Quartz Growth Conditions in Sandstones

Jahren, Jens S., Knut Bjørlykke, Department of Geology, University of Oslo, Oslo, Norway

Grain coatings of small quartz crystals (microquartz) are in many cases the main cause of good reservoir properties in deeply buried sandstones. Microquartz form from highly supersaturated pore waters as opal A and opal CT dissolve. At higher temperatures when there is no highly soluble silica present further quartz cementation has to come from disso­lution of quartz at grain contacts. The presence of clay minerals like illite could raise quartz solubility at grain contacts but this is thermodynamically not necessary for producing the required supersaturation for quartz cementation if pressure dissolution is effective during low temperature burial diagenesis. The 0.5 - 5 micrometer size range commonly found for microquartz coatings indicate that the cutoff saturation needed for growth of quartz cement in general is close to the solubility of the smallest microquartz crystals. The solubility of a 0.5 micrometer euhedral crystal will be 3% higher than for infinitely large quartz crystals (Ostwald-Freundlich equation). This would be the best estimate of the necessary quartz supersaturation for growth of quartz during burial diagenesis. Effective supply of silica could be due to illite at grain contacts from a catalytic effect on quartz dissolution but it is impor­tant to find out to what extent dissolution occurs at contacts between quartz grains with no clay coatings to evaluate the importance of illite in the pressure dissolution process. For this purpose nearly clay free samples from the early Cambrian Ringsaker Quartzite found in the Mjøsa area in Norway have been studied.