Microquartz Grain Coatings
and Inhibited Quartz Cementation: An EBSD Study of the Way Microquartz
Cement Grows
Worden, Richard H.1, S.C.
Haddad1, D.J. Prior1, P.C. Smalley2 (1)
University of Liverpool, Liverpool, United Kingdom (2) BP Exploration, Middlesex,
United Kingdom
Microcrystalline quartz coatings on detrital
quartz grains have been implicated as a cause of anomalously low quantities of
porosity-occluding quartz overgrowths in deeply buried sandstones. The nature
of microcrystalline quartz grain-coatings remains obscure despite them
apparently having a beneficial effect on reservoir quality. We have studied
microcrystalline quartz from the Upper Jurassic Miller field in the Central North Sea, UKCS, using cathodoluminescence
imaging and electron backscattered diffraction (EBSD) in a scanning electron
microscope. EBSD can reveal silica mineralogy and precise crystallographic
orientation at a resolution of 1 mm2 for entire polished thin sections. Macrocrystalline quartz overgrowths in Miller have exactly
the same crystallographic orientation as their host detrital
grain. Where microcrystalline quartz sits on a detrital
grain with no later overgrowth, microcrystalline quartz crystals are randomly misoriented by up to 4º relative to the detrital
quartz grain. In contrast, some grains have a microcrystalline quartz layer
covered by quartz overgrowth; in these cases the microcrystalline quartz is not
misoriented relative to the detrital
grain. It seems that where microcrystalline quartz is misoriented
it inhibits subsequent quartz cementation but where it is not misoriented it has no effect. The localised
misorientation of microcrystalline quartz cement
suggests rapid nucleation at screw dislocations exposed at detrital
grain surfaces. It seems that an inhibiting effect of microcrystalline quartz
on porosity-destroying quartz cement is fundamentally controlled by its
crystallographic orientation, itself controlled by the nature of detrital quartz grain surfaces and the degree of quartz supersaturation. Not all microcrystalline quartz preserves
reservoir quality.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California