​Heavy fuel oil (HFO) is primarily used as fuel in marine engines and boilers to generate electricity. Nuclear magnetic resonance (NMR) is a powerful analytical tool for structure elucidation, and in this study, 1H and 13C NMR spectroscopy were used for the structural characterization of two HFO samples. The NMR data were combined with elemental analysis and average molecular weight to quantify average molecular parameters (AMPs), such as the number of paraffinic carbon, naphthenic carbon, aromatic hydrogen, olefinic hydrogen, etc., in the HFO samples. Recent formulas published in the literature were used for calculating various derived AMPs, such as aromaticity factor (fa), C/H ratio, average paraffinic chain length (n̅), naphthenic ring number (RN), aromatic ring number (RA), total ring number (RT), aromatic condensation index (φ), and aromatic condensation degree (Ω). These derived AMPs help in understanding the overall structure of the fuel. A total of 19 functional groups were defined to represent the HFO samples, and their respective concentrations were calculated by formulating balance equations that equate the concentration of the functional groups with the concentration of the AMPs. Heteroatoms, such as sulfur, nitrogen, and oxygen, were also included in the functional groups. Surrogate molecules were finally constructed to represent the average structure of the molecules present in the HFO samples. This surrogate molecule can be used for property estimation of the HFO samples and also serves as a surrogate to represent the molecular structure for use in kinetic studies.​