Detrital and authigenic clay minerals in shales: A review on their identification and applications

Clay minerals are widely distributed and are important components in sedimentary rocks. The identification and quantification of detrital and authigenic clay minerals have been hot topics recently due to the needs of petroleum geology, paleoclimate, and sedimentary history research. Detrital clay minerals exhibit the phenomenon of discrete grain boundaries, while authigenic clay minerals have various specific particle shapes. The differences in crystal structure of different clay mineral origins are reflected in the polytype character of clay minerals, especially for illite and chlorite. Different origins of clay minerals inherently have different chemical characteristics. Instead of focusing only on conventional techniques (scanning electron microscopy, X-ray diffraction, and geochemical analysis), here we review established approaches and introduce emerging analytical techniques (automated mineralogy techniques and artificial intelligence techniques) that can complement current methods. In addition, the applications of these techniques in illite age analysis, paleoclimate reconstruction, and reservoir properties are discussed. Further, different state-of-the-art techniques for clay mineral identification and quantification (e.g., scanning electron microscopy for specific petrographic characterization, X-ray diffraction for bulk crystal structure identification, etc.) are discussed. We propose that combining individual and bulk techniques obtained at multiple scales can provide novel insights for deciphering various clay mineral origins. Understanding the characteristics of clay minerals from different origins is of great importance to scientific issues such as shale oil and gas exploration.