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).

Organic sulfur in coal can pollute the atmosphere due to the emission of SO x gas during coal combustion and utilization. Although the relative distributions of the different organosulfur functional groups change with progressive coalification (maturity), how these groups are incorporated in the macromolecule structures of coals and the role they play in the coalification process remain ambiguous. The purpose of this study, then, is to elucidate these relationships by using thermodynamic simulation with a reactive force field program, calculation of the C–S bond dissociation energies of four types of organosulfur functional groups, X-ray photoelectron spectroscopy (XPS) analysis of seven coal samples, and published data to determine the changes in the types and proportions of organosulfur functional groups in three coalification stages (0–1, 1–2, and >2% Ro). The results show that the bond stabilities follow the order Cal–S < S–S < Car–S. Disulfides and thioethers occur in lower contributions in the coalification stage of 0–2% Ro, while mercaptans present in a relatively higher proportion. The contribution of sulfone increases and exceeds a half in organosulfur at 1–2% Ro; the major role of sulfur turns from edge bond (0–1% Ro) to bridge bond (1–2% Ro) gradually. Thiophene decreases in the 0–1% Ro coalification stage and increases in relatively high proportions in the 1–2% Ro coalification stage. When the coalification stage is above 2% Ro, the contributions of sulfides, sulfone, and mercaptan decrease, while that of thiophene increases, and sulfur gradually acts as a thiophenic ring bond. These results demonstrate that the role of organosulfur in the macromolecular skeleton of coal changes with progressive coalification.