Modelling Quartz Cementation and Porosity in Reservoir Sandstones: Examples from the Norwegian Continental Shelf

Abstract

The volume of precipitated quartz cement and the resulting porosity loss in a quartzose sandstone can be calculated from the temperature history of the sandstone based on an equation relating the quartz precipitation rate per unit surface area and per unit time to temperature. In addition to temperature and time, the quartz surface area available for quartz cement precipitation will control the progress of quartz cementation within a given sandstone. Grain size, detrital grain mineralogy and abundance of grain coatings, factors which are controlled by provenance and depositional environment, are therefore also essential input parameters for modelling of quartz cementation. Computed quartz cement volumes and porosities were compared with measured values for Brent Group sandstone samples from two wells in the northern North Sea. Porosities and quartz cement volumes in these sandstones currently vary from 8 to 19% and from 6 to 28%, respectively, due to large variations in grain size, grain coating abundance and quartz clast content. Despite these compositional and textural variations, modelled and measured values for both quartz cement and porosity in most cases differ by less than a few percent. Mean measured porosity and quartz cement volume differ from mean modelled porosity and quartz cement volume by less than one percent in both wells.