Earth Science Frontiers ›› 2021, Vol. 28 ›› Issue (6): 56-65.DOI: 10.13745/j.esf.sf.2021.1.42

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Geochemical characteristics and distribution patterns of subsurface brines in the Qianjiang Depression, Jianghan Basin

NIU Xinsheng1,2, HUANG Hua3, ZHENG Mianping1,2   

  1. 1. Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China;
    2. MNR Key Laboratory of Saline Lake Resources and Environment, Beijing 100037, China;
    3. Research Institute of Exploration and Development, SINOPEC Jianghan Oilfield Company, Wuhan 430223, China
  • Received:2020-01-01 Revised:2020-06-01 Online:2021-11-25 Published:2021-11-25

Abstract: Subsurface brines of the Qianjiang Formation, Qianjiang Depression, Jianghan Basin are one type of strategic resource besides oil and gas for their economic values. However, chronic flooding in oilfield has certain influence to subsurface brines. In order to further assess this influence and investigate the distribution of trace elements in subsurface brines, brine samples from the 1st-4th brine beds were analyzed to study their geochemical characteristics and distribution patterns. The subsurface brines of the Qianjiang Formation are apparently affected by chronic flooding in oilfield. Although desalination occurred in general, subsurface brines still have high TDS contents, and the influence of desalination is limited. In the first to the fourth brine bed, arranged from top to bottom of brine reservoir, carbonate content and sodium sulfate subtype brine gradually decreased while chloride content gradually increased with increasing depth. Despite the influence of flooding on subsurface brines, the Li, B and Br contents in brines are still high, mostly reaching or exceeding the industrial grade threshold, with the grades highest in the second and third brine beds and decreasing from the fourth to the first. The nNa+/nCl- ratio of subsurface brines are close to 1, exhibiting the characteristics of lixiviation brine found in continental strata. The ρCl-/ρBr-ratio of brines revealed that the local subsurface brines are the mixing of lixiviation and sedimentary brines. The desulfurization coefficient and calcium/magnesium coefficient of brines revealed that the reservoir space sealing capacity is rather poor, and it improves from the first to the third brine bed, then worsen slightly in the fourth. Horizontally, Li, B and Br showed similar distribution characteristics. In the depression, the brine concentration centre showed obvious bottom-to-top migration characteristics. The area enclosed by Zhongshi, Haozikou, Wangchang and Zhouji Districts is the centre of high-concentration brines, whose distribution is obviously controlled by the distribution of brine reservoir.

Key words: Qianjiang Depression, subsurface brines, geochemistry, Li, B, Br, oilfield water

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