Capillary leakage of gas from buried structures can be quantified using Darcy migration concepts together with formulations for cap-rock permeability, relative permeability and entry pressures at low gas saturations. Capillary trapping has a static component defined by the entry pressure of the cap rock and a dynamic component defined by Darcy migration velocities and rates of migration into the trap. The importance of the dynamic trapping component increases with burial due to a corresponding reduction in cap-rock permeability. The low gas migration velocities through buried cap rocks make it unlikely that gas escape from these traps will be focused above their tops. Barriers within the cap-rock sequence may increase the gas saturation and increase the lateral width of the leakage zone. This may result in complex leakage routes into shallower structures. Source rocks that supply gas to deeply buried traps will start to become exhausted as the temperatures increase. This reduces the rate of gas supply into the traps. Dynamic traps which have trapped gas columns that exceed the capacity of the cap-rock entry pressures can then no longer support the same column heights, and they are reduced in size. The reduction in column heights may continue below the capacity defined by cap-rock entry pressures because of hysteresis effects. The reduction in seal capacity for deeply buried traps can be estimated if the gas fill history of the trap is known. Traps with a low vertical relief can be shielded from the dynamic seal destruction mechanism by the spill process. During periods of reduced or halted gas generation from the source rocks, dynamic traps will continue to leak for million of years. This delayed leakage may be an important source for the filling of shallower traps with gas long after the source rocks have been buried too deeply for generation to take place.