Based on the statistical data from a large number of gas pools of the Bohai Bay petroliferous basin, the author discusses the influence of cap rocks on the accumulation and distribution of nat ural gas and the relationship between the thickness of direct cap rocks and the height of the hy-drocarbon column of a gas pool and puts forward the concept of critic cap rock thickness. The difference in natural gas abundance in different areas or in different formations of the same area is mainly the result of the sealing effect of the cap rocks, and the height of the hydrocarbon column of a gas pool is not simply in direct proportion to the thickness of the cap rocks. Among many oil and gas pools in a given area where there are similar reservoir-forming geological conditions, the hydrocarbon column height is only in direct proportion to the minimum thickness, i. e. the critic thickness of the cap rocks, and in most cases the height of the hydrocarbon column in an oil or gas pool is much greater than the critic thickness of the cap rocks.