The source rock potential of the Upper Jurassic– lowermost Cretaceous in the Danish and southern Norwegian sectors of the Central Graben, North Sea

Abstract

The principal source rocks in the Danish and southern Norwegian Central Graben in the North Sea are the Upper Jurassic to lowermost Cretaceous marine shales of the Mandal and Farsund formations. The distribution of generation potential and thermal maturity is quite complex. Biomarkers of crude oils from the northern Danish Central Graben indicate correlation to the Bo Member of the upper Farsund Formation or to the lower part of the formation. Biomarker ratios of oils from the Danish Chalk reservoirs show a maturity gradation from south to north, with oils in the northern part being more mature. Vitrinite reflectance gradients show differences in depth of the ‘top oil window’. In areas characterized by HPHT conditions and thick overlying Chalk, petroleum generation is retarded and the oil window is displaced downwards by 200–350 m. Source rock quality varies. The upper part of the shales is generally considered the most oil-prone. However, analyses indicate that the lower part of the Farsund Formation may contain intervals with rich oil-prone shales. Furthermore, the petroleum generation of the Upper Jurassic shales below the Farsund Formation is often overlooked, but source rock data indicate that gas/oil-prone, and to some extent oil-prone, shales are present.