Microfacies and petroleum properties of coals as revealed by a study of North Sea Jurassic coals

Abstract

The influence of the composition and microstructural aspects of coal microfacies on petroleum properties were investigated in light of petrographic and geochemical studies. The first study concerns a collection of North Sea Jurassic coals of various maceral compositions. The second study is a pyrolysis-gas chromatography study on pure microlithotypes from a similar French Jurassic coal. The results show that hydrocarbon potential is logically increased by a high content in liptinite but more often in hydrogen-rich vitrinite. In the same way, the hydrocarbon potential is reduced by a high inertinite content, mainly due to pre-sedimentary oxidation of the original biomass, but also due to early diagenetic processes that lead especially to the formation of micrinite, or to post-sedimentary alterations. These trends indicate that total hydrocarbon potential is increased by an autochthonous origin of the organic matter that was deposited in a relatively closed paleoenvironment and, thus, protected from detrital and organo-detrital inputs. Finally, the oil potential of humic coals, as determined by open pyrolysis-gas chromatographic studies, as related to the weight of rock, is similar to classical source rocks. The structural aspects of the coal microfacies, which probably play a role in the release of oil from coal seams in natural conditions, are also discussed.