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.