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The Tarbert Formation on a north-south oriented structure along the eastern flank of the Viking Graben has been studied. The reservoir in the two wells studied is buried ~100 m deeper in the northern than in the southern well. The present reservoir temperature is ~130°C. The reservoir quality is good, but due to extensive illitization of kaolin, the northern well shows lower permeability values than the southern well, for similar porosity values. The initial composition of the analysed samples in the two wells is very similar. There are no significant differences in total clay content and both wells contain K-feldspars and kaolin. However, the diagenetic alteration is more advanced in the deeper well. Total porosity is about the same in both wells but, due to illitization, the ratio of micro- vs. macro-porosity is higher in the deeper well. Petroleum emplacement in the structure occurred in two phases: oil emplacement predates, whereas gas emplacement postdates, most of the quartz cementation. In the deeper well, illitization occurred after gas emplacement.