Structure and partial ordering of terrestrial kerogen: Insight from high-resolution transmission electron microscopy

Kerogen structure impacts shale gas behavior: adsorption, desorption, and diffusion. The structural partial order (the orientation and stacking extent) varies with source and maturation. The partial order of eight terrestrial kerogen samples were evaluated using high-resolution transmission electron microscopy. The fringe length, angle, and stacking distributions were quantified. For the majority of the vitrinite equivalent reflectance (VR_eq) range (1.1–1.9%), the aromatic structural units were similar in distribution: mainly composed of fringes of size equivalent to naphthalene, 2 × 2, and 3 × 3 rings. When the VR_eq was > 1.7%, there was a shift in the distribution towards the larger molecules and some fringes equivalent to 4 × 4 ring structures were present. A slight orientation was found in the sample having the lowest VR_eq value, with greater alignment in the VR_eq range 1.5–1.9% There was significantly higher alignment for the overmature samples. Stacking was limited with 92 to 99% of fringes being an individual “layer”. There was an increase in the stacking frequency with an increase of VR_eq > 1.8% with some stacks exceeding four layers. Thus, the increase in the structural order for the basic structural units was not uniform with maturation. There was a continuous but slight change in partial order when the reflectance was between 1.1 and 1.7% but a transition when the reflectance was ~1.8–2.0%. The transition in partial ordering from slight to more ordered was consistent with coalification jump transitions for coal maturation at Ro,m = 1.8–2.0%. These transformations are likely to impact the shale gas adsorption and its behaviors.