Lithostratigraphic definition of the Upper Jurassic – lowermost Cretaceous (upper Volgian–Ryazanian) organic-rich and oil-prone Mandal Formation in the Danish Central Graben, North Sea

Abstract

The Volgian–Ryazanian shale interval in the Norwegian Central Graben, North Sea, is assigned to the Mandal Formation. Despite shales with similar lithological and log characteristics are being present in the Danish Central Graben, the formation is not defined for this part of the Central Graben. This paper presents a new lithostratigraphic subdivision of the Upper Volgian–Upper Ryazanian interval in the Danish Central Graben, where the Mandal Formation is defined and replaces the uppermost part of the Farsund Formation in the ‘old’ lithostratigraphy. The Mandal Formation is recognized from its log character and is correlatable across the Danish Central Graben. The formation typically has high gamma-ray (GR) and resistivity readings accompanied by low velocity and density values. In basin centres it normally consists of a lower part showing an upward increase in GR readings (‘warming-up’ interval), an overlying interval with conspicuous high GR readings (Bo Member), and an upper heterogeneous shale interval. The base of the Mandal Formation can be seismically mapped in most of the Danish Central Graben. The upper boundary of the formation, corresponding to the Base Cretaceous Unconformity (BCU) at the transition to the Lower Cretaceous, can likewise be seismically mapped throughout the basin. The Mandal Formation is developed with thick successions in basin depocentres, while it is thin or absent along inversion structures and on structural highs. The organic-rich shales of the Mandal Formation were deposited in a restricted, low-energy, oxygen-deficient/anoxic marine environment distal from any significant terrestrial influence. Along the eastern Coffee Soil Fault in the Danish Central Graben, siliciclastic deposits constituting the gravity-flow sandstones of the Poul Member interfinger with the Mandal Formation. Gravity-flow deposits formed by turbidite currents are also common throughout the Mandal Formation in basin centres. The Mandal Formation and particularly the Bo Member are highly oil-prone and contain the best source rock quality of the entire Upper Jurassic – lowermost Cretaceous shale succession in the Danish Central Graben. Interbedded potential reservoir sandstones suggest the Mandal Formation may in places be considered as an interval with hydrocarbon prospectivity, especially along the Coffee Soil Fault.