Quantitative Stratigraphic Architecture, Depositional History and Progradation Rates of an Ancient Sand-Prone Subaqueous Delta (Sognefjord Formation, Troll Field, Norwegian North Sea)

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

In this study, we develop a new method to extract progradation rates from ancient shallow-marine clinoforms, and we use it to refine the depositional model of the Upper Jurassic Sognefjord Formation, which forms the main reservoir in the giant Troll Field (Norwegian North Sea). This provides a tool to improve reservoir characterisation and near-field exploration by enhancing prediction of reservoir distribution and character.

The stratigraphic architecture of the studied succession is constrained by integrating 3-D seismic data with a dense core and wireline-log dataset. The Sognefjord Formation is a 10-200 m thick clastic wedge, deposited in ca. 6 Myr, by a fully marine, westward-prograding deltaic system that was sourced from the Norwegian mainland. A series of 10-60 m thick, westerly-dipping subaqueous clinoform sets are developed within the Sognefjord Formation and these 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. Horizontal to descending trajectories are observed in each clinoform set, and the sets are stacked vertically. Coarse-grained subaqueous deltas provide a new interpretative template that may be applicable to other ancient clinoform-bearing shallow-marine sandstones with reservoir potential.

Quantification of clinoform age and progradation rates is constrained by 20 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. Our results indicate a fall in progradation rate (from 500 to 30 km/Myr), net sediment flux (from 90 to 10 km2/Myr) and simultaneous rise in sedimentation rate (from 15 to 70 m/Myr) towards the basin; we attribute these variations 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. As a result of this interaction of seabed bathymetry and oceanographic processes, the emplacement of deep-marine reservoirs on the basin floor is potentially hindered, even during periods of relative sea level fall.

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