Distribution of weight,density, and molecular weight in crude oil derived from computerized capillary GC analysis

Chromatographic separation of crude oil components has been performed on a 50 m Chrompack Sil 5 fused silica capillary column, temperature programmed from 10 to 300°C. Approximately 70 peaks eluting before n-nonane (n-C₉) and all normal paraffins to n-C₁₉ were identified by a retention time table. Peak areas were converted to weight % using 1-heptene as internal standard and relative response factors. Components were divided into boiling range groups from n-C₆ to n-C₁₉. Paraffinic, naphthenic, and aromatic distributions were obtained for the boiling point fractions up to n-C₉. Group average densities and molecular weights were calculated from pure component properties up to n-C₉. Knowing the density and molecular weight as functions of boiling range group to n-C₉, these functions can be extrapolated to n-C₁₉, based on data and correlations from more than 20 distillations of paraffinic North Sea oils adn condensates. The densities and molecular weights of the C₁₀+ and the C₂₀+ residues were calculated to satisfy a balance of weight, volume, and number of moles for the whole sample. Measurements on the residues were used to cross-evaluate the GC method against distillation. The difference in weight was estimated to be less than ± 0.5% in absolute terms for the C₁₀+ residue. The corresponding figure for C₂₀+ was ± 2%. Relatively, calculated densities are within ± 1%, molecular weights within ± 2% compared to direct measurements. Calculations were performed by a BASIC program in a GC interfaced computer.