Atomistic
representations of kerogen, which appropriately capture
chemical and physical properties, will aid in unconventional gas extraction
and in following maturation transformations. Here, the structure of
an overmature kerogen (from Longmaxi shale) was investigated using
HRTEM, 13C NMR, XPS, and CO2 sorption evaluations.
The fringe property distributions of length, angle, tortuosity, and
stacking were quantified from seven HRTEM micrographs. The fringe
lengths were estimated and were equivalent in size to benzene (31%),
naphthalene (22%), phenanthrene (10%), 2 × 2 rings (22%), and
3 × 3 rings (9%). The stacking was limited with 10% of fringes
being within a stack. However, the fringes were well aligned with
44% (range of 33–54%) of the total fringe length within a 45°
angle in the major direction. The aromaticity (fa
′) was 82.2%, with aliphatic and carbonyl carbons
accounting for 13.9 and 3.9%, respectively. The kerogen micropore
volume was 0.06 mL/g. An image-guided construction strategy, Fringe3D,
and Packmol were used to generate an atomistic structural representation
that was geometry-optimized (C853H601O45N13S4). The structure incorporates the elemental
compositions, carbon forms, heteroatom functionality, and aromatic
molecule alignment, stacking, and curvature to generate a chemically
and physically appropriate macromolecule (the model contains six macromolecules)
within a small unit volume (23.5 × 23.5 × 23.5 Å).
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