--> --> Cretaceous Tectono-stratigraphy of the Danish Central Graben, the Chalk Play Revisited

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Cretaceous Tectono-stratigraphy of the Danish Central Graben, the Chalk Play Revisited


The lithological composition of the Cretaceous Chalk of the North Sea Basin is homogenous and consists mainly of fine-grained calcareous plankton, with only locally admixtures of clays and coarser bioclasts. Traditionally, exploration strategy in these homogenous rocks was guided by acoustic impedance volumes that showed the location of high porosity units. With increased maturation of fields and the search for remaining hydrocarbon occurrences, it is now essential to better understand the high resolution stratigraphic architecture and the tectonic context to identify new play types. In this presentation we will present evidence for basin inversion, and pay particular attention to the rich pallet of seismic geomorphological features that characterises this tectonically active period. This study focussed on the Danish Central Graben and benefitted from a recently re-processed, regional 3D seismic dataset (6000 km2) combined with well-log and biostratigraphic information. Advanced seismic geomodel interpretation technology was applied to construct a seismic Relative Geological Time (RGT) model. Standard seismic attributes, displayed upon horizons from a 3D RGT model, are very effective in the illustration and interpretation of complex chalk depositional features. The evolution from a post-rift phase into a dramatic inversion phase and its impact on sediment distribution is documented at the basin scale. In uplifted areas local erosion and long phases of non-deposition occurred, whereas in areas of subsidence thick packages of chalk accumulated. The stratigraphic heterogeneity this created is captured in chronostratigraphic schemes. Along the newly created highs, mass waste deposition took place at the 10 km scale. Several different mass waste complexes have been documented in 3D, illustrating a number of typical features such as sediment creep, slumping, debris avalanches, olistoliths, debris flows and mudflows. Also traction current sedimentary features had a prominent role in the depositional system and left several thalwegs of different aspect ratios. Together these features present a very dynamic picture of chalk sedimentation. This integrated, regional 3D seismic geomorphological approach allows for interpretation of previously poorly understood seismic features in the context of the geological evolution of the area, and helps to redefine plays based on a better understanding and prediction of the porosity and permeability distribution in the chalk.