Oil & Gas Geology ›› 2023, Vol. 44 ›› Issue (4): 946-961.doi: 10.11743/ogg20230412

• Petroleum Geology • Previous Articles     Next Articles

Origin and sweet spots of typical low-resistivity oil reservoirs of fine-grained sedimentary rocks

Senlin YIN1(), Xu CHEN2, Yi YANG3, Tong ZHANG4, Huanghui CHENG4, Tao JIANG4, Ting XIONG5, Juanxia LIU3, Lipeng HE5, Xiaojiang YANG5   

  1. 1.Institute of Mud Logging Technology and Engineering,Yangtze University,Jingzhou,Hubei 430023,China
    2.School of Geosciences,Yangtze University,Wuhan,Hubei 430100,China
    3.China-France Bohai Geoservices Co. Ltd. ,CNOOC,Tianjin 300452,China
    4.Xinjiang Oilfield Company,PetroChina,Karamay,Xinjiang 834000,China
    5.Shenzhen Branch of CNOOC,Shenzhen,Guangdong 518067,China
  • Received:2023-02-02 Revised:2023-05-20 Online:2023-08-01 Published:2023-08-09

Abstract:

Fine-grained sedimentary rocks, in which low-resistivity oil and gas reservoirs are well developed, have become a significant target in unconventional hydrocarbon exploration. The study serves to reveal the genetic mechanism of typical low-resistivity oil reservoirs of fine-grained sedimentary rocks in applying the data from logging, seismic and thin-section electron microscopy (EM), as well as well facies-controlled reservoir parameter distribution, while proposing three sweet-spot distribution patterns under low resistivity. The research results are shown as follows. First, the origin of low-resistivity reservoirs mainly includes the large specific surface area, high content of mixed layer illites/smectites, well-developed micropore network and high bound water content, high-salinity water and high content of conductive minerals. Second, we propose three typical types of fine-grained sedimentary rocks with low resistivity, including tight low-resistivity oil reservoir with high mud content (i.e., tight reservoir of low resistivity), low-resistivity oil reservoir with high content of conductive minerals (i.e., conducive mineral-dominant reservoir of low resistivity) and water network low-resistivity oil reservoir with well-connected loose sandstones (i.e., well-connected water network conductive reservoir of low resistivity). Meanwhile, their genetic mechanisms are analyzed built on this. Third, the sweet-spot distribution patterns in subtle low-resistivity reservoirs are clarified. The tight reservoir of low resistivity has the sweet spots mainly developed in those of relatively higher resistivity; the conductive mineral-dominant type has the sweet spots mainly grown in the dissolved section of the dolomitic siltstone matrix and the section with well-developed dolomitic mud/shale fractures; meanwhile, the well-connected water network conductive reservoir of low resistivity has the sweet spots mainly developed in the ultra-fine-grained rocks with dual-mode pore structure. The results are of great significance to guiding the exploration of fine-grained sedimentary oil/gas reservoirs.

Key words: hydrocarbon sweet spot, conductive mineral, shale, tight reservoir, fine-grained sedimentary rock, low-resistivity oil reservoir, unconventional oil/gas exploration

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