Effects of biodegradation on crude oil compositions and reservoir profiles in the Liaohe basin, NE China

Abstract

Biodegradation of crude oil in reservoirs has major economic consequences. While the effects of biodegradation are empirically well known, the processes involved, the fraction of oil destroyed and the critical factors controlling degradation remain obscure. A suite of oils and reservoir extracts from the Liaohe basin, NE China, was analyzed to investigate the effects and controls of biodegradation on petroleum composition. The aims of this project were to further our understanding of the factors which influence the biodegradation of crude oils, and the processes by which these factors interact and to assess the role played by biodegradation in controlling the composition of a wide range of crude oils. The geological settings of the study area have been addressed in detail, including structural and stratigraphic characteristics, source rock and reservoir characteristics, tectonic evolution and hydrocarbon generation history, formation water composition. A single source and similar maturity make the study area an ideal site to investigate the constraint of biodegradation. Systematic changes seen as marked gradients in petroleum bulk composition, component concentrations and molecular indicators have been observed in biodegraded oil columns. The susceptibilities of several aromatic hydrocarbon, alkylcarbazole and alkylphenol isomers to biodegradation have been discussed for the first time in this thesis. The author also addressed the biodegradation effects on many classical geochemical evaluation procedures such as those involving facies and maturity assessmenat nd on the physical and chemical properties of petroleum. The variation in the degree of biodegradation at different sites of the column is controlled by water leg size, with a higher level of degradation being associated with a thicker water leg. The supply of nutrients from the water leg is thought to have a significant impact upon the degree of biodegradation. In addition to water leg size/nutrient supply, the compositional gradients are also controlled by the relative rate of mixing of fresh and degraded oils. The conceptual biodegradation model proposed couples geochemical and geological factors to provide a coherent approach to ascertain the impact of degradation on petroleums and to help to reliably predict biodegradation risk for an exploration target. Our data suggest that biodegradation occurs within a narrow region near the base of the oil column, probably at the oil water contact (OWC), and that in this reservoir there has been a late charge of oil to the top of the column. The mixing of oils through continuous charging and the diffusion of hydrocarbons towards the OWC, and the diffusion of metabolites of degraded oil away from the reaction site may be considered as the most important factor controlling the biodegradation process. Based on an oil charging-biodegradation model (Larter et al., 2003) mass balance calculation has been carried out and the results indicate at level 5 biodegradation up to 50% of petroleum mass has been depleted, beyond this level of biodegradation loss of oil mass is less significant but structural rearrangements are important.