Oil & Gas Geology ›› 2020, Vol. 41 ›› Issue (1): 116-131.doi: 10.11743/ogg20200111

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Pore structure and controlling factors of the ultra-deep Middle Permian dolomite reservoirs, northwestern Sichuan Basin

Tianbo Yang1,2,3(), Zhiliang He3,4,*(), Zhenkui Jin5, Juntao Zhang2,3, Shuangjian Li2,3   

  1. 1. College of Energy Resources, China University of Geosciences(Beijing), Beijing 100083, China
    2. Petroleum Exploration and Production Research Institute, SINOPEC, Beijing 100083, China
    3. Laboratory of Structural and Sedimentological Reservoir Geology, Petroleum Exploration and Production Research Institute, SINOPEC, Beijing 100083, China
    4. China Petroleum & Chemical Corporation, Beijing 100728, China
    5. College of Geosciences, China University of Petroleum(Beijing), Beijing 102249, China
  • Received:2019-07-29 Online:2020-02-01 Published:2020-01-19
  • Contact: Zhiliang He E-mail:3006180012@cugb.edu.com;hezl.syky@sinopec.com
  • Supported by:
    国家科技重大专项(2017ZX05005);国家自然科学基金重点资助项目(U1663209);中国科学院A类战略性先导项目(XDA14010201)

Abstract:

The Middle Permian dolomite reservoirs in the piedmont of Longmen Mountain, northwestern Sichuan Basin, were deeply buried and experienced long-term diagenetic evolution during burial, but abundant reservoir space is still well preserved at present, and even porous dolomite gas reservoirs have been discovered in such ultra-deep reservoirs. However, these reservoirs feature high heterogeneity, and the formation and preservation mechanism of reservoir space still need further study. Three types of pore structures are recognized via physical property analysis, mercury injection porosimetry and CT scanning. TypeⅠ is dominated by large pore and coarse throat with large pore-throat coordination number and high connectivity. Type Ⅱ has two sets of pore-throat combinations (larger pore-medium throat and medium pore-medium throat), and shows higher pore throat heterogeneity. Type Ⅲ is characterized by small pore-fine throat combination resulting from strong pressure solution. The difference in pore-throat structures is mainly controlled by rock fabric of precursor limestone and the overprint of various kinds of diagenesis. Type Ⅰ and Ⅱ reservoirs, usually originating from porous grainstones, went through thorough dolomitization, and were well developed in planar, automorphic-hypautomorphic dolomites. Nevertheless, in comparison with Type Ⅱ reservoirs, Type Ⅰ reservoirs are stronger in modification by burial dissolution at middle-late stages but weaker in over-dolomitization. Furthermore, relict dolomite and non-planar, xenomorphic dolomite are most likely to form Type Ⅲ reservoirs due to their weaker anti-pressure-solution capacity and stronger late over-dolomitization.

Key words: pore structure, dolomitization, dissolution, ultra-deep reservoir, Middle Permian, Sichuan Basin

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