Preparation of damage-free kerogen specimen for microscopy: Understanding the damage mechanisms induced by ion milling techniques

Abstract

Focused ion beam (FIB) milling techniques have been used for more than a decade to prepare organic-rich shale samples for electron microscopy. Nano- and micro-scale imaging of organic pores is used to assess pore volumes and hence estimate gas-in-place. Ion milling natural composites is particularly challenging due to drastic extremes in the physical properties of individual phases such as minerals and organics. Damage to the organic-rich phases occurs due to the large differences in thermal conductivities of minerals and organics that could reach an order of magnitude and the resulting buildup of heat in the organics. Furthermore, radiation damage via ion implantation may also alter the structural characteristics of the shale.Herein, the damage associated with preparing electron-transparent kerogen specimen using low kV FIB, cryo-FIB, and cryomicrotome, a new technique for shale kerogen, is quantified using experimental and modeling techniques. The degree of gallium implantation is quantified using scanning transmission electron microscopy (STEM), selected area electron diffraction (SAED), and energy dispersive X-ray spectroscopy (EDX) along with the stopping range of ions in matter (SRIM) model. Interestingly, although the use of cryogenic temperature removes the thermal effect, Ga+ ion implantation is observed in the specimen prepared by cryo-FIB.Furthermore, the magnitude of the thermal damage at a highly localized level was determined using nano-beam electron diffraction (NBED) performed at −196 °C. Time-resolved diffraction patterns indicate that despite the fact kerogen is among the most stable geopolymers, the aromatic components still degraded due to the local temperature increases. On the other hand, the cryomicrotome technique operating at liquid nitrogen temperature is demonstrated as a suitable method to prepare thin kerogen specimen while minimizing structural damage and preserving its native porosity and chemical composition.