A method of determining a chemical composition of crude oil, the method comprising: in a processor: i) selecting classes of hydrocarbon constituent molecules, the classes comprising a paraffins class, a napthenes class, and an aromatics class; the method being characterised by: ii) selecting, for each class, segment types and a corresponding segment number range for each segment type, wherein each segment type is a molecular structural repeating unit of each class, wherein: a) the segment types for the paraffins class comprise a secondary carbon segment contributing CH 2, a tertiary carbon segment contributing CH, and a quaternary carbon segment contributing C; b) the segment types for the naphthenes class comprise a cyclohexane side ring segment contributing C 4 H 6, a secondary carbon segment contributing CH 2, and a primary carbon segment contributing CH 3; and c) the segment types for the aromatics class comprise an aromatic side ring segment contributing C 4 H 2, a cyclohexane side ring segment contributing C 4 H 6, a secondary carbon segment contributing CH 2, and a primary carbon segment contributing CH 3; iii) identifying a probability distribution function for each segment type of each class and corresponding segment number range, wherein the probability distribution function has scale and shape parameters; iv) regressing characterization data of the crude oil with the identified probability distribution function for each segment type of each class and corresponding segment number range, thereby determining the scale and shape parameters of the probability distribution function of each segment type of each class and determining a weight percent of the paraffins class, the naphthenes class, and the aromatics class; and v) outputting the determined scale and shape parameters of the probability distribution function of the segment types of each class and the determined weight percent of the paraffins class, the naphthenes class, and the aromatics class as the determined chemical composition of the crude oil. i) selecting classes of hydrocarbon constituent molecules, the classes comprising a paraffins class, a napthenes class, and an aromatics class; the method being characterised by: ii) selecting, for each class, segment types and a corresponding segment number range for each segment type, wherein each segment type is a molecular structural repeating unit of each class, wherein: a) the segment types for the paraffins class comprise a secondary carbon segment contributing CH 2, a tertiary carbon segment contributing CH, and a quaternary carbon segment contributing C; b) the segment types for the naphthenes class comprise a cyclohexane side ring segment contributing C 4 H 6, a secondary carbon segment contributing CH 2, and a primary carbon segment contributing CH 3; and c) the segment types for the aromatics class comprise an aromatic side ring segment contributing C 4 H 2, a cyclohexane side ring segment contributing C 4 H 6, a secondary carbon segment contributing CH 2, and a primary carbon segment contributing CH 3; a) the segment types for the paraffins class comprise a secondary carbon segment contributing CH 2, a tertiary carbon segment contributing CH, and a quaternary carbon segment contributing C; b) the segment types for the naphthenes class comprise a cyclohexane side ring segment contributing C 4 H 6, a secondary carbon segment contributing CH 2, and a primary carbon segment contributing CH 3; and c) the segment types for the aromatics class comprise an aromatic side ring segment contributing C 4 H 2, a cyclohexane side ring segment contributing C 4 H 6, a secondary carbon segment contributing CH 2, and a primary carbon segment contributing CH 3; iii) identifying a probability distribution function for each segment type of each class and corresponding segment number range, wherein the probability distribution function has scale and shape parameters; iv) regressing characterization data of the crude oil with the identified probability distribution function for each segment type of each class and corresponding segment number range, thereby determining the scale and shape parameters of the probability distribution function of each segment type of each class and determining a weight percent of the paraffins class, the naphthenes class, and the aromatics class; and v) outputting the determined scale and shape parameters of the probability distribution function of the segment types of each class and the determined weight percent of the paraffins class, the naphthenes class, and the aromatics class as the determined chemical composition of the crude oil.