A dynamic biodegradation model suggested by petroleum compositional gradients within reservoir columns from the Liaohe basin, NE China

Abstract A suite of petroleum reservoir extracts from the Liaohe basin, NE China, was analyzed to investigate the effects and controls of biodegradation on petroleum composition. Systematic changes, seen as marked gradients in petroleum bulk composition, component concentrations and molecular indicators, have been observed in two biodegraded oil columns. In the Es3 column (1700–1850 m), biodegradation has removed substantial amounts of normal and isoprenoid alkanes. More severe biodegradation occurs in the shallower Es1 column (1500–1650 m), with both steranes and hopanes being partially removed. Temperature has an overall control on biodegradation in the two columns, a lower reservoir temperature leading to a higher degree of biodegradation. However, observed variation in the degree of biodegradation in the Es3 column at different sites 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 also are controlled by the relative rate of mixing of fresh oil charge. Gradients can only be conserved in biodegraded petroleum columns if the rate of diffusive mixing is similar to or lower than the rate at which the hydrocarbons are removed. A biodegradation model is proposed that couples geochemical and geological factors to provide a coherent approach for assessing the impact of degradation on petroleum. Our geochemical study indicates that biodegradation occurs mainly within a narrow region at or near the base of the oil column. The stable compositional gradients are produced after an initial induction period involving diffusive transport of alkanes to the oil-water contact (OWC) and diffusion of biodegradation products such as 25-norhopanes and other metabolites away from the reaction site. The mixing of continuously-charged oil with ‘residential’ biodegraded oil by diffusion may be considered as one of the most important factors controlling the biodegradation process. The conceptual model illustrates all of these processes and opens the possibility of model-driven prediction of oil properties and ‘sweetspot’ location in reservoirs.