Utilizing smear mounts for X-ray diffraction as a fully quantitative approach in rapidly characterizing the mineralogy of shale gas reservoirs

Abstract

X-ray diffraction (XRD) sample preparation methods were compared for fine grained reservoir rocks. The viability of using a hand ground, smear mount method was investigated compared to the widely used micronized, cavity mount method of sample preparation for quantitative phase analysis. Micronizing a sample before analyzing by XRD has been used successfully to reduce the average crystallite size to 10 μm. However, because of the fine grained nature of shale gas reservoirs, the average crystallite size is already below 10 μm. Therefore, the sample only requires disaggregation of larger particles which is easily accomplished by hand grinding. Samples were prepared using smear and cavity mount methods to compare the differences in quantitative phase abundances determined by Rietveld refinement. In addition, samples of known composition were prepared to assess the accuracy and precision of the methods. Quantitative analysis on whole rock samples shows excellent precision between the methods of sample preparation with an absolute error of ±2.25 wt.% at the 95% confidence level per individual phase. Quantitative analysis on artificially prepared samples using the smear mount method shows both excellent precision and accuracy with an absolute error of ±0.9 wt.% at the 95% confidence level per individual phase. A hand ground, smear mount method is therefore a quantitative and viable method for quickly assessing the mineralogy of shale gas reservoirs and fine grained rocks.