The Sleipner Effect: a subtle relationship between the distribution of diagenetic clay, reservoir porosity, permeability, and water saturation

Petrographic, mineralogical and geochemical core analysis of Palaeocene tur biditic sandstones in the Sleipner East gas-condensate reservoirs show the importance of diagenetic clay distribution on porosity, permeability, and w ater saturation. An observed high resistivity zone' (HRZ) corresponds to in tervals with low water saturation, a more restricted distribution of diagen etic clay (mainly chlorite), and up to 5% quartz cement. The underlying 'lo w resistivity zone' (LRZ) corresponds to intervals with more widely distrib uted diagenetic clay, which have lower degrees of quartz cementation, highe r porosity, and variably reduced permeability. Crosscutting relationships o f the HRZ/LRZ with mapped sedimentary depositional units, as well as fluid inclusion analysis data, suggest that the distribution of diagenetic clay w as affected by an earlier (late Miocene?) oil charge, and more extensive ch lorite formation in a palace-water zone. Recent gas condensate charge and s tructuring of these sandstones resulted in LRZ reservoirs with substantiall y higher water saturations than those in the HRZ.