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

Statoil a.s, 4035 Stavanger, Norway

^* E-mail: pana{at}statoil.com

(Received 1 December 1998; revised 18 April 1999)

Petrographic, mineralogical and geochemical core analysis of Palaeocene turbiditic sandstones in the Sleipner East gas-condensate reservoirs show the importance of diagenetic clay distribution on porosity, permeability, and water saturation. An observed ‘high resistivity zone’ (HRZ) corresponds to intervals with low water saturation, a more restricted distribution of diagenetic clay (mainly chlorite), and up to 5% quartz cement. The underlying ‘low resistivity zone’ (LRZ) corresponds to intervals with more widely distributed diagenetic clay, which have lower degrees of quartz cementation, higher porosity, and variably reduced permeability. Crosscutting relationships of the HRZ/LRZ with mapped sedimentary depositional units, as well as fluid inclusion analysis data, suggest that the distribution of diagenetic clay was affected by an earlier (late Miocene?) oil charge, and more extensive chlorite formation in a palaeo-water zone. Recent gas condensate charge and structuring of these sandstones resulted in LRZ reservoirs with substantially higher water saturations than those in the HRZ.

KEYWORDS: diagenesis, palaeocene sandstones, clay mineral distribution, quartz cement, hydrocarbon charge, fluid contacts

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