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Haszeldine, R Stuart1, Mark Wilkinson1
(1) University of Edinburgh, Edinburgh, United Kingdom

ABSTRACT: Does Micro-quartz Really Preserve Deep Porosity in Geopressured Sandstones?

Quartz cement fills porosity in sandstones, but cementation is generally considered to be inhibited by grain coatings of clay. It has been suggested that grain coating by micro-quartz also inhibits cementation by quartz. We have re-examined the original published data for Upper Jurassic North Sea sandstones (bioturbated marine shelf), and find that the recorded distribution of micro-quartz does not support its hypothetical role in porosity preservation.
In new work, we have compiled an extensive dataset from geopressured marine sandstones of the North Sea similar in age and facies (Fulmar Formation), down to 17,000ft (5.18km). SEM examination has checked grain surfaces, and thin-sections linked to poroperm data. This shows that: A) Within fields, geopressure has only a minor effect on porosity preservation; B) Within fields, two parallel gradients of porosity decline can exist, the upper of which comprises micro-quartz sandstones; C) Only an additional 5% porosity can be ascribed to micro-quartz and this is micro-porosity; D) Oil charge is a more important factor in porosity preservation.
There is only a small difference in porosity between sandstones which SEM examination shows to contain micro-quartz, and those which do not. Quartz cement volumes in these sandstones are generally less than predicted, except for layers low in clay, which do have cement quantities compatible with predictions. Quartz cement is considered to be inhibited by oil, and up to 20% detrital and authigenic clay content – which bioturbation has coated around the grains. Micro-quartz coating is an effect, not a cause, of porosity preservation.

 

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.