarse-grained subaqueous deltas provide a new interpretative template that may be applicable to other ancient clinoform-bearing shallow-marine sandstones with reservoir potential, whilst calculation of progradation rates provides a tool to improve reservoir characterisation and near-field exploration by enhancing prediction of reservoir distribution and character.