Geomorphology, Facies Character and Stratigraphic Architecture of an Ancient Sand-prone Subaqueous Delta: Upper Jurassic Sognefjord Formation, Troll Field, Offshore Norway

Patruno, Stefano; Hampson, Gary J.; Jackson, Chris

The integration of core sedimentology, seismic stratigraphy and seismic geomorphology has enabled interpretation of a sand-prone subaqueous delta in the Upper Jurassic Sognefjord Formation of the Troll Field, Horda Platform, offshore Norway. Mud-prone subaqueous deltas, characterised by a compound clinoform morphology, are common in many modern tide- and wave-influenced settings, but ancient examples are rarely reported. The Sognefjord Formation data demonstrate the criteria for recognition of subaqueous deltas in the stratigraphic record, as well as refining the depositional model of the main reservoir in the super-giant Troll oil and gas field.

Two 10-60 m thick, wave-dominated, regressive-transgressive packages bounded by major marine flooding surfaces are distinguished in the lower Sognefjord Formation. In the west, these packages consist mainly of cross-bedded sandstones; in the south-east they become dominated by finer-grained, hummocky cross-stratified sandstones. The presence of coarser, poorly sorted sandstones in the north-east indicates greater proximity to a fluvial sediment input point. No evidence of subaerial exposure is observed.

Each regressive-transgressive package corresponds to a set of seismically resolved, westerly-dipping clinoforms. The packages thicken westwards, until they reach a maximum where the clinoform "envelope" rolls over to define a topset-foreset-toeset geometry at the position of maximum regression. Both individual clinoforms and reflections bounding the clinoform sets are oriented sub-parallel to the Horda Platform edge (N005-N030). In the eastern half of the field, individual clinoforms are thin (10-30 m) and relatively gently dipping (1°-6°). Towards the west, clinoforms gradually become thicker (15-60 m) and steeper (5°-14°). Topsets are consistently well developed.

We interpret deposition by fully subaqueous, wave-dominated, linear clinoforms that prograded westwards across the Horda Platform. Subaqueous clinoforms were fed by a river outlet at the north-east and sculpted by the action of currents sub-parallel to the clinoform strike. By analogy with modern systems, the shoreline is inferred to have been marked by a subaerial delta clinoform, separated from the subaqueous clinoform by a subaqueous topset.

The resulting model of a coarse-grained subaqueous delta provides a new interpretative template that may have applications to other clinoform-bearing shallow-marine sandstones in the ancient stratigraphic record.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013