Stratigraphic Traps—Learnings and Mapping Techniques from Ringhorne West, Norwegian North Sea

Espejord, Kjell, Per Øyvind Seljebotn, Odd Tangen, ExxonMobil, Norway

Abstract: Exploration potential in stratigraphic traps in the North Sea has received a lot of attention the least years as limited remaining potential exist in the more conventional traps. The challenge in mapping these potentials has been to identify and document the trap­ping mechanism. The Ringhorne West discovery in the Utsira High area of the Norwegian North Sea was made during production drilling for the Jurassic section of the Ringhorne Development. Oil was encountered in a 3.5-meter sand of the Lower Palaeocene, deep marine Ty Formation sand deposits. This sand is resting directly on the chalk and occurs on the seismic section within the chalk reflection cycle. Depositional system and mapping of the pinch-out zone for this sand accumulation was achieved by utilising several interpreta­tion techniques rather different from normal horizon interpretation along a seismic reflector. NW-SE striking faults and flexes exert significant control on the sand thickness and distri­bution. The discovery also proved the existence of an intra Palaeocene seal, which was not earlier known in the area. The tuning thickness for the sand is about 30m in the area and 90% of the reserves is in reservoir significantly thinner than the tuning thickness. The sand mapping methodology has proved to be accurate enough to enable successful production wells placement in the sand close to the mapped pinch-out zone where the sand thickness averages 5 meters. The evaluation process of this discovery added significant learning and improved mapping techniques leading to the definition of this stratigraphic trap.