Investigation of hydrocarbon biodegradation from a downhole profile in Bohai Bay Basin: Implications for the origin of 25-norhopanes |
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| Institution: | 1. State Key Laboratory of Petroleum Resources and Prospecting, Department of Organic Geochemistry and Environmental Science, College of Geosciences, China University of Petroleum, 18 Fuxue Road, Changping, Beijing 102249, China;2. COGER, King Saud University, Riyadh 11451, Saudi Arabia;3. Department of Chemistry, Oregon State University, Corvallis, OR 97331, USA;4. Institute of Geological Science Research, Shengli Oilfield Company, Dongying 257015, China;1. Universidade Federal do Rio de Janeiro, Instituto de Química, LAGOA-LADETEC, Ilha do Fundão, Rio de Janeiro, RJ 21941-909, Brazil;2. Division of Geochemistry, PETROBRAS Research and Development Center (CENPES), PETROBRAS, Rua Horácio Macedo, 950, Ilha do Fundão, Rio de Janeiro, RJ 21941-915, Brazil;1. School of Energy Resource, China University of Geosciences, Beijing 100083, PR China;2. Department of Geoscience, University of Calgary, 2500 University Drive NW, Calgary, AB T2N 1N4, Canada;3. Key Lab of Petroleum Geochemistry, CNPC, Beijing 100083, PR China;4. State Key Lab for Enhancing Oil Recovery, Research Institute of Petroleum Exploration & Development, PetroChina, Beijing 100083, PR China;5. Wuxi Institute of Petroleum Geology, SINOPEC, 2060 Lihu Road, Wuxi, Jiangsu 213126, PR China;1. Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education, China University of Geosciences, Wuhan 430074, China;2. U.S. Geological Survey, Central Energy Resources Science Center, Denver, CO 80225, USA;3. Tianjin Branch of China National Offshore Oil Company Ltd, Tianjin 300452, China;4. State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Changping, Beijing 102249, China;5. Research Institute of Uranium Geology, Beijing 100029, China;1. Department of Geoscience, University of Calgary, Calgary T2N 1N4, Canada;2. Key Laboratory of Tectonics and Petroleum Resources, China University of Geosciences, Ministry of Education, Wuhan 430074, PR China;3. School of Geosciences, Yangtze University, Wuhan 430100, PR China;4. Wuxi Research Institute of Petroleum Geology, SEPRI, SINOPEC, Wuxi 214000, PR China;1. Instituto de Ciencias de la Tierra, Facultad de Ciencias, Universidad Central de Venezuela, Caracas, Venezuela;2. SINOPEC Key Laboratory of Petroleum Accumulation Mechanisms, Wuxi Institute of Petroleum Geology, SINOPEC, 2060 Lihu Road, Wuxi, Jiangsu Province 213126, China |
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| Abstract: | A suite of reservoir cores (oil sands) from a single well in Bohai Bay Basin, East China, displayed a progressive increase in petroleum biodegradation extent on the basis of bulk composition and 25-norhopane content. This fits with the proposal that subsurface petroleum biodegradation is dominantly an anaerobic process and usually occurs at the oil–water contact. It is likely that sequential microbial degradation of hydrocarbons under anoxic conditions does not occur in a true stepwise fashion, but is controlled by various factors such as concentration and solubility of hydrocarbons and their diffusion rate to the oil/water contact. In fact, 25-norhopanes were formed prior to the complete elimination of the acyclic, and mono- and bicyclic alkanes. An inverse response of the 22S/(22S + 22R) ratio between each extended 17α(H)-hopane and its corresponding 25-norhopane was observed as severe biodegradation occurred, supporting the proposal that the 25-norhopanes originate from demethylation of hopanes. Field observation revealed that biomarkers without extended alkyl side chains, such as oleanane, gammacerane and β-carotane, have significant resistance to biodegradation and can be used as naturally occurring “internal standards” to evaluate variations in other biomarkers. The results suggest that the quantity of 25-norhopanes showed a minor increase as the hopanes decreased significantly, i.e. only partial hopane conversion to the corresponding 25-norhopanes. Alternative degradation pathways for hopanes might occur in reservoirs, in addition to C-25 demethylation. |
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