Abstract:Use temperature, time, compaction, clay mineral composition and other rock influence parameters of diagenesis, constructing diagenesis index; good relationship between displacement pressure and diagenetic index of mudstone, realize the clay cap rock sealing capacity evaluation, especially on capillary pressure seal ability evaluation. Through the Tarim Lunnan Carboniferous mudstone applications, found high linear correlation between the diagenetic index and displacement pressure, which could explain the fitting of the diagenetic stage (diagenesis index) to predict the capillary pressure sealing ability effective, so avoid a lot of pressure mercury test to evaluate the ability of mudstone cap rock capillary seal, should be widely applied.