Formation evaluation of the Jurassic Stø and Nordmela formations in exploration well 7220/8-1, Barents Sea, Norway

Abstract: Evaluating reservoirs with alternating sequences of sand and shale meet the challenge of quantifying the effect from clay minerals. The clay minerals have an impact on logging tools and the resulting data. When not considering the effects, a formation evaluation will present incorrect results on e.g. porosity and water saturation leading to management decisions based on poor quality interpretations. One of the most challenging tasks for a petrophysicist is to determine the water saturation (Sw) in the formation. This is a crucial step because the assumption is that the hydrocarbon saturation are the remaining fluids whenever the water saturation is lower than 100% (1-Sw). This work present a formation evaluation on logging data from exploration well 7220/8-1, in the Barents Sea. It is based on the comparison between conventional evaluation techniques and techniques which compensate for the clay mineral effect. Results from Archie’s water saturation are compared to Indonesia water saturation and Archie´s equation containing an alternative formation resistivity based on data from the triaxial induction tool, referred to as “sand resistivity”. The conventional Archie´s method uses bulk resistivity affected by the conductive properties from both sand and lithologies built by clay minerals. Using sand resistivity as opposed to the bulk resistivity compensates for the clay mineral effect. The results show significantly lower water saturation when compensating for the clay mineral effects. The faster and cheaper sand resistivity approach could be validated by conducting special core analysis providing data for the Waxman-Smits water saturation equation. Comparable results would be in favor of the cheaper and faster method. The petrophysical effects on logging tools are tied to the regional geological evolution revealing the need for a multidisciplinary approach when evaluating potential reservoirs within the petroleum industry.