Muddy shale is a name given to those compacted clay deposits. Deposits become sedimentary rocks through the courses of compaction, dehydration, recrystallization and ion exchange. Being affected by the buried depth, geological age, temperature and ion circumstances etc. during its compaction and dehydration, clay minerals will change their pattern. Usually, shallow formations are younger, liable to swell and disperse. Deeper formations are relatively dense and hard, and is poorly swelling and dispersive. According to the complicated borehole problems which occured during drilling operation, shale is divided into sloughing shale, collapsing shale, mud creation shale, swelling shale and plastic shale. Moreover, gas bearing shale and abnormally high-pressured shale are also the important factors responsible for the complexity of the borehole problems.

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