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