Structural Controls on the Stratigraphic Architecture of Net-transgressive Shallow-Marine Reservoirs in a Salt-influenced Rift Basin: Middle-to-Upper Jurassic Egersund Basin, Norwegian North Sea

Mannie, Aruna; Jackson, Christopher A.; Hampson, Gary J.

The growth of salt structures in many basins exerts a first-order control on accommodation development, sediment distribution and basin physiography over relatively short length-scales. However, the influence of such structures on the stratigraphic architecture of salt-influenced rift basins has been poorly documented, particularly within net-transgressive strata that typically contain thin and laterally discontinuous sandstones of potential reservoir quality. Studies of net-transgressive strata in salt-influenced rift basins provide insight into controls on accommodation development, depositional process regime and facies character (i.e. wave- vs. tide- vs. river-dominated), and distribution of shallow-marine sandstones, thereby reducing reservoir risk during hydrocarbon exploration and production.

Analysis of Middle-to-Upper Jurassic net-transgressive strata of the Egersund Basin, Norwegian North Sea rift was conducted using an integrated dataset of seismic reflection, wireline log, biostratigraphic and core data. These strata consist of low-energy, wave-dominated offshore-to-shoreface and coal-bearing coastal plain deposits arranged in four parasequences that are aggradationally to retrogradationally stacked to form a net-transgressive succession up to 150 m thick, over 20 km in depositional strike (SW-NE) extent, and over 70 km in depositional dip (NW-SE) extent. Halokinesis and normal faulting influenced depositional thickness by creating accommodation on the flanks of salt structures, and in the hangingwalls of normal faults. Sediment supply was sufficiently high to result in thickness but not facies changes across active salt structures. An abrupt, downdip transition from shoreface sandstones to offshore mudstones only occurs across a large displacement, basement-involved normal fault, located where a fringing shoreline developed in the immediate footwall of the fault and deeper water conditions were present in the hangingwall.

The results of this study highlight that basin physiography, rate of accommodation creation and sediment influx are the main factors controlling facies distribution and thickness, variability in net:gross ratio, and extent of net-transgressive shallow marine reservoirs in salt-influenced rift basins, impacting reservoir presence and quality.

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