ABSTRACT

Precipitation rates for quartz cement in quartz-rich Jurassic sandstones from the Norwegian shelf have been determined by combining petrographic data, fluid-inclusion data, and temperature-history modeling. Thin-section petrography enables the number of moles of quartz cement precipitated in a sample and the surface area available for precipitation to be determined. Measurement of homogenization temperatures for fluid inclusions located at the boundaries between quartz clasts and quartz overgrowths permits the temperature of initial quartz cementation to be found, and this temperature may be translated to a date by constructing a temperature-history curve for each sandstone. Since quartz cementation has continued up to the present in the studied sandstones, precipitation rates for qua tz cement per unit time and surface area can be calculated.

Calculated precipitation rates for the 27 examined samples vary from 9.8 10^-21 moles/cm²·s to 1.9 10^-18 moles/cm²·s, and increase systematically with temperature from approximately 1 10^-20 moles/cm²·s at 80°C to approximately 5 10^-19 moles/cm²·s at 140°C. Although quartz cement is derived dominantly from dissolution of quartz at stylolites and to a smaller degree at grain contacts, no clear correlation between effective pressure and quartz precipitation r te was found. There is no obvious difference in quartz precipitation rates for hydrocarbon-saturated sandstones versus water-saturated sandstones.

The calculated precipitation rates enable an equation giving quartz precipitation rate as a function of temperature to be defined. Quartz cementation, and consequently also porosity evolution, in deeply buried quartz-rich sandstones can therefore be predicted quantitatively.