Well logs are often the only continuous acquisitions across targeted reservoir intervals. In unconventional shale reservoirs, the most important uncertainties for quantitative petrophysical well log interpretations are related to the clay and organic matter end-points but difficulties also arise from the complexity of the involved mineral mixtures, without a well differentiated lithology. Accurate quantitative mineralogical analyses are therefore essential for calibrating well log interpretations.
Our reference quantitative mineralogy method, called MinEval, integrates X-Ray Diffraction (XRD) analyses of crushed samples with physical and chemical laboratory measurements performed on the same samples. It is very accurate but labor-intensive and not well adapted when applied on drill cuttings due to drilling mud contaminations and/or fall-out materials.
In this paper, we present our new quantitative mineralogy workflow, based on the elemental mapping of thin-sections, core chips or resin-impregnated drill cuttings using a non-destructive Scanning Electron Microscopy method using Energy Dispersive X-Ray Spectroscopy (SEM-EDS). All the obtained SEM-EDS maps are post-treated using a dedicated software allowing the automatic 2D quantification of mineralogy through the deconvolution of the EDS spectra of each analyzed pixel using up to three reference or measured mineral spectra. In our laboratory, this method, called SEM-EDS QEMSCAN (Quantitative Evaluation of Minerals by Scanning Electron Microscopy), is systematically calibrated with reference quantitative mineralogical analyses.
Results of calibrated SEM-EDS QEMSCAN analyses performed on core samples and drill cuttings from the Vaca Muerta formation (Upper Jurassic, Neuquén Basin, Argentina) and Alum Shale formation (Lower Silurian to Upper Ordovician, Norwegian-Danish Basin, Denmark) are presented in this paper. They demonstrate that the SEM-EDS QEMSCAN workflow can produce fairly accurate quantitative mineralogy data for these two shale formations, in a time- and cost-efficient manner, through appropriate calibration on a limited set of samples using our reference MinEval quantitative mineralogy method. Calibration is crucial as non-calibrated SEM-EDS QEMSCAN mineralogy data, expressed in area%, are not directly equivalent to volume%.
Results obtained for Vaca Muerta Fm core samples show that the calibration of SEM-EDS QEMSCAN analyses is similar throughout the Neuquén Basin but is different from that required for Alum Shale samples. The calibration is therefore basin-dependent and should be carefully completed for each new play in order to produce reliable quantitative mineralogy data. Results obtained for Alum Shale samples also show that the calibration for drill cuttings can sometimes be different from that required for core samples. The calibrated SEM-QEMSCAN method is however particularly adapted to the analysis of drill cuttings as drilling mud contaminations and fall-out materials can be numerically removed to only analyze the mineralogical composition of the targeted formation intervals.