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Dessort, Daniel1, Joelle Hy-Billot1, Francois Montel1
(1) TOTAL, Pau, France

ABSTRACT: Quantitative Approach to Oil Biodegradation in Reservoirs

The biodegradation of oils in petroleum reservoirs is usually estimated either using qualitative scales which describe the sequence of attack of the petroleum molecules or molecular ratios between compounds more or less resistant to the biodegradation.
The modelling of the biodegradation of oils in reservoirs and the alteration of their physical properties (°API, viscosity...) requires the quantification of the biodegraded organic compounds. We present here the work which we carried out on this subject.
The evaluation of C5+ whole oil and sub-fractions transformed by natural biodegradation was calculated from the quantitative analysis of heavy refractory compounds occurring in oils having the same origin and maturity. The results made it possible: - to carry out relationships between the quantity of oil biodegraded and the evolution of the composition and physical properties of oils according to the biodegradation, - to define several families of fluids. The membership of these families depends closely on the composition (eg., type of source rock and maturity of oils before biodegradation, - to predict the evolution of the physical properties of the biodegraded oil if one knows to which family this oil belongs, - to quantity the secondary biogenic gas formed by biodegradation.
Moreover, the quantitative analysis of the heavy biomarkers (i.e. hopanoids) shows that their biodegradation starts as of the first stages oil biodegradation, which is not revealed by the measurement alone of the classical molecular ratios.


AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.