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<p>The Fengcheng Formation of Junggar Basin has been demonstrated as an essential target interval for fine-grained sedimentary rocks and shale oil exploration-exploitation in ancient alkaline lacustrine basin in China. However, the volcanic activities have influenced alkaline lacustrine environment to a large extent. Therefore, the characteristics and distributions of alkaline-volcanic lithofacies remain unclear. The cores, thin sections, scanning electron microscope (SEM) and wireline logs are employed to conduct research on the type, characteristics, distribution and evolution model of lithofacies in Fengcheng Formation. The results indicate that seven lithofacies are developed, including basalt, tuff aceous conglomerate/sandstone, massive/laminated/alkaline tuffaceous fine-grained sedimentary rocks, and alkaline rocks. Among them, basalt is mainly developed in lower part. The massive/laminated/alkaline tuffaceous fine-grained sedimentary rocks are mainly developed in middle part. The tuff aceous conglomerate/sandstone and alkaline rocks are mainly developed in upper part. The wireline responses of diff erent lithofacies/sub-lithofacies vary significantly in stacking patterns and value scope. However, the lithofacies/sub-lithofacies with similar genesis share similarity in wireline responses. The lithofacies/sub-lithofacies distribution generally show progradation in the northwest and retrogradation in the southwest. The lithofacies/sub-lithofacies scope of fine-grained sedimentary rocks are to a large extent related to tuff aceous conglomerate/sandstone distribution. In addition, the lithofacies/sublithofacies are influenced by paleo-environment (paleo-salinity, paleo-climate, paleo-water depth) changes and volcanic activities. The climate in early depositional period has turned into dry and hot. Subsequently, the scope of lacustrine basin starts to shrink, resulting in gradually-increasing salinity. The lithology during this period is dominated by dolomite and dolomitic rocks. However, the climate in late depositional period remains dry and hot. The lake level continues to fall and the salinity continues to increase. The alkaline minerals such as alkali are precipitated, marking the final formation of alkaline lakes and alkaline rocks..</p>

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The characteristics and distributions of alkaline-volcanic lithofacies of Fengcheng Formation in Mahu Sag, Junggar Basin

  • 期刊论文

  • 发表日期：

    2025年5月27日

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  • Zhi-qi Wu,
  • Fu-sheng Yu,
  • Jian Li,
  • Qiang Liu,
  • Xin Liu,
  • Nai-xiang He

摘要

The Fengcheng Formation of Junggar Basin has been demonstrated as an essential target interval for fine-grained sedimentary rocks and shale oil exploration-exploitation in ancient alkaline lacustrine basin in China. However, the volcanic activities have influenced alkaline lacustrine environment to a large extent. Therefore, the characteristics and distributions of alkaline-volcanic lithofacies remain unclear. The cores, thin sections, scanning electron microscope (SEM) and wireline logs are employed to conduct research on the type, characteristics, distribution and evolution model of lithofacies in Fengcheng Formation. The results indicate that seven lithofacies are developed, including basalt, tuff aceous conglomerate/sandstone, massive/laminated/alkaline tuffaceous fine-grained sedimentary rocks, and alkaline rocks. Among them, basalt is mainly developed in lower part. The massive/laminated/alkaline tuffaceous fine-grained sedimentary rocks are mainly developed in middle part. The tuff aceous conglomerate/sandstone and alkaline rocks are mainly developed in upper part. The wireline responses of diff erent lithofacies/sub-lithofacies vary significantly in stacking patterns and value scope. However, the lithofacies/sub-lithofacies with similar genesis share similarity in wireline responses. The lithofacies/sub-lithofacies distribution generally show progradation in the northwest and retrogradation in the southwest. The lithofacies/sub-lithofacies scope of fine-grained sedimentary rocks are to a large extent related to tuff aceous conglomerate/sandstone distribution. In addition, the lithofacies/sublithofacies are influenced by paleo-environment (paleo-salinity, paleo-climate, paleo-water depth) changes and volcanic activities. The climate in early depositional period has turned into dry and hot. Subsequently, the scope of lacustrine basin starts to shrink, resulting in gradually-increasing salinity. The lithology during this period is dominated by dolomite and dolomitic rocks. However, the climate in late depositional period remains dry and hot. The lake level continues to fall and the salinity continues to increase. The alkaline minerals such as alkali are precipitated, marking the final formation of alkaline lakes and alkaline rocks..