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Abstract

Quartz occurs widely as a cement in pre-Tertiary North Sea reservoir sands. Fluid inclusion and oxygen isotope data show that much of the cement formed in the Tertiary from 18O-rich waters with variable, but often high salinities. The volume of this type of water is limited and is unlikely to be more than the void space of the sedimentary basin. The mineralizing fluids also show considerable compositional heterogeneity over relatively small areas and depth ranges.

The composition and compositional heterogeneity of the mineralizing fluids limit the mechanisms by which silica transport and quartz cementation could have occurred. Meteoric recharge is ruled out. Mass balance considerations strongly suggest that compaction-driven flow is extremely unlikely to have led to the type of pervasive quartz cementation observed in the pre-Tertiary North Sea section. The compositional heterogeneity of the mineralizing fluids suggests that large-scale convective overturn is also an unlikely transport mechanism. It is probable that the silica required for quartz cementation was supplied locally and not by large-scale fluid flow. We believe that the fluid data restrict potential silica transport mechanisms to: (1) reservoir-scale convection and (2) diffusion. However, significant mass transport by either of these mechanisms has yet to be demonstrated in sedimentary basins.

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