Abstract: A computer method of characterizing chemical composition of crude oil and crude oil blends, includes determining respective segment type and segment number range of selected classes of hydrocarbon constituent molecules based on physical and chemical property data on each class of hydrocarbon constituent molecules and on crude oil physical and chemical property data. The method determines relative ratio of each class of hydrocarbon constituent molecules that forms a chemical composition representative of the subject crude oil, and therefrom characterizes chemical composition of the subject crude oil. The method/system displays to an end-user, the characterized chemical composition of the subject crude oil. Based on the identified distribution functions and the relative ratio of each class of hydrocarbon constituent molecules, the method estimates chemical composition of the crude oil. Estimates of physical and chemical properties of the crude oil are then based on the estimated chemical composition.