CHARACTERISTICS OF BREAKTHROUGH PRESSURE OF MESOZOIC-PALEOZOIC COVER IN JIANGSU LOWER YANGTZE AREA

Sets of cap formations have generated from Sinian to Tertiary in the Jiangsu Lower Yangtze area,such as-C₁mu,S₁g and K²p.Breakthrough pressure of mudstone caprock is mainly controlled by distribution of micropores and small throats.Statistics have shown that as burial depth increases,breakthrough pressure increases at first and then decreases,indicating the controlling effect of diagenesis on breakthrough pressure.Adsorption and desorption isotherms of microporosity have proved that caprock breakthrough pressure changes due to micropores structures of mudstone during different evolution stages.During the lower stage,mudstone is characterized by pore-type structure and breakthrough pressure increases with depth.During the higher stage,mudstone brittle increases and micro-fissures come into being,resulting in decreases of breakthrough pressure and sealing ability.