Abstract: In the southern margin of Junggar Basin，wellbore instability has been perplexing the on-site drilling operation，because of the extremely complex geological conditions. In recent years，oil-based drilling fluid，characterized by its high shale/mudstone inhibition and good thermal stability，has been applied in the southern margin，which yet fails to effectively solve the wellbore instability issue. In this paper，the experimental methods such as X-ray diffraction and scanning electron microscopy are used to analyze the physical and chemical properties of rock samples in the unstable well section in the southern margin，including the bulk rock and clay mineral composition，organic matter characteristics，and microstructure characteristics. By doing so，the causes of wellbore instability of high-temperature organic-matter-rich reservoirs in the southern margin are identified and the wellbore instability mechanism in the case of oil-based drilling fluid is clarified. The unstable formation features high content of clay minerals and organic matter，and extensive development of beddings and micro fractures. Oil-based drilling fluid （with pH > 9） enters the formation，driven by the capillary force，which results in high-temperature alkali corrosion and a large number of dissolution pores. These corroded features lead to an uneven stress distribution in rock and the rock compressive strength degrades. Consequently，wellbore instability occurs and the borehole wall collapses. Given the aforementioned understanding，technical countermeasures are proposed.

Key words: wellbore instability, high-temperature organic matter, oil-based drilling fluid, southern margin of Junggar Basin

摘要： 准噶尔盆地南缘地区因地质情况异常复杂，井壁失稳问题一直困扰现场钻井。近年来，油基钻井液因其强抑制、热稳定性好等优点在南缘地区得到了应用，但仍未有效解决井壁失稳问题。采用X-射线衍射法、扫描电镜实验等手段对南缘失稳井段岩样进行了全岩和黏土矿物组成特征、有机质特征、微观结构特征等理化性能的分析。得出了南缘地区高温有机质储层井壁失稳原因。明确了油基钻井液作用下井壁失稳机理是岩石黏土矿物含量高，有机质含量高，岩石层理、微裂缝发育，油基钻井液（pH值>9）通过毛管力作用进入地层发生高温碱蚀，形成大量溶蚀孔导致岩石应力分布不均，岩石抗压强度下降，井壁失稳垮塌。并针对此提出技术对策。

关键词: 井壁失稳, 高温有机质, 油基钻井液, 准噶尔盆地南缘