Liaohe Oilfield Crude Oil Biodegradation And Controlling Factors

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. A single source and similar maturity make the study area an ideal site to investigate the constraint of biodegradation. 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. Systematic changes seen as marked gradients in petroleum bulk composition, component concentrations and molecular indicators have been observed in biodegraded oil columns.In the aliphatic hydrocarbon fraction n-alkanes are the most susceptible to bacterial attack, isoprenoid distributions are distorted at moderate biodegradation levels (level 3 and above on Peters and Moldowan (1993) biodegradation scale). Hopanes are slightly more resistant to biodegradation and their concentrations increase at early stages of biodegradation (PM level 1-3). 25-norhopanes only occur in severely biodegraded situations (PM level > 5) and their concentrations increase with the degree of biodegradation. Concentrations of steranes also increase during early stages of biodegradation and they start to decrease at level 4. Biodegradation affects the different classes of aromatic hydrocarbons in different ways, while their isomer variation within a compound series occur slightly late (usually > PM level 4). Aromatic steroid hydrocarbons are highly resistant to biodegradation and their concentration increase gradually with an abrupt increase after PM 4 level biodegradation and control the composition of aromatic compound. Reservoir temperature is the primary control on the degree of biodegradation but there is no simple liner relationship between them. The water leg thickness and oil water contact situation are critical to degree of biodegradation. Biodegradation degree increases with increasing water leg thickness most likely due to mineral dissolved by water determining the nutrient supply to the microorganisms. Biodegradation is a dynamic process, where the diffusion of hydrocarbon to oil water contact (OWC), the diffusion of metabolites away from the reaction site to the oil column. Late charging and mixing occurred in the reservoir also control on the degree of biodegradation. This biodegradation model based on geological and geochemical factors opens the possibility of model-driven prediction of oil properties and sweetspots in reservoirs.