Control on cap rock seal capacity by sequence style in middle-upper Yangtze region

The marine strata in the middle-upper Yangtze region can be divided into 3 types:T,H and TH.Different sequence style has different effect on accumulation.Cap rock deposition and evolution are mainly controlled by the H style.As to muddy cap formations,the cap rocks in the 2nd grade transgre-ssion system tracts(TST)of the H style are good in quality.For gypsum cap formations,the cap rocks in good quality mainly locate in the 2nd grade high-stand system tracts(HST)of the H style,which has experienced fast transgression and slow regression.Meanwhile,the mudstones in the 3rd grade TST of the H style are also favorable for petroleum sealing.On the profile of Qifeng tunnel,Luoyixi Town,Guzhang City,Hunan Province,from HST to TST,breakthrough pressure and calcareousness content increase,while specific surface area and sandy content decrease.It illustrates that the seal capacity of muddy cap rocks in transgression system tracts is better than that in high-stand system tracts.