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Until recently, the discovered oil and gas accumulations in the Upper Cretaceous and Lower Tertiary Chalk Group were related to conventional structural traps formed as a response to movements of Permo-Triassic salt deposits or formed by Tertiary inversion. In 1999, a fast-track field development of the first non-structurally trapped oil accumulation, the Halfdan Field, was initiated, and the field was brought on production in March 2000. The field, discovered early in 1999, is located in the syncline between the Dan and Skjold Fields. The updip trapping mechanism is a stratigraphical pinch out of chalk with good reservoir properties. The discovery is the result of the application of advanced methods in processing, inversion to acoustic impedance and interpretation of 3D seismic data, integrated with detailed geological, petrophysical and reservoir engineering studies. Drilling of long reach horizontal wells in the flank area of the Dan Field provided the necessary data for regional mapping of the free water level required for fluid modelling. Information about layer thickness and matrix porosity was provided through stochastic forward modelling of seismic acoustic impedance data extracted from the 3D seismic survey.
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