Abstract

Coals, as massive concentrations of organic matter, are present in many petroleum prospective basins worldwide. The roles of these organic concentrates and their organic constituents (macerals) in the generation of crude oil in commercial quantities have been subjects of controversy for many years. To assess the contribution of coal and the various types of dispersed organic matter to accumulated hydrocarbons, pyrolysis-gas chromatographic analyses of individual macerals, bulk coals and organic rich rocks have been performed and the results compared. Qualitatively, only alginite produces a molecular range fingerprint that closely resembles crude oil. The fingerprints of exinite pyrolysates contain oil-range hydrocarbons; however, the fingerprints are not similar to a normal oil. The hydrocarbons generated by vitrinite in these experiments do not show distributions resembling those of crude oils. Semiquantitative evaluation of the fingerprints suggests that alginite generates significant quantities of oil-range hydrocarbons and undoubtedly could source crude oil. Exinites appear to generate smaller amounts of hydrocarbons, though not similar in character to normal crudes. If present in high concentrations, exinites may impart a significant overprint on the character of the crude. Quantitatively, vitrinite generates minor quantities of hydrocarbons and must therefore be discounted as a viable oil source. These observations support current geochemical concepts that algae and bacteria probably represent the common basis for all crude oils. The fact that C7–C22 n-paraffins, which are primarily the products of thermal decomposition of algae and bacteria, constitute a significant portion of unaltered crudes may explain the great similarity in the overall composition of crude oils worldwide. The hydrocarbon contribution of other macerals would superimpose specific chemical components on the overall crude oil character.