Print this pageAssessing In-Reservoir Microbial Degradation of Light Hydrocarbons Using
Stable
Carbon and Hydrogen Isotopic Signatures
Andrea Vieth and Heinz Wilkes
GeoForschungsZentrum Potsdam, D-14473 Potsdam, Germany
Biodegradation of light hydrocarbons in petroleum reservoirs leads to a
significant change in crude oil composition and highly influences its economic
value. To evaluate these microbial processes, stable isotopic signatures of
light hydrocarbons may be applied. The affect of biodegradation on the carbon
and hydrogen isotopic composition of light hydrocarbons has only been
investigated recently, showing that anaerobic biodegradation of alkylbenzenes
(BTEX) is associated with an enrichment of the heavier isotopes in the residual
compound fraction. Hydrogen isotope fractionation tends to be at least one order
of magnitude higher than carbon isotope fractionation, indicating higher
sensitivity in the assessment of microbial degradation processes. The change in
the isotopic composition can be used to quantify biodegradation by applying the
Rayleigh-equation. The Rayleigh-equation correlates the decrease in
concentration and the enrichment in the isotopic composition by an isotope
fractionation factor. This factor depends on the degradation mechanism and the
substrate molecule and can be derived from degradation experiments. Thus, the
use of carbon and hydrogen isotope ratios of hydrocarbons and other organic
compounds has become a sensitive tool in identification and quantification of
biodegradation processes in sedimentary systems. This will be demonstrated using
a sequence of initially to moderately biodegraded crude oils from the Gullfaks
oil field which was analysed for stable carbon and hydrogen isotopic composition
of the light hydrocarbons. This presentation will compare results from
laboratory degradation experiments to isotopic analysis of crude oil samples.
Quantification of microbial degradation in reservoirs will be possible by this
stable isotope
approach.
AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005