THERMOGRAVIMETRY EVOLVED WATER ANALYSIS (TG EWA) COMBINED WITH XRD FOR IMPROVED QUANTITATIVE WHOLE-ROCK ANALYSIS OF CLAY-MINERALS IN SANDSTONES/

Current methods of quantitative whole-rock clay mineral analysis of sa ndstones often provide little more than an estimate of clay mineral ab undances, especially where the total clay mineral content is <10 wt% o f the sandstone. More accurate determinations of clay mineral abundanc e in the whole rock can be made by combining thermogravimetry/evolved water analysis (TG/EWA) and X-ray diffraction (XRD) data. The TGA/EWA system incorporates a purpose built thermobalance linked to a water sp ecific infrared detector which is used to measure quantitatively the c lay mineral dehydroxylation water evolved from the whole rock when hea ted from 250 degrees C to 900 degrees C. This gives a measure of the t otal hydroxyl content of the clay minerals in the whole rock which, wh en combined with XRD analysis of a separated clay size-fraction, enabl es individual clay mineral abundances in the whole-rock sample to be d etermined. Results on artificial sand/clay mineral mixtures prepared w ith known amounts of different clay minerals (chlorite, illite and kao linite) show that the accuracy of the combined method is most influenc ed by the accuracy of the XRD data. Errors associated with TG/EWA were found to be negligible by comparison. A ease study is included in whi ch the technique has been used to determine accurately the illite abun dance in the Magnus Sandstone Reservoir, Northern North Sea.