Abstract: Through the evaluation of macroscopic sealing parameters(lithology,mudstone accumulative thickness and single mudstone thickness) and microscopic sealing parameters(permeability and displacement pressure),this paper gives a comprehensive analysis of the sealing capacity of the Tugulu Group in the southern margin of Junggar Basin.The results show that the Hutubi Formation is the main cap and comprises interbedded mudstone,silt mudstone and pelitic siltstone.The shale stratum ratio,average mudstone accumulative thickness and maximum single mudstone thickness are 80%-95%,337 m and 138 m respectively.Permeability and displacement pressure of the Tugulu Group mudstone range from 0.000 001 84×10^-3 μm² to 0.000 24×10^-3 μm² and 4.72 MPa to 44.85 MPa respectively,suggesting type Ⅰ to Ⅱ sealing capacity based on the criteria for cap rock evaluation.By restoring the dynamic evolution of displacement pressure of cap rocks,the Tugulu Group is capable of sealing oil and late gas generated by the Jurassic source rock,because the timing of sealing capacity forming is well in tune with the timing of hydrocarbon generation and expulsion.Uplifting and thrust faults evolved during the Hemalaya Movement did not destroy the sealing capacity of the Tugulu Group.Through these comprehensive analysis,we conclude that cap rocks of the Tugulu Group maintains the capacity of sealing large to middle gas field.

Key words: permeability, displacement pressure, dynamic evolution of cap rock sealing capacity, mudstone cap rock, Tugulu Group, Junggar Basin