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.