A method to determine the lower pressure limit on fault surface required by vertical seal of fault and its application

Based on the analysis of mechanism of vertical seal of faults and the influencing factors,and according to the theory of mudstone deformation,it is proposed that the plastic deformation limit of mudstone should be the lower limit on fault surface required by vertical seal of faults.Based on predecessors experimental data about mudstone plastic deformation,a method,with the application of the relationship between the plastic deformation ultimate of mudstone and the buried depth,to research on the changes of the lower limit of pressure on fault surface needed by the vertical seal of faults with the buried depth is established.The method is used to the quantitative evaluation of the vertical closure property of faults in Kq1 caprock,and the faults include Fault W1,W2,W3 and W4 in Wangjiatun gas field and Fault S1,S2,S3 and S4 in Sanzhan gas field in Songliao basin.The result shows that Fault W1,W2,W3 and W4 in Kq1 have vertical closure property,which accords with the fact of the existence of abnormal pore fluid pressure in Kq1 and the preservation of gas field at present;Fault S1,S2,S3 and S4 of Sanzhang gas field has also vertical closure property in Kq1,but the closure degree is lower than that in Wangjiatun gas field,and it accords with the fact that reserves in Sanzhan gas field are less than those in Wangjiatun gas field.It is indicated that it is feasible to apply the evaluation standard based on the method mentioned to the quantitative study on vertical closure property of faults.