The minimum trapping pressure ratio of hydrocarbon fluid inclusions can be defined by their isochore single value variations in P-T space projection and physical property of their isochore. Calcite veins

quartz secondary enlarged margins and fluid inclusions in particle cracks in sandstones of Yancheng Formation of YC21-1 structure in Qiongdongnan(southeastern Hainan) basin of the South China Sea indicate in geologic history that four phases of fluid inclusions were developed in that region; the capping bed was once burst during the third mobile phase of thermo-fluid inclusions which would be healed soon after. Therefore

no many free-state natural gas reservoirs could be formed there.