A suite of 17 oil samples from the Tertiary reservoirs (Palaeocene-Eocene) of the North Sea was subjected to routine geochemical analytical techniques in order to evaluate the degree of biodegradation. The methods included latroscan TLC-FID, gas chromatography of the saturated and aromatic hydrocarbon fractions, gas chromatography-mas spectrometry of the saturated hydrocarbon fraction a nd stable carbon and hydrogen isotope analyses. In addition, API gravities and weight percentages of asphaltene were also determined. Based on these data it was found that oils in the suite were derived from the Jurassic aged Kimmeridge Clay Formation originating from source rock horizons at similar maturity levels. The oils in the suite were taken from reservoirs with present day temperatures of 49-114 degrees C. Differences in the chemical composition of the oils (e.g. reduced relative amounts of saturated hydrocarbons, increased polar amounts and reduced relative concentrations of n-alkanes), were attributed to secondary alteration processes of which the most likely process to occur at temperatures less than 80 degrees C and consistent with the observed chemical characteristics of the oils is biodegradation. The differences in chemical composition are thus attributed to different degrees of biodegradation. The oils were ranked and classified in terms of the degree of biodegradation using a range of restricting parameters based on the gas chromatographic concentrations of different alkane species. Based on these parameters, oils were divided into four classes. These classes are (a) non to slightly biodegraded; (b) mildly biodegraded; (c) moderately biodegraded; and (d) severely biodegraded. in cases where the degree of biodegradation was found to have reached levels where unambiguous identification of alkanes was not possible, unresolved complex mixture (UCM) parameters were utilised to assess the extent of biodegradation. The degree of biodegradation was subsequently compared to the type of reservoir sandstone facies from which oils in this study were taken. It was found that at similar temperatures, oils in the Balder Formation (Upper Late Palaeocene) were biodegraded to a higher degree compared to oils from either The Forties (Lower Late Palaeocene) or Sele (Middle Late Palaeocene) Formation. The higher degree of biodegradation in Balder reservoirs is attributed to relatively greater openness to meteoric water invasion in these reservoirs. (C) 1997 Elsevier Science Ltd. All rights reserved.
