Oil & Gas Geology ›› 2020, Vol. 41 ›› Issue (1): 164-176.doi: 10.11743/ogg20200115

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Discovery of non-hydrothermal saddle-shaped dolomite in Leikoupo Formation, Western Sichuan Basin and its significance

Guorong Li1(), Zhengzhong Liu2, Zixiao Xie1, Yongmin Duan2, Sai He1, Meizhou Deng2, Yuchen Wang1, Yong Li2, Zhangzhi Wu1   

  1. 1. Chengdu University of Technology, Chengdu, Sichuan 610059, China
    2. Southwest Oil and Gas Branch of SINOPEC, Chengdu, Sichuan 610041, China
  • Received:2019-08-30 Online:2020-02-01 Published:2020-01-19
  • Supported by:
    国家科技重大专项(2017ZX05005-001);国家自然科学基金项目(41272150)

Abstract:

Since saddle-shaped dolomites are usually formed by hydrothermal fluid, multiple scholars have considered the saddle-shaped dolomite in the Leikoupo Formation, Western Sichuan Basin, as hydrothermal origin and ascribed the reservoir therein as a product of hydrothermal process. The study made an in-depth analysis of the petrology and geochemistry of saddle-shaped dolomite in the area, in order to further identify the origin of the dolomite and the genetic mechanism of the dolomite reservoir. Two origin types of saddle-shaped dolomite were identified-the hydrothermal saddle-shaped dolomite and the non-hydrothermal one. The former developed inside tectonic fractures emits bright orange red light or is non-luminant under cathodoluminescence, featuring relatively high content of δ18O and Sr-K-Fe-Mn elements, rich rare-earth elements, and negative-Eu abnormality. It's quite different from the matrix rocks, and may be a product of post-magmatic hydrothermal processes following intermediate to acidic magma activities in depth. However, the latter is a product of the dolomitization of medium-to-deep fluids inside the dolomite reservoir. Its saddle-shaped dolomite plaques in limestone and the saddle-shaped dolomite precipitates in dissolved pores or cavities in dolomite emit rose-bengal light under cathodoluminescence, featuring relatively lower content of δ18O and Sr-K-Fe-Mn elements, as well as deficiency in rare-earth element and Eu abnormality. They are quite similar to the surrounding dolomites, indicating that dolomitization fluids and substance have originated from within the same reservoir. The fact of limited distribution of the saddle-shaped dolomite of hydrothermal origin and wide distribution of the saddle-shaped. dolomite of non-hydrothermal origin in the Leikoupo Formation, indicates that the Leikoupo reservoir is not of hydrothermal origin. The relatively subdued activities of formation and hydrothermal fluids in burial have led to good preservation of effective reservoir space.

Key words: non-hydrothermal origin, reserving significance, saddle-shaped dolomite, Leikoupo Formation, Western Sichuan Basin

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