Quantitative impact of diagenesis on reservoir quality of the Triassic Chang 6 tight oil sandstones,Zhenjing area,Ordos Basin,China |
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| Affiliation: | 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. Petroleum Exploration and Production Research Institute, SINOPEC, Beijing 100083, China;1. School of Energy Resources, China University of Geosciences, Beijing, 100083, PR China;2. Key Laboratory of Shale Gas Exploration and Evaluation, Ministry of Land and Resources, Beijing, 100083, PR China;3. Shanxi Yanchang Petroleum (Group) Corp. Ltd., 100083, Xi''an, 710075, PR China;4. Zhongyuan Oil Field 2nd Oil Factory of Sinopec, Puyang, 457532, PR China;5. Shandong Provincial Geo-mineral Engineering Exploration Institute, Jinan, 250014, PR China;1. Northwest Oilfield Company, SINOPEC, Urumuqi, Xinjiang, 830011, China;2. Petroleum Exploration & Production Research Institute of Northwest Oilfield Company, SINOPEC, Urumuqi, Xinjiang, 830011, China;3. Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China;1. Department of Geosciences, China University of Petroleum, Beijing, 102249, China;2. State Key Laboratory of Petroleum Resource and Prospecting, China University of Petroleum, Beijing, 102249, China;3. Research Institute of Petroleum Exploration Development, Zhongyuan Oilfield Company, Sinopec, Puyang, 457001, China;1. School of Geoscience and Technology, Southwest Petroleum University, Chengdu 610500, China;2. College of Geosciences, China University of Petroleum, Beijing 102249, China;3. Research Institute of Petroleum Exploration &Development, PetroChina Changqing Oilfield Company, Xi''an 710018, China;4. Chengdu North Petroleum Exploration & Development Technology Company Limited, China ZhenHua Oil Co., Ltd, Chengdu 610500, China;1. State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development, Beijing 100083, China;2. College of Geosciences, China University of Petroleum, Qingdao 266580, China;3. Department of Earth and Environmental Sciences, The University of Texas at Arlington, Arlington, TX 76019, USA;4. Petroleum Exploration and Production Research Institute, SINOPEC, Beijing 100083, China;5. College of Geosciences, China University of Petroleum, Beijing 102249, China;6. School of Energy and Resources, China University of Geosciences, Beijing 100083, China;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 |
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| Abstract: | The Upper Triassic Chang 6 sandstone, an important exploration target in the Ordos Basin, is a typical tight oil reservoir. Reservoir quality is a critical factor for tight oil exploration. Based on thin sections, scanning electron microscopy (SEM), X-ray diffraction (XRD), stable isotopes, and fluid inclusions, the diagenetic processes and their impact on the reservoir quality of the Chang 6 sandstones in the Zhenjing area were quantitatively analysed. The initial porosity of the Chang 6 sandstones is 39.2%, as calculated from point counting and grain size analysis. Mechanical and chemical compaction are the dominant processes for the destruction of pore spaces, leading to a porosity reduction of 14.2%–20.2% during progressive burial. The porosity continually decreased from 4.3% to 12.4% due to carbonate cementation, quartz overgrowth and clay mineral precipitation. Diagenetic processes were influenced by grain size, sorting and mineral compositions. Evaluation of petrographic observations indicates that different extents of compaction and calcite cementation are responsible for the formation of high-porosity and low-porosity reservoirs. Secondary porosity formed due to the burial dissolution of feldspar, rock fragments and laumontite in the Chang 6 sandstones. However, in a relatively closed geochemical system, products of dissolution cannot be transported away over a long distance. As a result, they precipitated in nearby pores and pore throats. In addition, quantitative calculations showed that the dissolution and associated precipitation of products of dissolution were nearly balanced. Consequently, the total porosity of the Chang 6 sandstones increased slightly due to burial dissolution, but the permeability decreased significantly because of the occlusion of pore throats by the dissolution-associated precipitation of authigenic minerals. Therefore, the limited increase in net-porosity from dissolution, combined with intense compaction and cementation, account for the low permeability and strong heterogeneity in the Chang 6 sandstones in the Zhenjing area. |
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| Keywords: | Tight oil sandstones Diagenesis Reservoir quality Ordos basin |
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