Tracing microbial hydrocarbon transformation processes in a high temperature petroleum reservoir using signature metabolites

The consequences of microbial activity in petroleum reservoirs, such as deterioration in crude oil quality and technical problems in oil production were recognized a long time ago, while the metabolic capability and hydrocarbon transformation processes of microorganisms are less understood. In this study, petroleum hydrocarbons serving as substrates for microorganisms, as well as the organic acid composition of formation water were investigated for samples originating from an oilfield in Upper Austria with an elevated reservoir temperature around 78 °C. Many alkanoic, aromatic and dicarboxylic acids were found in the formation water, and which may derive from microbial degradation of hydrocarbons. To the best of our knowledge, polyalkylated succinic acids such as 2,2- and 2,3-dimethylsuccinic acid, 2-ethyl-3-methylsuccinic acid and 2,2,3-trimethylsuccinic acid have not been described in formation water before. Considering the degree of substitution, we conclude that many of the alkylated succinic acids did not derive from the direct activation of hydrocarbons via addition to fumarate. It is more likely that they were produced during the course of hydrocarbon transformation and intracellular catabolism of fatty acids, and possibly other compounds, and play a role in protection mechanisms against high temperature and solvent stress. The results provide clear evidence for ongoing biodegradation of hydrocarbons in a petroleum reservoir at elevated temperature close to the generally accepted threshold of 80 °C.