The impact of severe biodegradation on the molecular and stable (C,H, N,S) isotopic compositions of oils in the Alberta Basin,Canada |
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| Institution: | 1. Petroleum Reservoir Group, Department of Geoscience, University of Calgary, Canada;2. Applied Geochemistry Group, Department of Geoscience, University of Calgary, Canada;1. Key Lab of Petroleum Geochemistry, CNPC, Beijing 100083, China;2. State Key Lab for Enhancing Oil Recovery, Research Institute of Petroleum Exploration & Development, PetroChina, Beijing 100083, China;3. School of Energy Resource, China University of Geosciences, Beijing 100083, China;4. Department of Geoscience, University of Calgary, 2500 University Drive NW, Calgary, AB T2N 1N4, Canada;1. State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China;2. University of Chinese Academy of Sciences, Beijing 100049, China;1. School of Energy Resource, China University of Geosciences, Beijing 100083, China;2. Department of Geoscience, University of Calgary, 2500 University Drive NW, Calgary, AB T2N 1N4, Canada;3. Research Institute of Exploration and Development, PetroChina Liaohe Oil Company, Panjin, Liaoning, China;1. Key Lab of Petroleum Resources Research, Institute of Geology and Geophysics, CAS, Beijing 100029, China;2. Key Lab of Exploration Technologies for Oil and Gas Resources of Ministry of Education, Yangtze University, Wuhan 430100, China;3. Institute of Earth Sciences, Hebrew University, Jerusalem 91904, Israel;4. Liverpool University, Department of Earth, Ocean and Ecological Sciences, Liverpool, Merseyside L69 3GP, UK;1. Organic Paleobiogeochemistry, Max Planck Institute for Biogeochemistry, Jena, Germany;2. MARUM - Center for Marine Environmental Sciences, University of Bremen, Bremen, Germany;3. Institute of Geology and Palaeontology - Applied Geology, University of Münster, Münster, Germany;1. State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing, 102249, China;2. Greater Nile Petroleum Operating Company (GNPOC), Khartoum, Sudan |
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| Abstract: | Although the effects of biodegradation on the composition and physical properties of crude oil have been well studied, effects of in-reservoir petroleum biodegradation on molecular and isotopic compositions of crude oils are not yet clearly understood. The Alberta Basin, in western Canada, is one of the world’s largest petroleum accumulations and constitutes an ideal example of a natural suite of sequentially biodegraded oils. The basin hosts moderately to severely biodegraded petroleum, regionally distributed and in single, more or less continuous, oil columns. In this study, a series of oil samples from the Alberta heavy oil and oil sands provinces, with varying degrees of biodegradation, were analyzed to assess the impact of progressive biodegradation on the molecular and C, H, N, and S isotopic compositions of oils. The results of the molecular characterization of the hydrocarbon fraction of the studied oils show that the oils have suffered biodegradation levels from 2 to 10+ (toward the Alberta–Saskatchewan border) on the Peters and Moldowan scale of biodegradation (abbreviated PM 2 to PM 10) and from tens to hundreds on the Manco scale. Within single reservoirs, increasing biodegradation was observed from top to bottom of the oil columns at all sites studied. The whole oil stable isotopic compositions of the samples varied in the ranges δ13C = ?31.2‰ to ?29.0‰, δ2H = ?147‰ to ?133‰, δ15N = 0.3–4.7‰ and δ34S = 0.4–6.4‰. The maximum differences between δ values of samples (Δ) within single oil columns were Δ13C = 1.4‰, Δ2H = 7‰, Δ15N = 1.7‰ and Δ34S = 1.0‰. Regional variations in the isotopic compositions of oil samples from different wells (averaged values from top to bottom) were 1.2‰ for δ13C, 12‰ for δ2H, 4.1‰ for δ15N and 5.5‰ for δ34S and hence generally significantly larger variations were seen than variations observed within single oil columns, especially for N and S. It appears that even severe levels of biodegradation do not cause observable systematic variations in carbon, nitrogen or sulfur isotope composition of whole oils. This indicates that sulfur and nitrogen isotopic compositions may be used in very degraded oils as indicators for oil charge from different source rock facies. |
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