Controls on reservoir heterogeneity of tight sand oil reservoirs in Upper Triassic Yanchang Formation in Longdong Area,southwest Ordos Basin,China: Implications for reservoir quality prediction and oil accumulation |
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| Institution: | 1. State Key Laboratory of Petroleum Resource & Prospecting, College of Geosciences, China University of Petroleum, Beijing 102249, China;2. National Engineering Laboratory for Exploration and Development of Low-permeability Oil & Gas Field, Xi''an 710000, China;3. Research Institute of Exploration and Development, Changqing Oil Field, Xi''an 710000, China;4. Geophysical Research Institute, Bureau of Geophysical Prospecting, China National Petroleum Corporation, Zhuozhou, Hebei 072751, China;1. State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing 102249, China;2. College of Geosciences, China University of Petroleum-Beijing, Beijing 102249, China;3. Research Institute of Petroleum Exploration and Development, Tarim Oilfield Company, CNPC, Korla, 841000 Xinjiang, China;4. Department of Petroleum Engineering, University of Wyoming, Laramie, WY 82071, USA;1. State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing 102249, China;2. Department of Earth Sciences, Durham University, South Road, Durham DH1 3LE, UK;1. State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing 102249, PR China;2. College of Geosciences, China University of Petroleum, Beijing 102249, PR China;3. Research Institute of Petroleum Exploration and Development, Changqing Oilfield Company, PetroChina, Xi''an, SX 710021, PR China;1. College of Earth Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, China;2. Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266590, China;3. Exploration & Production Research Institute, SINOPEC, Beijing 100083, China |
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| Abstract: | Compared to conventional reservoirs, pore structure and diagenetic alterations of unconventional tight sand oil reservoirs are highly heterogeneous. The Upper Triassic Yanchang Formation is a major tight-oil-bearing formation in the Ordos Basin, providing an opportunity to study the factors that control reservoir heterogeneity and the heterogeneity of oil accumulation in tight oil sandstones.The Chang 8 tight oil sandstone in the study area is comprised of fine-to medium-grained, moderately to well-sorted lithic arkose and feldspathic litharenite. The reservoir quality is extremely heterogeneous due to large heterogeneities in the depositional facies, pore structures and diagenetic alterations. Small throat size is believed to be responsible for the ultra-low permeability in tight oil reservoirs. Most reservoirs with good reservoir quality, larger pore-throat size, lower pore-throat radius ratio and well pore connectivity were deposited in high-energy environments, such as distributary channels and mouth bars. For a given depositional facies, reservoir quality varies with the bedding structures. Massive- or parallel-bedded sandstones are more favorable for the development of porosity and permeability sweet zones for oil charging and accumulation than cross-bedded sandstones.Authigenic chlorite rim cementation and dissolution of unstable detrital grains are two major diagenetic processes that preserve porosity and permeability sweet zones in oil-bearing intervals. Nevertheless, chlorite rims cannot effectively preserve porosity-permeability when the chlorite content is greater than a threshold value of 7%, and compaction played a minor role in porosity destruction in the situation. Intensive cementation of pore-lining chlorites significantly reduces reservoir permeability by obstructing the pore-throats and reducing their connectivity. Stratigraphically, sandstones within 1 m from adjacent sandstone-mudstone contacts are usually tightly cemented (carbonate cement > 10%) with low porosity and permeability (lower than 10% and 0.1 mD, respectively). The carbonate cement most likely originates from external sources, probably derived from the surrounding mudstone. Most late carbonate cements filled the previously dissolved intra-feldspar pores and the residual intergranular pores, and finally formed the tight reservoirs.The petrophysical properties significantly control the fluid flow capability and the oil charging/accumulation capability of the Chang 8 tight sandstones. Oil layers usually have oil saturation greater than 40%. A pore-throat radius of less than 0.4 μm is not effective for producible oil to flow, and the cut off of porosity and permeability for the net pay are 7% and 0.1 mD, respectively. |
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| Keywords: | Sandstone diagenesis Pore structure characterization Reservoir heterogeneity Controlling factors Tight oil sand reservoirs Oil accumulation Ordos Basin |
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