Modeling quartz cementation and porosity in Middle Jurassic Brent Group sandstones of the Kvitebjorn field, Northern North Sea

Petrographic study of deeply buried Middle Jurassic Brent Group sandstones from the Kvitebjorn gas field in the Norwegian sector of the North Sea show s that quartz cement volumes range from less than 1% to almost 30% over sho rt distances, and porosity ranges from 5 to 30%. A clear correlation betwee n quartz surface area and quartz cement volume indicates that this variatio n is due to differences in quartz surface area available for quartz overgro wth formation, which, in turn, is a function of grain size, abundance of gr ain coatings, and quartz clast abundance. The correlation between quartz su rface area and quartz cement volume also suggests that the quartz cementati on process is a strongly precipitation rate-controlled process, and that qu artz cementation can indeed be modeled quantitatively by modeling the preci pitation step in the quartz cementation process. Using temperature history, detrital mineralogy, grain size, and grain coati ng abundance as input, quartz cement volumes were for 90% of the samples mo deled to within 4% or less of observed values with the EXEMPLAR(R) diagenet ic modeling program. Modeled porosities deviate from measured values by les s than 3% for 75% of the samples, and the difference between measured and m odeled porosities exceeds 5% for only two of the 40 samples. Deviations between modeled and measured quartz cement volumes do not correl ate with distance to nearest stylolite, but a tendency for underestimating quartz cement in samples with low quartz surface areas may possibly be pres ent. Comparison with results from modeling of quartz cementation in other s andstones shows that optimal fit between measured and modeled quartz cement volumes is not always obtained with the same values for the kinetic parame ters controlling quartz precipitation rate per unit surface area as a funct ion of temperature. The variation in optimal kinetics between data sets is probably partly due to inaccurate temperature histories, but improving the quartz surface area function may also reduce the range of optimal values fo r the kinetic parameters.