Direct Imaging of π–π Stacking and Its Mechanical Impact on the Kerogen

The hidden structures in any organic materials govern their functional properties for wide range applications. As to organic matter, named kerogen, its mechanical properties are of significant for hydrocarbon production through “fracking” in the source rocks. However, direct imaging of the hidden structure in kerogen at the level of individual molecules has not been possible using conventional transmission electron microscope (TEM). This challenge is in part due to the highly electron beam sensitivity and their irregular arrangement of low Z contrast elements, i.e., C, H and O in organic materials system. Therefore, new TEM method needs to be developed to systematically the structure-properties relations in kerogen.<br/>Previous experiments have demonstrated that the mechanical variations in kerogen are tight with the geochemical heterogeneities arising from the thermal maturation¹. For instance, measured elastic modulus and hardness of kerogen increased with higher maturity and aromaticity². Occasionally, some matured kerogen with high elastic modulus can demonstrate more ordered even graphite-like fringe features. Yet, direct correlation of maturity-induced microstructure and mechanical behavior of kerogen remains challenging owing to the difficulty to extracted kerogen from the source rock and the appropriate mechanical testing tool designed for small scale samples with high spatial resolution. In the past decades, severe simulations have aided to the understanding of mechanical behavior of kerogen&lt;span style="font-size:10.8333px"&gt;.&lt;/span&gt; The ratio of aromatic to aliphatic carbon has been suggested to governe the underlying fracture mechanism of kerogen. Tshe increased fraction of aromatic carbon entail the transition of ductile to brittle fracture mechanisms in kerogen through the localized deformation initiated within the polyaromatic planes^{&lt;span style="font-size:10.8333px"&gt;4&lt;/span&gt;}. In this regard, the presence of polyaromatic plane, which is also a form of π–π stacking, was also found to be energetically favorable in high maturity kerogen and is responsible for the high stiffness supporting the hypothesizes. Although simulation results are clear, the direct evidence of the underlying structures in kerogen and how they would evolve with thermal maturation and the strong impact on the mechanical behavior of kerogen have not yet been experimentally demonstrated.<br/>Toward this end, we introduce four-dimensional scanning transmission electron microscopic technique (4D-STEM) to experimentally reveal previously unseen microstructure, named π–π stacking, and their spatial extend in kerogen at the level of individual defects and nanoscale domains. The morphologies observed by this technique vary from π–π stacking that are relatively long with continued crystal orientations to those that are short with overlapping domains in mature and immature kerogen. In addition, <i>in situ</i> 4D-STEM tensile testing was applied to visualize how local structural order can govern the fracture behavior of kerogen under deformation and suggest the increasing π–π stacking give rise to the higher hardness and lower ductility. These findings highlight the advantages of new electron microscopy technique to probe the strong impact of the local structural order on the mechanical property of kerogen and potentially other organic materials.<br/>Reference<br/>1 Li, C.<i> et al.</i> Nanomechanical characterization of organic matter in the Bakken formation by microscopy-based method. <i>Marine and Petroleum Geology</i> <b>96</b>, 128-138 (2018).<br/>2 Yang, J., Hatcherian, J., Hackley, P. C. & Pomerantz, A. E. Nanoscale geochemical and geomechanical characterization of organic matter in shale. <i>Nature communications</i> <b>8</b>, 1-9 (2017).<br/>3 Wang, X., Huang, X., Gao, M. & Zhao, Y.-P. Mechanical response of kerogen at high strain rates. <i>International Journal of Impact Engineering</i> <b>155</b>, 103905 (2021).<br/>4 Bousige, C.<i> et al.</i> Realistic molecular model of kerogen’s nanostructure. <i>Nature materials</i> <b>15</b>, 576-582 (2016).