Alteration and multi-stage accumulation of oil and gas in the Ordovician of the Tabei Uplift,Tarim Basin,NW China: Implications for genetic origin of the diverse hydrocarbons |
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| Institution: | 1. Research Institute of Petroleum Exploration and Development, PetroChina, Beijing 100083, China;2. Research Institute of Petroleum Exploration and Development, Tarim Oilfield Company, PetroChina, Korla 841000, China;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. Department of Geoscience, University of Calgary, 2500 University Drive NW, Calgary, AB T2N 1N4, Canada;4. School of Energy Resource, China University of Geosciences, Beijing 100083, China;1. The State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Wushan, Guangzhou 510640, China;2. University of Chinese Academy of Sciences, Beijing 100049, China;3. Department of Earth Science, Zhejiang University, Hangzhou 310027, 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. Key Lab of Petroleum Geochemistry, CNPC, Beijing 100083, China;4. State Key Lab for Enhancing Oil Recovery, Research Institute of Petroleum Exploration & Development, PetroChina, Beijing 100083, China;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. Research Institute of Petroleum Exploration and Development, PetroChina, Beijing 100083, China;2. Colorado School of Mines, Golden, CO 80401, USA;3. Tarim Oilfield Research Institute of Exploration and Development, PetroChina, Xinjiang, Korla 841000, China;4. China University of Petroleum, Qingdao, Shandong 266580, China |
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| Abstract: | The Ordovician is the most important exploration target in the Tabei Uplift of the Tarim Basin, which contains a range of petroleum types including solid bitumen, heavy oil, light oil, condensate, wet gas and dry gas. The density of the black oils ranges from 0.81 g/cm3 to 1.01 g/cm3 (20 °C) and gas oil ratio (GOR) ranges from 4 m3/m3 to 9300 m3/m3. Oil-source correlations established that most of the oils were derived from the Mid-Upper Ordovician marine shale and carbonate and that the difference in oil properties is mainly attributed to hydrocarbon alteration and multi-stage accumulation. In the Tabei Uplift, there were three main periods of hydrocarbon accumulation in the late Caledonian stage (ca. 450–430 Ma), late Hercynian stage (ca. 293–255 Ma) and the late Himalayan stage (ca. 12–2 Ma). The oil charging events mainly occurred in the late Caledonian and late Hercynian stage, while gas charging occurred in the late Hercynian stage. During the late Caledonian stage, petroleum charged the reservoirs lying east of the uplift. However, due to a crustal uplifting episode in the early Hercynian (ca. 386–372 Ma), most of the hydrocarbons were transformed by processes such as biodegradation, resulting in residual solid bitumen in the fractures of the reservoirs. During the late Hercynian Stage, a major episode of oil charging into Ordovician reservoirs took place. Subsequent crustal uplift and severe alteration by biodegradation in the west-central Basin resulted in heavy oil formation. Since the late Himalayan stage when rapid subsidence of the crust occurred, the oil residing in reservoirs was exposed to high temperature cracking conditions resulting in the production of gas and charged from the southeast further altering the pre-existing oils in the eastern reservoirs. A suite of representative samples of various crude oils including condensates, lights oils and heavy oils have been collected for detailed analysis to investigate the mechanism of formation. Based on the research it was concluded that the diversity of hydrocarbon physical and chemical properties in the Tabei Uplift was mainly attributable to the processes of biodegradation and gas washing. The understanding of the processes is very helpful to predict the spatial distribution of hydrocarbon in the Tabei Uplift and provides a reference case study for other areas. |
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| Keywords: | Hydrocarbon alteration Multi-stage accumulation Biodegradation Oil density Tarim Basin |
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