Abstract: On the basis of the principles of existance of adsorption and condensation on the sur-face of micropores,the authors obtain the adsorption and desorption isotherms of microp-orosity through the measurements by using the ST-03 type specific surface pore diame-ter.By use of their geometry and the research results of adsorbed capacity of microporosi-ty,they decide the microporous geometry in the capping-beds of natural gas and make adistinction between pores and cracks in order to lay foundation for studying the primaryand post-primary migrations of oil and gas.