Accurate Quantitative Mineralogy in Gas Shales Based on Integrating Multiple Chemical and Physical Measurements with XRD Analyses: Improvement from Use of SEM μ-XRF Chemical Analysis

Fialips, Claire; Laurent, Jean-Paul; Labeyrie, Bernard; Burg, Valérie; Simeone, Patrick; Kinderstuth, Thierry; Girard, Jean-Pierre; Kluska, Jean-Michel; Umbhauer, François

Accurate bulk mineralogical analysis of gas shales, performed on statistically representative sets of core/cuttings samples, is required for proper interpretation/calibration of wireline logs and delineation of lithology. Since log response is sensible to clay content and type, particular care in the characterization and quantification of clay minerals is essential. In many laboratories, quantitative mineralogical analyses are based solely on powder X-ray diffraction (XRD). Although this technique is continuously improved, accurate quantification of clay minerals remains very challenging due to large chemical/structural variability of clays, very fine particle size, and tendency to preferential orientation during sample preparation. Consequently, mineralogical analyses relying exclusively on XRD data are often associated to significant uncertainty, with errors on total clay content reaching more than 20 wt.% in some cases.

The presented quantitative mineralogy methodology, called QM, has been specifically developed over the last 10 years to provide accurate and robust mineralogical compositions for any rock sample, including clay-rich rocks. The strength of this method is that it fully integrates standard XRD analyses with a suite of specific chemical and physical measurements. It relies on the systematic fulfillment of internal consistency between measured and calculated values of a minimum suite of 15 parameters for all related samples treated as one single data set. Recent analytical developments, based on μ-XRF chemical analysis of very small area in thin-sections using a SEM instrument equipped with a XRF detector, substantially improved quantification of clay minerals (especially chlorites and interstratified phases). The μ-XRF chemical maps are numerically treated to remove points corresponding to non-clay phases and projected on various diagrams to determine the average chemical formulae of clay phases. The so-determined clay compositions are used in our in-house developed numerical quantification process to optimize accuracy.

The strength of our approach lies in the great accuracy and robustness of results as the method integrates key elements to check internal consistency, hence reliability, of data over a complete statistically representative suite of samples rather than on an individual sample basis. Despite the labor-intensive nature of this approach, it provides great added value in quantitative mineralogy investigations.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013