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The present paper describes the basis for delineating a significant volume of non-structurally trapped oil within low permeable chalk at the flank of the mature Dan field offshore Denmark. Drilling of extensive long horizontal wells at the flank of the main field, including one that set a world record horizontal section, has resulted in the discovery of a thin high porous (>30%) Maastrichtian chalk unit. The best reservoir zone is developed with consistent high oil saturation in the order of 80–90%, and trapped between lower porosity predominantly water bearing Danian and Maastrichtian Chalk units. Full vertical and lateral pressure continuity is observed between the hydrocarbon and water bearing units on the main field. It was therefore surprising to find the oil in the better of the units extending down flank, and outside the structural closure far beyond what could be predicted from modeling of the saturation profiles observed in the crestal wells. Drilling of three wells with up to 19,600 ft long horizontal sections in the flank area, consistently found the free water level dipping and deepening significantly to the north-west of the field allowing hydrocarbon filling of the units of highest reservoir quality in this area. Drilling of long reach horizontal wells thus provided the data needed for regional mapping of the free water level required as input to fluid modeling. Information on layer thickness and matrix porosity, necessary for volumetric calculations, was provided from well logs and through stochastic forward modeling of seismic acoustic impedance data extracted from a regional 3D survey. Improved understanding of the hydro- and geodynamic induced fluid pattern, combined with seismic mapping and modeling, thus resulted in the identification of significant and yet undrained volumes at the flanks of a mature oil field.

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