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Abstract

The Cecilie field is located at the mouth of the Paleocene Siri Canyon near the Danish Central Graben. The field comprises a stratified succession of lower to upper Paleocene deep-marine sandstones and hemipelagic mudstones. Part of the field is occupied by a remarkable mounded structure, circular in planform and approximately 600 m (1968 ft) in diameter. Continuous coring through the mound and in the surrounding undisturbed sections provides significant data for the interpretation of this mounded feature. The Cecilie Mound gained its present shape by differential compaction over a dome-shaped injection of sand. In cores, the domeshaped injection is seen to have eroded and removed 25–30 m (82–98 ft) of the original stratigraphy preserved only outside the mound. Excavation of this attic chamber resembles magmatic stoping and occurred primarily by sand injection, splitting the overlying, semiconsolidated mudstones, so that clasts were spalled off. This resulted in the formation of common mudstone-clast conglomerates, which, along with flow structures formed by density segregation of the glauconitic sands, are important for the differentiation of injected and in-situ depositional sandstones. The close relationship of the Cecilie Mound to deep-seated faults in the Chalk Group indicates that initial fluidization occurred in response to the reactivation of the graben faults, breaching the overburden and releasing the overpressured pore waters of the Paleocene sands. Mounded injections like the Cecilie Mound have the potential to form hydrocarbon traps, and during production, the crosscutting nature of the mounded injection has ensured good vertical and lateral hydraulic communication in the otherwise depositionally stratified reservoirs.

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