Sedimentology and Sequence Stratigraphy of Shallow Marine-to-Deltaic Sandstone Reservoir Targets: Krossfjord and Fensfjord Formations, Troll Field, Norwegian North Sea

The sedimentological character and stratigraphic architecture of shallow marine reservoirs are strongly controlled by the physical processes that occur at and near the shoreline (e.g. wave- vs. tide- vs. fluvial-dominated). These processes can be further complicated by syn-depositional normal faulting, which controls subsidence, uplift and accommodation development. We present a subsurface case study from the Middle-to-Upper Jurassic Krossfjord and Fensfjord formations, Horda Platform, offshore western Norway. These formations are dominated by shallow marine to deltaic sandstones, which were sourced from the Norwegian mainland to the east and which pinch out basinwards to the west into offshore shales. The distribution, geometry, and connectivity of these sandbodies are poorly understood, as they have not been the focus of previous work. However, the formations form a significant oil and gas reservoir in the Troll and Brage fields, and a prospective reservoir in the area around the Gjøa Field.

Core, biostratigraphic and wireline log data are used to produce a consistent geological interpretation for the Krossfjord and Fensfjord formations in the Troll Field. In combination, the two formations define an overall regressive-to-transgressive wedge that is punctuated by marine shales of the Heather Formation. The facies associations identified in core represent wave- and tide-dominated deltaic, shoreline and shelf depositional environments. The Krossfjord Formation (c. 90 m thick) consists of three medium-to-coarse-grained, coarsening-upwards units that are each c. 20-30 m thick. The Fensfjord Formation (c. 140 m thick) is overall finer-grained than the Krossfjord Formation, and consists of three c. 20-30 m thick coarsening-upwards units in its lower part, and seven c. 5-20 m thick coarsening-upwards units in its upper part. Based on facies stacking patterns and abrupt shifts in facies, three regionally extensive flooding surfaces are identified. Analysis of 3D seismic reflection data indicates that both formations contain seismic-scale clinoforms, and that they both thin and pinch out towards the west. The recognition of pronounced variability in facies character and stratigraphic architecture emphasise the need for a robust understanding of the Krossfjord and Fensfjord formations in order to drive future exploration in these, and coeval, reservoirs.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California