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.