Abstract

We present an example of leaching by meteoric water in the Jurassic Magnus Sandstone Member of the northern North Sea. The interpretation is based on information from several independent techniques, including downhole natural gamma-ray spectrometry, detailed petrographic analysis, and cross-sectional time-slice reconstructions from seismic data. The initial lines of evidence that prompted further investigation into freshwater involvement in the Magnus Sandstone Member were brackish formation-water compositions and a potassium depletion shown by the gamma-ray spectrometry log, in sandstone beneath the unconformity at the base of the Cretaceous section. Subsequent detailed petrographic study confirmed that the potassium depletion had been brought about by potassium feldspar dissolution, resulting in precipitation of kaolinite and creation of secondary porosity, the latter also revealed by conventional core-analysis porosity and permeability measurements. Finally, cross-sectional time-slice reconstructions from a regional seismic line across the Magnus oilfield strongly suggested subaerial exposure of the Magnus Sandstone Member in Early Cretaceous time and consequent freshwater leaching of the Magnus Sandstone Member and generation of the unconformity at the base of the Cretaceous. The combination of several lines of evidence from the Magnus field supports the process of subaerial exposure and freshwater leaching to create secondary porosity in sandstones.

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