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Recognition of Preserved Beach-Ridges in the Etive Formation and Their Impact on the Dynamic Reservoir Behavior of the Lower Brent Group in the Oseberg East Field, Norwegian North Sea


Grünhagen, Henrike, Frede Bøen, Rolf Aasheim, Norsk Hydro ASA, Bergen, Norway


The well-known and frequently described Rannoch and Etive Formations represent the prograding shore-line deposits of the economically important Jurassic Brent Delta in North Sea. In the area of the Oseberg East Structure, offshore Norway these formations show an unexpected depositional pattern.

On geophysical attribute maps a set of 20-30 E-W trending stripes cover an area of more than 10x10km. Around thirty wells on Oseberg East and several further wells in the sur­rounding area confirm that the amplitude anomalies are caused by rhythmic thickness vari­ations within the Etive Formation. While shoreface sediments are generally deposited as homogenously thick sheet-sands, these bands of thick Etive formation are interpreted as preserved beach-ridges that have formed along an E-W trending paleo-shoreline during progradation of an extended strand plain. Seismic modeling indicates that infill of the result­ing paleo-topography with coals of the overlying Ness Formation contributes significantly to the seismic response.

The presence of the beach-ridges combined with significant vertical heterogeneities within the Rannoch and Etive formations are complicating oil production from that zone on Oseberg East. Attic oil is trapped in the beach-ridges while the underlying coarse-grained and high-permeable foreshore facies acts as ‘thief-zone’ that causes early water/gas break­through. This new understanding of the geometry and sedimentology of the Etive Formation on Oseberg East is beeing implemented into 3D facies, property and reservoir simulation models to investigate the formation’s dynamic behavior and to plan wells that can increase oil-recovery.