Development of far-infrared attenuated total reflectance spectroscopy for the mineralogical analysis of shales

Determining and understanding the geochemical properties of shale rocks is an important and essential aspect of hydrocarbon resource exploration and development. Although mid-infrared spectroscopy (MIR) has been widely used to characterize and provide chemical information of many different mineral components, mineral quantification of natural geological systems is still a major challenge that has not yet been satisfactorily achieved. Part of the problem is that many minerals which are frequently present in the shale rock in large amounts do not absorb and/or have characteristic absorption bands in the MIR region. We show for the first time that far-infrared spectroscopy (FIR) can be used to provide compositional information of the main mineral groups (i.e., carbonates, silicates and clays) in a shale. Furthermore, partial least square (PLS) methods were developed using seven mineral standards chosen to represent the most common minerals found in shale-type reservoir rocks. The developed PLS model was validated using 28 natural shale samples and a reasonable correlation was found between the FIR method and X-ray diffraction (XRD) analysis.