Spectro-Mechanical Characterizations of Kerogen Heterogeneity and Mechanical Properties of Source Rocks at 6 nm Spatial Resolution

Abstract

In situmeasurements of the chemical compositions and mechanical properties of kerogen help understand the formation, transformation, and utilization of organic matter in oil shale source rocks. However, the optical diffraction limit prevents attainment of nanoscale resolution using conventional spectroscopy and microscopy. Here, we developed peak force infrared (PFIR) microscopy for multimodal characterization of kerogen in organic shales. PFIR microscopy provides correlative infrared imaging, mechanical mapping, and broadband infrared spectroscopy capabilities with 6 nm spatial resolution within the frequency region of 2400−4000 cm −1 . We have observed nanoscale heterogeneity in the chemical composition, aromaticity, and the level of maturity of the kerogens from source rocks obtained from the Eagle Ford shale play in Texas. The level of aromaticity of the kerogen positively correlates with the local mechanical moduli of the surrounding inorganic matrix, offering insights into the effect of kerogen heterogeneity on the nanoscale mechanical properties of the source rock. Our method and investigation advances the understanding toward the origin and transformation of kerogen in geological settings.

Geomechanical characterisation of organic-rich calcareous shale using AFM and nanoindentation

“…This suggests that an upper limit on the resolution of the TAP525 is set at 25 GPa kerogen. The different mean moduli between samples probably reflect chemical differences in OM, supported by observations by Jakob et al (2019) who showed that there is significant chemo-mechanical variation even within a single sample. The effects of OM chemistry on mechanical properties are still an open question (Li et al 2018b, a;Goodarzi 2018), although some preliminary work has been done by Emmanuel et al (2016), who suggest that increasing maturity causes stiffening in organic material.…”

“…Such intra-sample variability therefore precludes inter-sample comparison unless the repeatability of AFM measurements can be demonstrated. The results of Jakob et al (2019) show that mechanical differences between OM in shales that are equally mature from the perspective of RockEval may still yield mechanical differences at the nanoscale that are the result of maturity-related heterogeneity. Therefore, demonstrating the repeatability of AFM results is crucial to interpret their mechanics reliably.…”

“…Various studies of OM in shale cite other studies to validate their results. However, Jakob et al (2019) have shown that intra-sample geochemical composition can vary significantly at the nanoscale. Such intra-sample variability therefore precludes inter-sample comparison unless the repeatability of AFM measurements can be demonstrated.…”

Revealing Phonon Polaritons in Hexagonal Boron Nitride by Multipulse Peak Force Infrared Microscopy

“…This method has been demonstrated on a variety of samples to provide spectroscopic imaging with a spatial resolution as high as 6 nm, as well as complimentary nano-mechanical information. [20,22] In this communication, we show that PFIR microscopy is a competitive imaging method for studying 2D polaritonic materials. PhPs of hexagonal boron nitride (h-BN) were revealed, and their dispersion relations were extracted, similar to studying h-BN by using the imaging capability of s-SNOM.…”

“…PFIR microscopy operates in the peak force tapping mode that can avoid sample damage. This method has been demonstrated on a variety of samples to provide spectroscopic imaging with a spatial resolution as high as 6 nm, as well as complimentary nano‐mechanical information …”