Nanoscale geochemical heterogeneity of organic matter in thermally-mature shales: an AFM-IR study

The composition and characteristics of organic matter within shales can reveal important information on hydrocarbon generation, migration and accumulation processes during catagenesis. It may also impact upon the chemical properties and mechanical behavior of shales acting as hydrocarbon reservoirs or caprocks/seals for carbon and subsurface energy storage. Here we apply advanced atomic force microscopy-based infrared spectroscopy (AFM-IR) to capture the in-situ chemical structure of nanoscale molecular composition of individual organic particles and map the distribution of typical functional groups nondestructively; something that cannot be achieved via conventional analysis since some methods damage the sample, some only collect bulk measurements and some do not achieve nano-scale resolution. The nanoscale geochemical characteristics of organic matter in thermally mature samples of a Baltic Basin shale, from Lithuania and the Bowland Shale, from the UK, obtained from AFM-IR, are compared in this study. Significant chemical heterogeneity can be observed in migrated solid bitumen. In contrast, graptolite, alginite and inertinite generally display homogeneous molecular composition. The potential role of laminae as barriers to the vertical migration of hydrocarbons is also apparent. This research provides insights into nanoscale geochemical information on different organic matter types in thermally-mature shales to enhance the understanding of molecular structure transformation of organic matter during hydrocarbon expulsion and migration processes, and also sheds light on implications of nanogeochemistry for enhanced gas recovery, carbon sequestration and underground hydrogen storage-related energy applications. We also characterised specific materials which have not been reported in other geological studies using AFM-IR. Some of them display similar characteristics to other organic matter particles in the samples, but the possibilities of unknown organic contaminants introduced to the sample during sample storage or preparation stages cannot be ruled out. A comprehensive analysis on this undefined organic material is provided. Standardized approaches during sample preparation and storage are required for future research to dislodge potential surface-attached contaminants.