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Facies Characterization and Depositional Architecture of the Fruholmen and Stø Formation, Barents Sea, Norway

Abstract

The Stø, Nordmela and Fruholmen formations are proven reservoirs in the Norwegian Barents Sea, Norway. In the Hoop Area, the formations are condensed in comparison to other locations in the Barents Sea, like in the Hammerfest Basin and along the Bjørnøyrenna Fault Complex. They represent the main reservoir interval at the Wisting discovery. Several wells were cored through the stratigraphic intervals, spanning from Triassic to Jurassic Periods. A detailed facies and rock property analyses of one of the cored wells reveal a number of lithofacies which control the reservoir quality in the area. One of the essential challenges of the potential field development is to understand the vertical facies distribution along the cored wells of condensed sections with several unconformities, and to use geological concepts in order to predict the lateral distribution. The developed facies sedimentological conceptual model is supported by core and log data. Modern and ancient analogs are included in order to forecast the architecture and geometries of the depositional elements. Thirty (30) descriptive lithofacies are recognized for the entire cored section. Overall nine (9) depositional subenvironments are distinguished: bay/embayment, tidal sandy deposits, tidal flat, multistorey fluvial channel, tidally distributary channel, middle shoreface, upper shoreface, transgressive lags and shallow marine platform deposits. The facies analysis suggests a wave dominated system at the Stø Formation. Reservoir facies are related to predominantly fine to medium grained massive and low to high angle cross bedded sandstones corresponding to upper shoreface. Moreover tidally distributary channels deposits are also interpreted in this geological unit. The predominantly fine grained sandstones are well sorted with rounded grains with presence of glauconite and organic matter laminations. Visual porosity is very good to excellent in these sandstones. The Fruholmen Formation consists of very fine to medium grained sandstones, associated with fluvial channels and tidal/embayment settings. Current rippled laminations and mud clasts are the most distinguished sedimentary features in this geological unit. Visual porosity is good. The final facies concept provides a predictive distribution of reservoir property. The most suitable scenarios have been laterally identified towards the Eastern area. This allows the optimal drainage of the reservoir in a potential field development scenario. It is confirmed better reservoir properties at the Stø Formation compared with the Fruholmen Formation (e.g. higher net-to-gross ratio). This is a key element of the control of the oil saturation distribution per reservoir units and is important input to the development strategy of the area.