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Characteristics and Origin of Tight Oil Accumulations in the Upper Triassic Yanchang Formation of the Ordos Basin, North-Central China
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This study is granted by the National Program on Key Basic Research Project (973 Program) (grant No. 2014CB239000), State Oil and Gas Major Project (grant No. 2011ZX05001) and CNPC Major Project (grant No. 2016B-0301-04), which are financially supported by the Ministry of Science and Technology of China. We thank the members of our research community for the data and ideas they contributed. We also thank Changqing Oilfield Company for providing data and assistance during our study. Moreover, we thank Ms. Wan Lei and Ms. Ni Yunyan for their work and helpful suggestions about our manuscript. Last but not the least, we thank the AGS editors and reviewers for their time and help.

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    Abstract:

    The Upper Triassic oil accumulations in the Ordos Basin is the most successful tight oil play in China, with average porosity values of less than 10% and permeability values below 1.0 mD. This study investigated the geological characteristics and origin of the tight oil accumulations in the Chang 6 member of the Upper Triassic Yanchang Formation in the Shanbei area based on over 50, 000 petrological, source-rock analysis, well logging and production data. The tight oil accumulation of the Chang 6 member is distributed continuously in the basin slope and the centre of the basin. The oil-water relationships are complex. Laumontite dissolution pores are the most important storage spaces, constituting 30%–60% of total porosity and showing a strong positive relationship with oil production. The pore-throat diameter is less than 1 μm, and the calculated critical height of the oil column is much larger than the tight sand thickness, suggesting that the buoyancy was probably of limited importance for oil migration. The pressure difference between the source rocks and sandstone reservoirs is inferred to have provided driving force for hydrocarbon migration. Two factors of source-reservoir configuration and laumontite dissolution contributed to the formation of the Chang 6 tight oil accumulations. Intense hydrocarbon generation and continuous sand bodies close to the hydrocarbon kitchen are the foundation for the large-scale oil distribution. Dissolution of feldspar-laumontite during the process of organic matter evolution generated abundant secondary pores and improved the reservoir quality.

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WU Songtao, ZOU Caineng, ZHU Rukai, YAO Jingli, TAO Shizhen, YANG Zhi, HAI Xiufen, CUI Jingwei, LIN Senhu.2016. Characteristics and Origin of Tight Oil Accumulations in the Upper Triassic Yanchang Formation of the Ordos Basin, North-Central China[J]. Acta Geologica Sinica(),90(5):1821-1837

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History
  • Received:March 05,2015
  • Revised:June 23,2015
  • Online: October 20,2016
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