The controls on the composition of biodegraded oils in the deep subsurface – Part 3. The impact of microorganism distribution on petroleum geochemical gradients in biodegraded petroleum reservoirs

• Published in: Organic geochemistry 2013-03, Vol.56, p.94-105
• Main Authors: Bennett, B., Adams, J.J., Gray, N.D., Sherry, A., Oldenburg, T.B.P., Huang, H., Larter, S.R., Head, I.M.
• Format: Article
• Language: English
• Subjects: Earth sciences; Earth, ocean, space; Exact sciences and technology; Hydrocarbons; Sedimentary rocks; Stratigraphy
• ISSN: 0146-6380; 1873-5290
• DOI: 10.1016/j.orggeochem.2012.12.011

► We report geochemistry and microbiology through a biodegraded oil reservoir. ► Biodegradation of oil is focussed in a bioreactor zone at the base of the oil column. ► Hydrocarbon biodegradation alters many compound classes at different rates. ► The numbers of organisms coincide with a zone of intense biodegradation. ► Enhanced bacterial numbers in the upper part of the oil water transition zone. A combined geochemical, geological and microbiological analysis of an actively biodegrading 24.5m thick oil column in a Canadian heavy oil reservoir has been carried out. The reservoir properties associated with the cored vertical well are characterised by a 15.75m thick oil column and an 8.75m zone of steadily decreasing oil saturation below the oil column, referred to as the oil–water transition zone (OWTZ), grading down into a thin water leg. The oil column exhibits systematic gradients in oil physical properties and hydrocarbon composition and shows variations in biodegradation level throughout the reservoir consistent with the notion that the biodegradation of oil is focussed in a bioreactor zone at the base of the oil column. Through the oil column, the dead oil viscosity measured at 20°C ranged from 50,000cP (0.05McP) at the top of the oil column to 1.4McP at the oil–OWTZ contact, and continued to increase to 10.5McP within the OWTZ. The saturated and aromatic hydrocarbons are characterised by systematically decreasing bulk fraction and component concentrations down through the oil column. Different compound classes decreased to levels below their detection limit at different depths within the OWTZ, defining a likely bioreactor extent of over 5m in depth with, for example, n-alkanes being reduced to their detection limit concentration at the bottom of the oil column/top of the OWTZ, while branched isoprenoid alkanes were not completely degraded until well into the OWTZ. Core samples from the oil column and the lower part of the OWTZ were estimated to contain ca. 104–105 bacterial cells/g, based on qPCR of bacterial 16S rRNA genes, while samples from a narrow interval in the OWTZ immediately below the oil column contained on the order of 106–107cells/g of sediment. Interestingly, these latter numbers are typical of those observed in active deep subsurface biosphere systems with the notion that microbial activity and abundance in the deep subsurface is elevated at geochemical interfaces. The numbers of organisms are not constant throughout the OWTZ. The high