Oil & Gas Geology ›› 2021, Vol. 42 ›› Issue (6): 1401-1413.doi: 10.11743/ogg20210614

• Petroleum Geology • Previous Articles     Next Articles

Development characteristics and formation mechanism of lacustrine dolomite reservoirs in the Paleogene inter-salt shale sequence, Qianjiang sag, Jianghan Basin

Junjun Shen1(), Guoliang Tao2, Kongquan Chen1,*(), Junjun Li3, Pengwan Wang4, Zhiming Li2, Qigui Jiang2, Jianghui Meng1   

  1. 1. Hubei Cooperative Innovation Center of Unconventional Oil and Gas, Yangtze University, Wuhan, Hubei 430100, China
    2. Wuxi Research Institute of Petroleum Geology, Petroleum Exploration and Production Research Institute, SINOPEC, Wuxi, Jiangsu 214126, China
    3. Zhejiang Oilfield Branch Company, PetroChina, Hangzhou, Zhejiang 310013, China
    4. Hangzhou Research Institute of Geology, PetroChina, Hangzhou, Zhejiang 610041, China
  • Received:2021-07-25 Online:2021-12-28 Published:2021-12-16
  • Contact: Kongquan Chen E-mail:shenhema@163.com;30760410@qq.com

Abstract:

This study deals with the 10th cyclotherm of the 4th oil layer group in the lower submember 3rd of the member of Qianjiang Formation (hereafter referred to as Qian 3L(4-10) cyclotherm), an inter-salt shale sequence in the Qianjiang sag, Jianghan Basin, with respect to the development characteristics and formation mechanism of dolomite reservoirs in the cyclotherm. The drilling, core and logging data serve as the research basis, in combination with analytical data from thin section identification, scanning electron microscopy (SEM), high pressure mercury intrusion porosimetry (MIP), X-ray diffraction, rock freezing and Rock-Eval pyrolysis, and gas chromatography-mass spectrometry (GC-MS) of saturated hydrocarbon. The results show that (1) the target layer features complicated mineral compositions of 4 lithic facies, including carbon-rich argillaceous dolomite, carbon-rich argillaceous limestone, carbon-rich dolomitic mudstone and carbon-rich dolomitic mudstone filled with glauberite. Among others, the carbon-rich argillaceous dolomite is considered the best, owing to its high abundance of organic matters in a range of 3.3% to 6.3%, high level of brittle minerals of 50% to 76%, low level of clay minerals of 18% to 33%, and high porosity of 10.8% to 26.3%; (2) The reservoir space of the dolomite reservoirs is dominated by intercrystalline pores, together with few dissolved pores and fractures, and these pores and fractures are characterized by poor structure and poor connectivity; (3) The macroscopic distribution of argillaceous dolomite reservoirs are significantly determined by sedimentary setting. For instance, the carbon-rich argillaceous dolomite mainly occurs in areas of moderate salinity during the middle phase of eustatic lake level descending. Microscopically, its pore development is markedly associated to compaction, while slightly to cementation and dissolution; (4) The weak compaction during the early diagenesis, the penecontemporaneous dolomitization and the hydrocarbon charging to reservoirs, all play a positive role in the preservation of intercrystalline pores, and are key to reservoir formation in the study area.

Key words: reservoir development characteristics, reservoir genesis, inter-salt shale sequence, dolomite, Qianjiang Formation, Qianjiang sag, Jianghan Basin

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