Geochemical compositional gradients in a mixed oil reservoir indicative of ongoing biodegradation

This study reports the results of organic geochemical characterisation of oil extracts from cores through an oil reservoir column within the Grane field, North Sea. The results show small gradients in composition attributed to ongoing biodegradation occurring at the oil–water contact. Biodegradation is affecting the labile constituents that have been replenished by recent charging of this mixed oil and there is no apparent ongoing biodegradation effect upon the resistant compounds associated with earlier oil charge. The use of principal component analysis has allowed compositional changes due to evaporation during storage to be distinguished from the effects of biodegradation, and has highlighted the C19+ n-alkanes/(hopanes+steranes) ratio as a biodegradation sensitive parameter that shows a gradient in composition through the oil column. The concentration of the C19+ n-alkanes though the oil column confirms that the n-alkanes are depleted towards the oil–water contact. This approach has shown that biodegradation related trends, indicative of ongoing biodegradation, can be identified in reservoirs that contain multiple altered oil charges even when affected by subsequent compositional changes during sample storage.