Abstract

The deeply buried (4 km, 160°C) quartz-rich sandstones of the Middle Jurassic Garn Formation in well 6507/7-1 offshore mid-Norway typically contain 20–30% quartz cement and only 3–9% porosity. The exception is a less than 1.5 m-thick interval at the top of the formation where quartz cement volumes typically drop to 8–16% and porosities are in the range 7–19%. The uppermost part of the Garn Formation is not isolated from the rest of the formation by any type of impermeable barrier; the sandstones are water-filled indicating that the change in cementation does not correspond to a hydrocarbon contact; and, there are no significant changes in grain size, grain coating abundance, grain mineralogy or other factors that could give a lower quartz surface area for development of quartz overgrowths within the uppermost 1.5 m of the sandstones. However, in the samples from the less cemented interval, illitization of authigenic kaolin is either not detectable or very restricted compared to deeper down in the cores where illitization of kaolin is usually complete. We propose that the cause of reduced quartz cementation in the top of the Garn Formation may be the coating of stylolites developed from primary illitic clay-rich and micaceous laminae with authigenic kaolin as quartz cementation proceeds and the authigenic kaolin-rich rock adjacent to the stylolites is dissolved. Although we do not have evidence indicating that this may be a common phenomenon of major importance for preservation of reservoir quality, our observations may help to unravel the mechanisms of quartz dissolution at stylolites. Specifically, we suggest that the lack of significant surface charge on kaolin compared to illite and mica may lead to a reduced thermodynamic drive for quartz dissolution at kaolin-quartz contacts. Similarly, lack of dissolution at clean quartz-quartz surfaces may be due to smaller and equal surface charges setting up oppositely directed electric fields that cancel each other, whereas the positive surface charge of carbonate minerals may actually enhance the electric fields from negatively charged illitic clay and mica, which may explain the extensive dissolution often seen at stylolites in limestones.

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