Molecular Structure Evaluation and Image-Guided Atomistic Representation of Marine Kerogen from Longmaxi Shale

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).