Transcripción del resumen del autor. Liquid oil seeps from organic-rich source rock intervals in the Eocene Armàncies Formation in the walls of the underground Riutort oil-shale mine in the SE Pyrenees. The mine was excavated at the beginning of the last century for oil shale extraction. For this study, oil samples were recovered from fractures in the mine walls, and from pools of water on the mine floor. Some oil is present at the bottom of these pools; oil also floats on the surface of the water in association with emulsions colonized by microbial mats. The oils have undergone variable degrees of biodegradation. The physical and chemical environment in the mine was studied in order to establish the controls on biodegradation processes. The results show that the degree of biodegradation depended on factors including the location of the oil (i.e. floating on the top of the water or from the bottom of a pool), and the addition of fresh seepage oil. The biodegradation observed mainly involved the progressive removal of n-alkanes, isoprenoids and some aromatics. Biodegradation was also assessed in terms of the sulphur content and by quantitative analyses of molecular markers in the aromatic fraction. These approaches indicated that at least 50% of the oil was lost as a result of biodegradation. Isotope studies were also undertaken but isotope signatures did not provide significant data. Microbiological data were consistent with data collected from chemical analyses. Evidence for the presence of hydrocarbon-degrading bacteria were obtained from laboratory studies.

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