In-situ nanoscale geochemical characterization of organic matter in shale by AFM-IR

Due to the fine-grained nature of shale, organic matter particles are generally micro- and nano-scale in size. Functional groups differ between different organic matter types and as such provide unique chemical information for organic matter. Micro-FTIR can provide direct measurement to characterize sample features at the micrometer scale. However, optical diffraction limits its application at the nanometer scale. As a non-destructive high-resolution scanning probe technique, atomic force microscopy (AFM) is very powerful in nanoscale research and has been widely used in the fields of polymers, semiconductors, electrochemistry and biology. To provide a better combination of AFM’s unique advantages with nanoscale chemical analysis, the AFM-IR technique has been developed in recent years and also attracted the attention of geologists to explore the application in geological materials.

In this research, AFM-IR which is a quite new technique in geological research was used to investigate the in-situ geochemical characteristics of organic matter in shale. Nanoscale molecular composition of individual organic particles was captured nondestructively, and the distribution of typical functional groups was displayed via 2D IR mapping. In our samples, both alginite and inertinite display chemical homogeneity. The former is dominated by oxygenated and aliphatic contents which indicates a higher hydrocarbon generation potential, whereas the latter is dominated by aromatic carbon. In contrast, migrated solid bitumen particles show compositional heterogeneities at the nanometer scale as some are aromatic-rich and others are aliphatic-rich. Finally, linking this advanced nanochemical technique to potential applications in subsurface energy was explored. This research demonstrates that AFM-IR is a powerful tool to examine the in-situ nanoscale geochemical characteristics of different organic matter types, which can also provide implications for energy applications.