Assessing 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