Cathodoluminescence of quartz cements in Brent Group sandstones, Alwyn South, UK North Sea

Abstract

Abstract Quartz cements can, at best, enhance sandstone reservoir quality through strengthening the clastic framework against compaction or, at worst, degrade poroperm properties by occluding pore spaces and constricting fluid pathways. An understanding of the controls on distribution and timing of quartz cementation can therefore aid exploration and production strategy. In the Middle Jurassic Brent Group sandstones in Alwyn South authigenic quartz, kaolinite and illite development has resulted in severe deterioration of reservoir properties. Further resolution is obtained using a scanning electron microscope (SEM) adapted for cathodoluminescence (CL) mode observations. CL images show a clear distinction between detrial and authigenic quartz. In detail quartz overgrowths are complex and multiply zoned and a recurrent sequence of three major and two minor CL zones is distinguished. Zones become increasingly euhedral from core to rim and reflect variation in crystal growth rates and silica supply during several pulses of silicification. Heterogeneous zones and dissolution-boundary development shows that quartz precipitation is flux-controlled. Correlation of CL zones with clay mineral inclusions allow cement paragenesis to be refined and correlations with fluid inclusion data allow the conditions of quartz precipitation to be constrained.