RESEARCH OF QUANTITATIVE RELATIONS BETWEEN SEALING ABILITY AND THICKNESS OF CAP ROCK

The ability of caprock sealing free hydrocarbons includes two aspects:one is displacement pressure of the bottom caprock and other is absorption resistance of capillary for water in caprock.The absorption resistance has something to do with lithology,degree of lithogenesis and thickness of caprock.The smaller the grain size of caprock is, the higher the content of clay mineral is, the stronger the degree of lithogensis is and the thicker the caprock is, the stronger the absorption resistance and the sealing ability of caprock are.Based on the mechanism of low velocity seepage of pore fluid in caprock, the absorption resistance is equal to twice as much as ultra pressure value of fluid in the pore of caprock.Calculated results reveal that the absorption resistance of Qingshankou formation in Sanzhao depression of Songliao Basin is approximately twice as much as the displacement pressure of the base for the caprock i.e.The real sealing ability of the caprock is approximately three times as big as the known ability in the past.The research result can be used to explain why Quaternary system caprocks with high porosity, high permeability, and ultra low displacement pressure have sealed a great quantity organic gas in many basins in our country.