Localized sand bodies can be found encased within widespread Upper Jurassic shales, preserved on the flanks of eroded paleo-highs in the North Viking Graben. They are often situated in structural lows, poorly represented by well control and highly heterogeneous. Moreover, they are difficult to identify using conventional seismic and their location even harder to predict using geological modelling techniques. However, these excellent quality sands provide stratigraphic traps that can be quantitatively mapped using borehole calibrated 3D seismic acoustic impedance.

The Statfjord Nord 3D seismic survey was shot and processed during the last decade. Recently, borehole calibrated processing was performed on the 3D survey to obtain seismic acoustic impedance from each seismic trace. An objective method was used requiring no a priori information to constrain the solution and hence was not dependent upon a geological model.

The 3D seismic acoustic impedance permitted discrimination of Upper Jurassic sands from adjacent shales by virtue of their unique acoustic signature. Sand effective porosity was computed for a subset of the 3D volume using relationships obtained from an acoustic-petrophysical study carried out using well data. The 3D acoustic impedance was converted to velocity, thus allowing the thickness to be obtained for each seismic time sample. This quantitative calibration of 3D seismic made it possible to objectively map porosity-thickness of Upper Jurassic sands.

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