PRECIPITATION RATES FOR QUARTZ CEMENT IN SANDSTONES DETERMINED BY FLUID-INCLUSION MICROTHERMOMETRY AND TEMPERATURE-HISTORY MODELING OLAV WALDERHAUG Rogaland Research, P.O. Box 2503 Ullandhaug, 4004 Stamnger, Norway ABsraacr: 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. Meusurement of homogenization temperatures for fluid inclusions located at the boundaries between quartz clasts and quartz over- growths permits the temperature of initial quartz cementation to be found, and this temperature may be translated to a date by constructing a tem- perature-history curve for each sandstone. Since quartz cementation has continued up to the present in the studied sandstones, precipitation rates for quartz cement per unit time and surface area can be calculated. Calculated precipitation rates for the 27 examined samples vary from 9.8 x 10 -z~ moles/cm2.s to 1.9 x 10 -'8 moles/cm2-s, and increase sys- tematically with temperature from approximately 1 x 10 2o moles/era 2 s at 80°C to approximately 5 × 10 -~9 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 rate 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 pre- cipitation rate as a function of temperature to be defined. Quartz cemen- tation, and consequently also porosity evolution, in deeply buried quartz- rich sandstones can therefore be predicted quantitatively. INTRODUCTION Measurement of homogenization temperatures for aqueous and hydro- carbon fluid inclusions in quartz overgrowths has contributed greatly to the understanding of quartz cementation in sandstones. Homogenization temperatures place constraints on the temperatures of quartz cementation and possible sources of quartz cement (e.g., Pagel 1975; Malley et al. 1986; Konnerup-Madsen and Dypvik 1988; Bufley et al. 1989; Walderhaug 1990; Ehrenberg 1990), and are also useful for evaluating the e~ect of hydrocarbons on quartz cementation (Walderhaug 1990; Nedkvitne et al. 1993; Saigal et al. 1992). Moreover, combining the fluid-inclusion data with petrographic data and temperature-history modeling also makes cal- culation of quartz precipitation rates possible. Determining these rates is of crucial importance for quantitative modeling of quartz cementation (Oelkers et al. 1992; Oelkers et al. in press) and consequently for prediction of porosities in sandstone petroleum reservoirs. In this paper, rates of quartz cementation as a function of temperature are calculated for 27 samples from l I Jurassic sandstones from different parts of the Norwegian continental shelf based on homogenization temperatures, temperature- history curves, and petrographic data from Walderhaug (1990), Walder- haug and Fjeldskaar (! 993), and Walderhaug (1994). Comparison of cal- culated precipitation rates with pressure and hydrocarbon saturation data also permits evaluation of the influence of effective pressure and hydro- carbon emplacement on quartz precipitation rates. SAMPLE MATERIAL The sandstones included in this study are the most prolific Jurassic reservoir sandstones in the Norwegian sector of the North Sea and offshore mid-Norway, i.e., the sandstones of the Brent Group (Graue et al. 1987), the Stattjord Formation (Roe and Steel 1985; Buza and Unneberg 1987), the Ula Formation (Bailey et al. 1981), the Garn Formation (Gjdberg et al. 1987; Harris 1989), and the Tilje Formation (DaUand et al. 1988; Gjelberg et al. 1987).The sampled wells, formations, and depths are listed in Table 1, and well locations are shown in Figure 1. With the exception of the Garn Formation in 6506/12-4 and in 6507/10-1, all sampled sand- stones are oil- or gas-filled. The Stat0ord Formation was deposited in a fluvial setting; the other sandstones were deposited in shallow-marine environments. ANALYTICAL METHODS Fluid Inclusions Homogenization temperatures were measured with a Linkam THM 600 heating stage and a Linkam TMS 90 control unit. The heating stage was calibrated with Merck standards with melting points of 70°C and 135°C. Homogenization of inclusions was observed with a Nikon 40 × extra- long-working-distance objective and 15 x eyepieces. In addition, the op- tical path contained a 1.25 x lens giving a total magnification of 750 x during observation of inclusion homogenization. Hydrocarbon inclusions were distinguished from aqueous inclusions by their fluorescence.Heating runs were made with heating rates of 6°C/minute until homogenization was considered to be imminent. The temperature was thereaffterraised by steps of 0.3-0.5°C and held constant for 30-.60 seconds between each temperature increase. Homogenization temperature was defined as the mean of the highest temperature where a gas bubble was present and the lowest temperature where a gas bubble was not observed. After homog- enization was considered to have taken place, temperature was lowered slightly in order to observe if the gas bubble reformed. If a bubble did reform, this was considered to imply that homogenization had not taken place, since inclusions usually required at least 3-10°C of cooling below their homogenization temperatures before gas bubbles reformed. By ap- plying this method, precise determinations of homogenization tempera- tures for inclusions as small as 3.-.4tam could be obtained; i.e., reproduc- ibility of measurements is approximately _+ I°C. Measurements of homogenization temperatures were made on both pohshed thin sections with thicknesses of 30 tam and 50 tam mounted on l mm thick glass plates and on polished rock slices approximately 100 um thick. No systematic differences in homogenization temperatures were observed for measure- ments made on polished rock slices versus measurements made on thin sections. All samples were prepared using the same equipment and tech- niques and by the same technician. Maximum temperatures during sample preparation are less than 70"C, far lower than the formation temperatures for the samples. Petrography All samples used for fluid-inclusion microthermometry were examined with a polarizing microscope and point counted with at least 300 points per thin section. Grain sizes were determined by measuring the maximum diameter of 20 quartz grains in each sample and calculating the mean maximum diameter for the 20 measured grains. To check that the studied inclusions really were located in quartz cement and not in quartz clasts, cathodoluminescence photographs were taken of the grains containing the inclusions. Cathodoluminescence photographs were taken after homoge- Journal or seDIMeNtary ResearcH, VoL. A64, NO. 2, Amt, 1994, P. 324-333 Copyright © 1994, SEPM (Society for Sedimentary Geology) 1073-I30Xt94/0A64-324/$03.00